Miniaturized two-fold dilution method for enumerating total microbial load and coliforms in raw milk

A rapid and simple method for enumerating total aerobic plate counts (APC) and coliforms in raw milk was developed and compared with conventional plating method. Following two-fold serial dilution of samples in a 96 well rnicrotiter plate, double strength of two different modified media for APC or coliforms was added to each well. The final positive well (purple to yellow color) was determined and converted to dilution factors. The dilution factor of each sample was converted to Log₁₀ DF (Dilution factors) and compared to actual microbial numbers Log₁₀ CFU/mL. The results of 2-fold dilution method (Log₁₀ DF) were strongly correlated to conventional plating method (Log₁₀ CFU/mL) (P < 0.05). The correlation of the scatterplot of spread plating and 2-fold dilution method indicated a high level of agreement between two methods (R² = 0.921 for total counts and R² = 0.916 for forms from raw milk). This 2-fold dilution method is an easy, rapid, and economical method for enumeration of total microbial loads and coliforms in raw milk.