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Most probable numbers for evaluation of coli-aerogenes test by fermentation tube method. Public Health Repts., 49, 393-405. LEIFSON, E. 1936 New selenite enrichment media for the isolation of typhoid and paratyphoid (Salmonella) bacilli.

Five analysts participated in a study to evaluate the following aspects of the 13th edition of Standard Methods for the Examination of Dairy Products (SMEDP): (a) analyst variation in overall Standard Plate Counts (SPC), and (b) analyst duplication of bacterial colony counts on agar places. Each analyst prepared 24 samples of pasteurized, homogenized milk during a successive 8-day period (i.e., 3 samples/day), and then the analysts estimated the numbers of bacterial colonies for these, as well as other analysts' plates, initially after 48 h of incubation, and then 1 h later and 24 h later. Statistically significant differences in colony enumerations were found between analysts in preparation of agar plates on 3 days. Significant differences were also noted between analysts for bacterial counts of agar plates. Mean bacterial estimates of certain analysts ranged between 565 and 948, and fluctuated greatly between the initial, 1-h, and 24-h determinations. These results indicate that the “standards of accuracy” currently specified in SMEDP are not realistic, i.e., (a) among-analyst variation of 18.2% compared to 10%, and (b) within-analyst variation of 7.7% compared to 5% in SMEDP.

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Ten media, commonly used for the detection and isolation of streptococci of sanitary significance in water, dairy and other food products, were compared to establish a plating medium for the enumeration of enterococci in dairy products. To make all media suitable for comparison by the agar plate method, agar was added to those media which initially were recommended for use as broths. Criteria used in selecting the medium were high recovery, selectivity but not undue inhibition of enterococci and ease in obtaining and interpreting results. The recovery data of three different platings of enterococcus cultures were statistically analyzed. In this manner, one medium was eliminated on the basis of low recovery. Six of the remaining nine media were eliminated because they permitted the growth of non-enterococcus cultures. Two of the three media then remaining were eliminated because they allowed one S. bovis culture to grow. In addition, these media showed considerable variation in size and color of enterococcus colonies. The medium selected, the Citrate azide medium of Reinbold, Swern and Hussong (13), was modified by increasing the azide concentration. This did not result in undue inhibitory effects. It was further tested by obtaining recovery data for 158 known enterococcus cultures. High selectivity was demonstrated by showing that 408 colonial isolates from plates of raw milk, cheese and butter could be identified as enterococci.