Influence of Penicillin and other Antibiotics on Lactic Streptococci in Starter Cultures used in Cheddar Cheesemaking

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Abstract
Complete inhibition of acid production by 6 starter cultures was obtained with 0.5 U. of penicillin per ml. of milk, strong inhibition with 0.1 U. and moderate with 0.05 U. Penicillin was the most inhibitory of 6 antibiotics tested with subtilin next; chloromycetin was the least potent. 45 strains of lactic streptococci isolated from starter cultures were completely inhibited in skim milk in amts. ranging from 0.2 to 0.4 U. per ml. In whey broth most of the cultures were completely inhibited by 0.05 to 0.1 U.; several were sensitive to 0.00625 and 0.0125 U. A culture of Leuconostoc citrovorum tested required 1.6 U. for complete inhibition. Penicillinase at the rate of 0.02 mg./ 100 ml. of milk permitted almost normal acid production by a starter at a concn. of penicillin (0.5 U./ml.) which stopped fermentation completely and permitted 50% acid production in presence of 1 U./ml. Cysteine in amts. ranging from 1 to 10 mg. per 100 ml. milk was ineffective against penicillin, as was pasteurization. A penicillin-resistant starter culture was developed which coagulated milk in the presence of 3 U. of penicillin per ml. and retained its resistance after 20 passages in absence of penicillin. Calculations are included on the amts. of penicillin which may be found in milk used for cheesemaking.