Pseudomonas aeruginosa is an opportunistic pathogen that causes
infections in a variety of settings. Many P. aeruginosa isolates are infected by filamentous Pf bacteriophage integrated into the bacterial chromosome as a prophage. Pf virions can be produced without lysing P. aeruginosa. However, cell lysis can occur during
superinfection, which occurs when Pf virions successfully infect a host lysogenized by a Pf prophage. Temperate phages typically encode
superinfection exclusion mechanisms to prevent host lysis by virions of the same or similar species. In this study, we sought to elucidate the
superinfection exclusion mechanism of Pf phage. Initially, we observed that P. aeruginosa that survive Pf
superinfection are transiently resistant to Pf-induced plaquing and are deficient in twitching motility, which is mediated by type IV pili (T4P). Pf utilize T4P as a
cell surface receptor, suggesting that T4P are suppressed in bacteria that survive
superinfection. We tested the hypothesis that a Pf-encoded
protein suppresses T4P to mediate
superinfection exclusion by expressing Pf
proteins in P. aeruginosa and measuring plaquing and twitching motility. We found that the Pf
protein PA0721, which we termed Pf
superinfection exclusion (PfsE), promoted resistance to Pf
infection and suppressed twitching motility by binding the T4P
protein PilC. Because T4P play key roles in biofilm formation and virulence, the ability of Pf phage to modulate T4P via PfsE has implications in the ability of P. aeruginosa to persist at sites of
infection. IMPORTANCE Pf bacteriophage (phage) are filamentous viruses that infect Pseudomonas aeruginosa and enhance its virulence potential. Pf virions can lyse and kill P. aeruginosa through
superinfection, which occurs when an already infected cell is infected by the same or similar phage. Here, we show that a small, highly conserved Pf phage
protein (PA0721, PfsE) provides resistance to
superinfection by phages that use the type IV pilus as a
cell surface receptor. PfsE does this by inhibiting assembly of the type IV pilus via an interaction with PilC. As the type IV pilus plays important roles in virulence, the ability of Pf phage to modulate its assembly has implications for P. aeruginosa pathogenesis.