McMaster Researchers Uncover Bacteria’s Temporary Stealth Mode Against Phage Attacks
Researchers at McMaster University have made a groundbreaking discovery in the ongoing battle between bacteria and their viral predators. A new study reveals how the opportunistic pathogen Pseudomonas aeruginosa can activate a temporary “stealth mode” to evade bacteriophages—viruses that specifically target and destroy bacteria. Published on March 30, 2026, in the prestigious journal mBio, the research highlights the sophisticated adaptability of bacteria, posing both challenges and opportunities for emerging phage therapies.
This finding emerges from the laboratory of Professor Linda L. Burrows at McMaster’s Michael G. DeGroote Institute for Infectious Disease Research (IIDR), underscoring the university’s pivotal role in tackling antimicrobial resistance (AMR) through innovative microbiology. As Canadian universities lead efforts to combat superbugs, McMaster’s work provides critical insights into bacterial defense mechanisms that could refine future treatments.
The Role of Type IV Pili in Bacterial Virulence and Phage Targeting
Pseudomonas aeruginosa is notorious for causing severe infections in lungs, skin, blood, and the gastrointestinal tract, particularly in vulnerable patients with cystic fibrosis, burns, or compromised immune systems. Central to its pathogenicity are type IV pili (T4P)—hair-like appendages on the bacterial surface that function like “tiny grappling hooks,” enabling twitching motility across surfaces, biofilm formation, and adhesion to host tissues.
Bacteriophages, or phages, exploit these pili as receptors. Phages such as PO4, a lytic podovirus from the phiKMV family, latch onto extended pili during retraction, injecting their genetic material to hijack and lyse the host cell. This specificity makes T4P-targeting phages promising for precision therapy against AMR pathogens like P. aeruginosa, which accounts for about 18% of Gram-negative bacterial infections in Canadian surveillance data.
However, bacteria evolve resistance rapidly, mirroring antibiotic resistance dynamics. McMaster’s study dissects this arms race, showing that while many resistance mutations abolish pilus function—potentially attenuating virulence—some allow temporary evasion without permanent loss.
Decoding the Experimental Approach at McMaster
To map resistance, the team exposed P. aeruginosa PAO1—a well-characterized strain lacking CRISPR interference—to phage PO4 on agar plates, isolating 128 resistant mutants (PRMs). Whole-genome sequencing identified 28 unique mutations across 16 T4P genes, including frameshifts, nonsense mutations, single nucleotide polymorphisms (SNPs), insertions/deletions, and one large deletion spanning pilS to pilY1.
Phenotypic assays assessed twitching motility (via agar stab inoculation), pilus expression (SDS-PAGE/Western blots), intracellular pilin processing (time-course blots), phage susceptibility (plaque/spot assays), and protease secretion (skim milk clearance). Structural modeling with AlphaFold3 visualized mutation impacts. Reversion potential was tested by passaging mutants without phage pressure and sequencing revertants.
All PRMs lost motility, confirming T4P disruption. Most lacked surface pili despite intracellular PilA accumulation, but three retained pili: PilT Δ568-571 (retraction defect), PilB D388A (ATPase dominant-negative), and notably, PilD12 (four-amino-acid duplication).
The PilD Duplication: Core of Stealth Mode
The standout “stealth mode” mutant featured a 12-bp duplication in pilD, encoding the essential prepilin peptidase/methyltransferase. This insertion (F184-V187) in a helix distant from active sites delayed PilA processing, leading to uncleaved pilin accumulation by 6-8 hours post-subculture. This buildup inhibited the PilSR two-component system, suppressing new prepilin expression and limiting functional T4P assembly—conferring resistance without total loss.
A PilS N323A mutation, desensitizing to unprocessed pilins, restored motility and susceptibility. Smaller insertions (PilD3) allowed partial function; larger ones (PilD6/9) mimicked loss. Crucially, 45% of PilD12 colonies reverted to wild-type post-phage removal via slip-strand mispairing—a codon-modified variant did not revert, proving mechanism.
This reversible resistance explains potential clinical relapses, as bacteria “hide” pili temporarily then reactivate virulence.
Phage Steering: Therapeutic Silver Lining
Unlike irreversible antibiotic resistance, T4P loss often reduces fitness and virulence, aligning with “phage steering”—phages evolve bacteria into less harmful forms. Professor Burrows notes: “Phages don’t necessarily need to kill their targets to prevent or alleviate infection.” However, temporary modes like PilD12 complicate this, urging multi-phage cocktails targeting diverse receptors (e.g., T4P + LPS).
In Canada, phages remain unapproved but accessed via compassionate Special Access Program (SAP). McMaster links to both approvals: phages for S. epidermidis PJI (Ottawa Hospital, trained by Gerry Wright) and E. coli UTI (Burrows lab isolation).Read the full mBio paper here.
McMaster University’s Vanguard in Canadian Phage Innovation
McMaster’s IIDR positions the university as Canada’s phage research hub. Burrows’ lab, alongside Wright’s, advances phage-bacteria dynamics, gel-based phage delivery, and resistance evolution. Recent works include phage-antibiotic synergy and accessible storage methods.
Collaborations with NRC and U of T amplify impact, fostering a national ecosystem. Student trainees like Hanjeong Harvey (first author), Tanisha Lahane, and Veronica Tran exemplify McMaster’s mentorship, transitioning to PhDs or industry.
Pseudomonas aeruginosa: Canada’s AMR Priority Pathogen
In Canada, P. aeruginosa drives significant AMR burden, with 2,840 deaths attributable and 13,300 associated in 2021 alone. Surveillance shows stable but high resistance rates; CARSS 2025 highlights it among priority pathogens, with MDR prevalence varying regionally (e.g., higher in hospitals).
Universities like McMaster bridge gaps, informing policy via evidence on resistance trade-offs.
Navigating Resistance Challenges in Phage Therapy Development
Bacterial resilience mirrors AMR crisis; Canada projects 40% first-line resistance by future years. Phage cocktails mitigate single-receptor escape, but regulatory hurdles persist—Health Canada’s SAP limits scale-up. Canadian research emphasizes personalized matching, high-throughput platforms.
McMaster’s findings urge reversion monitoring, multi-target strategies.
Future Outlook: Phage Therapy at Canadian Universities
With Phage Canada symposia and U of T hubs, momentum builds. McMaster eyes clinical trials, integrating stealth insights for safer therapies. Broader implications: reduced antibiotic reliance, preserving efficacy amid 26-40% resistance rise.
Stakeholders—from CIHR funders to hospitals—view universities as innovators, training next-gen researchers.
Training Tomorrow’s Infectious Disease Experts at McMaster
Graduate programs in Biochemistry & Biomedical Sciences equip students like Tran: “This was completely unexpected... capable of developing temporary phage resistance.” McMaster’s co-op, IIDR seminars foster interdisciplinary skills for AMR careers.
- Hands-on WGS, phenotypic assays build expertise.
- Collaborations with clinical partners translate bench-to-bedside.
- Alumni lead phage initiatives nationwide.
