Microbiology of Aerosols E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
An introduction to the microbiology of bioaerosols and their impact on the world in which we live The microbiology of aerosols is an emerging field of research that lies at the interface of a variety of scientific and health-related disciplines. This eye-opening book synthesizes the current knowledge about microorganisms--bacteria, archaea, fungi, viruses--that are aloft in the atmosphere. The book is written collaboratively by an interdisciplinary and international panel of experts and carefully edited to provide a high-level overview of the emerging field of aerobiology. Four sections within Microbiology of Aerosols present the classical and online methods used for sampling and characterizing airborne microorganisms, their emission sources and short- to long-distance dispersal, their influence on atmospheric processes and clouds, and their consequences for human health and agro-ecosystems. Practical considerations are also discussed, including sampling techniques, an overview of the quantification and characterization of bioaerosols, transport of bioaerosols, and a summary of ongoing research opportunities in the field. Comprehensive in scope, the book: * Explores this new field that is applicable to many disparate disciplines * Covers the emission of bioaerosols to their deposit, covering both quantitative and qualitative aspects * Provides insights into social and environmental effects of the presence of bioaerosols in the atmosphere * Details the impact of bioaerosols on human health, animal and plant health, and on physical and chemical atmospheric processes Written by authors internationally recognized for their work on biological aerosols and originating from a variety of scientific fields collaborated on, Microbiology of Aerosols is an excellent resource for researchers and graduate or PhD students interested in atmospheric sciences or microbiology.
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Table of Contents
Cover
Title Page
Copyright
List of Contributors
Preface
Hunting fog
It all happens up there …
Cela se passe là-haut …
Part I: Bioaerosols, Sampling, and Characterization
Chapter 1.1: Main Biological Aerosols, Specificities, Abundance, and Diversity
1.1.1 Introduction
1.1.2 Pollen
1.1.3 Fungi
1.1.4 Bacteria
1.1.5 Archaea
1.1.6 Viruses
References
Chapter 1.2: Sampling Techniques
1.2.1 Introduction
1.2.2 Passive and surface sampling
1.2.3 Filtration
1.2.4 Inertia-based samplers: sedimentation samplers, impactors, cyclones
1.2.5 Impingement
1.2.6 Electrostatic sampling
References
Chapter 1.3: Quantification and Characterization of Bioaerosols (offline techniques)
1.3.1 Cultures and metabolic/phenotypic characterization of microbial isolates
1.3.2 Microscopy and flow cytometry
1.3.3 Nucleic acid-based methods
1.3.4 Chemical and biological tracers
1.3.5 Biological activity-based methods
References
Chapter 1.4: Online Techniques for Quantification and Characterization of Biological Aerosols
1.4.1 Introduction
1.4.2 Single-particle fluorescence spectroscopy
1.4.3 Bioaerosol mass spectrometry
1.4.4 Other real-time bioaerosol detection techniques
Acknowledgments
References
Part II: Sources and Transport of Microbial Aerosols
Chapter 2.1: Bioaerosol Sources
2.1.1 Introduction
2.1.2 Emission mechanisms
2.1.3 Measuring emission fluxes
2.1.4 Impact of aerosol sources on the concentration and diversity of airborne microbial communities in the near-surface atmosphere
2.1.5 Identifying predictors of bioaerosol emission and airborne community composition
2.1.6 Conclusion
References
Chapter 2.2: Short-Scale Transport of Bioaerosols
2.2.1 Introduction
2.2.2 Particle dynamics and deposition processes
2.2.3 Transport processes and dispersal scales
2.2.4 Survival of microorganisms during transport
2.2.5 Modeling tools for the transport of microbial aerosols
2.2.6 Dispersal patterns
2.2.7 Conclusion
References
Chapter 2.3: Global-Scale Atmospheric Dispersion of Microorganisms
2.3.1 Historical context
2.3.2 Mechanisms of dispersion
2.3.3 Microorganisms associated with long-range dispersion
2.3.4 Residence time, transport history, and emission models
2.3.5 Implications for planetary exploration
Acknowledgments
References
Part III: Impacts of Microbial Aerosols on Atmospheric Processes
Chapter 3.1: Impacts of Bioaerosols on Atmospheric Ice Nucleation Processes
3.1.1 Introduction
3.1.2 Measurements of ice-nucleating particles
3.1.3 Findings from laboratory experiments, field collections, and field studies
3.1.4 Atmospheric implications
3.1.5 Conclusion and future needs
References
Chapter 3.2: Impacts on Cloud Chemistry
3.2.1 Introduction
3.2.2 Chemical composition of clouds
3.2.3 Clouds as oxidative reactors
3.2.4 Clouds as spaces of biodegradation
3.2.5 Interactions with cloud oxidants
3.2.6 Clouds as spaces of organic compound functionalization
3.2.7 Conclusion
References
Part IV: Impacts of Bioaerosols on Human Health and the Environment
Chapter 4.1: Health Impacts of Bioaerosol Exposure
4.1.1 Introduction
4.1.2 Hazardous potential of bioaerosols
4.1.3 Infectious diseases associated with bioaerosols
4.1.4 Toxic and hypersensitivity disease-associated bioaerosols
4.1.5 Biological agents used for bioterrorism
4.1.6 Conclusion
References
Chapter 4.2: Impacts of Microbial Aerosols on Natural and Agro-ecosystems: Immigration, Invasions, and their Consequences
4.2.1 Introduction
4.2.2 Colonization of virgin and extreme habitats
4.2.3 Invasion of agriculture
4.2.4 Opportunities for research
References
Index
End User License Agreement
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Guide
Cover
Table of Contents
Preface
Part I: Bioaerosols, Sampling, and Characterization
Begin Reading
List of Illustrations
Chapter 1.2: Sampling Techniques
Figure 1.2.1 Digitel DHA-80 high-volume filter sampler operating from the roof of a building. (Photos: Anna Kunert, Max Planck Institute for Chemistry, Mainz, Germany.)
Figure 1.2.2 Quartz fiber filter in its holder before (A) and after 1 week of aerosol sampling (B) with the Digitel DHA-80 high-volume sampler. (Photos: Anna Kunert, Max Planck Institute for Chemistry, Mainz, Germany).
Figure 1.2.3 Examples of inertia-based samplers used for collecting bioaerosols. A, High-throughput “jet” spore and particle sampler: this sedimentation sampler is used for collecting spores and pollens (Burkard Scientific). (Courtesy of Burkard Scientific.) B, String collector: a passive impactor used for collecting cloudwater droplets (57). (Reproduced with permission from the Royal Society of Chemistry.) C, Rotating arm collector: an active impactor used for collecting pollens and spores. (Courtesy of Jim Deacon, The University of Edinburgh.) D, Seven day volumetric spore trap: an active impactor used for monitoring spores and pollens. (Courtesy of Burkard Scientific.) E, Cloud droplet impactor: an active impactor designed for collecting cloudwater at high wind speeds (58). (Reproduced with the permission of Backhuys Biological Books.) F, Six-stage Andersen sampler: an active cascade impactor with impaction plates consisting of nutritive agar medium (59). (Reproduced with the permission of American Society for Microbiology.) G, Coriolis® μ: a centrifugal liquid impactor. (Courtesy of Bertin Technologies.) H, Reuter Cyclone Sampler (RCS®) and agar strips: a centrifugal impactor on agar medium. (Courtesy of Merck-Millipore.) See text for details on the different categories of samplers.
