A Quantitative Differential Method for Counting Mixed Cultures of Bacteria

Abstract

Accurate differential counts of mixed cultures of bacteria are obtained by a modification of the standard plating technique. The plating medium is poured in sterile Petri dishes, allowed to harden and dried in an incubator 14-18 hrs. Dilutions of the culture to be counted are made in the usual manner. One milliliter of the dilution is placed on the warm agar and evenly flooded over the surface by tilting the plate. When dry the plates are incubated in the usual manner. Colony distribution is uniform, usually more uniform than in the standard plate. Statistical analyses of counts of pure cultures by both methods indicates that the flooded plate method gives results of an accuracy equal to that of the standard method.