Colistin- and octenidine-induced selective pressure on Pseudomonas aeruginosa clinical isolates in vitro: Antimicrobial activity and mechanisms of cross-resistance

• Octenidine is a very effective antiseptic against multidrug-resistant Pseudomonas aeruginosa clinical isolates • Decreased susceptibility to octenidine can arise upon selective pressure through dysregulation of efflux pump encoding genes • Such mutants could only obtained using sub-inhibitory octenidine concentrations which are far below the ones used in clinical practice • Decreased susceptibility to octenidine did not induce co-lateral decreased susceptibility to chlorhexidine or colistin Infection prevention using antiseptics is crucial to control transmission of multidrug-resistant Gram-negative pathogens. This in vitro study aimed to evaluate: (1) the susceptibility to octenidine of 104 Pseudomonas aeruginosa (including multidrug-resistant) clinical isolates (OCT); (2) whether resistant mutants can be obtained upon selective pressure using increasing concentrations of OCT and colistin (COL); (3) genetic basis of underlying resistance mechanisms; (4) co-lateral effects on various antibiotics and chlorhexidine (CHX); (5) efficacy of OCT-based products in mutant strains according to EN13727. OCT showed high activity against all isolates. Long-term exposure of four P. aeruginosa strains to OCT resulted in five mutants exhibiting predominantly mutations in genes involved in regulation of efflux-pumps. No co-lateral effect of OCT-mutants on CHX, COL or other antibiotics was detected. OCT as well as OCT-based products were still fully effective in all OCT-mutants under “dirty conditions” within 1 min contact-time. Using COL as selective molecule, four mutants exhibiting decreased susceptibility to COL were recovered, presenting mutations in pmrB . Cross-resistance to gentamicin was observed in two mutants, but not to OCT, CHX and other antibiotics. Observed rare mutations triggered in P. aeruginosa by OCT upon in vitro long-term exposure had no impact on the efficacy of OCT and did not select for other antiseptic or antibiotic cross-resistance.