Active efflux mechanisms for antimicrobial resistance

Proton-dependent multidrug efflux systems

Multidrug efflux systems display the ability to transport a variety of structurally unrelated drugs from a cell and consequently are capable of conferring resistance to a diverse range of chemotherapeutic agents. This review examines multidrug efflux systems which use the proton motive force to drive drug transport. These proteins are likely to operate as multidrug/proton antiporters and have been identified in both prokaryotes and eukaryotes. Such proton-dependent multidrug efflux proteins belong to three distinct families or superfamilies of transport proteins: the major facilitator superfamily (MFS), the small multidrug resistance (SMR) family, and the resistance/ nodulation/cell division (RND) family. The MFS consists of symporters, antiporters, and uniporters with either 12 or 14 transmembrane-spanning segments (TMS), and we show that within the MFS, three separate families include various multidrug/proton antiport proteins. The SMR family consists of proteins with four TMS, and the multidrug efflux proteins within this family are the smallest known secondary transporters. The RND family consists of 12-TMS transport proteins and includes a number of multidrug efflux proteins with particularly broad substrate specificity. In gram-negative bacteria, some multidrug efflux systems require two auxiliary constituents, which might enable drug transport to occur across both membranes of the cell envelope. These auxiliary constituents belong to the membrane fusion protein and the outer membrane factor families, respectively. This review examines in detail each of the characterized proton-linked multidrug efflux systems. The molecular basis of the broad substrate specificity of these transporters is discussed. The surprisingly wide distribution of multidrug efflux systems and their multiplicity in single organisms, with Escherichia coli, for instance, possessing at least nine proton-dependent multidrug efflux systems with overlapping specificities, is examined. We also discuss whether the normal physiological role of the multidrug efflux systems is to protect the cell from toxic compounds or whether they fulfil primary functions unrelated to drug resistance and only efflux multiple drugs fortuitously or opportunistically.

GLOBAL CHANGE AND HUMAN SUSCEPTIBILITY TO DISEASE

▪ Abstract  Although the loss of good health is inherently unpredictable, human behavior at the individual and societal levels profoundly influences the incidence and evolution of disease. In this review, we define the human epidemiological environment and describe key biophysical, economic, sociocultural, and political factors that shape it. The potential impact upon the epidemiological environment of biophysical aspects of global change—changes in the size, mobility, and geographic distribution of the human population; land conversion; agricultural intensification; and climate change—is then examined. Human vulnerability to disease is strongly and deleteriously influenced by many of these ongoing, intensifying alterations. We then examine threats to human defenses against disease, including immune suppression, loss of biodiversity and indigenous knowledge, and the evolution of antibiotic resistance. Effective responses will require greatly enhanced attention by and collaboration among experts in diverse academic disciplines, in the private sector, and in government worldwide.

Contributions of individual mechanisms to fluoroquinolone resistance in 36 Escherichia coli strains isolated from humans and animals

Twenty-eight human isolates of Escherichia coli from Argentina and Spain and eight veterinary isolates received from the Ministry of Agriculture Fisheries and Foods in the United Kingdom required 2 to > 128 micrograms of ciprofloxacin per ml for inhibition. Fragments of gyrA and parC encompassing the quinolone resistance-determining region were amplified by PCR, and the DNA sequences of the fragments were determined. All isolates contained a mutation in gyrA of a serine at position 83 (Ser83) to an Leu, and 26 isolates also contained a mutation of Asp87 to one of four amino acids: Asn (n = 14), Tyr (n = 6), Gly (n = 5), or His (n = 1). Twenty-four isolates contained a single mutation in parC, either a Ser80 to Ile (n = 17) or Arg (n = 2) or a Glu84 to Lys (n = 3). The role of a mutation in gyrB was investigated by introducing wild-type gyrB (pBP548) into all isolates; for three transformants MICs of ciprofloxacin were reduced; however, sequencing of PCR-derived fragments containing the gyrB quinolone resistance-determining region revealed no changes. The analogous region of parE was analyzed in 34 of 36 isolates by single-strand conformational polymorphism analysis and sequencing; however, no amino acid substitutions were discovered. The outer membrane protein and lipopolysaccharide profiles of all isolates were compared with those of reference strains, and the concentration of ciprofloxacin accumulated (with or without 100 microM carbony cyanide m-chlorophenylhydrazone [CCCP] was determined. Twenty-two isolates accumulated significantly lower concentrations of ciprofloxacin than the wild-type E. coli isolate; nine isolates accumulated less then half the concentration. The addition of CCCP increased the concentration of ciprofloxacin accumulated, and in all but one isolate the percent increase was greater than that in the control strains. The data indicate that high-level fluoroquinolone resistance in E. coli involves the acquisition of mutations at multiple loci.

Tetracycline resistance determinants: mechanisms of action, regulation of expression, genetic mobility, and distribution

Tetracycline-resistant bacteria were first isolated in 1953 from Shigella dysenteriae, a bacterium which causes bacterial dysentery. Since then tetracycline-resistant bacterial have been found in increasing numbers of species and genera. This has resulted in reduced effectiveness of tetracycline therapy over time. Tetracycline resistance is normally due to the acquisition of new genes often associated with either a mobile plasmid or a transposon. These tetracycline resistance determinants are distinguishable both genetically and biochemically. Resistance is primarily due to either energy-dependent efflux of tetracycline or protection of the ribosomes from the action of tetracycline. Gram-negative tetracycline efflux proteins are linked to repressor proteins which in the absence of tetracycline block transcription of the repressor and structural efflux genes. In contrast, expression of the Gram-positive tetracycline efflux genes and some of the ribosomal protection genes appears to be regulated by attenuation of mRNA transcription. Specific tetracycline resistance genes have been identified in 32 Gram-negative and 22 Gram-positive genera. Tetracycline-resistant bacteria are found in pathogens, opportunistic and normal flora species. Tetracycline-resistant bacteria can be isolated from man, animals, food, and the environment. The nonpathogens in each of these ecosystems may play an important role as reservoirs for the antibiotic resistance genes. It is clear that if we are to reverse the trend toward increasingly antibiotic-resistant pathogenic bacteria we will need to change how antibiotics are used in both human and animal health and food production.

