ENUMERATING CHROMOGENIC AGAR PLATES USING THE COLOR QCOUNT AUTOMATED COLONY COUNTER

Abstract

ABSTRACT This study compared colony counts of mixed bacterial cultures on chromogenic spread plates using manual and Color QCount (Spiral Biotech, Inc., Norwood, MA) automated methods. Inoculum levels spanned 30–300 cfu/mL, 26 agar types were used and 581 plates were analyzed. Plates were prepared according to manufacturers' instructions, manually counted once by two scientists and counted in duplicate automatically. The correlation coefficient comparing automated and manual counts for the pooled population of data was 0.987. The slope and intercept for the linear regression line were 1.0067 and 0.031, respectively. The mean log value difference between automated and manual counts for pooled data was−0.042. The mean log value differences between manual and automated counts demonstrated that 83.8% of plates analyzed were within 0.1 log, and 98.2% were within 0.2 log. These results demonstrate that the Color QCount automatic counter is a suitable alternative to the standard method of manually counting colored colonies on chromogenic agar plates.PRACTICAL APPLICATIONSA variety of microbiology applications require users to count bacterial colonies on agar plates. Colonies are counted to 1) assess levels of contamination in the environment, water, food and dairy products; 2) conduct preservative and efficacy studies; and 3) determine the quality of pharmaceutical and medical products with microbial limits testing. There are several sources of variability in plating including inoculating errors, sampling errors, incubation errors, and counting errors. Errors in counting may worsen with user fatigue.Using an automated colony counter to complete the tedious job of manually counting bacterial colonies on agar plates will not only improve efficiency in microbiology laboratories by saving valuable time for alternative tasks but also may improve the accuracy and consistency of the counts.