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Differences in Sensitivity of Lactic Starters to Antibiotics

Bacteriology Division, Department of Agriculture, Ottawa, Canada¹

ABSTRACT

It is well known that the presence in milk of minute amounts of antibiotics residual from mastitis therapy may seriously interfere with lactic acid fermentations. Of the various antibiotics available, penicillin is still the most widely used [and also the most easily identified through the disc assay method (15)]. Penicillin has been reported (9, 15, 17) to inhibit the lactic starter organisms more than aureomycin, but our tests in 1952 (7) with two starters indicated that the latter compound was more inhibitory. While the Cornell workers (15), working with a single starter, reported a starter activity of 79% in milk containing 0.3 aureomycin per milliliter, we obtained only 17%.

It has been suggested (7) that differences in the sensitivity of the respective starters employed might explain these discrepant findings. Katznelson and Hood (9) found partial inhibition of four of 45 unidentified strains isolated from starters in skimmilk containing 0.00625 units penicillin per milliliter, while seven required 0.2 units. Hunter (6) found that in addition to variation within the species, four strains of Streptococcus lactis generally required 0.25–0.3 units penicillin per milliliter to bring about the same reduction in acid development as was achieved by 0.1 units with four strains of S. cremoris.

¹ Contribution No. 357.