The Type A Phages of Salmonella typhimurium: Observations on Temperate Phage and Lysogenesis

Publisher Website
Google Scholar
Abstract
When a standard suspension of Salmonella typhimurium Q1, an indicator strain sensitive to the type A phages recovered from lysogenic strains of this organism, was exposed to concentrations of these phages giving a phage: bacteria ratio of approximately 1:10 (thus ensuring that, with rare exceptions, bacteria became infected with single phage particles) it was found that definite percentages of the bacteria were either lysogenized or productively infected (lysed). These percentages were constant for each particular phage type, but varied widely in the 11 members of the group. It was concluded that each phage consisted of a mixed population of particles, some capable, as single infections, of producing lysogenization ([alpha] particles) and others lacking this property ([beta] particles). An increase in the phage: bacteria ratio, resulting in multiple infections of single bacteria, led to an increase in lysogenization at the expense of productive infection. The number of [alpha] particles present in any particular concentration of phage was calculated from the figures determined at low phage: bacteria ratios (limit dilution). With some phages, when bacteria became infected with more than one particle by exposure to rising phage concentrations, the number lysogenized was compatible with the hypothesis that [alpha] particles are dominant over [beta] particles, and that every bacterium infected by an [alpha] particle is lysogenized. This hypothesis was however invalidated by the fact that, with other phages, either the number of bacteria lysogenized was in excess of the available [alpha] particles, indicating that bacteria had been lysogenized by infection with two or more [beta] particles, or the number of [alpha] particles was grossly in excess of the number of bacteria lysogenized. The implications of these findings are discussed. Very high concentrations of these temperate phages produced lysis-from-without, and in some cases appeared to induce lysis of bacteria which had first been lysogenized. In all cases in which such experiments were carried out, exposure to low temperatures (20[degree]), to high temperatures (42[degree]), to the salts of certain organic acids, and to anaerobiosis had to significant effect on lysogenization.