Estimation of viable airborne microbes downwind from a point source

The aeromicrobiome: the selective and dynamic outer-layer of the Earth's microbiome

The atmosphere is an integral component of the Earth's microbiome. Abundance, viability, and diversity of microorganisms circulating in the air are determined by various factors including environmental physical variables and intrinsic and biological properties of microbes, all ranging over large scales. The aeromicrobiome is thus poorly understood and difficult to predict due to the high heterogeneity of the airborne microorganisms and their properties, spatially and temporally. The atmosphere acts as a highly selective dispersion means on large scales for microbial cells, exposing them to a multitude of physical and chemical atmospheric processes. We provide here a brief critical review of the current knowledge and propose future research directions aiming at improving our comprehension of the atmosphere as a biome.

A tracing method of airborne bacteria transmission across built environments

Disease transmission across built environments has been found to be a serious health risk. Airborne transmission is a vital route of disease infection caused by bacteria and virus. However, tracing methods of airborne bacteria in both lab and field research failed to veritably express the transporting process of microorganism in the air. A new tracing method of airborne bacteria used for airborne transmission was put forward and demonstrated its feasibility by conducting a field evaluation on the basis of genetic modification and bioaerosol technology. A specific gene fragment (pFPV-mCherry fluorescent protein plasmid) was introduced into nonpathogenic E. coli DH5α as tracer bacteria by high-voltage electroporation. Gel electrophoresis and DNA sequencing proved the success of the synthesis. Genetic stability, effect of aerosolization on the survival rate of tracer bacteria, and the application of the tracer bacteria to the airborne bacteria transmission were examined in both lab and field. Both the introduced plasmid stability rates of tracer E. coli in pre-aerosolization and post-aerosolization were above 95% in five test days. Survival rate of tracer E. coli at 97.5% ± 1.2% through aerosolization was obtained by an air-atomizer operated at an air pressure of 30 Psi. In the field experiment, the airborne transmission of E. coli between poultry houses was proved and emitted E. coli was more easily transmitted into self-house than adjacent house due to the ventilation design and weather condition. Our results suggested that the tracing method of airborne bacteria was available for the investigation of airborne microbial transmission across built environments.

Atmospheric dispersion modelling of bioaerosols that are pathogenic to humans and livestock - A review to inform risk assessment studies

In this review we discuss studies that applied atmospheric dispersion models (ADM) to bioaerosols that are pathogenic to humans and livestock in the context of risk assessment studies. Traditionally, ADMs have been developed to describe the atmospheric transport of chemical pollutants, radioactive matter, dust, and particulate matter. However, they have also enabled researchers to simulate bioaerosol dispersion. To inform risk assessment, the aims of this review were fourfold, namely (1) to describe the most important physical processes related to ADMs and pathogen transport, (2) to discuss studies that focused on the application of ADMs to pathogenic bioaerosols, (3) to discuss emission and inactivation rate parameterisations, and (4) to discuss methods for conversion of concentrations to infection probabilities (concerning quantitative microbial risk assessment). The studies included human, livestock, and industrial sources. Important factors for dispersion included wind speed, atmospheric stability, topographic effects, and deposition. Inactivation was mainly governed by humidity, temperature, and ultraviolet radiation. A majority of the reviewed studies, however, lacked quantitative analyses and application of full quantitative microbial risk assessments (QMRA). Qualitative conclusions based on geographical dispersion maps and threshold doses were encountered frequently. Thus, to improve risk assessment for future outbreaks and releases, we recommended determining well-quantified emission and inactivation rates and applying dosimetry and dose-response models to estimate infection probabilities in the population at risk.

