Active efflux mechanisms for antimicrobial resistance

Ineffectiveness of topoisomerase mutations in mediating clinically significant fluoroquinolone resistance in Escherichia coli in the absence of the AcrAB efflux pump

Fluoroquinolone-resistant mutants, selected from a wild-type Escherichia coli K-12 strain and its Mar mutant by exposure to increasing levels of ofloxacin on solid medium, were analyzed by Northern (RNA) blot analysis, sequencing, and radiolabelled ciprofloxacin accumulation studies. Mutations in the target gene gyrA (DNA gyrase), the regulatory gene marR, and additional, as yet unidentified genes (genes that probably affect efflux mediated by the multidrug efflux pump AcrAB) all contributed to fluoroquinolone resistance. Inactivation of the acrAB locus made all strains, including those with target gene mutations, hypersusceptible to fluoroquinolones and certain other unrelated drugs. These studies indicate that, in the absence of the AcrAB pump, gyrase mutations fail to produce clinically relevant levels of fluoroquinolone resistance.

Bacterial resistance to disinfectants: present knowledge and future problems

Bacterial resistance to antibiotics is a long-established, widely-studied problem. Increasingly, attention is being directed to the responses of various types of microbes to biocides (antiseptics, disinfectants and preservatives). Different groups of bacteria vary in their susceptibility to biocides, with bacterial spores being the most resistant, followed by mycobacteria, then Gram-negative organisms, with cocci generally being the most sensitive. There are wide divergencies within this general classification. Thus, (i) spores of Bacillus subtilis are less susceptible to biocides than those of Clostridium difficile: (ii) Mycobacterium chelonae strains may show high resistance to glutaraldehyde and M. avium intracellulare is generally less sensitive than M. tuberculosis; (iii) Gram-negative bacteria such as Pseudomonas aeruginosa, Providencia spp and Proteus spp may be difficult to inactivate; (iv) enterococci are less sensitive than staphylococci to biocides and antibiotic-resistant strains of Staphylococcus aureus might show low-level biocide resistance. The mechanisms involved in biocide resistance to biocides are becoming better understood. Intrinsic resistance (intrinsic insusceptibility) is found with bacterial spores, mycobacteria and Gram-negative bacteria. This resistance might, in some instances, be associated with constitutive degradative enzymes but in reality is more closely linked to cellular impermeability. The coats(s) and, to some extent, the cortex in spores, the arabinogalactan and possibly other components of the mycobacterial cell wall and the outer membrane of Gram-negative bacteria limit the concentration of active biocide that can reach the target site(s) in these bacterial cells. A special situation is found with bacteria present in biofilms, which can be considered as being an intrinsic resistance mechanism resulting from physiological (phenotypic) adaptation of cells. Acquired resistance to biocides may arise by cellular mutation or by the acquisition of genetic elements. Plasmid/transposon-mediated resistance to inorganic and organic mercury compounds by hydrolases and reductases has been extensively studied. Plasmid-mediated resistance to some other biocides in Gram-negative bacteria and in staphylococci has been described, but its significance remains uncertain. As to the future, there is a need to establish conclusively whether there is a clear-cut linkage between antibiotic and biocide resistance in non-sporulating bacteria and whether biocides can select for antibiotic resistance. Additionally, the responses to biocides of new and emerging pathogens must be assessed. At the same time, continuing research is necessary to establish further the underlying mechanisms of resistance and to provide more efficient means of bacterial inactivation.

Cysteine-scanning mutagenesis around transmembrane segment VI of Tn10-encoded metal-tetracycline/H(+) antiporter

Each amino acid in putative transmembrane helix VI and its flanking regions, from Ser-156 to Thr-185, of a Cys-free mutant of the Tn10-encoded metal-tetracycline/H(+) antiporter (TetA(B)) was individually replaced by Cys. All of the cysteine-scanning mutants showed a normal level of tetracycline resistance except for the S156C mutant, which showed moderate resistance, indicating that there is no essential residue located in this region. All 20 mutants from S159C to W178C showed no reactivity with N-ethylmaleimide (NEM), whereas the mutants of the flanking regions from S156C to H158C and F179C to T185C were highly or moderately reactive with NEM. These results indicate that like transmembrane helices III and IX, the transmembrane helix VI comprising residues Ser-159-Trp-178 is totally embedded in the hydrophobic environment.

