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 Table III.

Incubation temperatures and conditions for viable (culturable) microorganisms.

Fungi

25 oC or Room temperature with natural light

Bacteria, environmental

25 to 30 oC

Bacteria, human-source

35 to 37 oC

Bacteria, thermophilic Actinomycetes

50 to 56 oC

Laboratory media blanks and field media blanks must be handled in the same manner as samples.

b.

Enumeration (1)

Total Concentration (Colony Forming Units Per Cubic Meter)

The total concentration of culturable microorganisms is calculated by dividing the volume of air sampled into the total number of colonies observed on the plate. A colony is a macroscopically visible growth of microorganisms on a solid culture medium. Concentrations of culturable bioaerosols normally are reported as colony forming units (CFU) per unit volume of air. CFU is the number of microorganisms that can replicate to form colonies, as determined by the number of colonies that develop. (2)

Adjusted Concentration (CFU/m3) Often, it is difficult to identify multiple colonies at one location on a plate because of the lack of differential colony morphology or because the chemicals secreted by one microorganism might inhibit the growth of other microorganisms at that same location [Burge et al. 1977]. In addition, some organisms produce large, spreading colonies while others produce microcolonies. Also, the morphology of the colony of one microorganism may completely obscure that of another, and a fast-grower might obscure a slow-grower. In these cases, a statistical adjustment of the observed number of colonies is needed to account for the probability that more than one particle impacted the same site [Andersen 1958; Leopold 1988; Macher 1989]. The adjusted concentration of culturable microorganisms is calculated by dividing the volume of

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NIOSH Manual of Analytical Methods