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Background  Resistance Nodulation Division (RND) efflux pumps of  Escherichia coli  extrude antibiotics and toxic substances before they reach their intended targets. Whereas these pumps obtain their energy directly from the proton motive force (PMF), ATP-Binding Cassette (ABC) transporters, which can also extrude antibiotics, obtain energy from the hydrolysis of ATP. Because  E. coli  must pass through two pH distinct environments of the gastrointestinal system of the host, it must be able to extrude toxic agents at very acidic and at near neutral pH (bile salts in duodenum and colon for example). The herein described study examines the effect of pH on the extrusion of ethidium bromide (EB).    Methodology/Principal Findings    E. coli  AG100 and its tetracycline induced progeny AG100 TET  that over-expresses the  acrAB  efflux pump were evaluated for their ability to extrude EB at pH 5 and 8, by our recently developed semi-automated fluorometric method. At pH 5 the organism extrudes EB without the need for metabolic energy (glucose), whereas at pH 8 extrusion of EB is dependent upon metabolic energy. Phe-Arg β-naphtylamide (PAβN), a commonly assumed inhibitor of RND efflux pumps has no effect on the extrusion of EB as others claim. However, it does cause accumulation of EB. Competition between EB and PAβN was demonstrated and suggested that PAβN was preferentially extruded. A K m  representing competition between PAβN and EB has been calculated.    Conclusions/Significance  The results suggest that  E. coli  has two general efflux systems (not to be confused with a distinct efflux pump) that are activated at low and high pH, respectively, and that the one at high pH is probably a putative ABC transporter coded by  msbA , which has significant homology to the ABC transporter coded by  efrAB  of  Enterococcus faecalis , an organism that faces similar challenges as it makes its way through the toxic intestinal system of the host.

pH Modulation of Efflux Pump Activity of Multi-Drug Resistant Escherichia coli: Protection During Its Passage and Eventual Colonization of the Colon