Changes in the Bacteriophage Sensitivity of Lactic Streptococci

Twenty-three fast acid-producing pure cultures of lactic streptoco~,ci, propagated daily at 22 C. in skimmilk fortified with 0.75% nonfat milk solids, were tested periodically in litmus milk for changes in sensitivity to pro'fried bacteriophage filtrates. The following changes occurred in the 10-too. 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. (.i) Three cultures became resistant to their homologous bacteriophages and then were resistant to a mixture of those available. [n all cases studied, except one, the bacteriophages were adsorbed only bv sensitive cultures. The adsorption behavior of the bacteriophages, the types of change observed, and the methods used to prevent changes indicate that changes in sensitivity resulted from displaeemen~ of sensitive bacterial populations by resistant populations that arose from rare mutant bacteria. The rare nmtants had a selective advantag'e and (in all eases except one) altered barteriophage-adsorption characteristics (assumed to reflect changes in surface structure).