Viruses known as bacteriophages, or phages, play a critical role in the human gut by selectively targeting and destroying bacteria. These viruses function by injecting their genetic material into a bacterial host, hijacking its cellular machinery, and forcing the production of new phages until the host cell eventually bursts. While this process is highly effective, bacteria have historically evolved defenses against these viral attacks, mirroring how they develop resistance to traditional antibiotics.
A study published Thursday in Nature Microbiology suggests that phages possess their own sophisticated evolutionary countermeasures. Researchers at Michigan State University have identified specific, previously underappreciated regions within bacteriophage genomes that serve as genetic “hotspots.” These regions allow the viruses to repeatedly alter key genes as they replicate, effectively hedging their evolutionary bets. Rather than producing a population of genetically identical copies, the phages generate a diverse collection of offspring, increasing the likelihood that some variants will survive and continue to infect their bacterial hosts.
“This changes our understanding of how phages evolve,” explained co-author Chris Waters, a faculty member in Michigan State University’s Ecology, Evolution, and Behavior program. Waters noted that instead of merely producing mass copies of themselves, the phages are utilizing these mutation hotspots to create a “zoo” of varied offspring, which he describes as “essentially hedging their bets.”
The research team reached these findings by studying bacteriophage T2, which targets E. coli, after introducing a bacterial defense system capable of recognizing and destroying invading phage DNA into the bacteria. In laboratory settings, the researchers observed that within a few hours, the phages consistently began to overcome the bacterial defenses. When the team sequenced these resistant viruses, they discovered repeated mutations in a gene identified as agt. This gene contains a stretch of repetitive DNA that functions as a contingency locus, where the DNA-copying machinery often slips during replication.
This mechanism results in a reversible frameshift mutation, allowing the gene's instructions to be read differently. Some phages gain an extra repeat while others lose one, resulting in a population with diverse abilities to evade bacterial defenses. The researchers found that these repetitive regions accumulated mutations thousands of times faster than the rest of the viral genome. Furthermore, the study confirmed that this mechanism is not limited to a single virus, as similar contingency loci were identified in E. coli phage T4 and are widespread across diverse E. coli phages.
This discovery arrives at a crucial time as scientists and policymakers seek alternatives to antibiotics. While phage therapy dates back to the 1920s, interest waned following the widespread availability of drugs like penicillin. However, the escalating crisis of antimicrobial resistance has reignited interest in the field. Although phages are highly specific—offering an advantage over antibiotics that often kill beneficial bacteria—that same specificity can be a weakness if bacteria evolve resistance. By understanding these evolutionary tricks, researchers hope to develop more resilient phage therapies to combat the global antibiotic resistance crisis, aiming to minimize the impact of bacterial resistance even if it cannot be completely eliminated.
Using experimental evolution and genome sequencing, the researchers found similar contingency loci in E. coli phage T4. They also found that simple sequence repeats were widespread across diverse E. coli phages, although their abundance varied between genes with different functions.
The appeal is partly down to specificity. Whereas many antibiotics can kill beneficial bacteria alongside the pathogen they target, individual phages can be highly selective for particular bacterial species or strains.