Figure 1.2.4 Representation of the cut-off diameters of each stage of the cascade Andersen sampler, and corresponding sites of deposition in the human respiratory tract. (Courtesy of Jim Deacon, The University of Edinburgh.)
Figure 1.2.5 Schematic diagrams of (A) the conventional AGI-30 impinger and (B) the BioSampler®, improved for reducing reaerosolization and evaporation (106).
Chapter 1.3: Quantification and Characterization of Bioaerosols (offline techniques)
Figure 1.3.1 Colony-forming units on potato-dextrose-yeast extract agar (PDYA) medium from air sampled with a Casella CEL Airborne Bacteria Sampler (slit sampler). (Photo: T.C.J. Hill, Colorado State University, Colorado, USA.)
Chapter 1.4: Online Techniques for Quantification and Characterization of Biological Aerosols
Figure 1.4.1 General schematic overview of the single-particle fluorescence spectrometer SPFS instrument. (Reproduced from Pan et al. (47) with the permission of Elsevier.)
Figure 1.4.2 Application of the ultraviolet aerodynamic particle sizer (UV-APS) during a field study in remote Amazonia. Size distributions show similarity of results using fluorescent particles from the UV-APS and direct characterization using scanning electron microscopy, elemental analysis, and fungal staining. FBAP, fluorescent biological aerosol particle; OA, organic aerosol ; PBAP, primary biological aerosol particle. (Reproduced from Huffman et al. (83).)
Figure 1.4.3 Application of the wideband integrated bioaerosol sensor (WIBS) data summarizing blimp trip across the USA showing particle category breakdowns. AZ, Arizona; CA, California; E, East; FL, Florida; LA, Los Angeles; NM, New Mexico; S, South; TX, Texas; W, West. (Reproduced from Perring et al. (100) with the permission of John Wiley and Sons.)
Figure 1.4.4 Clusters of individual particles collected in Las Cruces, NM, by the SPFS instrument. (Reproduced from Pinnick et al. (143) with permission from Elsevier.)
Figure 1.4.5 Picture and schematic overview of the BAMS instrument. (Reproduced with permission from Steele et al. (156).)
Chapter 2.1: Bioaerosol Sources
Figure 2.1.1 Outdoor sources of bioaerosols.
Chapter 2.2: Short-Scale Transport of Bioaerosols
Figure 2.2.1 A general scheme for the transport of microbial particles.
Figure 2.2.2 Deposition velocity and particle diameter, after the single-layer surface model of Raupach et al. (14). The case represented here is for a vegetation of height 0.06 m and leaf area index 1. The shaded area shows the range of deposition velocities calculated for a range of wind speeds giving friction velocities between 0.35 and 1.40 m s
–1
. The solid line is the predicted terminal velocity. Diameter ranges for four classes of microbial particles are shown.
Chapter 2.3: Global-Scale Atmospheric Dispersion of Microorganisms
Figure 2.3.1 Global wind circulation patterns. 1, Hadley cell; 2, Ferrel cell; 3, Polar cell. (Figure courtesy of NOAA's National Weather Service, Southern Region Headquarters, Fort Worth, Texas. Figure located at http://www.srh.noaa.gov/jetstream/global/circ.htm and http://www.srh.noaa.gov/jetstream/global/jet.htm)
Figure 2.3.2 Desert dust lofted into the atmosphere over the Sahara by storm activity. (Photo courtesy of NASA's Earth Observatory (http://earthobservatory.nasa.gov/IOTD/view.php?id=84400).)
Figure 2.3.3 Dust devils lifting exposed agricultural soils into the atmosphere southwest of Ritzville in Adams County, WA, USA (27 September 2015). (Photo credit: D.W. Griffin.)
Figure 2.3.4 Scanning electron microscope image of organic detritus collected by membrane filtration during an African dust event that impacted the northern Caribbean in the summer of 2000. Light asymmetric particles like this can serve as atmospheric rafts and prolong the atmospheric suspension of associated microorganisms. (Photo credit: D.W. Griffin.)
Figure 2.3.5 Annual mean near-surface number concentrations of bacteria, fungal spores, and pollen, simulated with a global aerosol model (195). (Reproduced with permission from IOP Publishing.)
Chapter 3.1: Impacts of Bioaerosols on Atmospheric Ice Nucleation Processes
Figure 3.1.1 Measurements of bounding curves for precipitation sampling of ice-nucleating particles (INPs) (adapted from Petters and Wright (7) overlain on aerosol measurements of INPs from DeMott et al. (6)). The agreement of bounding curves with land-sourced particle data on the upper end and sea spray aerosol INPs on the lower end is remarkable and potentially revealing of a specific impact of regional INPs on precipitation. The warm temperature “bubble” of INPs over land is often seen and is undoubtedly of biological origin. Sea spray aerosol INPs are also implicated as biogenic, and yet these are clearly different in their behaviors. This warns against describing all biological INPs as a single type or as always being of superior efficiency at modest supercooling.
Figure 3.1.2 Recently discovered ice-nucleating (IN) entities. (A) IN particles directly isolated from soil beneath this sagebrush shrubland (34); both particles inset in the main micrograph nucleated at –7°C (bar lengths are 10 µm). Sagebrush (arrowed) tissue also ice nucleates at –12°C (34). (B) IN entities potentially contributing to the pool of bio-INPs emitted from oceans, especially so during the decay phase of phytoplankton blooms. (C) IN bacterium
Pseudomonas putida/fluorescens
isolate BF81Fb (19) (reproduced with the permission of the American Society for Microbiology) and
Isaria farinosa
(one of three IN fungi) isolated from the air at this ponderosa pine forest (117). (D) IN fungi isolated from the air above and soil of this native grassland. Shown are an IN
Penicillium
isolate (157), grown from boundary layer air, and
Mortierella alpina
, which can release copious numbers of cell-free IN proteins active between –5°C and –6°C, isolated from the soil (41).
Figure 3.1.3 Use of next-generation sequencing of the 16S rRNA gene to profile the bacteria present in aerosol downwind of corn harvesting in Nebraska (see sampling details in Garcia et al. (9)). Twenty percent of bacteria were potential ice-nucleating (IN) species, but sequencing of
ina
genes, which code for the active protein, revealed that
Pantoea agglomerans
, which accounted for 11% of all bacterial sequences, was the primary IN species.
Chapter 3.2: Impacts on Cloud Chemistry
Figure 3.2.1 Oxidation processes of organic compounds with two carbon atoms in the gas and aqueous phases and exchanges of these chemical compounds between the two phases (“mass transfer”). LMC, large multifunctional compound. (Modified from Ervens et al. (91). Reproduced with the permission of John Wiley and Sons.)
