Single and double gel-diffusion techniques were employed to examine serologically coagulase-positive staphylococci from cheese for enterotoxigenicity. Super
A method is described for determining the number of bacteria in a solution by the use of a machine which deposits a known volume of sample on a rotating agar plate in an ever decreasing amount in the form of an Archimedes spiral. After the sample is incubated, different colony densities are apparent on the surface of the plate. A modified counting grid is described which relates area of the plate of volume of sample. By counting an appropriate area of the plate, the number of bacteria in the sample is estimated. This method was compared to the pour plate procedure with the use of pure and mixed cultures in water and milk. The results did not demonstrate a significant difference in variance between duplicates at the α = 0.01 level when concentrations of 600 to 12 × 10
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bacteria per ml were used, but the spiral plate method gave counts that were higher than counts obtained by the pour plate method. The time and materials required for this method are substantially less than those required for the conventional aerobic pour plate procedure.
The spiral plate count method is a semiautomated plating technique that greatly reduces manpower and material costs normally associated with the pour plating technique. In this collaborative study, 8 laboratories compared the spiral and pour plating techniques, using 4 samples each of 3 products : frozen pumpkin pie, frozen chicken pot pie, and shampoo. The results show that 10 of the 12 comparisons of means of the pour and spiral methods were not significantly different; 2 values were significant at α = 0.01. Overall, the components of variance were less than that of the current milk standard, and the replicate per cent coefficient of variation was satisfactory. This study indicates that the spiral plate method is an acceptable alternative to the pour plate method; the spiral plate method has been adopted as official first action.
The spiral plating procedure is a rapid method for determining bacteriological counts. Results from a collaborative study indicate that the procedure should be useful in milk analysis. Typical milk samples (homogenized milk, raw milk, chocolate drink, 2% milk, and 20% cream) were sent to six analysts to be examined by standard plate count (SPC) and spiral plate count (SPLPC). Analysis of duplicate samples shows that the SPC and SPLPC values did not differ at the a = 0.01 level. Components of variance for replicate determinations among laboratories and laboratory-sample interaction were computed. The standard deviation was 0.109 compared to the 0.110 estimate reported for SPC in state laboratories. Results from the SPLPC method compared favorably to the results of conventional (SPC) pour procedure.
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