Osmotic conditions shape fitness gains and resistance mechanisms during E. coli and T4 phage co-evolution

Bacteriophages (phages) – viruses that infect bacteria – offer a promising approach to combating antimicrobial resistance in the gut by targeting specific harmful bacteria while preserving the wider microbiota. This study examines how variations in osmolality (solute concentration), which can occur in the gut due to factors like food intolerances and laxative use, affect the in-vitro co-evolution of T4 phage and its host E. coli. When evolved independently, we observed large fitness gains in both bacteria (growth rate) and phages (productivity), especially in higher osmolalities. Phage productivity in particular increased dramatically – up to two orders of magnitude. Simple mathematical modelling was used to explore possible explanations for this observation. During co-evolution, fitness gains were more modest, and phage resistance emerged in all co-evolved bacteria populations. Resistance mechanisms varied by osmolality: in lower osmolalities, mutations affected phage binding sites, conferring strong resistance, while in higher osmolalities, mutations led to colonic acid overproduction and a mucoid phenotype with weaker resistance. This phenotype is linked to increased virulence, underscoring the need to consider environmental factors when designing phage therapies.