Phage morons play important roles in controlling Pseudomonas aeruginosa phenotypes

The viruses that infect bacteria, known as phages, play critical roles in controlling bacterial populations in many diverse environments, including the human body. This control stems not only from phages killing bacteria, but also the formation of lysogens. In this state the phage replication cycle is suppressed, and the phage genome is maintained in the bacterial cell in a form known as a prophage. Prophages often carry genes that benefit the host bacterial cell since increasing survival of the host cell by extension also increases the fitness of the prophage. These highly diverse and beneficial phage genes, which are not required for the life cycle of the phage itself, have been referred to as “morons” as their presence adds “more on” the phage genome in which they are found. While individual phage morons have been shown to contribute to bacterial virulence by a number of different mechanisms, there have been no systematic investigations of their activities. Using a library of phages that infect two different clinical isolates of P. aeruginosa , PAO1 and PA14, we compared the phenotypes imparted by the expression of individual phage morons. We identified morons that inhibit twitching and swimming motilities and observed inhibition of production of virulence factors such as rhamnolipids and elastase. This study demonstrates the scope of phage-mediated phenotypic changes and provides a framework for future studies of phage morons. Importance Environmental and clinical isolates of the bacterium Pseudomonas aeruginosa frequently contain viruses known as prophages. These prophages can alter the virulence of their bacterial hosts through the expression of nonessential genes known as “morons”. In this study we identified morons in a group of Pseudomonas aeruginosa phages and characterized the effects of their expression on bacterial behaviours. We found that many morons confer selective advantages for the bacterial host, some of which correlate with increased bacterial virulence. This work highlights the symbiotic relationship between bacteria and prophages and illustrates how phage morons can help bacteria adapt to different selective pressures and contribute to human diseases.