Estimation of infectious risks in residential populations exposed to airborne pathogens during center pivot irrigation of dairy wastewaters

In the western United States where dairy wastewaters are commonly land applied, there are concerns over individuals being exposed to airborne pathogens. In response, a quantitative microbial risk assessment (QMRA) was performed to estimate infectious risks after inhalation exposure of pathogens aerosolized during center pivot irrigation of diluted dairy wastewaters. The dispersion of pathogens (Campylobacter jejuni, Escherichia coli O157:H7, non-O157 E. coli, Listeria monocytogenes, and Salmonella spp.) was modeled using the atmospheric dispersion model, AERMOD. Pathogen concentrations at downwind receptors were used to calculate infectious risks during one-time (1, 8, and 24 h) and multiday (7 d at 1 h d(-1)) exposure events using a β-Poisson dose-response model. This assessment considered risk of infection in residential populations that were 1 to 10 km from a center pivot operation. In the simulations, infectious risks were estimated to be the greatest in individuals closest to the center pivot, as a result of a higher pathogen dose. On the basis of the results from this QMRA, it is recommended that wastewaters only be applied during daylight hours when inactivation and dilution of airborne pathogens is highest. Further refinement of the dispersion and dose-response models should be considered to increase the utility of this QMRA.

Airborne Microorganisms From Livestock Production Systems and Their Relation to Dust

Large amounts of airborne microorganisms are emitted from livestock production. These emitted microorganisms may associate with dust, and are suspected to pose a risk of airborne infection to humans in vicinity and to animals on other farms. However, the extent to which airborne transmission may play a role in the epidemic, and how dust acts as a carrier of microorganisms in the transmission processes is unknown. The authors present the current knowledge of the entire process of airborne transmission of microorganisms-from suspension and transportation until deposition and infection-and their relation to dust. The sampling and the mitigation techniques of airborne microorganisms and dust in livestock production systems are introduced as well.

Screening-level risk assessment of Coxiella burnetii (Q fever) transmission via aeration of drinking water

A screening-level risk assessment of Q fever transmission through drinking water produced from groundwater in the vicinity of infected goat barnyards that employed aeration of the water was performed. Quantitative data from scientific literature were collected and a Quantitative Microbial Risk Assessment approach was followed. An exposure model was developed to calculate the dose to which consumers of aerated groundwater are exposed through aerosols inhalation during showering. The exposure assessment and hazard characterization were integrated in a screening-level risk characterization using a dose-response model for inhalation to determine the risk of Q fever through tap water. A nominal range sensitivity analysis was performed. The estimated risk of disease was lower than 10(-4) per person per year (pppy), hence the risk of transmission of C. burnetii through inhalation of drinking water aerosols is very low. The sensitivity analysis shows that the most uncertain parameters are the aeration process, the transport of C. burnetii in bioaerosols via the air, the aerosolization of C. burnetii in the shower, and the air filtration efficiency. The risk was compared to direct airborne exposure of persons in the vicinity of infected goat farms; the relative risk of exposure through inhalation of drinking water aerosols was 0.002%.

BOARD-INVITED REVIEW: fate and transport of bioaerosols associated with livestock operations and manures

Airborne microorganisms and microbial by-products from intensive livestock and manure management systems are a potential health risk to workers and individuals in nearby communities. This report presents information on zoonotic pathogens in animal wastes and the generation, fate, and transport of bioaerosols associated with animal feeding operations and land applied manures. Though many bioaerosol studies have been conducted at animal production facilities, few have investigated the transport of bioaerosols during the land application of animal manures. As communities in rural areas converge with land application sites, concerns over bioaerosol exposure will certainly increase. Although most studies at animal operations and wastewater spray irrigation sites suggest a decreased risk of bioaerosol exposure with increasing distance from the source, many challenges remain in evaluating the health effects of aerosolized pathogens and allergens in outdoor environments. To improve our ability to understand the off-site transport and diffusion of human and livestock diseases, various dispersion models have been utilized. Most studies investigating the transport of bioaerosols during land application events have used a modified Gaussian plume model. Because of the disparity among collection and analytical techniques utilized in outdoor studies, it is often difficult to evaluate health effects associated with aerosolized pathogens and allergens. Invaluable improvements in assessing the health effects from intensive livestock practices could be made if standardized bioaerosol collection and analytical techniques, as well as the use of specific target microorganisms, were adopted.

