Stable coexistence of Citrobacter rodentium with a lytic bacteriophage during in vivo murine infection

Bacteriophages are ubiquitously present in bacterial communities, yet phage-bacteria interactions in complex environments like the gut remain poorly understood. While antibiotic resistance is driving a renewed interest in phage therapy, most studies have been conducted in in vitro systems, offering limited insight into the complexity of such dynamics in physiological contexts. Here, we use Citrobacter rodentium (CR), a natural mouse-restricted enteric pathogen and well-established model for human enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC) infections, to investigate phage-pathogen interactions in vivo . We isolate and characterise Eifel2, a novel lytic phage infecting CR, and generate anti-phage specific antibodies that enable the visualisation of phage infections in vitro . In a murine model of CR infection, oral administration of Eifel2 led to robust phage replication in the gut without reducing the bacterial burden or infection-associated inflammation, confirming the establishment of a stable coexistence in the gut. Despite the emergence of a sub-population of phage-resistant CR mutants in vivo , they did not undergo clonal expansion, indicating that additional selective pressures impaired their widespread dissemination in the gut. Together, our findings demonstrate that imaging approaches can capture key infection stages in vitro , while in vivo models are essential for capturing the complexity of phage-bacteria interactions. This work highlights the importance of studying phage therapy in host-pathogen contexts that include a normal microbiota and a suitable host environment, where dynamic co-existence rather than eradication may define therapeutic outcomes.