FACTORS AFFECTING MICROBIOLOGICAL COLONY COUNT ACCURACY FOR BIOAEROSOL SAMPLING AND ANALYSIS

The effects of the following variables on the occurrence of colony mas king (the indistinguishable merging or overlap of sufficiently close c olonies) were evaluated experimentally using the bacterium Bacillus su btilis: spore density on a collection surface, concentration of nutrie nts in the culture medium, sample incubation time, and ability of an o bservation system to distinguish overlapped colonies. Increasing spore surface density and incubation time increased colony masking, whereas lowering nutrient concentration decreased colony diameter and, theref ore, masking but also limited spore germination and growth. Overall, f ull-strength medium was best for accurate counting of early microcolon ies examined with the aid of a microscope, whereas half or quarter-str ength medium was better for counting older readily observable macrocol onies. Masking bias was determined for varying spore surface densities and colony diameters and was applied to two widely used slit-to-agar bioaerosol impactors. Appropriate collection rimes have been determine d for these samplers to minimize colony masking for expected bioaeroso l concentrations. It was found, for example, that 6-min samples collec ted from all environment with an air concentration of 10(3) CFU m(-3) would result in colony surface densities, for 3-mm colonies, of 1.5 an d 3.9 microorganisms cm(-2) for the two samplers with respective maski ng biases of <10% and <20%.