a). The typing of Salmonella thompson by means of bacteriophage

SUMMARY: By the use of seven phage preparations, forty-one of fifty smooth strains of Salmonella thompson were classified into eleven types. The remaining nine strains were insusceptible to all the test phages; mouse passage did not render them susceptible. Acquired phage resistance was responsible for many of the strains being regarded as different phage types. A study of phage-resistant variants cast doubt on the complete validity of Bail's cross-resistance tests. The danger of using lysogenic cultures for the propagation of phages for the study of phage mutation were noted. Most of the S. thompson phages were fully active against strains of S. cholerue-suis but not for other Salmonella sp. that were tested. In a recent study (Williams Smith, 1951) a number of bacteriophages were isolated from lysogenic strains of S. thompson. The purpose of this present paper is to report observations that were made while evolving a method of typing strains of S. thompson by the use of these phages. MATERIALS AND METHODS The method of isolation, purification, and propagation of the phages used i