Phage-Associated Lysins Affecting Group N and Group D Streptococci

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Abstract
Phage lysins were prepared from high titer phage lysates of 3 strains of Streptococcus lactis, one of which was purified 500-fold by treatment with cold acetone, chromatography on Amberlite-CG50 ion-exchange resin, and fractional precipitation from potassium phosphate buffers. The lysins were activated by monovalent cations and were similar in physical properties. Viable streptococci of groups-N and -D were lysed, but not those in groups-A, -B and -C. Different strains of group-N streptococci were lysed at different rates, the rate of lysis being characteristic for individual phage lysins. A lytic enzyme, prepared from purified phage particles by freezing and thawing or by ultrasonic treatment, showed the same pH optimum and specificity as the corresponding phage lysin. Streptococci were lysed-from-without in the presence of high multiplicities of phage. The type of lysin produced in phage-infected culture was genetically determined by the phage. Lysis of streptococci by phage lysin was inhibited by an unidentified substance present in crude phage lysates. The lysis of different strains of streptococci by participate phage, phage-tail enzyme and phage lysin occurred at the same relative rates. This is interpreted as evidence for the localization of the phage-lysin substrate within the phage receptor site. Possible reasons for the different specificities of phage lysins from streptococci of groups-N and -C are discussed.