Multidrug efflux in intrinsic resistance to trimethoprim and sulfamethoxazole in Pseudomonas aeruginosa

Pseudomonas aeruginosa possesses at least two multiple drug efflux systems which are defined by the outer membrane proteins OprM and OprJ. We have found that mutants overexpressing OprM were two- and eightfold more resistant than their wild-type parent to sulfamethoxazole (SMX) and trimethoprim (TMP), respectively. For OprJ-overproducing strains, MICs of TMP increased fourfold but those of SMX were unchanged. Strains overexpressing OprM, but not those overexpressing OprJ, became hypersusceptible to TMP and SMX when oprM was inactivated. The wild-type antibiotic profile could be restored in an oprM mutant by transcomplementation with the cloned oprM gene. These results demonstrate that the mexABoprM multidrug efflux system is mainly responsible for the intrinsic resistance of P. aeruginosa to TMP and SMX.

Emerging resistance to antibiotics: impact on respiratory infections in the outpatient setting

OBJECTIVE

To review the major mechanisms of antibiotic resistance and provide an overview of currently available oral antibiotics. This article discusses the impact of antibiotic resistance on respiratory infections (eg, otitis media, sinusitis, and pneumonia) encountered in the outpatient setting and appropriate management strategies for these infections are proposed.

DATA SOURCES

A Medline search was performed to identify recent references from the English language pertinent to this topic. Proposed management schemes for infections due to antibiotic-resistant bacteria were derived from current literature as well as the opinion of the authors.

RESULTS

Currently available oral antibiotics including penicillins, cephalosporins, macrolides, trimethroprim/sulfamethoxazole, and clindamycin continue to provide effective therapeutic options despite the increasing importance of antibiotic resistance in bacteria causing respiratory tract infections in the outpatient setting. The effectiveness of a given agent may no longer be assured, leading to the need for development of updated management strategies for patients experiencing treatment failure with antibiotics. Diagnostic strategies (ie, tympanocentesis) and novel treatment algorithms (eg, use of amoxicillin in combination with amoxicillin/clavulanate) for patients experiencing clinical failures potentially due to antibiotic-resistant bacteria are proposed.

CONCLUSIONS

The increasing prevalence of antibiotic resistant bacteria (especially penicillin-resistant Streptococcus pneumoniae) is leading to new approaches to the management of common respiratory infections in the outpatient setting. Additional experience and careful clinical trials are necessary to identify optimal management strategies for these infections.

Transmembrane glutamic acid residues play essential roles in the metal-tetracycline/H+ antiporter of Staphylococcus aureus

Three transmembrane aspartyl residues play essential roles in the transposon Tn10-encoded metal-tetracycline/H+ antiporter (Tet(B)) [Yamaguchi, A. et al. (1992) J. Biol. Chem. 267, 7490-7498]. The tetK gene-encoding tetracycline resistance protein (Tet(K)) of Staphylococcus aureus mediates metal-tetracycline/H+ antiport similarly to Tet(B); however, it has no transmembrane aspartyl residue. On the other hand, Tet(K) has three glutamyl residues, Glu-30, Glu-152 and Glu-397, in the putative transmembrane regions. In the present work, tet(K) gene was expressed in Escherichia coli and the transport activity was measured in everted membrane vesicles. When these glutamyl residues were replaced with Gln, the tetracycline transport activity was almost completely lost, indicating the important roles of these residues in Tet(K). In the case of Glu-397, even the charge-conserved mutation to Asp caused complete loss of the activity. On the other hand, the mutation of Glu-30 and Glu-152 to Asp resulted in significant retention of transport activity. These results are similar to those on the mutation of the three transmembrane aspartyl residues in Tet(B), indicating that the transmembrane glutamyl residues in Tet(K) play roles similar to those of the transmembrane aspartyl residues in Tet(B).

Gyrase mutations in laboratory-selected, fluoroquinolone-resistant mutants of Mycobacterium tuberculosis H37Ra

To characterize mechanisms of resistance to fluoroquinolones by Mycobacterium tuberculosis, mutants of strain H37Ra were selected in vitro with ofloxacin. Their quinolone resistance-determining regions for gyrA and gyrB were amplified and sequenced to identify mutations in gyrase A or B. Three types of mutants were obtained: (i) one mutant (TKp1) had no mutations in gyrA or gyrB; (ii) mutants that had single missense mutations in gyrA, and (iii) mutants that had two missense mutations resulting in either two altered gyrase A residues or an altered residue in both gyrases A and B. The TKp1 mutant had slightly reduced levels of uptake of [14C]norfloxacin, which was associated with two- to fourfold increases in the MICs of ofloxacin, ciprofloxacin, and sparfloxacin. Gyrase mutations caused a much greater increase in the MICs of fluoroquinolones. For mutants with single gyrA mutations, the increases in the MICs were 4- to 16-fold, and for mutants with double gyrase mutations, the MICs were increased 32-fold or more compared with those for the parent. A gyrA mutation in TKp1 secondary mutants was associated with 32- to 128-fold increases in the MICs of ofloxacin and ciprofloxacin compared with the MICs for H37Ra and an eight-fold increase in the MIC of sparfloxacin. Sparfloxacin was the most active fluoroquinolone tested. No sparfloxacin-resistant single-step mutants were selected at concentrations of > 2.5 micrograms/ml, and high-level resistance (i.e., MIC, > and = 5 micrograms/ml) was associated with two gyrase mutations. Mutations in gyrB and possibly altered levels of intracellular accumulation of drug are two additional mechanisms that may be used by M. tuberculosis in the development of fluoroquinolone resistance. Because sparfloxacin is more active in vitro and selection of resistance appears to be less likely to occur, it may have important advantage over ofloxacin or ciprofloxacin for the treatment of tuberculosis.

