Moraxella catarrhalis AcrAB-OprM Efflux Pump Contributes to Antimicrobial Resistance and Is Enhanced during Cold Shock Response

Abstract

ABSTRACTMoraxella catarrhalisis a common pathogen of the human respiratory tract. Multidrug efflux pumps play a major role in antibiotic resistance and virulence in many Gram-negative organisms. In the present study, the role of the AcrAB-OprM efflux pump in antibiotic resistance was investigated by constructing mutants that lack theacrA,acrB, andoprMgenes inM. catarrhalisstrain O35E. We observed a moderate (1.5-fold) decrease in the MICs of amoxicillin and cefotaxime and a marked (4.7-fold) decrease in the MICs of clarithromycin foracrA,acrB, andoprMmutants in comparison with the wild-type O35E strain. Exposure of theM. catarrhalisstrains O35E and 300 to amoxicillin triggered an increased transcription of all AcrAB-OprM pump genes, and exposure of strains O35E, 300, and 415 to clarithromycin enhanced the expression ofacrAandoprMmRNA. Inactivation of the AcrAB-OprM efflux pump genes demonstrated a decreased ability to invade epithelial cells compared to the parental strain, suggesting thatacrA,acrB, andoprMare required for efficient invasion of human pharyngeal epithelial cells. Cold shock increases the expression of AcrAB-OprM efflux pump genes in all threeM. catarrhalisstrains tested. Increased expression of AcrAB-OprM pump genes after cold shock leads to a lower accumulation of Hoechst 33342 (H33342), a substrate of AcrAB-OprM efflux pumps, indicating that cold shock results in increased efflux activity. In conclusion, the AcrAB-OprM efflux pump appears to play a role in the antibiotic resistance and virulence ofM. catarrhalisand is involved in the cold shock response.