Many bacteria, including the opportunistic pathogen Pseudomonas aeruginosa, are chronically infected by viruses that can reproduce without killing the cell. These viruses, called filamentous phages, typically do not harm the bacteria and can provide certain benefits, such as increased antimicrobial resistance.
In a study published Oct. 2 in Current Biology, University of Pittsburgh researchers reported that they discovered a genetic mutation in a phage—called Pseudomonas filamentous (Pf) phage—of P. aeruginosa that allows bacteria to weaponize the phage to outcompete other bacterial cells that aren’t infected with the mutated phage. However, this benefit does not last because the spread of this defective virus also leads to the production of cheater miniphages—phages that lack important genes—which ultimately cause host bacteria to lose any of the benefits the phage once provided.
This entire cycle requires as little as 24 hours and suggests that P. aeruginosa may be subject to frequent cycles that affect their evolution and susceptibility to drugs. In the future, the research team, led by Vaughn Cooper, professor of microbiology and molecular genetics, School of Medicine, aims to engineer these viruses to better control chronic drug-resistant infections.
Nanami Kubota, a graduate research assistant in the Department of Microbiology and Molecular Genetics, School of Medicine, who was lead author of the study, explained the research in her own words.
What did your research find?
P. aeruginosa, the bacteria in the study, are often infected with Pf phage—a virus, i.e., bacteriophage (or “phage” for short). Pf phages are beneficial to their bacterial host by making the bacterium more tolerant of antibiotics. These string-like phages can exist inside the bacterial cell as DNA, either inside the bacteria’s DNA or separately in a circularized form. A mutation in one of the phage genes (Pf phage repressor gene) turns the bacteria into a phage factory, kickstarting rampant phage DNA replication and production of phage proteins, including the string-like, protective shell that houses the phage DNA.
Bacteria with this mutant phage can use these phages to kill off other bacterial competitors, but this heightened phage production is unstable. Because there are a lot of phages being produced, cheater miniphages (phages with shorter DNA that can replicate but do not contribute to making proteins for the protective shell) emerge and take advantage of the heightened phage production to selfishly shuttle their own DNA into the protective shell made by other cooperative phages. This rapidly leads to a scenario where cheater miniphages overtake the population, causing a phage “tragedy of the commons” where bacteria are only infected by cheater miniphages.
However, because these cheaters cannot make the protective shell, bacteria lose the ability to use their phages against other bacteria, and eventually, the phages go extinct.
Will this help patients? If so, how?
P. aeruginosa can cause infection, especially in immunocompromised or immunosuppressed people. Understanding how the bacteria and its phage influence each other’s evolution is important to understanding whether we can take advantage of this bacteria-phage alliance and turn them against each other to make bacterial infections easier to treat. For example, Pf phages are beneficial to their bacterial host because the phages can protect their bacteria from antibiotics. Can we take advantage of these cheater miniphages to drive Pf phages to extinction, thereby making the bacteria more sensitive to drugs?
What are your next steps?
We wanted to understand whether the combination of this repressor mutation and antibiotic treatment would alter how bacteria evolve drug resistance. If the phage contributes to drug tolerance, then the presence of cheater miniphages should reduce bacterial survival in the presence of antibiotics. To test this, we evolved bacteria carrying the repressor mutation under antibiotic pressure to see whether they could adapt and survive the combined effects of the cheater miniphage and the drug.