Culture medium brand choice impacts colony swarming behavior among industrial Bacillus isolates and the accuracy of aerobic plate counts.

Aerobic plate count assays are an industry standard method for the enumeration of microbial products. Colony swarming among industrial Bacillus isolates on solid medium can impact a counting technician's interpretation of colony count, promote inter-technician variance and reduce the agreement of plate counts with growth-independent enumeration methods. In the present study, we examined swarming behavior among four industrial Bacillus species as a function of culture medium brand choice. Colony diameter for three Bacillus species was found to be influenced by culture medium brand, as was colony count interpretation among three out of four plate counting technicians. Estimations of Bacillus endospore concentration were likewise influenced by culture medium brand, leading to an increased incidence of QC failure for replicate samples of a Bacillus-based microbial product as a function of brand choice. Results suggest that culture medium brand choice may be an additional source of variance when plate counting is used for the enumeration of Bacillus-based microbial products. We recommend that plating medium brand availability in testing laboratories be considered by microbial product manufacturers when considering sources of variance in customer and regulatory laboratories.