Aerial Dispersal and Epiphytic Survival of Pseudomonas syringae during a Pretest for the Release of Genetically Engineered Strains into the Environment

Prospective experimental field evaluation of genetically engineered microorganisms, such as microbial pest control agents, raises issues of how to properly ascertain their fate and survival in the environment. Field trials with recombinant organisms must reflect requirements for sampling and monitoring. Field trials were conducted at Tulelake, Calif., to monitor the numbers of viable cells of a nonrecombinant strain of Pseudomonas syringae that entered the atmosphere and landed on plants and soil during and after an aerosol spray application. An exponential decrease in numbers of viable cells deposited at increasing distances from three sprayed plots was observed. The relative rate of survival of cells sprayed directly on plants was more than 10 times higher than that of cells dispersed through the air to similar adjacent plants. Results are being used to gain experience with the characteristics of a release site that influence containment or dispersal and to develop appropriate sampling methodologies for evaluating survival and dispersal characteristics of genetically engineered bacteria released into the environment. The ability to make predictions about microbial dispersal and survival will reduce the uncertainties associated with environmental releases of recombinant organisms.

Inoculum Density-Dependent Mortality and Colonization of the Phyllosphere by Pseudomonas syringae

Pseudomonas syringae inocula containing cell concentrations ranging from 10 to 10 cells per ml were applied to the primary leaves of bean plants. The plants were incubated under conditions of high temperature and illumination and low relative humidity. Bacterial mortality rates and the proportional population decline of the inoculum were lowest at the highest inoculum concentrations. Addition of a high concentration of heat-killed cells to the inoculum containing a low concentration of viable cells significantly reduced both the mortality rate and the proportional population decline of the viable cells. The mechanisms underlying this density-dependent mortality may include cooperative protective effects of extracellular factors, such as bacterial extracellular polysaccharides, and physical protection by neighboring cells. Although epiphytic populations derived from inoculum concentrations of 10 or 10 cells per ml tended toward 10 CFU/g, the presumed carrying capacity of the leaf, populations derived from lower inoculum concentrations never achieved this carrying capacity. Assuming that epiphytic populations of P. syringae reside in discrete protected sites, our results suggest that at low inoculum concentrations, following a period of environmental stress, the number of viable cells may have dropped to zero in some sites; hence, the carrying capacity of the leaf could not be achieved.

Simulation of airborne microbial droplet transport

The framework for a simulation model which describes the dispersion of individual droplets of water containing viable microbes is presented. The model accounts for physical, chemical, biological, and measured meteorological parameters of each droplet at each of many short time steps. Repeating the modeling process for many droplets will simulate a cloud of droplets. The model is compared with the Tulelake, Calif., release in 1988 and found to show very similar patterns of deposition within 30 m (the maximum observation distance of the source. A hypothesis for the survival sequence in the microbe-containing droplets is discussed.

Molecular bacterial community analysis of clean rooms where spacecraft are assembled

Molecular bacterial community composition was characterized from three geographically distinct spacecraft-associated clean rooms to determine whether such populations are influenced by the surrounding environment or the maintenance of the clean rooms. Samples were collected from facilities at the Jet Propulsion Laboratory (JPL), Kennedy Space Flight Center (KSC), and Johnson Space Center (JSC). Nine clone libraries representing different surfaces within the spacecraft facilities and three libraries from the surrounding air were created. Despite the highly desiccated, nutrient-bare conditions within these clean rooms, a broad diversity of bacteria was detected, covering all the main bacterial phyla. Furthermore, the bacterial communities were significantly different from each other, revealing only a small subset of microorganisms common to all locations (e.g. Sphingomonas, Staphylococcus). Samples from JSC assembly room surfaces showed the greatest diversity of bacteria, particularly within the Alpha- and Gammaproteobacteria and Actinobacteria. The bacterial community structure of KSC assembly surfaces revealed a high presence of proteobacterial groups, whereas the surface samples collected from the JPL assembly facility showed a predominance of Firmicutes. Our study presents the first extended molecular survey and comparison of NASA spacecraft assembly facilities, and provides new insights into the bacterial diversity of clean room environments .