Figure 3.2.2 Main metabolic routes for the biotransformation of formaldehyde.
Figure 3.2.3 Transformation rates of organic compounds present in a real cloud sample collected at the Puy de Dôme (France) station resulting from microbial activity (black) or radical chemistry (white).
List of Tables
Chapter 2.1: Bioaerosol Sources
Table 2.1.1 Microbial taxa that can be used as specific indicators of bioaerosol sources (3, 5–8, 71).
Chapter 2.2: Short-Scale Transport of Bioaerosols
Table 2.2.1 Examples of Eulerian and Lagrangian atmospheric models for the transport of biological particles at different scales.
Chapter 2.3: Global-Scale Atmospheric Dispersion of Microorganisms
Table 2.3.1 Summary of recent global and mesoscale model studies of bioaerosol emission and dispersion.
Table 2.3.2 Tools for characterizing aerosol transport history.
Chapter 3.1: Impacts of Bioaerosols on Atmospheric Ice Nucleation Processes
Table 3.1.1 Direct and indirect measurements of the relative contribution of biogenic ice-nucleating particles (INPs) to ambient aerosols.
Microbiology of Aerosols
Edited by
Anne-Marie DelortPierre Amato
Université Clermont Auvergne, CNRS, Institut de Chimie de Clermont-Ferrand (ICCF), France
This edition first published 2018
© 2018 John Wiley & Sons, Inc.
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Library of Congress Cataloging-in-Publication Data
Names: Delort, Anne-Marie, editor. | Amato, Pierre, editor.
Title: Microbiology of aerosols / edited by Anne-Marie Delort, Pierre Amato.
Description: Hoboken, NJ : John Wiley & Sons, 2017. | Includes bibliographical references and index. |
Identifiers: LCCN 2017023659 (print) | LCCN 2017035569 (ebook) | ISBN 9781119132295 (pdf) | ISBN 9781119132301 (epub) | ISBN 9781119132288 (cloth)
Subjects: LCSH: Air-Microbiology. | Atmospheric aerosols.
Classification: LCC QR101 (ebook) | LCC QR101 .M52 2017 (print) | DDC 579/.175-dc23
LC record available at https://lccn.loc.gov/2017023659
Cover Design: Wiley
Cover Image: Courtesy of Pierre Amato
List of Contributors
P. AmatoUniversité Clermont AuvergneCNRSInstitut de Chimie de Clermont-FerrandClermont-FerrandFrance
P. Blais LecoursCentre de Recherche de l'Institut Universitaire de Cardiologie et de Pneumologie de QuébecPQCanada
E. BriseboisDépartement de Biochimiede Microbiologie et de Bioinformatique—Université LavalInstitut Universitaire de Cardiologie et Pneumologie de QuébecPQCanada
Y. BrunetUMR 1391 ISPACentre INRA Bordeaux-AquitaineVillenave d'OrnonFrance
I. CanetUniversité Clermont AuvergneCNRSInstitut de Chimie de Clermont-FerrandClermont-FerrandFrance
N. ChaumerliacUniversité Clermont AuvergneCNRSLaboratoire de Métérologie Physique/OPGCClermont-FerrandFrance
F. ConenDepartment of Environmental SciencesUniversity of BaselBernoullistrasseBaselSwitzerland
L. DeguillaumeUniversité Clermont AuvergneCNRSLaboratoire de Météorologie Physique/OPGCClermont-FerrandFrance
A.-M. DelortUniversité Clermont AuvergneCNRSInstitut de Chimie de Clermont-FerrandClermont-FerrandFrance
P.J. DeMottDepartment of Atmospheric Science (Atmospheric Chemistry)Colorado State UniversityFort CollinsColoradoUSA
M. DraghiCentre Scientifique et Technique du Bâtiment (CSTB)Division Agents Biologiques et AérocontaminantsChamps-sur-MarneMarne-la-ValléeFrance
C. DuchaineDépartement de Biochimiede Microbiologie et de Bioinformatique—Université LavalInstitut Universitaire de Cardiologie et Pneumologie de QuébecCanada
J. Fröhlich-NowoiskyMax Planck Institute for ChemistryMultiphase Chemistry DepartmentMainzGermany
A. GalèsLBEINRANarbonneFrance
C. Gonzalez-MartinInstituto Universitario de Enfermedades Tropicales y Salud Publica de CanariasUniversidad de La LagunaSan Cristobal de La LagunaTenerifeIslas CanariasSpain
D.W. GriffinU.S. Geological SurveySt. PetersburgFloridaUSA
T.C.J. HillDepartment of Atmospheric Science (Atmospheric Chemistry)Colorado State UniversityFort CollinsColoradoUSA
C. HooseKarlsruhe Institute of TechnologyInstitute of Meteorology and Climate ResearchGebaudeKarlsruheGermany
J.A. HuffmanDepartment of Chemistry and BiochemistryUniversity of DenverDenverColoradoUSA
G. MainelisDepartment of Environmental SciencesSchool of Environmental and Biological SciencesRutgersThe State University of New JerseyNew BrunswickNew JerseyUSA
D. MarsolaisDépartement de Médecine—Université LavalQuébecPQCanada
O. MöhlerKarlsruhe Institute of Technology (KIT)Institute of Meteorology and Climate ResearchKarlsruheGermany
C.E. MorrisPlant Pathology Research UnitINRAMontfavetFranceand Department of Plant Sciences and Plant PathologyMontana State UniversityBozemanMontanaUSA
M.-L. NadalartistCité Internationale des ArtsParisFrance
P. RenardUniversité Clermont AuvergneCNRSInstitut de Chimie de Clermont-FerrandClermont-FerrandFrance
E. RobineCentre Scientifique et Technique du Bâtiment (CSTB)Division Agents Biologiques et AérocontaminantsChamps-sur-MarneMarne-la-ValléeFrance
D.C. SandsDepartment of Plant Sciences and Plant PathologyMontana State UniversityBozemanMontanaUSA
J. SantarpiaWMD Counterterrorism and ResponseSandia National LaboratoriesAlbuquerqueNew MexicoUSA
S.C. SaragonipoetParisFrance
D.J. SmithNASA Ames Research CenterSpace Biosciences Research BranchMoffett FieldCaliforniaUSA
M. ThibaudonRéseau National de Surveillance Aérobiologique (RNSA)BrussieuFrance
M. VaïtilingomUniversité Clermont AuvergneCNRSInstitut de Chimie de Clermont-FerrandClermont-FerrandFrance
V. VinatierUniversité Clermont AuvergneCNRSInstitut de Chimie de Clermont-FerrandClermont-FerrandFrance
N. WéryLBEINRANarbonneFrance
Preface
Despite its proximity, the air we breathe is an environment where the biology and its spatial and temporal variability remains poorly known and understood. Many airborne biological particles, bioaerosols, are transported in the atmosphere: animal and plant fragments, pollens, fungal spores, bacteria, viruses, proteins, etc. They are present among all particle size ranges in the atmosphere, from submicron to hundreds of micrometers, at concentrations of up to billions per cubic meter of air. Among the large variety of bioaerosols, microorganisms in particular are raising interest from the scientific community. Indoor and at short spatial scale, these are mostly regarded as potential allergens and pathogens to humans. Outdoors, they can be transported over long distances up to high altitudes, and their presence is also related to epidemiology and biogeography. In addition, airborne biological particles and microorganisms probably contribute to atmospheric physical and chemical processes, such as cloud formation, precipitation, and the processing of chemical compounds. The microbial cells surviving their travel in the atmosphere are also new incomers to natural and agricultural surface ecosystems, which they will eventually colonize or where they will compete with established communities.
