Changes in the Bacteriophage Sensitivity of Lactic Streptococci

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Abstract
Twenty-three fast-acid-producing pure cultures of lactic streptococci, propagated daily at 22[degree]C in skim milk fortified with 0.75% nonfat milk solids, were tested periodically in litmus milk for changes in sensitivity to purified bacteriophage filtrates. The following changes occurred in the ten-month test period: (1) Six cultures became resistant to their homologous bacteriophages and sensitive to one bacteriophage. (2) Three resistant cultures became sensitive to one bacteriophage. (3) Six cultures became mixtures and required two bacteriophages to stop acid production. (4) Three cultures became resistant to their homologous bacteriophages and then were resistant to a mixture of those available. In all cases studied, except one, the bacteriophages were adsorbed only by sensitive cultures. The adsorption behavior of the bacteriophages, the types of change observed, and the methods used to prevent changes indicate that the changes in sensitivity resulted from displacement of sensitive bacterial populations by resistant populations that arose from rare mutant bacteria. The rare mutants had a selective advantage and (in all cases except one) altered bacteriophage-adsorption characteristics (assumed to reflect changes in surface structure).