Accuracy and Speed in Counting Agar Plates ¹

To determine the accuracy of colony counts made by analysts, agar plates were photographed. The agar plates and photographs were compared to obtain a true count (photocount) which was used to determine analyst accuracy over selected count ranges. Analyst accuracy was also determined by comparing analyst's counts to the mean of the counts obtained by several analysts ("established standard"). The "established standard" compared favorably with the photocount. Analysts' counts were within 5% of the photocount and "established standard" on 60 and 68% of plates having 30-300 colonies and 60 and 67% on plates having 20-200 colonies, respectively. Average counting time for plates in the 10-100, 20-200, 30-300, and 40-400 colony count ranges was 18, 30, 41, and 52 sec, respectively. Plates having 20-200 colonies were as suitable for counting as plates having 30-300 colonies and were counted with a time-saving of 27%.