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SUMMARY: A temperate phage of Pseudomonas aeruginosa (P. pyocyanea), on addition to a non-lysogenic indicator strain of the organism, produced a lysogenic culture with a high spontaneous rate of free phage production. A virulent mutant, serologically related to the temperate phage, was obtained from this lysogenic culture ; this mutant did not give rise to lysogenically infected bacteria. Both virulent and temperate phages, acting on an indicator strain, had latent periods of 90 min., and the multiplication of each was inhibited by a phenanthridine added immediately after adsorption. This indicated that the initial stage of infection is the same for both types of phage. The production of free phage (i.e. the prophage to phage change) by the lysogenic culture derived from the temperate phage was not inhibited by the drug. Filtrates of lysogenic strain C7 X of Pseudomonas aeruginosa yielded two types of plaques when assayed against an indicator strain (Dickinson & Codd, 1952). One type was clearly visible after 18 hr. incubation of the assay plate while the other gave hazy plaques, only visible after 48 hr. In view of Boyd's (1951) work on lytic and symbiotic phages of Salmonella typhimurium, the results of further investigations into the case of P. aeruginosa are of interest. Lwoff & Gutman (1950) and Lwoff, Siminovitch & Kjeldgaard (1950), using Bacillus rneguterium, considered that whether or not phage was reduced to prophage in a given bacterium was primarily a function of host resistance. Lieb (1953) analysed the factors involved in the establishment of lysogenesis in a strain of Escherichia coli and suggested that the cell response was due to physiological conditions. Lwoff (1953) has recently reviewed the subject of lysogeny. The present work compares the behaviour of a temperate phage with a virulent mutant. Definitions. In this paper the terms proposed by Jacob, Lwoff, Siminovitch & Wollman (1953) are used, in particular: Temperate phage, meaning a phage capable of producing a lysogenic system with a given host. Virulent phage, meaning a phage incapable of producing a lysogenic system with a given host. Cultures are denoted by C and a culture, e.g. ClO, made resistant to a given phage, Pc, is denoted by ClO/c. The total phage count of a culture means the number of infectious centres, including free phage. Thefree phage count is that obtained when all bacteria have been removed.

The Behaviour of a Temperate Phage of Pseudomonas aeruginosa compared with that of a Serologically related, Virulent Mutant