Detection of residual penicillins in milk by using a Bacillus stearothermophilus disk assay.

A paper disk method based on a procedure described by the International Dairy Federation is presented for the detection of 4 beta-lactam antibiotics (penicillin G, ampicillin, cephapirin, and cloxacillin) in whole milk. Bacillus stearothermophilus var. calidolactis, prepared as a stable spore suspension, was used as the test organism. Levels of 0.005 unit penicillin G/ml, 0.005 microgram ampicillin or cephapirin/ml, and 0.05 microgram cloxacillin/ml were readily detected in whole milk. Results were produced in 3-4 hr. The method offers several advantages, including greater simplicity, sensitivity, and rapidity, over methods now commonly used to detect residual levels of these drugs in milk.