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Influence of Penicillin and other Antibiotics on Lactic Streptococci in Starter Cultures used in Cheddar Cheesemaking¹

Central Experimental Farm, Ottawa, Canada

ABSTRACT

Complete inhibition of acid production by six starter cultures was obtained with 0.5 unit penicillin per ml. of milk, strong inhibition with 0.1 unit and moderate with 0.05 unit.

Penicillin was the most inhibitory of the six antibiotics tested with subtilin next; chloromycetin was the least potent.

All 45 strains of lactic streptococci isolated from starter cultures were completely inhibited by penicillin in skim milk in amounts ranging from 0.2 to 0.4 unit per ml. In whey broth most of the cultures, were completely inhibited by 0.05 to 0.1 unit; several were sensitive to 0.00625 and 0.0125 unit. A culture of Leuconostoc citrovorum tested required 1.6 units for complete inhibition.

Penicillinase at the rate of 0.02 mg. per 100 ml. of milk permitted almost normal acid production by a starter at a concentration of penicillin (0.5 unit per ml.) which stopped fermentation completely and permitted 50 per cent acid production in the presence of 1.0 unit per ml.

Cysteine in amounts ranging from 1.0 to 10.0 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 units penicillin per ml. and retained its resistance after 20 passages in absence of penicillin.

Calculations are included on the amounts of penicillin which may be found in milk used for cheesemaking.

¹ Contribution no. 289 (Journal Series) from Division of Bacteriology and Dairy Research, Science Service, Department of Agriculture, Ottawa.

² Bacteriologist.

³ Chief, Dairy Research.