Molecular mechanisms of higher MICs of antibiotics and quaternary ammonium compounds for Escherichia coli isolated from bacteraemia

Abstract

A previous study identified an association between high MICs of quaternary ammonium compounds (QACs) and antibiotic resistance. The current aim was to investigate the genetic background of this association. Of 153 Escherichia coli clinical strains, seven were selected for their low or high MICs of antibiotics and/or QACs. Integron resistance gene contents were identified by sequencing after PCR amplification. The genes encoding the efflux pump AcrA/TolC and its regulatory regions marA, marO, marR, soxS and rob were sequenced. The gene expression of acrA, tolC, marA, marOR, soxS and rob was assessed by quantitative real-time PCR. MICs in the presence and absence of the efflux pump inhibitor phenyl-arginine-β-naphthylamide (PAβN) were compared. Of the seven strains, five were resistant to amoxicillin, amoxicillin/clavulanic acid and/or co-trimoxazole (trimethoprim/sulfamethoxazole) and/or had high MICs of ciprofloxacin and QACs. Four of the five harboured a class 1 integron (intI1). In three of these four strains, the presence of dfrA/sul1 and qacEΔ1 gene cassettes correlated with resistance to co-trimoxazole and high MICs of QACs. In all of the five strains, overexpression of tolC, marOR and soxS was always associated with higher MICs of antibiotics and/or QACs. PAβN reduced the MICs of ciprofloxacin and QACs, suggesting that extrusion of ciprofloxacin and QACs from bacteria depends on the AcrAB-TolC system. To our knowledge, this report is the first to describe dual involvement of the AcrAB-TolC system and class 1 integrons in clinical E. coli strains.