In this volume, current scientific knowledge about the different aspects of the research on microbial aerosols is synthesized; this consists of four parts:
the classical and latest developments of bioaerosol sampling and characterization methods
the emission of bioaerosols and their dispersion on short to large scales
the impacts of bioaerosols on microphysics and chemistry in the high atmosphere and in clouds
the consequences of bioaerosols for human health and the environment.
We warmly acknowledge all the authors, an interdisciplinary and international panel of experts on the multiple facets of this fascinating topic, for their precious contribution and their effort in constructing this book collectively. The result is a high-level and up-to-date piece of work, which, we hope, will serve as a reference book for researchers and Master's degree to PhD students entering the emerging field of aerobiology. Finally, we are happy to thank two French artists for their humanistic bird's-eye-view contributions introducing this book.
Anne-Marie Delort and Pierre Amato, editors
Hunting fog
At an early age, I started watching the rockets taking off from my grandfather's launch site. He didn't work for NASA though. At the age of 60, he had decided to trade his illegal hunting rifle for rockets.
The Pyrenean chamois that he had been chasing is not an easy animal to catch. One has to follow its track along craggy mountain paths. It is frail and light-footed, but also fearful and elusive. It always travels in groups. You can't ever let it see you—or smell you. You should know the territory better than your prey: the prevailing winds, and also the breezes, as well as their trajectories through the rocks.
I used to believe that the one they called the dahu1 was this trophy, this chamois that one absent-minded day ended up getting shot by my grandfather, before being skinned and stretched out on a wooden board to be exposed in the living room.
When he could no longer attend to the fate of dahus, when he could no longer set off on exploration and measure himself against faster animals, my grandfather decided to stay with us.
On his territory.
He tended to our stomachs in a wiser, slower manner, taking care of the harvest that needed so much attention. Fortunately, he had acquired during the war a slower, tenacious type of endurance, carved out over months and maybe years, and he was now able to answer vigilantly to all of the soil's demands.
The love he felt for his land was quickly overpowered by the distrust and hatred toward everything that could harm it: caterpillars, lice, mildew, mushrooms … .
But fighting all these diseases and vermin from the land was nothing compared with the swift and cataclysmic power of the weather. One summer in 1959, after a destructive rain of hail, my grandfather fell seriously ill.
He then joined another war, an insidious one, harder to comprehend. A war against anything that could come from the sky.
I don't mean a war of religion. I mean a war with no prey or ally. A war against the wind, the rain, and the importunate storms. A war for himself, which he justified by claiming it for mankind.
He would shoot rockets toward the sky as others throw bottles into the sea, because he wanted to be heard.
He attacked visible yet unpredictable targets, all potentially guilty by intent.
Alone, one knee to the ground, he would aim precisely for the heart of these masses, clouds, huge pachyderms and would set their evanescent and ephemeral souls free.
It took me a while to understand the enormous explosion that I could hear a few minutes after the launch. Even though I knew that it was all a game, I kept looking up, hoping to finally see the fireworks. But the missile would disappear in a loud and powerful discharge.
One day when I was watching him fire a rocket, the base moved during the launch, propelling it horizontally.
It spun, hit against the front of the building, bounced against a tree, and dug into the leaves before exploding over the lake a few yards away.
It was an astonishing vision when, running toward the lake, I saw the surface covered with silvery particles. The water seemed rough, thicker; as so many bubbles on a boiling pot of soup, fishes were coming up, one by one, revealing their bellies about to burst.
The lake was shiny, frozen, and lifeless. Immaculate.
My grandfather never recovered.
After that afternoon, we built him a chimney, small but firmly fixed, so he could keep on shooting at clouds.
Sitting on his chair near the pipe, fingers on the trigger, he is on the lookout: for the sky, the wind, and the humidity.
Only he knows when to shoot to avoid the hail or dry the storms.
Now he has gone, and the Météo-France2 has taken over; they call to tell us when to light up the fireplace or kill the fire. But Météo-France is far less precise than my grandfather.
Marie-Luce Nadal, Artist and PhD Candidate, SACRe, PSL Research University (Paris Sciences et Lettres)
1
The dahu is a legendary mountain creature.
2
Météo-France is the French national meteorological service.
It all happens up there …
It all happens up there, inside the large floating bellies of the clouds. Looking at them, what we believe we see is their ample motion. We follow their peaceful progression. Or are they disturbing? They scud along. We imagine them to be full of a remarkable accumulation of an uncertain substance. Sometimes we think they must be heavy, even though they may be inflated only with light water vapor, whose fine and dispersed moisture weighs little, and sometimes we sense that they are much weightier, laden with water that has already formed into droplets ready to fall back down on our heads, maturing into precipitations in the form of rain, snow, hailstones, maturing into our fogs and our monsoons.
We watch the passing clouds. Ascending and sliding sideways, they seem to come to life at the point where those two intersecting motions meet. We might notice also some changes in shape as they roll, twist, contract, or stretch out. Even when combined with these other changes, the sideways slipping motion still continues, and it is this vast translatory movement in the sky that we witness most frequently: with our nose in the air, we see them traveling along up there, dressed in their suit of light, bathed in more or less pronounced shades of grey, or sparkling tons of white. That is the sight that we see, and the motion that we follow, because the other movement, that is, the ascending motion, which is how they gradually come into being, is invisible to our eyes. At the start of the journey, there was a light moisture, a vapor, a breath, which managed to rise up, to detach itself from the ground, or to take flight from the foam of a wave. It ascends, the molecules bond together, and the huge floating mass comes into being. It is the source of our dreams, and the focus of our enchanted contemplation. It questions us too: who are they? But really, who are they?
At the end of the journey, up above, the moment finally arrives when the shape that we had been following with our eyes begins to dissipate. Whether by thinning out, by dislocation, or by dissolving, its gargantuan architecture is steadily dismantled, and its bulk, once so heavy, simply melts away. Like invisible balloons falling back down to Earth, the clouds then come to rest on the page where our children are drawing pictures, and are reborn beneath their chubby fingers, which press hard on paper as they delineate these puffy giants' cheeks.
We are able to capture virtually nothing of the clouds. They are but transition and transformation. We can never grasp the definition of their being. Terms belatedly applied to their shapes enable us to improve our way of looking at them. Yet we must also accept that, each time, these names refer to a kind of transient that enjoys only a brief existence—that of the ephemeral lifespan of a form also characterized by its color, however fleeting, and an altitude at which it lingers, however briefly … .
