Phages Produce Persisters

Bacteria primarily encounter stress, and, arguably, their greatest threats are phages. It is often assumed that those bacteria that escape phage attack have mutated; however, another possibility is that a subpopulation forms the dormant persister state, in a manner similar to that demonstrated for bacterial cells undergoing nutritive, oxidative, and antibiotic stress. Persister cells do not undergo mutation and survive lethal conditions by ceasing growth transiently. Slower growth and dormancy play a key physiological role as they allow host phage defense systems more time to clear the phage infection. Here we investigated how bacteria survive lytic phage infection by isolating surviving cells from the plaques of T2, T4, and lambda (cI mutant) virulent phages. We found that bacteria in plaques can escape phage attack both by mutation (i.e., become resistant) and without mutation (i.e., become persistent). Specifically, whereas T4-resistant and lambda-resistant bacteria with over a 100,000-fold less sensitivity were isolated from plaques with obvious genetic mutations (e.g., causing mucoidy), cells were also found after T2 infection that undergo no significant mutation, retain wild-type phage sensitivity, and survive lethal doses of antibiotics. Corroborating this, adding T2 phage to persister cells resulted in 137,000-fold more survival compared to that of addition to exponentially-growing cells. Phage treatments with Klebsiella pneumonia and Pseudomonas aeruginosa also generated persister cells. Hence, along with resistant strains, bacteria also form persister cells during phage infection.