ABSTRACT
Eleven analysts tested contaminated reconstituted (1:10) dry milk powders for penicillin residues using spores of Bacillus stearothermophilus var. calidolacris (Delvotest P method). Three types of responses were noted: positive, negative, and questionable. Prediction equations indicated that 95% of the time, analysts unfamiliar with the technique could detect positive results if penicillin concentrations in samples were 0.010 unit/ml or higher and positive and questionable results if the penicillin concentrations were 0.008 unit/ml. Increasing the reconstitution ratio from 1/11 to 1/4 increased the chances of detecting penicillin in milk powder. Penicillinase added to reconstituted penicillin-contaminated milks in all instances produced negative responses.
ABSTRACT
A disc assay method for testing sulfa drugs and antibiotics in milk was developed wherein Bacillus megaterium ATCC 9855 was used as the test organism and Mueller-Hinton agar was used as the test substrate. Incubation was at 37 C for 4 to 5 hr. The test procedure is an improvement over the Bacillus subtilis-Antibiotic Medium No. 1 method, as described in Standard Methods for the Examination of Dairy Products, in that it is sensitive to eight sulfa drugs and to bacitracin without a significant change in sensitivity to eight other antibiotics commonly used for mastitis therapy.