SPIRAL SYSTEM AND LASER COLONY SCANNER FOR ENUMERATION OF MICROORGANISMS

Abstract

ABSTRACT Aerobic Plate Count (APC) and spiral plate (Spiral) methods using both manual counting (MC) and laser counting (LC) procedures were compared for pure bacterial, yeast, and mold cultures and raw milk samples. All four combinations of methods (APC‐MC, APC‐LC, Spiral‐MC, and Spiral‐LC) gave similar log10 counts of studied pure microbial cultures, producing results that were not different for the purposes of practical microbiology. With bacterial and yeast cultures, counts differed by less than half a logarithmic cycle (the range of difference = ‐0.26 to +0.42), the range of difference being ‐0.03 to +0.62 for Aspergillus flavus and Penicillium camemberti molds. An exception was noticed with Rhizopus oligosporus mold when plates were read by laser due to large (10–15 mm) colony size. The difference between the readings made manually and by laser colony scanner was about one logarithmic cycle with both APC and Spiral methods. Statistical analyses of the manually read results of bacterial and Saccharomyces cerevisiae spiral plates showed no differences at the 0.05 level of significance between the readings made by four or five persons. For raw milk samples, Spiral‐MC and Spiral‐LC methods gave higher microbial numbers than APC‐MC (63% of samples) and APC‐LC (54% of samples) at 0.05 level of significance (p<0.001). Of the LC results, 75% were within a ± 0.5 logarithmic cycle range when compared to MC results, of which 98% were within the same range. All results were still within a ± 1.0 logarithmic range. When APC and Spiral for raw milk samples were analyzed, LC gave higher microbial numbers compared to MC: 71% of the APC‐LC results were higher than APC‐MC results, and 85% of Spiral‐LC results were higher than Spiral‐MC results at 0.05 level of significance (p<0.001). 77% of the results by either plating technique were within a ± 0.5 logarithmic range.