one viable bacterium). An additional limitation is that some bacteria are non-culturable on artificial laboratory media, which could result in an underestimation of the quantity of viable bacteria present.

CFUs are determined using a standard culturing protocol (i.e., growth media, time, temperature), dispersal of a fixed quantity (i.e., volume, weight, or sample collected from a fixed volume of air) of sample on the growth media, and counting the number of visible growth patterns of the same type. By diluting or disrupting the sample and then repeating the standard protocol, it is possible to release bacteria that are clumped together to determine the total number of viable agent entities in the sample.

2.1.2
Viruses

Viruses are minute infectious agents that lack an independent metabolism and are able to replicate only within a living host cell. An individual viral particle (a virion) consists of nucleic acid (either DNA or RNA) and a protein capsid shell that contains and protects the nucleic acid; the shell may be multilayered. The host range of viruses is extremely broad; virions may infect bacteria (i.e., bacteriophages), plants, animals, and humans. Individual virions range from approximately 20 to 200 nanometers in size. However, without special treatment virions are usually associated with larger particles. Since viruses depend on a viable host cell for replication, naturally derived virions are often found in a mixed matrix with material in which they were grown. Viruses can be cultivated under controlled conditions in animals, eggs, and cell cultures. The material associated with virus-containing aerosol particles depends on how the virus was grown and processed before aerosolization. Therefore, aerosol particles containing viruses can be very diverse in size, chemical make-up and number of virions contained in an aerosol particle. Further complexity arises from the variation in potential host responses to viruses presented in such diverse size distributions and chemical matrices. Examples of viral pathogens and their associated diseases are:

  • Variola major—smallpox

  • SARS associated corona virus—severe acute respiratory syndrome (SARS)

  • Ebola virus—ebola hemorrhagic fever

Aerosol particles containing viruses can be characterized using physical and chemical means as well as biological activity. Physical and chemical characterization methods are used to identify the presence of virus component materials. Viral nucleic acid is routinely analyzed by polymerase chain reaction (PCR). Capsid surface features are routinely identified by molecular recognition assays involving antibodies and other recognition moieties. These techniques do not assess whether the viruses are biologically active. (The ability of a virus to infect and replicate in a cell can be compromised by many environmental factors, including heat, sunlight, and humidity; different viruses are vulnerable to different environmental conditions.) Knowing whether the aerosol particle can establish an infection in a host cell culture or animal model is critical to determining the health hazard. Methods that are specific to a number of types of virus have been developed to characterize biological activity. These methods employ challenge of a known susceptible host cell culture, whole animal model, or other suitable host tissue, such as



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