Hiding in plain sight: McMaster study shows bacteria can activate ‘stealth mode’ to evade attacks from phages

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Hiding in plain sight: McMaster study shows bacteria can activate ‘stealth mode’ to evade attacks from phages


Lori Burrows (front) with graduate student Veronica Tran in the Burrows Laboratory at McMaster.


As antimicrobial resistance (AMR) erodes the effectiveness of antibiotics, viruses that prey exclusively on bacteria — called phages — are gaining traction as a potential therapeutic alternative. But phages are far from a perfect fail-safe, as bacteria can evolve resistance to them, too. 

In a new study, published March 30, 2026 in the journal mBio, researchers at McMaster University found that Pseudomonas aeruginosa, a common drug-resistant bacterium that causes lung, skin, blood, and gastrointestinal infections, can evade attacks from phages by entering a temporary “stealth mode.” 

These bacteria are covered in hair-like fibres called pili, which McMaster Professor Lori Burrows describes as tiny “grappling hooks” that help them crawl across surfaces and latch onto tissues. Those same hooks, she says, are also key to Pseudomonas’ ability to cause disease. 

But as important as they are to their survival, pili can also be a major liability for Pseudomonas. 

“Some phages can exploit these fibres to infect bacteria,” says Burrows, the principal investigator on the new study. “As Pseudomonas extend their pili outward to move, phages can latch on — and when the pili retract, those phages hitch an easy ride to the surface of the bacteria, where they infect and ultimately kill their host.” 

To overcome this existential threat, many strains of Pseudomonas have evolved ways to resist such attacks. In fact, in the presence of pili-targeting phages, the bacteria can outright stop the production or function of their pili so that the phages have no way to infect them.   

“Just like with antibiotics, bacteria can also evolve resistance to phages,” explains Veronica Tran, a graduate student in Burrows’ lab and first author on the new paper. “But unlike antibiotic resistance, phage resistance — in certain cases — can actually work in our favour.”  

Indeed, because pili play such a crucial role in how Pseudomonas infects and spreads, coaxing the bacteria into disarming themselves can achieve the same goal as eliminating them altogether.  

Burrows, the associate director of McMaster’s Michael G. DeGroote Institute for Infectious Disease Research, says this process is called “phage steering” — the notion that the viruses don’t necessarily need to kill their targets to prevent or alleviate infection, as long as they can push bacteria into becoming less harmful versions of themselves.  

But the new study suggests that phage steering may be more complicated than initially thought. 

Of the 28 different phage-resistant Pseudomonas mutants studied by the group, almost all behaved exactly as expected, showing varying degrees of reduced or lost pili function. But one mutant stood out: once the phages were removed from its environment, almost half of 100 different test samples returned to full pili function. 

“This was completely unexpected,” says Tran, who worked closely on the project with McMaster Biomedical Discovery & Commercialization student Tanisha Lahane. “It shows that Pseudomonas is capable of developing temporary phage resistance. In a clinical setting, bacteria with the ability to regain their virulence after treatment has ended could lead to recurring infections.”  

These findings, Burrows says, underscore the resilience and resourcefulness of bacteria, and are a reminder that even promising therapies like phages need to be handled carefully in the clinic — both now and in the future. Burrows notes that it is also the reason that phages are usually used as mixtures or “cocktails,” since it’s much harder for bacteria to escape from multiple kinds of phages simultaneously. 

While phages are not currently an approved therapy in Canada, they can be prescribed on compassionate grounds for patients with life-threatening antibiotic-resistant infections. To date, Health Canada has sanctioned phage therapy only twice, and both cases can be traced back to McMaster.  

The first involved a life-threatening prosthetic joint infection caused by drug-resistant Staphylococcus epidermidis. The successful phage treatment course, administered at The Ottawa Hospital, was led by infectious disease physician Marisa Azad, who trained in the laboratory of McMaster Professor Gerry Wright. In the second case, where phages cleared a chronic urinary tract infection in a Toronto-area patient, clinical samples were sent to Burrows’ own lab at McMaster, where her team successfully identified phages capable of killing the specific strain of Escherichia coli causing the infection.  

“Both cases show that phages can be a viable therapeutic option for patients with drug-resistant infections,” Burrows says. “That’s why it’s important to expand our understanding of how bacteria can adapt to and resist phages, so that if — or when — phage therapy becomes more widely available in Canada, we don’t find ourselves facing a resistance crisis like we have with antibiotics.


Article taken from, copyright bleongs to : https://healthsci.mcmaster.ca/hiding-in-plain-sight-mcmaster-study-shows-bacteria-can-activate-stealth-mode-to-evade-attacks-from-phages/

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