[Statistical study of the optimal conditions of the spiral plating method for counting bacterial numbers in river...

The application of the spiral plating method, a rapid and labor-saving technique for enumerating bacteria in food, to evaluate bacteria in river water, was examined. Standard plate counts and the numbers of heterotrophic bacteria, coliforms and Flavobacterium spp. in the water were tested by the spiral plating method, the results were compared with those of the conventional method and the optimal conditions for using the spiral plating method were considered and selected to fit the conventional methods. The optimal conditions selected were as follows: by laser colony counter after incubation at 35 degrees C for 38h. for standard plate counts; by laser colony counter after incubation at 25 degrees C for 48h. for the numbers of heterotrophic bacteria and Flavobacterium spp.; by colony viewer counting reddish colonies > or = 0.25mm in diameter after incubation at 35 degrees C for 22h. for the number of coliforms. The numbers of bacteria, except for coliforms, determined by this spiral plating method were found to be closely related to those from conventional methods (r > or = 0.91), and the replicating variances of both methods were not significant. The counts of bacteria by laser colony counter gave results similar to those by colony viewer counting by visual inspection. This spiral plating method saved time, money and labor in the evaluation of bacteria in river water as comparable to that in food. These results indicate that the spiral plating method can be used in place of conventional methods in evaluating the number of bacteria in river water.