We have always found it difficult to follow clouds, and to conceptualize them. The airplane introduced us to their vertical dimension, whose existence we had not suspected from below, and to the gigantic stature developed by their upward billowing. In this way, we gained a more accurate picture of their vast bulk. We also came to appreciate that they evolve between two or three different states of being, quivering internally with droplets of water that hesitate here and there, either to continue life in liquid form or, on the contrary, to pack tightly together into small prism or needle-shaped crystals, unless the two states briefly coexist.
And so today, we understand that an invisible other is in action in the clouds—that we must now enter into their own being, deep into the grasping of a flux that we can see even less well from below, the flux which drives their inner life, this great traffic of within, these sweeping motions in which minute particles of matter bathe, combine, and recombine, where fragments of our deserts, our meadows, and our oceans can be found, and, who knows, where one day we may be able to identify a few specks of dust from our own skins, some atoms of our own breath, anything that the wind is strong enough to carry aloft, anything which, however tiny it may be down here, then coalesces in the sky into formations of gigantic proportions.
Perched atop conveniently located hills, we probe these bellies, and take samples of this medium in which ceaseless mutations are unfolding. Is it movement, is it both direction and speed of travel that control the current state of being of the clouds, for individuals have left one aggregate behind to go and establish themselves inside another?
Is it the change of components that builds up a new kingdom, as compounds break down? Is it the predominance of specific elements that prevails and reshapes the properties of a given chunk of a cloud? We would like to grasp them more clearly, we need something to match the wanderings of our minds, a chronology of these transformations perhaps, or, for one phase of the process, an indication of the forces that prevail, even temporarily, because chemical constituents obey their own laws, but, now, molecules will be affected if exposed to stronger light, and will also react if subjected to a fading away of radiation. And then the wind gets involved, smashes up the existing masses, divides clusters of dust particles, and binds others together. It would be useful to have some large machinery, some sort of giant MRI scans, that could provide us with sections of their inner state of being. For the moment, the large machinery we have at our disposal is that of measurements. With those curves, diagrams, average values, and sharpened analysis, we are gradually building up our interpretations … .
In view of the specific nature of this constantly changing object, the study of clouds, perhaps more than any other branch of research, requires us to engage in a peculiar mental exercise, and to undergo the experience of an elusiveness perpetually renewed. Yet this feeling of the transient, which troubles our minds, once we accept it, then becomes the spring for our lively questioning; it too is continuously restored. In the quest that drives us, we are conducting an exhilarating experiment.
Today, to be sure, Caspar David Friedrich, with one foot firmly planted on a rock, stands observing the flying clouds, admiring the infinite combinations of these celestial constructions, then, his lungs replete with ethereal air, he strides back down to his laboratory.
Sara Chantal Saragoni, poet, Paris, France
Cela se passe là-haut …
Cela se passe là-haut, dans les grands ventres flottants des nuages. Nous croyons voir d'eux d'amples déplacements, nous suivons leurs circulations tranquilles. Ou bien inquiétantes. Ils passent. Nous nous les figurons tout pleins d'une accumulation phénoménale d'une substance à la vérité incertaine. Tantôt nous les devinons lourds certes, pourtant seulement gonflés de vapeur encore légère, par le peu de poids d'une humidité dispersée, tantôt nous les pressentons bien plus pesants, chargés d'une eau déjà formée en gouttelettes prêtes à redescendre vers nous en devenant nos précipitations de pluie, de neige ou de grêlons, nos brouillards et nos moussons.
Nous observons leurs passages. Ascension, et translation, ils semblent prendre vie au croisement de ces deux mouvements. À quoi peut bien s'ajouter quelque figure d'enroulement, de torsion, de contracture ou d'étirement. Même combiné à ces autres figures, le glissement latéral se poursuit toujours, et c'est à ce mouvement majeur de translation dans le ciel que nous assistons le plus souvent : le nez en l'air, nous les voyons là-haut voyager en habit de lumière, dans ces gris plus ou moins prononcés, et ces blancheurs étincelantes. C'est cela que nous voyons, ce déplacement que nous suivons, car l'autre mouvement, le premier, celui de l'ascension par lequel ils se sont progressivement constitués, nous ne le voyons pas. Au tout début du voyage, il y eut une humidité légère, c'était une vapeur, une haleine, elle a pu monter, s'extraire de la terre ou bien prendre son envol depuis l'écume d'une vague. Elle s'élève, les molécules se rallient, et le grand être flottant se constitue, source de nos rêves, lieu de nos contemplations enchantées. De nos interrogations aussi : qui sont-ils, mais qui sont-ils donc ?
Au bout du voyage, là-haut, arrive pourtant le moment où la forme que l'on suivait des yeux s'efface. Par amenuisement, par dislocation, par dissolution, la formidable architecture se défait, son poids, un temps si lourd, s'évanouit. Invisibles ballons revenant au sol, les nuages viennent alors se poser sur la page où dessinent nos enfants, ils renaissent sous les doigts potelés qui appuient très fort sur la feuille pour cercler d'un trait les joues des géants.
Tout d'eux ou presque nous échappe. Ils ne sont que transition, transformation. Nous ne parvenons jamais à fixer la définition de leur être. Les dénominations si tardives de leurs formes nous aident un peu à progresser dans le regard que nous leur portons. Mais ils faut accepter que ces noms désignent à chaque fois une sorte de « transitoire » qui ne se maintient qu'un temps, celui de la durée éphémère d'une forme que caractérisent également une couleur, mais passagère, une altitude de séjour, mais provisoire … Nous avions des difficultés à les suivre, à les « penser ». L'avion nous avait découvert une dimension verticale que nous ne soupçonnions pas d'en bas, un gigantisme développé dans la hauteur. Par là nous avions accédé à une connaissance un peu plus juste de leurs grands corps. Nous avions compris aussi qu'ils naviguent entre deux ou trois états de vie, tout tremblant intérieurement de gouttelettes d'eau qui hésitent ici et là sur le point de poursuivre leur vie sous forme liquide, ou au contraire de se resserrer en petits cristaux de prismes ou d'aiguilles, à moins que les deux états brièvement ne coexistent.
Et voilà qu'aujourd'hui nous comprenons qu'un autre invisible est en action dans les nuages, qu'il nous faut désormais entrer dans leur être propre, dans un mouvement que d'en bas nous voyons moins encore, et qui est celui qui anime leur vie intérieure, ce grand trafic du dedans, ces amples remuements où baignent et se combinent et se recombinent les infimes matières, où se rencontrent des parcelles de nos déserts de nos prairies ou de nos océans, et qui sait, où nous identifierons peut-être un jour quelques poussières de nos propres peaux, quelques atomes de nos souffles, tout ce que le vent a la force de hisser, tout ce qui, minuscule ici, s'accumule là-haut en formations alors gigantesques.
Perchés sur les dômes de reliefs bien situés, nous auscultons les ventres, nous prélevons des échantillons de ce milieu où se jouent d'inlassables mutations. Est-ce le mouvement, sont-ce les orientations et la vitesse des déplacements qui commandent l'état présent, des individus ayant quitté tel agrégat pour aller s'établir en un autre ?
