We cloned a gene, abeM, for a multidrug efflux pump from Acinetobacter baumannii using Escherichia coli as the host. Sequence analysis revealed that AbeM is a member of the MATE family of pumps. AbeM was found to be an H ؉ -coupled multidrug efflux pump and a unique member of the MATE family.Acinetobacter spp. are emerging as a major cause of nosocomial infections or in outbreaks of cross-infection, particularly in intensive care units, where the use of antimicrobial agents is greatest and the host is most susceptible. Acinetobacter baumannii is one of the Acinetobacter spp. that is the most frequently isolated from human infections (3). Certain strains of A. baumannii are now resistant to many commonly prescribed antibiotics (including imipenem and fluoroquinolones), and the multiple-drug resistance is often responsible for the failure of antibiotic therapy.Among the drug resistance mechanisms in A. baumannii, degradation of drugs seems to be a major cause because of the production of relevant enzymes in this organism (1,13,18,24). Modification of drugs is also common in this organism due to the synthesis of specific modifying enzymes (19,20). Alterations of the drug target due to mutations in gyrA and parC have been associated with high levels of resistance to fluoroquinolones (21,22). Recently, decreased expression of the outer membrane proteins has been reported to be involved in resistance to some drugs (6,7,12). Studies of drug resistance mediated by an efflux pump(s) are very few; only one efflux system, AdeABC, that mediates multidrug resistance has been identified and studied in detail in this organism (9, 14). Here we report on the gene cloning and biochemical characterization of a multidrug efflux pump from A. baumannii.Gene cloning. A. baumannii ATCC 19606 (generously provided by Shigeo Yamamoto, Okayama University) and Escherichia coli KAM32 which lacks the major multidrug efflux pumps AcrAB and YdhE (5) were used in this study. To identify a putative multidrug efflux transporter(s) in A. baumannii, we tried to clone the gene(s) responsible for multidrug resistance. Fragments of chromosomal DNA from strain ATCC 19606 were prepared and ligated to a vector plasmid, pBR322. The resulting recombinant plasmids that were obtained (five candidates) enabled E. coli KAM32 cells to grow in the presence of 10 g/ml of ethidium bromide. We chose one of the plasmids, pABE6, and constructed several deletion plasmids. The shortest deletion plasmid that conferred resistance to ethidium bromide, pABE618, was further analyzed. As a result of DNA sequencing, we found one open reading frame, designated abeM. Several promoter-like sequences and a Shine-Dalgarno sequence preceded the abeM gene. The deduced AbeM protein consists of 448 amino acid residues, is very rich in hydrophobic amino acid residues, and possesses 12 hydrophobic regions.Recently, the complete genome sequence of Acinetobacter ADP1 was published (2). The AbeM protein showed sequence homology (79% identity, 94% similarity) with the sequence of ACIAD0429, ...