Standardized Method for Determining the Effect of Various Antibiotics on Lactococcal Cultures ¹

Antibiotic resistance of 44 lactic starter culture strains currently used by the dairy industry has been examined in Mueller- Hinton medium, supplemented with glucose (5 g/L) and yeast extract (5 g/L), by the agar disk diffusion assay. Results with various classes of antibiotics used to treat cattle mastitis indicated that lactococcal cultures should be diluted 20% and incubated at 30°C for 16 to 18 h in this medium for reliable results. The degree of antibiotic susceptibility was compared to standards established by the National Committee for Clinical Laboratory Standards. The procedure outlined in the present study is recommended as a routine method for testing antibiotic resistance of lactococci. This represents a more logical drug sensitivity approach than the use of noncheese starter bacteria to test for antibiotics in milk.