Est-ce la modification des constituants qui, par dégradation des composés, édifie un nouveau règne ? Est-ce la présence en nombre d'éléments spécifiques qui l'emporte pour remanier les propriétés de telle portion de nuage ?
On voudrait y voir mieux, il nous faudrait on ne sait quoi qui réponde au balancement de nos esprits, une chronologie de ces transformations peut-être, ou encore, pour une phase, l'indication des forces qui prévalent, même temporairement, car les constituants chimiques vivent selon leur loi, mais voilà les molécules aux prises avec une lumière accrue qui agit, ou au contraire soumises à la diminution, tout aussi efficiente, de ce rayonnement, et puis le vent s'en mêle, bousculant les agglomérations existantes, dégrafant tels amas de poussières, en recombinant d'autres, On voudrait de grandes machines, des IRM géants nous donnant les coupes de leur état de vie.
Pour le moment, la grande machine dont nous disposons est celle des chiffres, avec leurs courbes, leurs moyennes, l'analyse qui s'affine, la construction progressive des interprétations…
Par la spécificité de son objet mouvant, l'étude des nuages, plus que d'autres recherches peut-être, commande que nous nous tenions dans un exercice singulier, que nous supportions l'expérience d'un éphémère perpétuellement reconduit. Mais ce sentiment du passager qui bouscule nos esprits, une fois admis, devient le formidable ressort d'une vitalité de questionnement elle aussi continuellement restaurée. Dans la quête qui nous anime, nous faisons cette expérience exaltante.
Aujourd'hui à n'en pas douter, Caspar David Friedrich, un pied sur le roc, regarde un temps les nuages, admire la combinatoire infinie des constructions du ciel, puis, les poumons gonflés d'air, il redescend à grandes enjambées vers son laboratoire.
Sara Chantal Saragoni, poète, Paris, France
Part IBioaerosols, Sampling, and Characterization
Chapter 1.1
Main Biological Aerosols, Specificities, Abundance, and Diversity
P. Amato1, E. Brisebois2, M. Draghi3, C. Duchaine2, J. Fröhlich-Nowoisky4, J.A. Huffman5, G. Mainelis6, E. Robine3 and M. Thibaudon7
1Université Clermont Auvergne, CNRS, Institut de Chimie de Clermont-Ferrand, Clermont-Ferrand, France
2Département de Biochimie, de Microbiologie et de Bioinformatique—Université Laval, Institut Universitaire de Cardiologie et Pneumologie de Québec, PQ, Canada
3Centre Scientifique et Technique du Bâtiment (CSTB), Division Agents Biologiques et Aérocontaminants, Champs-sur-Marne, Marne-la-Vallée, France
4Max Planck Institute for Chemistry, Multiphase Chemistry Department, Mainz, Germany
5Department of Chemistry and Biochemistry, University of Denver, Denver, Colorado, USA
6Department of Environmental Sciences, School of Environmental and Biological Sciences, Rutgers, The State University of New Jersey, New Brunswick, New Jersey, USA
7Réseau National de Surveillance Aérobiologique (RNSA), Brussieu, France
1.1.1 Introduction
Biological aerosols, or bioaerosols, are ubiquitous in the Earth's atmosphere. By definition, the term “aerosol” refers to liquid or solid (or both) particles passively suspended in a gaseous medium. Bioaerosols are often defined broadly as material derived from biological systems, implying that they are composed of organic material (mainly C, H, O, and N), generally as a mixture of proteins, lipids, and sugars. Here, we will further restrict the discussion to primary biological aerosols (PBAs), i.e., biological material directly emitted to the atmosphere as particles from the surface (1, 2). This definition thus excludes so-called secondary particles, which form in the atmosphere through gas-to-particle conversion (e.g., condensation of semi-volatile organic compounds oxidized in the atmosphere). It is estimated that PBAs typically represent 5–50% of the total number of atmospheric particles >0.2 µm in diameter (1, 3) and can constitute an even higher fraction of particulate mass in many environments. The diversity of bioaerosols reflects that of life. Thus, PBAs include a wide variety of objects with differing origins, shapes, and sizes, from a few nanometers to hundreds of microns: plant and animal debris, pollen grains, fragments of biofilm, spores and cells of bacteria and fungi, and viruses, as well as their fragments and excretions.
Whereas the transmission of human pathogens between individuals through breathing, coughing, and sneezing has long been known, recent findings have shown that humans release their own microbial cloud as they harbor diverse microbes in and on their bodies. Approximately 106 human-associated microbes are emitted into the surrounding air every hour by each individual, which can particularly influence air quality in indoor environments (4–6). Outdoor, human activities such as composting facilities and wastewater treatment plants can generate locally high amounts of potentially hazardous biological aerosols (7–10), while plant canopies have been identified as the strongest natural source of PBAs, far exceeding oceans and seas (11–13).
In order to specifically detect, quantify, and eventually recover and characterize bioaerosols within a mixed population of airborne particles, it is important to have knowledge of some of their properties. Bioaerosols share certain features that allow them to be detected and categorized as biological particles, and they also have specificities that allow differentiation. For epidemiological and ecological reasons, the main, and most studied categories of bioaerosols are pollen, fungi, bacteria (and until recently archaea), and viruses. These are briefly described below; relevant references are provided therein for further details.
1.1.2 Pollen
Pollen (from the Greek πάλη (pale): flour or dust) is the male fertilizing element of the flowers of higher plants. It consists of ovoid-shaped grains with a diameter of a few tens of microns, contained in the anther at the end of the flower stamen. The counterpart of the pollen grain in lower plants (algae, mosses, ferns, prothalli) is the male gametophyte. Hence, pollen is not directly related to microbiology, i.e., the study of microscopic organisms, and is outside the scope of this book. Nevertheless, pollen grains are important biological components of the atmosphere and cannot be totally ignored, so they are briefly introduced here. The references provided can be consulted for more detail.
Pollen species that utilize wind for dispersion are referred to as anemophilous: these include all gymnosperm (fir, pine, etc.) and some angiosperm trees (notably oak, beech, birch, hazelnut, chestnut, willow, and poplar), and grasses. For anemophilous species, pollen grains must fall on a female gamete “by chance” to initiate fertilization and so plants must produce and release enormous quantities of pollen, which can be very abundant in the air during the flowering season.
The size of pollen grains is, on average, around 30–40 µm, but this can vary widely by plant species. For example, pollen from Myosotis spp. can be as small as 7 µm, whereas Cucurbitaceae pollen grains can reach diameters of more than 100 µm (14). Many allergens are present in pollen grains of numerous plant species, such as birch, hornbeam, hazel, ash, olive, poplar, cypress, sycamore, alder, grasses, ragweed, plantain, and wall pelitory. Pollen grains are composed of one or several cells enclosed within two concentric layers. The outer wall, exposed to the environment (exine), is a very complex arrangement of sporopollenin, a biopolymer consisting primarily of short-chain dicarboxylic acids, fatty acids, and alkanes (15) that is extremely resistant to mechanical, physical, and chemical assault (15–17). This provides extreme longevity to pollen grains, which can maintain their integrity for thousands of years and be used for geological dating and paleontological investigations of past climates and ecosystems (18–21). The inner layer of the pollen wall (intine) is made of cellulose.