Bioaerosols from land-applied biosolids: issues and needs

Bioaerosols are a vehicle for the dissemination of human and animal pathogens. Because of land-filling costs and the ban on ocean dumping of municipal biosolids, land application of biosolids and animal manure is increasing all over the globe. There is no doubt that the creation, generation, and disposal of human and animal wastes increases the aerosolization potential of a wide variety of microbial pathogens and related pollutants. In an attempt to address public health issues associated with the land application of municipal biosolids, the U.S. National Research Council (Washington, D.C.) published a report on this issue in 2002. This paper focuses on the current information and technology gaps related to estimating the public health risks associated with bioaerosols during the land application of biosolids.

Effect of phenotypic plasticity on epiphytic survival and colonization by Pseudomonas syringae

The bacterial epiphyte Pseudomonas syringae MF714R was cultured on agar or in broth or collected from colonized leaves; it was then inoculated onto greenhouse-grown bean plants incubated in a growth chamber at low relative humidity or in the field or onto field-grown bean plants. Cells cultured in liquid medium survived the least well after inoculation of leaf surfaces under all conditions. Cells cultured in solid medium exhibited the highest percent survival and desiccation tolerance in the growth chamber but generally survived less well in the field than did cells harvested from plants. Cells harvested from plants and inoculated onto plants in the field usually exhibited the highest percent survival, started to increase in population earlier, and reached a higher number than did cells cultured in vitro. Differences in field survival were apparently not attributable to differential UV tolerance. The observed effects of phenotypic plasticity on epiphytic survival and colonization should be considered in risk assessment studies, in studies of bacterial epidemiology, and in the use of microbial antagonists for biological pest control.

Trajectory of aerosol droplets from a sprayed bacterial suspension

Simulated droplet trajectories of a polydispersed microbial aerosol in a laminar air flow regimen were compared with observed dispersal patterns of aerosolized Bacillus subtilis subsp. niger spores in quasilaminar airflow. Simulated dispersal patterns could be explained in terms of initial droplet sizes and whether the droplets evaporated to residual aeroplanktonic size before settling to the ground. For droplets that evaporated prior to settling out, a vertical downwind size fractionation is predicted in which the microbial residue of the smallest droplets settles the least, and is found in the airstream at about sprayer height, and progressively larger droplet residues settle to progressively lower heights. Observations of spore particle size distributions downwind from a spray source support the simulation. Droplet and particle size distributions near the source had three size fractions: one containing large, presumably nonevaporated droplets of greater than or equal to 7 microns in diameter, and two smaller fractions, with diameters of 2 to 3 microns (probably the residue of droplets containing more than one spore) and 1 to 2 microns (probably the residue from single-spore droplets). As predicted by the simulation, the aerosol settled and progressed downwind, with the number of small droplets and particles increasing in proportion to the height and distance downwind.

Aeromicrobiology--a review

This article reviews the presence of microorganisms in air and their sources, the relation of airborne dust and endotoxin, the sources of atmospheric microbial contamination in food-processing plants, the mechanisms of airborne particle deposition, the importance of airborne microbes, the survival of microorganisms in air, methods of air sampling, airborne microbial populations in food-processing plants, control of airborne microorganisms in food-processing plants, and the general issue of microorganisms in air and their impact on food safety. The purpose was to bring together scattered information about airborne microorganisms and review their importance in food protection and sanitation.

Model to predict aerial dispersal of bacteria during environmental release

Risk assessment for genetically engineered bacteria sprayed onto crops includes determination of off-site dispersal and deposition. The ability to predict microbial dispersal patterns is essential to characterize the uncertainty (risk) associated with environmental release of recombinant organisms. Toward this end, a particle dispersal model was developed to predict recovery of bacteria on fallout plates at various distances and directions about a test site. The microcomputer simulation incorporates particle size distribution, wind speed and direction, turbulence, evaporation, sedimentation, and mortality, with a time step of 0.5 s. The model was tested against data reported from three field applications of nonrecombinant bacteria and two applications of recombinant bacteria. Simulated dispersal of 10(5) particles was compared with reported deposition measurements. The model may be useful in defining appropriate populations of organisms for release, methods of release or application, characteristics of a release site that influence containment or dispersal, and in developing an appropriate sampling methodology for monitoring the dispersal of organisms such as genetically engineered bacteria.

