Abstract Aims To investigate the priming effects of sub‐inhibitory concentrations of biocides on antibiotic resistance in bacteria. Methods and results Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus were exposed to sub‐inhibitory concentrations of biocides via a gradient plate method. Minimum inhibitory concentration (MIC) and antibiotic susceptibility were determined, and efflux pump inhibitors (thioridazine and chlorpromazine) were used to investigate antibiotic resistance mechanism(s). Escherichia coli displayed a twofold increase in MIC (32–64 mg l−1) to H2O2 which was stable after 15 passages, but lost after 6 weeks, and P. aeruginosa displayed a twofold increase in MIC (64–128 mg l−1) to BZK which was also stable for 15 passages. There were no other tolerances observed to biocides in E. coli, P. aeruginosa or S. aureus; however, stable cross‐resistance to antibiotics was observed in the absence of a stable increased tolerance to biocides. Sixfold increases in MIC to cephalothin and fourfold to ceftriaxone and ampicillin were observed in hydrogen peroxide primed E. coli. Chlorhexidine primed S. aureus showed a fourfold increase in MIC to oxacillin, and glutaraldehyde‐primed P. aeruginosa showed fourfold (sulphatriad) and eightfold (ciprofloxacin) increases in MIC. Thioridazine increased the susceptibility of E. coli to cephalothin and cefoxitin by fourfold and twofold, respectively, and both thioridazine and chlorpromazine increased the susceptibility S. aureus to oxacillin by eightfold and fourfold, respectively. Conclusions These findings demonstrate that sub‐inhibitory concentrations of biocides can prime bacteria to become resistant to antibiotics even in the absence of stable biocide tolerance and suggests activation of efflux mechanisms may be a contributory factor. Significance and Impact of the Study This study demonstrates the effects of low‐level exposure of biocides (priming) on antibiotic resistance even in the absence of obvious increased biocidal tolerance.
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