The infection of Pseudomonas aeruginosa by RNA pilus phage PP7: The adsorption organelle and the relationship between phage sensitivity and the division cycle

Abstract

Evidence was obtained in favor of the proposal that FP pili are the adsorption organelle for the RNA containing Pseudomonas aeruginosa phage PP7. A pili-less mutant derived from the phage's piliated host was found to be phage resistant and unable to adsorb phage. Spheroplasts prepared from either the mutant or the sensitive strain could be infected by naked PP7 RNA with equal efficiency, demonstrating that the phage resistance of the mutant was due to a defect in the adsorption-penetration mechanism and not due to an inability to replicate the phage. Two types of experiments with synchronously dividing cells demonstrated that the phage's host was susceptible to infection only during a brief phase of the cellular growth cycle, beginning shortly before division. In the first, exposure of an asynchronously growing culture to phage infection for most of one generation time selected for cells in approximately the same growth phase as evidenced by the observation that the survivors grew synchronously for three generations after the phage exposure was terminated. In the second type of experiment, a culture which had been induced to divide synchronously by a period of starvation showed cyclic fluctuations in phage susceptibility with the maxima occurring just before cell division. Because of the short duration of the phage-susceptible phase, the susceptible cells extant in an exponentially growing culture constituted only a small minority of the cell population. Quantitative models were constructed in which phage susceptibility was defined as a particular phase of the cellular growth cycle. These models correctly predicted the rate at which phage-resistant cells became phage susceptible, were infected and accumulated as infected cells after phage had been added to an exponentially growing culture.