We found that cells of Vibrio parahaemolyticus possess an energy-dependent efflux system for norfloxacin. We cloned a gene for a putative norfloxacin efflux protein from the chromosomal DNA ofV. parahaemolyticus by using an Escherichia coli mutant lacking the major multidrug efflux system AcrAB as the host and sequenced the gene (norM). Cells of E. coli transformed with a plasmid carrying the norMgene showed elevated energy-dependent efflux of norfloxacin. The transformants showed elevated resistance not only to norfloxacin and ciprofloxacin but also to the structurally unrelated compounds ethidium, kanamycin, and streptomycin. These results suggest that this is a multidrug efflux system. The hydropathy pattern of the deduced amino acid sequence of NorM suggested the presence of 12 transmembrane domains. The deduced primary structure of NorM showed 57% identity and 88% similarity with that of a hypothetical E. coli membrane protein, YdhE. No reported drug efflux protein in the sequence databases showed significant sequence similarity with NorM. Thus, NorM seems to be a novel type of multidrug efflux protein. We cloned the ydhE gene from E. coli. Cells ofE. coli transformed with the cloned ydhE gene showed elevated resistance to norfloxacin, ciprofloxacin, acriflavine, and tetraphenylphosphonium ion, but not to ethidium, when MICs were measured. Thus, it seems that NorM and YdhE differ somehow in substrate specificity.

NorM of Vibrio parahaemolyticus apparently is a new type of multidrug efflux protein, with no significant sequence similarity to any known transport proteins. Based on the following experimental results, we conclude that NorM is an Na ؉ -driven Na ؉ /drug antiporter. Drug resistance, especially multidrug resistance, is presently a serious problem in hospitals. Drug efflux from cells is one of the major mechanisms of drug resistance in both prokaryotes and eukaryotes (11,12,15,18,26). Many drug efflux systems are known to exist in the biological world, and these transporters can be divided into four families: the major facilitator (MF) family, the small multidrug resistance (SMR) family, the resistance nodulation cell division (RND) family, and the ATP binding cassette family (4,6,17). Membrane transporters of the MF family possess 12 to 14 transmembrane domains. Transporters of the SMR family are rather small and usually possess four transmembrane domains. Transporters of the RND family require multiple components to function effectively. An electrochemical potential of H ϩ across cell membranes seems to be the driving force for drug efflux by members of the MF, SMR, and RND families of transporters (13,18,28,29). ATP is utilized as the energy donor in members of the ATP binding cassette family of multidrug efflux pumps (3, 26).The electrochemical potential of H ϩ across cell membranes is established mainly by the respiratory chain in aerobic or facultative anaerobic bacteria. The electrochemical potential of H ϩ across the membrane is converted to that of Na ϩ by Na ϩ /H ϩ antiporters (25,27). Both of the electrochemical potentials of H ϩ and Na ϩ across cell membranes can be utilized to drive solute uptake in bacterial cells. Solutes are taken up into cells by an H ϩ /substrate symport mechanism or an Na ϩ / substrate symport mechanism (19). An electrochemical potential of H ϩ is also utilized to drive extrusion of substrate from cells. Most multidrug efflux pumps in bacteria are driven by H ϩ , which is a mechanism for H ϩ /drug antiport (18). However, no Na ϩ -driven extrusion system for drugs, i.e., no Na ϩ / drug antiporter, has been reported for bacterial cell membranes. Although an Na ϩ /Ca 2ϩ exchanger (16) and an Na ϩ / urea antiporter (9) have been reported for animal cells, no Na ϩ /drug antiporter has been reported for animal cells. Vibrio parahaemolyticus, a slightly halophilic marine bacterium, is one of the major causes of food poisoning in Japan and many other countries (14). This microorganism requires Na ϩ for its growth (2). Energy metabolism and energy coupling in membranes of this microorganism are unique (20). Cells of V. parahaemolyticus possess a primary respiratory Na ϩ pump (24) and Na ϩ -coupled membrane processes, such as an Na ϩ / solute symporter (21,22,24) and an Na ϩ -driven flagellar motor (1). We thought that Na ϩ /drug antiporters might exist in this marine organism.If an Na ϩ /drug antiporter were to exist, it would be anticipated that (i) Na ϩ would stimulate drug efflux fro...

Two new genes (mexXY) similar to mexAB,mexCD, and mexEF and mediating multidrug resistance were cloned from the chromosome of Pseudomonas aeruginosa. Elevated ethidium extrusion was observed withEscherichia coli cells harboring the plasmid carryingmexXY. This MexXY system confers higher resistance to fluoroquinolones than the MexAB and MexCD systems, and E. coli TolC or P. aeruginosa OprM is necessary for the function of the MexXY system.

We detected chloramphenicol/H+ antiport activity in membrane vesicles of Escherichia coli and cloned a gene for the antiporter from chromosomal DNA of E. coli. Introduction of the gene into E. coli cells conferred resistance to chloramphenicol and ethidium. A slight increase in resistance to acridine orange was also observed. Elevated chloramphenicol efflux and ethidium efflux were observed in cells harboring a plasmid carrying the gene. Addition of chloramphenicol to the assay mixture reduced the efflux of ethidium. Elevated chloramphenicol/H+ antiport activity was observed in membrane vesicles prepared from cells harboring the plasmid. The pH optimum for the activity was 6.5. We sequenced the gene and deduced the amino acid sequence of its product. A sequence homology search revealed that it was same as that of Cmr (or MdfA). Thus, it became clear that Cmr (MdfA) is the chloramphenicol(and ethidium)/H+ antiporter.