Some Observations on Lysogenic Strains of Salmonella

The proportion of lysogenic cultures in five species of salmonella examined varied from 84% in Salmonella thompson to 0% inS. pullorum. When estimating the amount of phage present in lysogenic cultures of S. thompson it was possible to obtain a plaque count approximately equal to the number of bacteria present in these cultures when the susceptible culture used for determining the counts was used relatively dilute. When used less dilute the counts were lewer and equal to those obtained when the bacteria were killed by heating to 56 for 30 min. This lower figure is an estimate, of the free phage in the culture. Plaque size varied inversely as the amount of bacteria in the susceptible culture. When lysogenic cultures were used for counting a related phage, plaque size was diminished by partial interference. Two types of relationship were noted between the growth rate of bacteria and phage in artificially induced lysogenie cultures of S. thompson. In one, the number of free phage particles was always directly related to the number of viable bacteria present; in the other, the phage particles increased rapidly in number during the logarithmic phase of bacterial multiplication and then decreased rapidly due presumably to their absorption by the bacteria. The active infection of the lysogenic cultures with an unrelated phage usually resulted in an increased-yield of the phage with which the culture was latently infected. In one instance the latent infection was transformed into an active one. All the artificially induced resistant variant strains of S. thompson were lysogenic. It was impossible to make some cultures permanently resistant to two different phases as the resulting cultures were unstable and soon showed signs of active infection. Others were made with a difficulty not experienced during the making of singly resistant strains. Most of these were latently infected only with the first phage to which they had been made resistant: others carried both phages.