Comparison of results for commercially available microbiological media plates with results for standard bacteriologic testing of bovine milk

Abstract

To compare results for 3 commercially available microbiological media plates with those for standard bacteriologic testing of bovine milk. Milk samples from postpartum cows and cows with a high somatic cell count (SCC) or clinical mastitis (CM). Sample-ready Staphylococcus culture medium (SRSC) plates were used to detect Staphylococcus aureus in milk samples obtained from postpartum cows and cows with a high SCC or CM. Rapid coliform count (RCC) plates were used to detect coliforms in milk samples obtained from cows with CM. Aerobic count (AC) plates were used to detect streptococci in CM samples. Fresh mastitic milk samples were frozen and then thawed to evaluate the effects of freezing for the SRSC and RCC plates. The effects of dilution (1:10) of samples were determined. Agreement of results between the commercially available plates and standard bacteriologic testing was evaluated. The ability of SRSC plates to detect S aureus in milk samples was highest with diluted samples from postpartum cows and cows with a high SCC or CM. Sensitivity of the RCC plate for detection of coliforms was highest with diluted mastitic milk samples. The AC plates had a poor positive predictive value for detection of streptococci in mastitic milk samples. Freezing increased S aureus detection. Overall, the SRSC and RCC plates were accurate, were easy to use, and yielded results comparable to those of standard bacteriologic testing for the detection of S aureus and coliforms in bovine milk.