Host-Controlled Modification of Rhizobium Bacteriophage

Abstract

Host-controlled phenotypic variation of host specificity was observed with two rhizobiophage strains, 0Ll and 0L5, following growth on six strains of Rhizobium legumino8arum and R. trifolii. The six hosts could be assigned to four groups, each group representing a different pattern of host specificity. Initial adaptation of 0L5 to hosts L2, L7, and L25 appeared to involve mutation, although replication in cells of these hosts generally involved additional phenotypic restriction. Restricted and unrestricted forms of a phage did not differ significantly in their ability to adsorb to several hosts. One-step analysis of the L25-specific form of 0L5 grown in L25 cells indicated a low average burst size of approximately one unrestricted plaque-forming unit in a small proportion of cells which were able to produce infective centres on L4. One-cycle analysis of L4-specific 0L5 modified by growth in L25 confirmed the phenotypic nature of phage variation in this phage-host system, and indicated that specificity for L4 was not replicated in L25.