Bacteria are prokaryotic microorganisms that lack many of the subcellular structures found in fungal, plant, and animal cells. They are usually single celled and reproduce by simple division. Most bacteria are saprophytes and require a source of complex carbon compounds (i.e., nonliving organic material); they decay substrates both aerobically and anaerobically. In general, bacteria require more water for active growth than the fungi and are usually the dominant organisms in water reservoirs with a pH of greater than 7. Some bacteria form spores that are extremely resistant to environmental stressors.
Many organic substrates can be degraded by bacteria, which form biofilms on surfaces that are continuously wet. Biofilm ecosystems also support the growth of algae, protozoa, and fungi. The bacteria are classified by cell shape, staining properties, spore production, metabolic characteristics, and the human diseases they cause.
Bacteria occupy a wide range of reservoirs both outdoors and indoors. Gram-negative bacteria often predominate in outdoor reservoirs on living leaf surfaces and are able to survive at least brief periods of transit in the air. A wide variety of bacteria can be found in soil and in natural bodies of water. Some Bacillus species and the thermophilic actinomycetes (e.g., Faenia rectivirtigula, Thermoactinomyces spp.) will grow only at temperatures between 45° and 60° C. They are found mainly in environments that have become warm from insolation, geothermal conditions, or self-heating.
Outdoor bacteria become airborne with the disturbance of substrates—for example, with the movement of air or rain or with human activity, especially activities related to farming and refuse handling. Indoor reservoirs that allow growth and dissemination of allergenic bacterial aerosols include water-containing appliances such as portable humidifiers, large humidification systems, organic material stored or accumulating indoors, cooling fluids in machining plants, and other such moist areas. Bacillus species tend to accumulate in house dust. Thermophilic organisms occupy indoor reservoirs such as humidifiers attached to heating systems, refrigerator drip pans, evaporative cooler media, clothes dryer exhausts, and other such places characterized by organic material, water, and warm temperatures. Air movements, inadvertent human activity, and activities that allow direct handling of contaminated material (e.g., in removal or cleaning procedures) are common dissemination factors for indoor bacteria.