Active efflux of fluoroquinolones in Mycobacterium smegmatis mediated by LfrA, a multidrug efflux pump

The lfrA gene cloned from chromosomal DNA of quinolone-resistant Mycobacterium smegmatis mc2-552 conferred low-level resistance to fluoroquinolones when present on multicopy plasmids. Sequence analysis suggested that lfrA encodes a membrane efflux pump of the major facilitator family (H. E. Takiff, M. Cimino, M. C. Musso, T. Weisbrod, R. Martinez, M. B. Delgado, L Salazar, B. R. Bloom, and W. R. Jacbos, Jr., Proc. Natl. Acad. Sci. USA 93:362-366, 1996). In this work, we studied the role of LfrA in the accumulation of fluoroquinolones by M. smegmatis. The steady-state accumulation level of a hydrophilic quinolone, norfloxacin, by M. smegmatis harboring a plasmid carrying the lfrA gene was about 50% of that by the parent strain but was increased to the same level as that of the parent strain by addition of a proton conductor, carbonyl cyanide m-chorophenylhydrazone. Norfloxacin efflux mediated by LfrA was competed for strongly by ciprofloxacin but not by nalidixic acid. Furthermore, we showed that portions of norfloxacin accumulated by starved cells were pumped out upon reenergization of the cells, and the rates of this efflux showed evidence of saturation at higher intracellular concentrations of the drug. These results suggest that the LfrA polypeptide catalyzes the active efflux of several quinolones.

Multidrug resistance in enteric and other gram-negative bacteria

In Gram-negative bacteria, multidrug resistance is a term that is used to describe mechanisms of resistance by chromosomal genes that are activated by induction or mutation caused by the stress of exposure to antibiotics in natural and clinical environments. Unlike plasmid-borne resistance genes, there is no alteration or degradation of drugs or need for genetic transfer. Exposure to a single drug leads to cross-resistance to many other structurally and functionally unrelated drugs. The only mechanism identified for multidrug resistance in bacteria is drug efflux by membrane transporters, even though many of these transporters remain to be identified. The enteric bacteria exhibit mostly complex multidrug resistance systems which are often regulated by operons or regulons. The purpose of this review is to survey molecular mechanisms of multidrug resistance in enteric and other Gram-negative bacteria, and to speculate on the origins and natural physiological functions of the genes involved.

Chromosomal tetA(L) gene of Bacillus subtilis: regulation of expression and physiology of a tetA(L) deletion strain

Deletion of the tetA(L) chromosomal region of Bacillus subtilis in a strain designated JC112 increased the strain's sensitivity to low tetracycline concentrations. It also resulted in phenotypic changes that correlate with the previously found role of TetA(L) in mediating electrogenic NA+/H+ antiport. Growth of JC112 was impaired relative to that of the wild type at both pH 7.0 and 8.3; Na(+)- and K(+)-dependent pH homeostases were impaired at alkaline pH. The phenotype of JC112 was complemented by plasmid-borne tetA(L) and related tet(K) genes; the antiport activity conferred by the tet(K) gene had an apparently higher preference for K+ over Na+ than that conferred by tetA(L). The data were consistent with TetA(L) being the major Na+(K+)/H+ antiporter involved in pH homeostasis in B. subtilis as well as a significant Na+ extrusion system. The phenotype of JC112 was much more pronounced than that of an earlier transposition mutant, JC111, with a disruption in the putative tetA(L) promoter region. Northern (RNA) blot analysis of tetA(L) RNA from wild-type and JC111 strains revealed the same patterns. That JC111 nevertheless exhibited some Na+ and alkali sensitivity may be accounted for by disruption of regulatory features that, in the wild type, allow increased tetA(L) expression under specific conditions of pH and monovalent cation concentration. Evidence for several different regulatory effects emerged from studies of lacZ expression from the transposon of JC111 and from a tetA(L)-lacZ translational fusion introduced into the amyE locus of wild-type and JC112 strains.

Multidrug resistance proteins QacA and QacB from Staphylococcus aureus: membrane topology and identification of residues involved in substrate specificity

The closely related multidrug efflux pumps QacA and QacB, from the bacterial pathogen Staphylococcus aureus, both confer resistance to various toxic organic cations but differ in that QacB mediates lower levels of resistance to divalent cations. Cloning and nucleotide sequencing of the qacB gene revealed that qacB differs from qacA by only seven nucleotide substitutions. Random hydroxylamine mutagenesis of qacB was undertaken, selecting for variants that conferred increased resistance to divalent cations. Both QacA and the QacB mutants capable of conferring resistance to divalent cations contain an acidic residue at either amino acid 322 or 323, whereas QacB contains uncharged residues in these positions. Site-directed mutagenesis of qacA confirmed the importance of an acidic residue within this region of QacA in conferring resistance to divalent cations. Membrane topological analysis using alkaline phosphatase and beta-galactosidase fusions indicated that the QacA protein contains 14 transmembrane segments. Thus, QacA represents the first membrane transport protein shown to contain 14 transmembrane segments, and confirms that the major facilitator superfamily contains a family of proteins with 14 transmembrane segments.

Na+/H+ antiport activity conferred by Bacillus subtilis tetA(L), a 5' truncation product of tetA(L), and related plasmid genes upon Escherichia coli

An Escherichia coli transformant expressing the Bacillus subtilis tetA(L) gene from a weak promoter was challenged by growth on medium with low, increasing tetracycline concentrations. Changes in the substrate preference ratios of the TetA(L)-mediated resistances and antiports were examined in view of recent findings suggesting that TetA(L) catalyzes efflux of Na+ in exchange for protons in addition to having the ability to catalyze metal-tetracycline/H+ antiport. After growth of the transformant on 1 microgram or more of tetracycline per ml for 12 to 15 h, the tetA(L) gene in the plasmid was found to be disrupted by an IS10 element 50 bp from the 5' end of the coding sequence. This disrupted recombinant plasmid, pKB1, conferred greater tetracycline resistance and higher levels of membrane metal-tetracycline/proton antiport than the original plasmid, pJTA1, but conferred lower NA+ resistance and Na+/H+ antiport levels than the original plasmid. The results indicate that the 5' end of the gene is necessary for optimal Na+/H+ antiport but that some such activity as well as robust tetracycline/H+ antiport persists in its absence. Two plasmid genes, tet(K) and qacA, were compared with tetA(L) vis-à-vis their abilities to enhance the Na+/H+ antiporter activity of everted vesicles from E. coli transformants. tet(K), which is more closely related to tetA(L), catalyzed 22Na+ uptake by energized vesicles, whereas the less closely related qacA gene did not.

