Fitness costs of phage-driven resistance mutations in Salmonella Enteritidis populations

ABSTRACT In the post-antibiotic era, bacteriophages have emerged as viable alternatives for combating antibiotic-resistant infections. Analogous to the emergence of antibiotic resistance, bacteria can also develop resistance to phages, a process often accompanied by a reduction in bacterial fitness. In this study, we investigated the mechanisms behind the development of phage resistance in Salmonella enterica serovar Enteritidis strain WJ48 during laboratory coevolution with phage GRNsp8, as well as the associated fitness costs and community dynamics. Three types of phage-resistant mutants, VP81, VP82, and VP84, were isolated. VP81 exhibited a frameshift mutation in the gene encoding glycosyltransferase and displayed partial resistance to GRNsp8. Both VP82 and VP84 carried frameshift mutations in btuB , conferring complete resistance to GRNsp8. Deletion of btuB abolished phage adsorption and conferred complete resistance. Meanwhile, complementation with btuB restored susceptibility. In vitro competition assays showed that the btuB -deletion strain was competitively disadvantaged relative to the wild-type strain within 24 h, exhibiting a relative fitness value of