The efficacy of antimicrobial agents is decreased in a polymicrobial environment

Abstract The airways of people with cystic fibrosis (CF) often harbour diverse polymicrobial communities. These airway infections can be impossible to resolve though antibiotic intervention, even though isolates of the individual species present are susceptible to the treatment when tested in vitro. This suggests that susceptibility to antimicrobial agents may be altered in the presence of other microbial species. In this work, we investigate how polymicrobial cultures of key CF-associated species respond to challenge with species-specific antimicrobial agents; colistin (targets Pseudomonas aeruginosa), fusidic acid (targets Staphylococcus aureus) and fluconazole (targets Candida albicans). We found that, compared with growth in axenic cultures, the target organism was protected (sometimes by several orders of magnitude) from the effect(s) of the antimicrobial agent when grown in a polymicrobial culture. This decreased antimicrobial efficacy in polymicrobial cultures was found to have both phenotypic and inherited components. Whole genome sequencing of the colistin-resistant P. aeruginosa isolates revealed single nucleotide polymorphisms and indels in genes encoding lipopolysaccharide (LPS) biosynthesis or pilus biogenesis. Colistin resistance associated with loss-of-function mutations in the LPS biosynthetic gene, wzy, could be complemented by expression of the wild-type wzy gene in trans. Our findings indicate that the polymicrobial nature of the CF airways is likely to have a significant impact on the clinical response to antimicrobial therapy.