Antimicrobial resistance patterns in long term geriatric care. Implications for drug therapy

There is a high prevalence of bacterial infections in long term care facilities (4.4 to 16.2%). This, together with the fact that antimicrobial resistance is a big concern in current medical practice, makes infection control so important in nursing home care. This article covers the mechanisms of antibacterial resistance and focuses on 4 major antibacterial-resistant bacteria. Vancomycin is the treatment of choice for methicillin-resistant Staphylococcus aureus (MRSA). Colonisation with MRSA is not uncommon in nursing homes and eradication is probably not necessary. Any clinically important enterococcal infection should be tested for high-level resistance. An infectious disease consultation should be sought for vancomycin-resistant enterococcal infections. Gram-negative bacilli have developed multi-resistance. Susceptibility testing can identify the most appropriate therapy. Multiresistance should also be considered when treating Streptococcus pneumoniae. Overall, handwashing is highly recommended. Barrier precautions, minimising hospitalisations and avoiding unnecessary personnel rotation can reduce the chance of resistance spread.

Ins and outs of antimicrobial resistance: era of the drug pumps

Over the past five years, concerns have heightened over the escalating numbers of pathogenic micro-organisms isolated that are resistant to many antibiotics and drugs. This phenomenon poses major problems in the treatment of patients with hospital- or community-acquired infections caused by bacteria, fungi, or parasitic organisms. Microbial cells have acquired resistances to specific antibiotics and drugs by mechanisms that include antibiotic inactivation, target alteration, or drug exclusion. More recently, the importance of another mechanism, that of drug expulsion, has been recognized as contributing significantly to antibiotic and drug resistance in microbes. Drug expulsion, mediated by membrane-associated drug efflux pumps, can protect cells from a range of toxic compounds and therefore may confer single-step multidrug resistance. It is imperative that new drugs be designed or discovered that will poison the pumps or bypass the efflux mechanisms.

Salmonella typhimurium acrB-like gene: identification and role in resistance to biliary salts and detergents and in murine infection

Salmonella serotype typhimurium transpositional mutants altered in resistance to biliary salts and detergents were isolated previously. We have characterized further the LX1054 mutant strain, the most sensitive of them. The chromosomal DNA segment flanking transposon insertion was cloned and sequenced. The highest level of identity was found for the acrB (formerly acrE) gene of Escherichia coli, a gene encoding a drug efflux pump of the Acr family. LX1054 exhibited a reduced capacity to colonize the intestinal tract. After passages in mice, the mutant strain lost the sensitive phenotype. In vitro, a resumption of growth appeared after 17 h of culture in medium with cholate or other tested biological or chemical detergents. Then, the acquired resistant phenotype seemed stable. The data suggested a role of S. typhimurium acrB-like gene in resistance to biliary salts and detergents and in mice intestinal colonization. However, the local and transient sensitivity observed in vivo, and the in vitro adaptations suggest that several detergent-resistance mechanisms operate in S. typhimurium.

Efflux pump of the proton antiporter family confers low-level fluoroquinolone resistance in Mycobacterium smegmatis

Due to the resurgence of tuberculosis and the emergence of multidrug-resistant strains, fluoroquinolones (FQ) are being used in selected tuberculosis patients, but FQ-resistant strains of Mycobacterium tuberculosis have rapidly begun to appear. The mechanisms involved in FQ resistance need to be elucidated if the effectiveness of this class of antibiotics is to be improved and prolonged. By using the rapid-growing Mycobacterium smegmatis as a model genetic system, a gene was selected that confers low-level FQ resistance when present on a multicopy plasmid. This gene, lfrA, encodes a putative membrane efflux pump of the major facilitator family, which appears to recognize the hydrophilic FQ, ethidium bromide, acridine, and some quaternary ammonium compounds. It is homologous to qacA from Staphylococcus aureus, tcmA, of Streptomyces glaucescens, and actII and mmr, both from Streptomyces coelicoler. Increased expression of lfrA augments the appearance of subsequent mutations to higher-level FQ resistance.

Inactivation of chloramphenicol by O-phosphorylation. A novel resistance mechanism in Streptomyces venezuelae ISP5230, a chloramphenicol producer

Plasmid pJV4, containing a 2.4-kilobase pair insert of genomic DNA from the chloramphenicol (Cm) producer Streptomyces venezuelae ISP5230, confers resistance when introduced by transformation into the Cm-sensitive host Streptomyces lividans M252 (Mosher, R. H. Ranade, N. P., Schrempf, H., and Vining, L. C. (1990) J. Gen. Microbiol. 136, 293-301). Transformants rapidly metabolized Cm to one major product, which was isolated and purified by reversed phase chromatography. The metabolite was identified by nuclear magnetic resonance spectroscopy and mass spectrometry as 3'-O-phospho-Cm, and was shown to have negligible inhibitory activity against Cm-sensitive Micrococcus luteus. The nucleotide sequence of the S. venezuelae DNA insert in pJV4 contains an open reading frame (ORF) that encodes a polypeptide (19 kDa) with a consensus motif at its NH2 terminus corresponding to a nucleotide-binding amino acid sequence (motif A or P-loop; Walker, J. E., Saraste, M., Runswick, M. J., and Gay, N. J. (1982) EMBO J. 1, 945-951). When a recombinant vector containing this ORF as a 1.6-kilobase pair SmaI-SmaI fragment was used to transform S. lividans M252, uniformly Cm-resistant transformants were obtained. A strain of S. lividans transformed by a vector in which the ORF had been disrupted by an internal deletion yielded clones that were unable to phosphorylate Cm, and exhibited normal susceptibility to the antibiotic. The results implicate the product of the ORF from S. venezuelae as an enzymic effector of Cm resistance in the producing organism by 3'-O-phosphorylation. We suggest the trivial name chloramphenicol 3'-O-phosphotransferase for the enzyme.