Estimating downwind concentrations of viable airborne microorganisms in dynamic atmospheric conditions

A Gaussian plume model has been modified to include an airborne microbial survival term that is a best-fit function of laboratory experimental data of weather variables. The model has been included in an algorithm using microbial source strength and local hourly mean weather data to drive the model through a summer- and winter-day cycle. For illustrative purposes, a composite airborne "virus" (developed using actual characteristics from two viruses) was used to show how wind speed could have a major modulating effect on near-source viable concentrations. For example, at high wind speeds such as those occurring during the day, or with short travel times, near-source locations experience high viable concentrations because the microorganisms have not had time to become inactivated. As the travel time increases, because of slow wind speed or longer distances, die-off modulation by sunshine, relative humidity, temperature, etc., potentially becomes increasingly predominant.

Recovery of Aspergillus fumigatus aerospora from municipal sewage sludge composting operations in the state of Maine

An investigation was conducted to determine the effect of composting activity on ambient and transient viable Aspergillus fumigatus aerospora at three composting operations (Westbrook, Bangor, Old Town) in the State of Maine. Meteorological conditions and seasonality were also considered. The purpose of the study was to determine whether composting operations caused increased public health risks due to the generation and dispersal of elevated A. fumigatus aerospora concentrations. Ambient aerospora concentrations at the Westbrook facility did not change during the first year of operation, nor did they differ from concentrations recovered before the composting facility was constructed. Ambient aerospora at Bangor and Old Town were also present at background concentrations. Background aerospora concentrations (less than 50 CFU/m3) were determined at control sites. Spatially and temporally restricted increases in transient A. fumigatus aerospora concentrations were observed. The greatest increases (up to 1 X 10(4) CFU/m3) were associated with wood chip manipulation activities. Aerospora concentrations always returned to background levels within 1 hr after the monitored activities ceased. Meteorological conditions which tended to reduce dust concentrations also decreased A. fumigatus aerospora yields. Evidence regarding seasonality as a factor affecting aerospora concentrations was equivocal. The results of the study demonstrated that residents in the vicinity of the Westbrook, Bangor and Old Town composting operations were not exposed to increased concentrations of A. fumigatus aerospora.

Die-away kinetics of aerosolized bacteria from sprinkler application of wastewater

A methodology for estimating, under field conditions, the microbial die-away constant (lambda) is presented. This constant may be used in predicting the aerosolized pathogenic microorganism concentrations downwind from a wastewater spray or aeration site by means of modified atmospheric diffusion equations. The mean lambda of Escherichia coli for very early morning runs was 8.8 X 10(-3)s-1, and that for afternoon runs was 6.6 X 10(-2)s-1.

Dispersal of Aspergillus fumigatus from Sewage Sludge Compost Piles Subjected to Mechanical Agitation in Open Air

Aerosolization of the thermophilous fungal opportunist Aspergillus fumigatus from mechanically agitated compost piles was examined at a pilot-scale sewage sludge composting facility and two other selected test sites. Aerosols of A. fumigatus downwind from stationary compost piles were insignificant in comparison with those downwind from agitated piles. These aerosols were generated by a front-end loader moving and dropping compost. Aerial concentrations of the fungus at distances downwind from the point of emission were used to determine an emission rate for A. fumigatus associated with the moving operations. The maximum emission rate, 4.6 � 10 6 A. fumigatus particles per s, was used to calculate predicted concentrations in an unobstructed plume with restrictive, neutral, and dispersive atmospheric mixing conditions up to 1 km downwind from the emission source.

Estimation of downwind viable airborne microbes from a wet cooling tower-Including settling

In recent years, reuse of municipal waste water as the coolant in drift-producing cooling towers at electrical generating plants has become increasingly common. A hueristic model is presented that can be used to estimate the concentrations of viable airborne microbes in the drift from a wet cooling tower given the concentration of microbes in the cooling tower. The purpose of this presentation is to allow the nonmeteorologist to understand the factors affecting airborne concentration and to make crude estimates of ground-level concentrations of airborne microorganisms. Concentrations are calculated using a standard meterological method, the Gaussian dispersion model, in which terms have been included for droplet settling and microbial death rate.