SUMMARY: The distribution of adapted particles of Vi-phage II has been examined. Fluctuation tests in which phage A was grown for a single cycle in type A of showed that phage E1 particles conformed to a Poisson distribution in a series of small samples and cannot, therefore, be spontaneous mutants of phage A. Particles of phage D1, on the other hand, showed a clonal distribution in a similar series of samples and are thus spontaneous host-range mutants of phage A. Phage E1 reverted to phage A during a single cycle of growth in type A organisms, which confirms that phage E1 is a phenotypic modification of phage A. In contrast, phage D1 was unaltered by a single cycle of propagation in type A organisms and its mutant nature is thus verified. Phage 29 has also been shown to be a host-range mutant of Vi-phage II. The adsorption of phage A to type E1 is lethal to the bacteria, of which only a small proportion support phage growth. The general applicability of these findings to the numerous adaptations of Vi-phage II is discussed.


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