The Lactococcal lmrP gene encodes a proton motive force-dependent drug transporter

To genetically dissect the drug extrusion systems of Lactococcus lactis, a chromosomal DNA library was made in Escherichia coli and recombinant strains were selected for resistance to high concentrations of ethidium bromide. Recombinant strains were found to be resistant not only to ethidium bromide but also to daunomycin and tetraphenylphosphonium. The drug resistance is conferred by the lmrP gene, which encodes a hydrophobic polypeptide of 408 amino acid residues with 12 putative membrane-spanning segments. Some sequence elements in this novel membrane protein share similarity to regions in the transposon Tn10-encoded tetracycline resistance determinant TetA, the multidrug transporter Bmr from Bacillus subtilis, and the bicyclomycin resistance determinant Bcr from E. coli. Drug resistance associated with lmrP expression correlated with energy-dependent extrusion of the molecules. Drug extrusion was inhibited by ionophores that dissipate the proton motive force but not by the ATPase inhibitor ortho-vanadate. These observations are indicative for a drug-proton antiport system. A lmrP deletion mutant was constructed via homologous recombinant using DNA fragments of the flanking region of the gene. The L. lactis (delta lmrP) strain exhibited residual ethidium extrusion activity, which in contrast to the parent strain was inhibited by ortho-vanadate. The results indicate that in the absence of the functional drug-proton anti-porter LmrP, L. lactis is able to overexpress another, ATP-dependent, drug extrusion system. These findings substantiate earlier studies on the isolation and characterization of drug-resistant mutants of L. lactis (Bolhuis, H., Molenaar, D., Poelarends, G., van Veen, H. W., Poolman, B., Driessen, A. J. M., and Konings, W. N. (1994) J. Bacteriol. 176, 6957-6964).

OprK and OprM define two genetically distinct multidrug efflux systems in Pseudomonas aeruginosa

Multidrug-resistant derivatives of Pseudomonas aeruginosa PAO1 were obtained after stepwise selection on tetracycline or erythromycin. Two phenotypes were generated. The tetracycline-resistant mutant (TETR) was phenotypically similar to OprM-overexpressing strains. This group displayed cross-resistance to quinolones, chloramphenicol, and all beta-lactams tested except imipenem, with no changes in the erythromycin MICs for the strains. Sodium dodecyl sulfate-polyacrylamide gels showed the overproduction of an outer membrane protein in the range of 50 kDa and a 46-kDa inner membrane protein. The erythromycin-resistant mutant (ERYR) kept its susceptibility to all beta-lactams tested with the exception of cefpirome, but it was resistant to chloramphenicol, quinolones, and tetracycline and was hypersusceptible to imipenem. This mutant also exhibited overexpression of a 50-kDa outer membrane protein that was different from OprM and of a 43-kDa inner membrane protein. The phenotype of ERYR was comparable to those of OprK- and OprJ-overexpressing strains. These strains were therefore classified as the OprK-like group. Transduction of the oprK::omega-Hg mutation of strain K613 (K. Poole, K. Krebes, C. McNally, and S. Neshat, J. Bacteriol. 175:7363-7372, 1993) into the multidrug-resistant strains resulted in the loss of multidrug resistance and the acquisition of hypersusceptibility in the OprM group, while the phenotype of the OprK-like group was unaffected. These experiments demonstrated the existence of two genetically distinct efflux systems in P. aeruginosa. The identities of the operons encoding the two efflux systems and their physiological roles are discussed.

Reconstitution of the metal-tetracycline/H+ antiporter of Escherichia coli in proteoliposomes including F0F1-ATPase

The tetracycline resistance gene (tetA) was cloned downstream of the lac promoter. When expression of the tetA gene in E. coli cells carrying the lac Iq gene was induced with isopropyl beta-D-thiogalactopyranoside, the tetracycline resistance protein (TetA) was overproduced, amounting to about 30% of the integral cytoplasmic membrane protein. Essentially pure TetA protein could be obtained by solubilization with 1.25% n-octyl-beta-D-glucopyranoside and one-step purification by DEAE Sepharose CL-6B column chromatography. The TetA protein was incorporated into proteoliposomes with F0F1-ATPase. The proteoliposomes exhibited [3H]tetracycline transport dependent on ATP hydrolysis. The specific activity was about 2 nmol/mg protein/min. The proteoliposomes also showed H+ efflux coupled with tetracycline influx. Tetracycline/H+ antiport by proteoliposomes reconstituted with the Ser-65-->Cys mutant TetA protein was inhibited by N-ethylmaleimide. These results proved for the first time that the tetracycline/H+ antiport is only mediated by the TetA protein.

Evidence for an efflux pump in multidrug-resistant Campylobacter jejuni

Mechanisms of drug resistance in Campylobacter jejuni were investigated. Mutant strains 34PEFr, which was resistant to pefloxacin (128-fold increase in the MIC), and 34CTXr, which was resistant to cefotaxime (32-fold increase in the MIC) and which was derived from the susceptible parent 34s, were obtained by serial passages on pefloxacin and cefotaxime gradient plates, respectively. Both mutants showed cross-resistance to erythromycin, chloramphenicol, tetracycline, beta-lactams, and quinolones. While the quinolone resistance of strain PEFr could be explained by a mutation at codon 86 of the gyrA gene, the multidrug resistance phenotype of both strains was further investigated. Accumulation of pefloxacin, ciprofloxacin, and minocycline was measured by fluorometry and was found to be lower in the mutant strains than in the parent strain. Preincubation of the cells with carbonyl cyanide m-chlorophenylhydrazone, however, completely abolished this difference. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of outer membrane preparations from both mutant strains showed overexpression of two proteins of 55 and 39 kDa which were absent from the outer membranes of the wild-type strain. These results indicate that in C. jejuni 34PEFr and 34CTXr, multidrug resistance is associated with an efflux system with a broad specificity.

Tetracycline/H+ antiport and Na+/H+ antiport catalyzed by the Bacillus subtilis TetA(L) transporter expressed in Escherichia coli

The properties of TetA(L)-dependent tetracycline/proton and Na+/proton antiport were studied in energized everted vesicles of Escherichia coli transformed with a cloned tetA(L) gene (pJTA1) from Bacillus subtilis. Inhibition patterns by valinomycin and nigericin indicated that both antiports were electrogenic, in contrast to the tetracycline/proton antiport encoded by gram-negative plasmid tet genes. Tetracycline uptake in the everted system was dependent upon a divalent cation, with cobalt being the preferred one. The apparent Km for tetracycline was markedly increased at pH 8.5 versus pH 7.5, whereas the Vmax was unchanged. The much higher apparent Km for Na+ decreased at pH 8.5 relative to that at pH 7.5, as did the Vmax. Na+ did not affect tetracycline uptake, nor did Co2+ and/or tetracycline affect Na+ uptake; complex patterns of inhibition by amiloride and analogs thereof were observed.

