Phage resistance of Bordetella avium due to an altered cell wall results in increased susceptibility to the polypeptide antibiotics colistin and polymyxin B

Infections caused by multidrug-resistant bacterial pathogens are becoming increasingly frequent, so alternative therapeutic strategies such as phage therapy are gaining in importance. However, one of the main challenges in phage therapy is the rapid emergence of phage-resistant bacteria. Hence, this study investigated the occurrence of phage resistance in Bordetella avium, the causative agent of bordetellosis in poultry of all ages, with a special focus on the association between bacterial phage resistance and antibiotic resistance. Here, we describe the isolation and analysis of four B. avium mutants that showed resistance to B. avium phages, with decreased adsorption leading to phage resistance of the mutants. SDS-PAGE analysis revealed a lipopolysaccharide (LPS) structure with altered nuclear patterns and a missing O-antigen moiety in the phage-resistant B. avium mutants. Whole-genome sequence analysis revealed mutations in the genetic loci BAV2233 and BAV2235, encoding lipopolysaccharide glucosyltransferase family 4 proteins, and BAV0511, encoding Vi polysaccharide biosynthesis UDP-N-acetylglucosamine C-6 dehydrogenase TviB protein, which are involved in LPS biosynthesis. Additionally, antimicrobial susceptibility testing against nine antibiotics using the agar dilution method showed that LPS changes in phage-resistant mutants affected the susceptibility of B. avium to two of the antibiotics examined. Specifically, the MIC values of the polymyxin antibiotics colistin and polymyxin B decreased by up to five two-fold dilutions, depending on the isolate considered. An association was observed between the length of the polysaccharide chain in LPS and the susceptibility of B. avium to colistin and polymyxin B, with shorter chains indicating higher susceptibility. Collective findings illuminate a beneficial aspect that may support the use of B. avium phages. Based on these results, genetically modified phages based on these B. avium phages would be a promising direction for future phage applications as an alternative to the use of antimicrobial agents.