STANDARDIZED METHOD FOR DETERMINING THE EFFECT OF VARIOUS ANTIBIOTICSON LACTOCOCCAL CULTURES

Antibiotic resistance of 44 lactic starter culture strains currently u sed 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 ag ar disk diffusion assay. Results with various classes of antibiotics u sed to treat cattle mastitis indicated that lactococcal cultures shoul d be diluted 20% and incubated at 30-degrees-C for 16 to 18 h in this medium for reliable results. The degree of antibiotic susceptibility w as compared to standards established by the National Committee for Cli nical Laboratory Standards. The procedure outlined in the present stud y is recommended as a routine method for testing antibiotic resistance of lactococci. This represents a more logical drug sensitivity approa ch than the use of noncheese starter bacteria to test for antibiotics in milk.