Multidrug resistance in Klebsiella pneumoniae: a novel gene, ramA, confers a multidrug resistance phenotype in Escherichia coli

Spontaneous multidrug-resistant (Mdr) mutants of Klebsiella pneumoniae strain ECL8 arose at a frequency of 2.2 x 10(-8) and showed increased resistance to a range of unrelated antibiotics, including chloramphenicol, tetracycline, nalidixic acid, ampicillin, norfloxacin, trimethoprim and puromycin. A chromosomal fragment from one such mutant was cloned, and found to confer an Mdr phenotype on Escherichia coli K12 cells that was essentially identical to that of the K. pneumoniae mutant. Almost complete loss of the OmpF porin in the E. coli transformant, and of the corresponding porin in the K. pneumoniae mutant, was observed. The presence of the Mdr mutation in K. pneumoniae or the cloned K. pneumoniae ramA (resistance antibiotic multiple) locus in E. coli also resulted in active efflux of tetracycline, and increased active efflux of chloramphenicol. After transformation of a ramA plasmid into E. coli, expression of chloramphenicol resistance occurred later than expression of resistance to tetracycline, puromycin, trimethoprim and nalidixic acid. The ramA gene was localized and sequenced. It encodes a putative positive transcriptional activator that is weakly related to the E. coli MarA and SoxS proteins. A ramA gene was also found to be present in an Enterobacter cloacae fragment that has previously been shown to confer an Mdr phenotype, and it appears that ramA, rather than the romA gene identified in that study, is responsible for multidrug resistance. The ramA gene from the wild-type K. pneumoniae was identical to that of the mutant strain and also conferred an Mdr phenotype on E. coli, indicating that the mutation responsible for Mdr in K. pneumoniae had not been cloned.

Mechanisms of membrane toxicity of hydrocarbons

Microbial transformations of cyclic hydrocarbons have received much attention during the past three decades. Interest in the degradation of environmental pollutants as well as in applications of microorganisms in the catalysis of chemical reactions has stimulated research in this area. The metabolic pathways of various aromatics, cycloalkanes, and terpenes in different microorganisms have been elucidated, and the genetics of several of these routes have been clarified. The toxicity of these compounds to microorganisms is very important in the microbial degradation of hydrocarbons, but not many researchers have studied the mechanism of this toxic action. In this review, we present general ideas derived from the various reports mentioning toxic effects. Most importantly, lipophilic hydrocarbons accumulate in the membrane lipid bilayer, affecting the structural and functional properties of these membranes. As a result of accumulated hydrocarbon molecules, the membrane loses its integrity, and an increase in permeability to protons and ions has been observed in several instances. Consequently, dissipation of the proton motive force and impairment of intracellular pH homeostasis occur. In addition to the effects of lipophilic compounds on the lipid part of the membrane, proteins embedded in the membrane are affected. The effects on the membrane-embedded proteins probably result to a large extent from changes in the lipid environment; however, direct effects of lipophilic compounds on membrane proteins have also been observed. Finally, the effectiveness of changes in membrane lipid composition, modification of outer membrane lipopolysaccharide, altered cell wall constituents, and active excretion systems in reducing the membrane concentrations of lipophilic compounds is discussed. Also, the adaptations (e.g., increase in lipid ordering, change in lipid/protein ratio) that compensate for the changes in membrane structure are treated.

Mechanisms of membrane toxicity of hydrocarbons

Microbial transformations of cyclic hydrocarbons have received much attention during the past three decades. Interest in the degradation of environmental pollutants as well as in applications of microorganisms in the catalysis of chemical reactions has stimulated research in this area. The metabolic pathways of various aromatics, cycloalkanes, and terpenes in different microorganisms have been elucidated, and the genetics of several of these routes have been clarified. The toxicity of these compounds to microorganisms is very important in the microbial degradation of hydrocarbons, but not many researchers have studied the mechanism of this toxic action. In this review, we present general ideas derived from the various reports mentioning toxic effects. Most importantly, lipophilic hydrocarbons accumulate in the membrane lipid bilayer, affecting the structural and functional properties of these membranes. As a result of accumulated hydrocarbon molecules, the membrane loses its integrity, and an increase in permeability to protons and ions has been observed in several instances. Consequently, dissipation of the proton motive force and impairment of intracellular pH homeostasis occur. In addition to the effects of lipophilic compounds on the lipid part of the membrane, proteins embedded in the membrane are affected. The effects on the membrane-embedded proteins probably result to a large extent from changes in the lipid environment; however, direct effects of lipophilic compounds on membrane proteins have also been observed. Finally, the effectiveness of changes in membrane lipid composition, modification of outer membrane lipopolysaccharide, altered cell wall constituents, and active excretion systems in reducing the membrane concentrations of lipophilic compounds is discussed. Also, the adaptations (e.g., increase in lipid ordering, change in lipid/protein ratio) that compensate for the changes in membrane structure are treated.

Importance of lipooligosaccharide structure in determining gonococcal resistance to hydrophobic antimicrobial agents resulting from the mtr efflux system

Levels of gonococcal resistance to antimicrobial hydrophobic agents (HAs) are controlled by the mtr (multiple transferrable resistance) system, composed of the mtrRCDE genes. The mtrR gene encodes a transcriptional repressor that appears to regulate expression of the upstream and divergent mtrCDE operon. The mtrCDE genes encode membrane proteins analogous to the MexABOprK proteins of Pseudomonas aeruginosa that mediate export of structurally diverse antimicrobial agents. In this study we found that a single base pair deletion in a 13 bp inverted repeat sequence within the mtrR promoter resulted in increased resistance of gonococci to both crystal violet (CV) and erythromycin (ERY) as well as to the more lipophilic non-ionic detergent Triton X-100 (TX-100). However, this cross-resistance was contingent on the production of a full-length lipooligosaccharide (LOS) by the recipient strain used in transformation experiments. Introduction of this mutation (mtrR-171) into three chemically distinct deep-rough LOS mutants by transformation resulted in a fourfold increase in resistance to TX-100 compared with a 160-fold increase in an isogenic strain producing a full-length LOS. However, both wild-type and deep-rough LOS strains exhibited an eightfold increase in resistance to CV and ERY as a result of the mtrR-171 mutation. This suggests that gonococci have different LOS structural requirements for mtr-mediated resistance to HAs that differ in their lipophilic properties. Evidence is presented that gonococci exclude HAs by an energy-dependent efflux process mediated by the mtr system.

