ReviewMolecular approaches for the detection and monitoring of microbial communities in bioaerosols: A review
Graphical abstract
Introduction
Microorganisms are ubiquitous in the environment and play key functional roles in nearly all ecosystems (Jaenicke, 2005). Bioaerosols originate from all types of environments, including the atmosphere, soil, freshwater, and oceans, and their dispersal into air is temporally and spatially variable. Airborne dissemination is likely a natural and necessary part of the life cycle of many microorganisms (Morris et al., 2008). Bioaerosols are generally defined as aerosols or particulate matter of microbial, plant or animal origin and includes a wide range of antigenic compounds, microbial toxins, and viruses; the term bioaerosol is often used synonymously with organic dust (Douwes et al., 2003, Peccia and Hernandez, 2006). Bioaerosols are emitted from terrestrial, soil, forest, and desert dust, agricultural and composting activities, urban areas, wetlands, coastal, and marine environments (Gandolfi et al., 2013, Jaenicke, 2005). Modern industrial activities (e.g., waste sorting, organic waste collection, composting, agricultural production, food processing, livestock raising, and wastewater treatment systems) also emit large amounts of bioaerosols, resulting in abundant exposure to biological agents (Brodie et al., 2007, Douwes et al., 2003). According to Matthias-Maser et al. (2000), the proportion by volume of biological material among total airborne particulates is 28%, 22%, and 10% in remote continental, populated continental and remote maritime environments, respectively. It has been estimated that 16% to 80% of the mass of primary atmospheric aerosols is from biological sources (Jaenicke, 2005).
The components of bioaerosols range in size. Pollens from anemophilous plants have typical diameters of 17–58 μm, fungal spores are typically 1–30 μm in diameter, bacteria are typically 0.25–8 μm in diameter, and viruses are typically less than 0.3 μm in diameter. Fragments of plants and animals may vary in size. Biological material does not necessarily occur in the air as independent particles. Shaffer and Lighthart (1997) determined that the majority of bacteria at inland sites are associated with particles of aerodynamic diameter greater than 3 μm. Bacteria may occur as agglomerations of cells or may be dispersed into the air on plants or animal fragments, on soil particles, on pollen, or on spores that have become airborne. Bioaerosols are a ubiquitous component of the atmosphere; a large number of these particles are small-sized microorganisms. Airborne bacterial and fungal cells can reach concentrations of ~ 103 and ~ 105 cells/m3, respectively. Aerosolized bacteria and fungi are present in at altitudes of up to 10–20 km in the troposphere and even altitudes of 20 to 40 km above sea level in the stratosphere (Fahlgren et al., 2011).
Accumulating evidence indicates an important role of bioaerosols in the atmospheric environment (Brodie et al., 2007, Douwes et al., 2003, Georgakopoulos et al., 2009, Peccia et al., 2008). Bioaerosols contribute to atmospheric physical and chemical processes (Fig. 1) (Deguillaume et al., 2008, Jaenicke, 2005). Strong correlations between the variations in atmospheric bacterial community structures over time and the physical and chemical characteristics of air masses have been observed (Fierer et al., 2008, Maron et al., 2005). Ariya and Amyot (2004) suggested that bioaerosols play significant roles in atmospheric chemistry and physics by altering the chemistry of the atmosphere via microbiological degradation, thus modifying the chemical composition of other organic compounds upon collision or contact and driving chemistry at environmental interfaces, such as the air-particle interface. Recent studies have demonstrated that bioaerosols can become attached to ambient particles and have significant climatic effects, acting as cloud condensation nuclei and ice nuclei that can initiate precipitation (Amato et al., 2005, Bauer et al., 2002, Christner et al., 2008, Morris et al., 2008, Sattler et al., 2001). One study determined that approximately 33% of the ice-crystal residues in cloud-condensation nuclei and ice nuclei were biological particles (Pratt et al., 2009).
However, little is known about the composition of atmospheric bioaerosols and how it varies by location or meteorological conditions. Airborne microorganisms are very difficult to assess accurately under field conditions due to factors such as the collection efficiency of the selected sampler (Henningson and Ahlberg, 1994), variations in the robustness of different species of microorganisms, and the difficulty of differentiating strains of the same species (Griffin et al., 2001). The relationship between environmental conditions and bacterial aerial dispersal indicates that microbial compositions could increase the health risk due to pathogens or allergenic components of unclassified environmental bacteria. Bioaerosols may also cause climate change (Brodie et al., 2007). Bioaerosols likely do not survive for long durations due to atmospheric conditions, including wind, moisture, and UV exposure. However, concerns about bioaerosol exposures have increased in recent years because exposure to biological agents in both indoor and outdoor environments has been associated with a wide range of adverse health effects, including respiratory diseases, allergies and even cancer (Douwes et al., 2003, Shelton et al., 2002).
Although modern developments in the fields of microbiology, meteorology, and environmental science have opened up new possibilities for the study of bioaerosols, the field is dominated by a remarkable lack of knowledge and an abundance of speculation. Very few observations have been published comparing the aerial environment with other environments, such as water and soil, and the lack of standard methods, environmental guidelines, and databases complicates the interpretation and comparison of results. Because of this limited microbiological information, the microbial analysis in bioaerosols continues to be explored at high resolution, and new technologies, especially culture-independent approaches, should be exploited for a better understanding of the clinical context of microorganism in bioaerosols. This review provides information on what is currently known about effects of bioaerosols on the public health and describes various sampling and microbial analysis methods to characterize microorganisms.
Section snippets
Adverse effects of bioaerosols on human health
Bioaerosol consist of pathogenic or non-pathogenic live or dead bacteria and fungi, viruses, high-molecular-weight allergens, bacterial endotoxins, mycotoxins, peptidoglycans, β(1 → 3)-glycans, pollen, and plant fibers, among other components (Douwes et al., 2003). The best-characterized adverse human health effects of bioaerosol exposure are respiratory symptoms. Lierl and Hornung (2003) observed a strong association between elevated outdoor pollen concentration levels in spring and summer and
Sampling methods
Current bioaerosol research is primarily focused on the monitoring and control of ambient or target bioaerosols. The effective monitoring of bioaerosols requires efficient collection of microorganisms from the air. An appropriate technique for air sample analysis must also be selected (Alvarez et al., 1995). A wide variety of bioaerosol sampling methods are available, and no standard protocols have been established. A number of sampling devices have been developed for particulate matter (PM)
Conclusions
Recent studies of bioaerosols have applied various methodologies with differing scopes and abilities to the elucidation of microbial community structures, depending on the questions being addressed. The most profound impact of bioaerosols is as the causative agent of disease. Recent increases in bioaerosol emissions and concerns about pathogenic bacteria in bioaerosols have motivated substantial biotechnological advances in the molecular methods and approaches used to detect pathogens.
Acknowledgments
This work was supported by the National Research Foundation (NRF) of Korea via a grant (No. 2011-0030040) funded by the Korea government (MSIP). In addition, we thank GAIA project from Korea Ministry of Environment for the performance of this study.
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