GROWTH OF SALMONELLAE IN SKIMMILK WHICH CONTAINS ANTIBIOTICS1

Abstract

Portions of sterile skimmilk were fortified with 0.1, 0.5, and 1.0 unit of penicillin; 1.0, 10, and 100 μg streptomycin; and 0.1, 1.0, and 10 μg tetracycline per milliliter. Such skimmilks were inoculated with salmonellae, incubated at 21 or 37 C, and tested with the plate count at 3-hr intervals during an 18-hr incubation. As much as 1.0 unit of penicillin per milliliter of skimmilk had little, if any, inhibitory effect on Salmonella typhimurium, Salmonella newbrunswick, Salmonella dublin, or Salmonella heidelberg at either temperature. Streptomycin, at concentrations of 1.0 and 10 μg per milliliter failed to inhibit growth of S. typhimurium or S. newbrunswick at either temperature. Presence of 100 μg streptomycin per milliliter of skimmilk markedly inhibited S. newbrunswick at both temperatures and S. typhimurium at 21 C and completely inhibited the latter bacterium at 37 C. Neither of the salmonellae were inhibited by 0.1 μg tetracycline per milliliter of skimmilk but both bacteria were somewhat inhibited by 1.0 μg antibiotic per milliliter, particularly at 37 C, and were completely inhibited with a concentration of 10 μg tetracycline per milliliter. Concentrations of any of the three antibiotics tested that can inhibit lactic starter cultures had no apparent effect on salmonellae.