The tetracycline efflux protein encoded by the tet(K) gene from Staphylococcus aureus is a metal-tetracycline/H+ antiporter

The tet(K) gene from Staphylococcus aureus was highly expressed in Escherichia coli by an alteration of its initiation codon from TTG to ATG and its ribosome-binding sequence from GAGG to GGAGG [Noguchi, N. et al. (1994) Biol. Pharm. Bull. 17, 352-355]. The inverted membrane vesicles prepared from the tet(K)-expressing cells showed respiration-dependent [3H]tetracycline transport comparable to the vesicles from the tet(B)-expressing cells. The affinity of Tet(K) vesicles to tetracycline was the same as that of Tet(B) vesicles, whereas the former Vmax value was about 60% of the latter one. Contrary to Tet(B) vesicles, Tet(K) vesicles showed no significant minocycline uptake, which was consistent with the low minocycline resistance of the Tet(K)-producing cells. The tetracycline transport mediated by Tet(K) vesicles was coupled with proton transport and the translocation of 60Co2+ ions as well as in Tet(B) vesicles. This observation indicates that the class K tetracycline resistance determinant from Gram-positive bacteria also encodes a metal-tetracycline/H+ antiporter that is functionally similar to that encoded by tet(B), although there is a considerable difference in the primary sequences and the putative topologies of these Tet proteins.

Molecular characterization of the staphylococcal multidrug resistance export protein QacC

The QacC polypeptide is a member of a family of small membrane proteins which confer resistance to toxic compounds. The staphylococcal qacC gene confers resistance to toxic organic cations via proton-dependent export. The membrane topology of the QacC polypeptide was investigated by constructing and analyzing a series of qacC-phoA and qacC-lacZ fusions. From these analyses, most of the predicted features of the QacC protein were verified, although data regarding the possible orientation of the COOH region were not conclusive. The role of the sole cysteine residue, Cys-42, in QacC was studied by using the sulfhydryl reagent N-ethylmaleimide and site-directed mutagenesis. N-Ethylmaleimide was shown to inhibit qacC-mediated ethidium export. Multiple amino acid substitutions were made for Cys-42, and mutations at this location had various effects on resistance specificity. This suggests that the Cys-42 residue may be located near a region of QacC that is involved in substrate recognition. Mutagenesis of conserved residues in QacC indicated that Tyr-59 and Trp-62 also play an essential structural or functional role in QacC.

EmrR is a negative regulator of the Escherichia coli multidrug resistance pump EmrAB

The emrAB locus of Escherichia coli encodes a multidrug resistance pump that protects the cell from several chemically unrelated antimicrobial agents, e.g., the protonophores carbonyl cyanide m-chlorophenylhydrazone (CCCP) and tetrachlorosalicyl anilide and the antibiotics nalidixic acid and thiolactomycin. The mprA gene is located immediately upstream of this locus and was shown to be a repressor of microcin biosynthesis (I. del Castillo, J. M. Gomez, and F. Moreno, J. Bacteriol. 173:3924-3929, 1991). There is a putative transcriptional terminator sequence between the mprA and emrA genes. To locate the emr promoter, single-copy lacZ operon fusions containing different regions of the emr locus were made. Only fusions containing the mprA promoter region were expressed. mprA is thus the first gene of the operon, and we propose that it be renamed emrR. Overproduction of the EmrR protein (with a multicopy vector containing the cloned emrR gene) suppressed transcription of the emr locus. A mutation in the emrR gene led to overexpression of the EmrAB pump and increased resistance to antimicrobial agents. CCCP, nalidixic acid, and a number of other structurally unrelated chemicals induced expression of the emr genes, and the induction required EmrR. We conclude that emrRAB genes constitute an operon and that EmrR serves as a negative regulator of this operon. Some of the chemicals that induce the pump serve as its substrates, suggesting that their extrusion is the natural function of the pump.

Role of outer membrane barrier in efflux-mediated tetracycline resistance of Escherichia coli

Accumulation of tetracycline in Escherichia coli was studied to determine its permeation pathway and to provide a basis for understanding efflux-mediated resistance. Passage of tetracycline across the outer membrane appeared to occur preferentially via the porin OmpF, with tetracycline in its magnesium-bound form. Rapid efflux of magnesium-chelated tetracycline from the periplasm was observed. In E. coli cells that do not contain exogenous tetracycline resistance genes, the steady-state level of tetracycline accumulation was decreased when porins were absent or when the fraction of Mg(2+)-chelated tetracycline was small. This is best explained by assuming the presence of a low-level endogenous active efflux system that bypasses the outer membrane barrier. When influx of tetracycline is slowed, this efflux is able to reduce the accumulation of tetracycline in the cytoplasm. In contrast, we found no evidence of a special outer membrane bypass mechanism for high-level efflux via the Tet protein, which is an inner membrane efflux pump coded for by exogenous tetA genes. Fractionation and equilibrium density gradient centrifugation experiments showed that the Tet protein is not localized to regions of inner and outer membrane adhesion. Furthermore, a high concentration of tetracycline was found in the compartment that rapidly equilibrated with the medium, most probably the periplasm, of Tet-containing E. coli cells, and the level of tetracycline accumulation in Tet-containing cells was not diminished by the mutational loss of the OmpF porin. These results suggest that the Tet protein, in contrast to the endogenous efflux system(s), pumps magnesium-chelated tetracycline into the periplasm. A quantitative model of tetracycline fluxes in E. coli cells of various types is presented.

