The susceptibility to penicillin of a wide range of single strain cultures of lactic streptococci was determined. Approximately 50% of the strains studied exhibited resistance to penicillin at the 0.1 unit/ml level. Eight of these cultures were adapted to grow in the presence of 0.5 unit penicillin/ml, but they were unable to produce acid at a rate or quantity equal to that of their sensitive parent strains. Cultures obtained from naturally resistant isolates produced more acid in the presence of low penicillin concentrations than their sensitive parent counterparts, but less than that produced by “trained” resistant strains from the same parent origin.

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Author notes

1Article No. 18:60, Department of Dairy Technology, The Ohio State University. Supported by a grant from the United States Public Health Service.

2Present address: New South Wales Department of Agriculture, N. S. W., Australia

3Present address: Johnson & Johnson, New Brunswick, New Jersey