Despite the fact that pollen grains are more or less spherical, their aerodynamic diameter (i.e., the diameter equivalent of a spherical particle with unit density) is typically lower than their geometric diameter. Indeed, some species such as Pinus spp. have developed systems for helping flotation in the air, such as air bladders or aerostats, and their surface is often sculpted with patterns of ridges and pores specific to the species. Hence, pollen size and shape and exine structure (stratification, surface sculptures and granules, number and arrangement of apertures, etc.) are used for identifying species by microscopic observation (22, 23). Their properties of fluorescence and light diffraction can also be used to specifically detect pollen in aerosols and to identify certain species (16, 24–30).
1.1.3 Fungi
Fungi can be unicellular or multicellular eukaryotic microorganisms, i.e., they have a complex intracellular organization with a well-delimited nucleus containing their genetic material and several types of organelles. They probably appeared 1.5 billion years ago. Around 105 species have been described but data acquired from several molecular methods have predicted that as many as 5.1 million fungal species may exist (31).
Fungi are ubiquitous organisms in the environment: in plants, soil, animals, water, indoors, etc. The majority of them are saprophytes living on dead organisms such as decaying plants or animals and on non-living organic substances such as food, paper, and fabrics. Many species are symbionts (endo- and ectomycorrhizae), meaning that they are important primary actors in the cycling of carbon, nitrogen, and other nutrients in the biosphere, and they are thus greatly involved in composting. Other fungi are important pathogens or parasites that obtain nutrients from their living host, such as species of Ustilago and Urocystis (smut), Puccinia (rust) and Erysiphales (mildews) on plants, and Candida and Cryptococcus species, among many others, on humans and animals. Thus the presence of fungi in the air has many epidemiological, agricultural, and ecological consequences, as well as meteorological impacts (see Sections 3 and 4).
Most vegetative forms of fungi are filamentous (hyphae aggregated, forming the mycelium). Their normal development comprises a vegetative phase of growth and nutrition, and, almost simultaneously, a reproductive phase in which spores are formed. Spore release can be part of the sexual or asexual stage of the life cycle. These spores are designed to be dispersed, often through the air, so they have developed resistance to desiccation and other environmental and atmospheric stresses. Their shape and size (typically between 2 and 56 µm in diameter) vary between species. Under favorable water and nutrient conditions, spores deposited on surfaces in indoor environments will germinate, mycelia will develop, and the substrate will be colonized (32–34). This growth usually ends with a massive production of spores, released into the air through intrinsic, or natural, mechanisms and through external events such as human or animal activities (35). As outlined in Elbert et al. (36), certain types of fungal spores are preferably emitted under humid conditions such as actively wet discharged asco-and basidiospores, which are emitted with the help of osmotic pressure or surface tension effects. Active discharge mechanisms also eject various organic molecules and inorganic ions that can be used as tracers of fungal spores in the atmosphere (37, 38). In contrast, dry discharged spores are preferably emitted under dry conditions and the emission is mostly wind-driven.
The global emission rate for fungal spores is estimated to be ~50 Tg a–1 (2, 36). Most airborne fungal spores are within the breathable fraction of aerosols (e.g., <8 µm (39)) and 30–35% of species are estimated to be potentially allergenic for humans (40). In the air at near-ground level, the abundance of fungal spores typically ranges from ~103 to ~106 m–3, with large spatial and temporal variations linked in some cases with meteorological or environmental variables (39–49). Extremely high concentrations of up to ~109 spores m–3 of air exist indoors and near strong sources of aerosols, such as composting facilities, farms, or greenhouses (50, 51), and the lowest concentrations (~103 spores m–3 or less) are observed at high altitudes and in polar areas (43, 45, 52, 53). Around 5% of the number of aerosols in the coarse mode (<10 µm) are fungal spores (54). With an average carbon biomass of ~300 ng m–3, they account for a significant fraction of the organic carbon in coarse aerosols at urban/suburban sites, i.e., ~1–10% of the total organic carbon mass in PM10 aerosols (particulate matter <10 µm), and up to 60% in the PM2–10 aerosols (54). At high altitude and in clouds (~1500 m. a.s.l.), the 102–104 spores m–3 still represent around 1.5% of the organic carbon in aerosols larger than 0.2 µm (43, 49).
Culture-based methods are often used to investigate airborne fungal diversity (40, 42). Indeed spores generally retain relatively high culturability in the atmosphere (often >10% of the total fungal spore number), but only 17% of the known fungal species can be grown in culture (55). The application of DNA-based methods allows a better characterization of airborne fungal species richness, which is estimated to be around 1200 (56). There is a high similarity between the species found commonly outdoors and indoors (57). Frequent species include Ascomycota (Cladosporium spp., Penicillium spp., Aspergillus spp., Botrytis spp. are among the most abundant) and Basidiomycota (Agaricomycetes class, Cryptococcus spp., and Dioszegia spp. most notably) (44, 47, 49–53, 56, 58–66). Except from spores, infested materials can also release various fungal aerosols: hyphal fragments as well as toxic and allergenic particles (mycotoxins and beta-glucans adsorbed on particles of material and of fungi (67, 68) that can be smaller than spores); according to laboratory studies, their number is always higher than the number of intact spores released from contaminated surfaces (69–71). Fungi in the air can be detected through the presence of biomarkers, like ergosterol, arabitol, or mannitol, that enter their composition (37, 72, 73). Volatile organic compounds (VOCs) are also emitted by fungi during growth (74).
Many fungi potentially responsible for health issues in humans, animals, and plants are disseminated by atmospheric means (51, 60). The inhalation of fungal particles (spores, mycelium fragments) or their airborne metabolites (mycotoxins, VOCs) may lead to irritating and nonspecific symptoms in sensitive persons. Allergens can be released from spores under humid conditions, such as during thunderstorms or after cell damage (75–77). Moreover, prominent airborne fungi such as Cladosporium herbarum and Alternaria alternata have been found to release higher amounts of allergens after germination (78). Fungal fragments such as cell walls or cytoplasmic material are easily suspended in the air and inhaled as fine particulate matter (78). Secondary metabolites (e.g., mycotoxins), components of fungal cell walls (e.g., (1–3)-β-D-glucan), and proteases have been reported to induce toxic, immunological, and inflammatory reactions (77).