A novel glycylcycline, 9-(N,N-dimethylglycylamido)-6-demethyl-6-deoxytetracycline, is neither transported nor recognized by the transposon Tn10-encoded metal-tetracycline/H+ antiporter

A novel tetracycline derivative, DMG-DMDOT [9-(N,N-dimethylglycylamido)-6-demethyl-6-deoxytetracycline] , is one of the glycylcyclines which have a broad antibacterial spectrum, including many tetracyclineresistant bacteria (R.T. Testa, P.J. Petersen, N.V. Jacobus, P.-E. Sum, V.J. Lee, and F.P. Tally, Antimicrob. Agents Chemother. 37:2270-2277, 1993). The mechanism by which DMG-DMDOT overcomes efflux-based tetracycline resistance was investigated. Tetracycline-resistant Escherichia coli cells carrying an R plasmid encoding the tet(B) gene, which encodes the typical tetracycline efflux pump [TetA(B)] of gram-negative bacteria, were as susceptible to DMG-DMDOT as was the tetracycline-susceptible host. When mid-log-phase cells carrying the tet(B) gene were incubated with a subbactericidal concentration of DMG-DMDOT (0.5 micrograms/ml) for 2 h, a significant amount of the TetA(B) protein was detected in the cell membrane by Western blotting (immunoblotting) with an anti-carboxyl-terminal antibody, similar to the case in which tetracycline was used as the inducer, indicating that the tet repressor, TetR, can recognize DMG-DMDOT as an efficient inducer. Everted membrane vesicles prepared from cells producing the TetA(B) protein showed absolutely no transport activity for DMG-DMDOT. Furthermore, the presence of excess DMG-DMDOT had no effect on the tetracycline transport activity of the everted vesicles, indicating that DMG-DMDOT is not recognized as a substrate by the TetA(B) protein.

Bacterial resistance to uncouplers

Uncoupler resistance presents a potential challenge to the conventional chemiosmotic coupling mechanism. In E. coli, an adaptive response to uncouplers was found in cell growing under conditions requiring oxidative phosphorylation. It is suggested that uncoupler-resistant mutants described in the earlier literature might represent a constitutive state of expression of this "low energy shock" adaptive response. In the environment, bacteria are confronted by nonclassical uncoupling factors such as organic solvents, heat, and extremes of pH. It is suggested that the low energy shock response will aid the cell in coping with the effects of natural uncoupling factors. The genetic analysis of uncoupler resistance has only recently began, and is yielding interesting and largely unexpected results. In Bacillus subtilis, a mutation in fatty acid desaturase causes an increased content of saturated fatty acids in the membrane and increased uncoupler resistance. The protonophoric efficiency of uncouplers remains unchanged in the mutants, inviting nonorthodox interpretations of the mechanism of resistance. In E. coli, two loci conferring resistance to CCCP and TSA were cloned and were found to encode multidrug resistance pumps. Resistance to one of the uncouplers, TTFB, remained unchanged in strains mutated for the MDRs, suggesting a resistance mechanism different from uncoupler extrusion.

Proton motive force-driven and ATP-dependent drug extrusion systems in multidrug-resistant Lactococcus lactis

Three mutants of Lactococcus lactis subsp. lactis MG1363, termed EthR, DauR, and RhoR, were selected for resistance to high concentrations of ethidium bromide, daunomycin, and rhodamine 6G, respectively. These mutants were found to be cross resistant to a number of structurally and functionally unrelated drugs, among which were typical substrates of the mammalian multidrug transporter (P-glycoprotein) such as daunomycin, quinine, actinomycin D, gramicidin D, and rhodamine 6G. The three multidrug-resistant strains showed an increased rate of energy-dependent ethidium and daunomycin efflux compared with that of the wild-type strain. This suggests that resistance to these toxic compounds is at least partly due to active efflux. Efflux of ethidium from the EthR strain could occur against a 37-fold inwardly directed concentration gradient. In all strains, ethidium efflux was inhibited by reserpine, a well-known inhibitor of P-glycoprotein. Ionophores which selectively dissipate the membrane potential or the pH gradient across the membrane inhibited ethidium and daunomycin efflux in the wild-type strain, corresponding with a proton motive force-driven efflux system. The ethidium efflux system in the EthR strain, on the other hand, was inhibited by ortho-vanadate and not upon dissipation of the proton motive force, which suggests the involvement of ATP in the energization of transport. The partial inhibition of ethidium efflux by ortho-vanadate and nigericin in the DauR and RhoR strains suggest that a proton motive force-dependent and an ATP-dependent system are expressed simultaneously. This is the first report of an ATP-dependent transport system in prokaryotes which confers multidrug resistance to the organism.

Microbial multidrug-resistance ABC transporters

Multidrug resistance in tumor cells is often caused by the increased efflux of a wide variety of drugs, mediated by P glycoprotein, a member of the superfamily of ATP-binding cassette (ABC) transporters. The genes encoding members of this superfamily have also been isolated from drug-resistant microorganisms, and the role of microbial ABC transporters in drug resistance is being investigated.

Epidemiology of tetracycline-resistance determinants

Resistance to tetracycline is generally due either to energy-dependent efflux of tetracycline or to protection of the bacterial ribosomes from the action of tetracycline. The genes that encode this resistance are normally acquired via transferable plasmids and/or transposons. Tet determinants have been found in a wide range of Gram-positive and Gram-negative bacteria and have reduced the effectiveness of therapy with tetracycline.

Mycobacterium tuberculosis: drug-resistance mechanisms

Tuberculosis has resurged during the past decade in many industrialized countries, and strains of Mycobacterium tuberculosis that are resistant to one or more of the main antituberculous drugs are emerging. The molecular basis of mycobacterial drug resistance is now beginning to be understood. Resistance derives from mutations in chromosomal genes leading to overproduction, alteration or loss of the drug target.

Resistance to drugs targeting protein synthesis in mycobacteria

Many antibiotics exert their effects by interfering with protein synthesis. Studies of the molecular mechanisms of antibiotic resistance in clinical strains of mycobacteria have revealed mutations in ribosomal RNAs. This type of acquired resistance was previously unknown in bacterial pathogens and was made possible because mycobacteria have only a single set of rRNA genes.