Substrate specificity of the OqxAB multidrug resistance pump in Escherichia coli and selected enteric bacteria.

OBJECTIVES A plasmid-encoded multidrug efflux pump, OqxAB, identified in Escherichia coli of porcine origin, was tested for substrate specificity against selected antibiotics, detergents and disinfectants. The ability of horizontal transfer to food-borne pathogens of the Enterobacteriaceae family was also investigated. METHODS The MICs of selected substrates were determined with a broth dilution assay using two isogenic E. coli strains, except for the presence of the oqxAB operon. A derivative of the plasmid encoding OqxAB (pOLA52) was constructed and horizontal transfer to Salmonella Typhimurium, Klebsiella pneumoniae, Kluyvera sp. and Enterobacter aerogenes was investigated. The effect of the presence of the OqxAB pump on susceptibility for selected compounds was investigated using broth dilution assays. RESULTS The OqxAB pump conferred antimicrobial resistance or reduced susceptibility towards a variety of substrates in E. coli. These included animal growth promoters, antimicrobials, disinfectants and detergents. pOLA52 could readily be transferred to enterobacterial pathogens. Transconjugants showed reduced susceptibility towards chloramphenicol, ciprofloxacin and olaquindox. CONCLUSIONS The plasmid-encoded OqxAB pump has a wide substrate specificity and can be transferred between Enterobacteriaceae conferring reduced susceptibility to a multitude of substrates. These results could indicate some dependence on the outer membrane proteins present in the different species.