Tolerance to the antimicrobial peptide colistin in <i>Pseudomonas aeruginosa</i> biofilms is linked to metabolically active cells, and depends on the <i>pmr</i> and <i>mexAB‐oprM</i> genes

TLDR

Biofilm‐forming bacteria are known to tolerate antimicrobial agents, and while antimicrobial peptides are promising therapeutics, the mechanisms underlying biofilm tolerance to these peptides remain poorly understood. In Pseudomonas aeruginosa biofilms, a metabolically active subpopulation develops colistin tolerance through pmr‑mediated lipopolysaccharide modification and mexAB‑oprM efflux, whereas low‑metabolic cells are killed; mutants lacking these pathways fail to form tolerant subpopulations, and combining colistin with ciprofloxacin or tetracycline eradicates all biofilm cells.

Abstract

Summary Bacteria living as biofilm are frequently reported to exhibit inherent tolerance to antimicrobial compounds, and might therefore contribute to the persistence of infections. Antimicrobial peptides are attracting increasing interest as new potential antimicrobial therapeutics; however, little is known about potential mechanisms, which might contribute to resistance or tolerance development towards these compounds in biofilms. Here we provide evidence that a spatially distinct subpopulation of metabolically active cells in Pseudomonas aeruginosa biofilms is able to develop tolerance to the antimicrobial peptide colistin. On the contrary, biofilm cells exhibiting low metabolic activity were killed by colistin. We demonstrate that the subpopulation of metabolically active cells is able to adapt to colistin by inducing a specific adaptation mechanism mediated by the pmr operon, as well as an unspecific adaptation mechanism mediated by the mexAB‐oprM genes. Mutants defective in either pmr ‐mediated lipopolysaccharide modification or in mexAB‐oprM ‐mediated antimicrobial efflux were not able to develop a tolerant subpopulation in biofilms. In contrast to the observed pattern of colistin‐mediated killing in biofilms, conventional antimicrobial compounds such as ciprofloxacin and tetracycline were found to specifically kill the subpopulation of metabolically active biofilm cells, whereas the subpopulation exhibiting low metabolic activity survived the treatment. Consequently, targeting the two physiologically distinct subpopulations by combined antimicrobial treatment with either ciprofloxacin and colistin or tetracycline and colistin almost completely eradicated all biofilm cells.