Summary
A bioRxiv preprint reports that trimethoprim reduced phage production and toxin production in laboratory experiments with a clinical Shiga toxin-producing E. coli isolate. The effect persisted alongside DNA-damaging antibiotics that normally increase phage activity.
A bioRxiv preprint reports that trimethoprim suppressed the activity of toxin-carrying phages in Shiga toxin-producing Escherichia coli (STEC), reducing phage production by approximately 100-fold in laboratory experiments. The effect persisted even when the bacteria were exposed to DNA-damaging antibiotics that normally increase phage activity.
The researchers, Ayesha Umair and Alexander P. Hynes of McMaster University, also observed reduced toxin production in a clinical STEC isolate. The findings point to trimethoprim as a possible lead for limiting the toxin amplification that can make some STEC infections dangerous, although the work is currently a preclinical laboratory study reported in a research preprint.
Why phages matter in STEC infections
STEC cause disease partly through Shiga toxins. The genes encoding these toxins are carried by bacteriophages, viruses that infect bacteria. When the phages become active, they can replicate inside the bacterial cell and promote the production and release of toxin-related genetic material.
This biology creates a treatment problem. The authors explain that some antibiotics damage bacterial DNA and activate bacterial stress responses, which can increase phage activity and, in turn, toxin production. For patients at particular risk of severe complications, including children and people who are immunocompromised, the potential for increased toxin production is a major reason the authors describe antibiotic treatment as contraindicated in this setting. Supportive care and fluid replacement are therefore central to current management described in the preprint.
The new work tested whether phage activity could instead be suppressed while the bacteria were exposed to conditions that normally stimulate it.
How trimethoprim affected phage replication
The experiments found that both acid exposure and trimethoprim strongly reduced phage production, by roughly two orders of magnitude. Trimethoprim retained this suppressive effect in the presence of antibiotics that increase phage activity.
The researchers linked the effect to a stage of the phage life cycle. Trimethoprim allowed the earliest steps of phage induction—the process that activates a dormant phage—to occur, but then completely blocked replication of the phage genome. Without genomic replication, the phage cannot efficiently produce new virus particles.
The suppression also appeared to operate independently of the canonical bacterial SOS response, a well-established DNA-damage response involved in phage activation. The authors found that trimethoprim’s effect partially depended on a glutamate-dependent bacterial acid-stress response, suggesting that the drug changes the bacterial environment in a way that interrupts phage replication.
The researchers then extended the experiments to a clinical STEC isolate. In that model, trimethoprim reduced toxin production, including when DNA-damaging antibiotics were present. This connects the molecular observation—blocked phage replication—to the clinically important output of lower toxin production in the tested strain.
What the finding means
The study identifies a potential strategy for separating antibiotic treatment from the phage-driven toxin increase that complicates STEC infections. Rather than targeting the toxin directly, trimethoprim appears to interfere with the virus-like genetic element responsible for carrying and amplifying toxin-related genes.
The evidence is limited to laboratory experiments, including work with one clinical STEC isolate, so the result is a research lead rather than an established treatment approach. Its relevance to patients, different STEC strains, dosing, safety and clinical outcomes would require further investigation. The report is a bioRxiv preprint and has not been presented here as clinical evidence.