In particular, fungal growth in indoor environments such as water-damaged homes, schools, children's daycare centers, offices, and hospitals creates severe sanitary problems and a potential human health risk. In northern Europe and North America, it is estimated that between 20% and 40% of buildings are contaminated by indoor molds (74). Flannigan (79) reviewed methods for indoor sampling of airborne fungi. Fungi can secrete various hydrolytic enzymes, so they can colonize almost any damp or wet material, such as carpeting, upholstered furniture, gypsum wallboard, ceiling tiles, wood products, shower walls and curtains, and potted plants (80–82). Although central heating, ventilation, and air-conditioning systems with in-duct filters will remove many airborne spores, fungi can grow on air filters or on insulation lining the interior of air-handling units or air ducts. Long-term exposure to fungal propagules and allergens may cause severe, debilitating disease, and fatal infections, such as asthma, allergic diseases, alveolitis, and invasive pulmonary disease, and have an impact on other chronic pulmonary diseases, for instance chronic obstructive pulmonary disease. Mold allergies account for 25–30% of all allergic asthma cases (83). Between 3% and 10% of adults and children worldwide are affected by fungal allergies, as verified by skin tests (84).
1.1.4 Bacteria
Bacteria are unicellular prokaryotic microorganisms, i.e., their genetic material is not enclosed within a nucleus, and they have no or few organelles. Their shape varies from spherical in coccoid cells (Micrococcus spp., Staphylococcus spp.) to thin or thicker rods (Pseudomonas spp., Bacillus spp.). Cell diameter is typically around 1 µm, but ultrasmall cells <0.1 µm in diameter exist in some species, notably some Sphingomonas spp. and Arthrobacter spp., retrieved from polar ice (85), and Rickettsia spp., intracellular parasites of eukaryotic cells. Giant bacteria also exist, such as filamentous bacteria (Beggiatoa spp.), which can be up to 120 µm wide and several millimeters long; these have notably been found in anoxic deep-sea sediments (86). Some species of bacteria (Bacillus spp., most notably) can form spores intended to resist extreme conditions (temperature, ultraviolet, oxidation, chemical assault), allowing dormant survival for extended periods of time, potentially up to thousands years. These can “germinate” and develop when the conditions become favorable.
Bacteria cells are all composed of a lipid bilayer that surrounds the intracellular space, which contains the genetic material and most of the metabolic machinery. Transport proteins and electron transport systems like the respiratory chain producing biochemical energy, notably, are embedded within the membrane. Surrounding it, the cell wall protects cells against mechanical assault and osmotic variations. The cell wall is composed of peptidoglycan, a sugar polymer of N-acetyl-glucosamine and N-acetyl-muramic acid linked together by peptide bonds constituted notably by d-amino acids, a unique feature in the living world. Two different categories of bacteria have been defined, depending on the structure of their wall, as revealed by their reaction to Gram differential staining. Gram-positive bacteria (e.g., Actinobacteria and Firmicutes phyla) have a thick peptiglycan cell wall and no outer membrane, whereas Gram-negative bacteria (e.g., all Proteobacteria and Bacteroidetes phyla) have a thinner layer of peptidoglycan, surrounded by the outer membrane, a lipid bilayer in contact with the extracellular environment.
The genetic material of bacteria consists of a single circular chromosome, and eventually plasmids that individuals of some species can exchange with others by conjugation. Horizontal gene transfer in bacteria can also be achieved by transformation (integration of exogenous genetic material from the environment) or transduction (acquisition of genetic material through the intervention of a bacteriophage virus). The size of the genome of bacteria ranges from ~110 kbp to ~10 Mbp, which is around 1000 times smaller than the human genome (~3.2 Gbp). Nevertheless, the trophic modes exhibited by bacteria for generating biochemical energy and biomass from their environment are extremely diverse and include all the known modes of functioning: chemotrophs oxidizing inorganic (chemolithotrophs) or organic (chemoorganotrophs) molecules as sources of electrons and energy and phototrophs taking energy from light and oxidizing organic or inorganic substrates as sources of electrons (photoorganotrophs or photolithotrophs, respectively). The source of carbon also defines trophic groups: autotrophy when the source is carbon dioxide (CO2) or heterotrophy when carbon is taken up from organic compounds (as a reference, humans are chemoorganoheterotrophic organisms, plants are photolithoautotrophic organisms). Anaerobic methanogens are chemolithoautotrophs that use CO2 as a source of electrons and hydrogen (H2) as a source of energy, and releasing methane; nitrate reducers (denitrifiers) are chemotrophs that use nitrates as the terminal acceptors of electrons, i.e., they respire nitrates. Because of their versatility, bacteria have colonized all the environmental niches of the planet, including the most extreme: deep oceans, glaciers, hot springs, etc. The total number of bacteria on Earth was estimated to be ~1030 cells, an amount of carbon nearly equivalent to that of plants (87). In the atmosphere at the global scale, the total number of bacteria aloft within the first 3 km of altitude was estimated to be around ~1019 (87). Despite the small fraction of the organic carbon they represent in aerosols (<~0.01% (43)), they have important environmental and epidemiological impacts (88–95) (see Sections 3 and 4).
In nature, bacteria form biofilms on surfaces. Biofilms are composed of exopolysaccharide matrices where cells are embedded; these matrices protect cells against environmental assault and facilitate adhesion and molecular dialog (quorum-sensing) (96, 97). Biofilm formation can be problematic for industry and medicine notably, and biofilms are often responsible for health issues (98, 99). The surface of plants is also covered by biofilms of commensal and phytopathogenic organisms (100–102). The canopy is thus a major source of microbial aerosols outdoors (12). As most airborne bacterial cells are generally found aggregated together, they probably derive from biofilms (103, 104), which likely favor survival (105, 106).
The typical concentration of bacteria in the air near the ground ranges from ~102 to ~106 cells m–3 (e.g., 107–115). As for fungi, there are large spatial variations: the lowest concentrations are found at high altitude and in polar regions (43, 45, 116–118), while the highest concentrations are detected indoors and in areas disturbed by human activities (51, 119–121). There is a very high temporal variability in bacteria number and composition in the air following diurnal and seasonal periodicities: their concentration is in general higher during the warm periods of the year than in winter, and during the day than during the night due to upward fluxes lofting cells from surfaces (11, 39, 46, 47, 58, 108, 113, 122–128). The influence of meteorological factors (wind speed, humidity, or temperature) on bacteria abundance in the air was reported in some studies (e.g., 46, 60, 111, 114, 129), but this seems to be highly dependent on the sampling site. No general relationship with meteorological variables, i.e., applicable anywhere on the planet, has been identified so far.
Aloft for typically 2–10 days (130), bacteria cells can travel over thousands of kilometers (127, 131–136) (see Section 2). Living specimens were recovered from altitudes of several tens of kilometers above ground level (137, 138). This attests to the high resistance of certain species or strains to cold, ultraviolet, and other stresses that can be encountered in the atmosphere (139, 140). Many airborne bacteria outdoors originate from plants and soils (58, 89, 141, 142), where they probably acquired some level of adaptation to atmospheric stresses. However, the vision of the biodiversity differs from one study to another, partly due to differences in methods. So far, the patterns of biodiversity in the airborne communities appear to be very variable and have not been clearly linked to environmental variables. Among the groups frequently identified outdoors, Proteobacteria often dominate (45, 47, 49, 109, 114, 116, 118, 141, 143, 144), notably Pseudomonas