Identification of structural and functional domains of the tetracycline efflux protein TetA(P) from Clostridium perfringens

The Clostridium perfringens tetracycline-resistance protein, TetA(P), is an integral inner-membrane protein that mediates the active efflux of tetracycline from the cell. TetA(P) acts as an antiporter, presumably transporting a divalent cation-tetracycline complex in exchange for a proton, and is predicted to have 12 transmembrane domains (TMDs). Two glutamate residues that are located in predicted TMD 2 were previously shown to be required for the active efflux of tetracycline by TetA(P). To identify additional residues that are required for the structure or function of TetA(P), a random mutagenesis approach was used. Of the 61 tetracycline-susceptible mutants that were obtained in Escherichia coli, 31 different derivatives were shown to contain a single amino acid change that resulted in reduced tetracycline resistance. The stability of the mutant TetA(P) proteins was examined by immunoblotting and 19 of these strains were found to produce a detectable TetA(P) protein. The MIC of these derivatives ranged from 2 to 15 microg tetracycline ml(-1), compared to 30 microg tetracycline ml(-1) for the wild-type. The majority of these mutants clustered into three potential loop regions of the TetA(P) protein, namely the cytoplasmic loops 2-3 and 4-5, and loop 7-8, which is predicted to be located in the periplasm in E. coli. It is concluded that these regions are of functional significance in the TetA(P)-mediated efflux of tetracycline from the bacterial cell.

Reversal of tetracycline resistance mediated by different bacterial tetracycline resistance determinants by an inhibitor of the Tet(B) antiport protein

Active efflux is a useful strategy by which bacteria evade growth inhibition by antibiotics. Certain semisynthetic tetracycline (TC) analogs, substituted at the 13th carbon at C-6 on ring C of the TC molecule, blocked TC efflux as revealed in everted membrane vesicles from class B TC-resistant (Tcr) Escherichia coli (M. L. Nelson, B. H. Park, J. S. Andrews, V. A. Georgian, R. C. Thomas, and S. B. Levy, J. Med. Chem. 36:370-377, 1993). A representative C-13-substituted analog, 13-cyclopentylthio-5-OH-TC (13-CPTC), was shown to competitively inhibit TC translocation by the Tet(B) protein, blocking the uptake of TC into vesicles and therefore the efflux of TC from whole cells. Against Tcr E. coli, 13-CPTC, when used in combination with doxycycline, produced synergistic inhibition of growth. 13-CPTC was shown to increase the uptake of [3H]TC into the resistant cells. 13-CPTC alone was a potent growth inhibitor against TC-susceptible (Tcs) and Tcr Staphylococcus aureus and enterococci specifying class K or class L efflux-dependent TC resistance mechanisms or, unexpectedly, the class M ribosomal protection mechanism. These findings indicate that derivatives of TC, identified by their ability to block the Tet(B) efflux protein, can restore TC activity against Tcr bacteria bearing either of the two known resistance mechanisms. Blocking drug efflux and increasing intracellular drug concentrations constitute an effective approach to reversing TC resistance and may be generally applicable to other antibiotics rendered ineffective by efflux proteins.

Use of a genetic approach to evaluate the consequences of inhibition of efflux pumps in Pseudomonas aeruginosa

Drug efflux pumps in Pseudomonas aeruginosa were evaluated as potential targets for antibacterial therapy. The potential effects of pump inhibition on susceptibility to fluoroquinolone antibiotics were studied with isogenic strains that overexpress or lack individual efflux pumps and that have various combinations of efflux- and target-mediated mutations. Deletions in three efflux pump operons were constructed. As expected, deletion of the MexAB-OprM efflux pump decreased resistance to fluoroquinolones in the wild-type P. aeruginosa (16-fold reduction for levofloxacin [LVX]) or in the strain that overexpressed mexAB-oprM operon (64-fold reduction for LVX). In addition to that, resistance to LVX was significantly reduced even for the strains carrying target mutations (64-fold for strains for which LVX MICs were >4 microg/ml). We also studied the frequencies of emergence of LVX-resistant variants from different deletion mutants and the wild-type strain. Deletion of individual pumps or pairs of the pumps did not significantly affect the frequency of emergence of resistant variants (at 4x the MIC for the wild-type strain) compared to that for the wild type (10(-6) to 10(-7)). In the case of the strain with a triple deletion, the frequency of spontaneous mutants was undetectable (<10(-11)). In summary, inhibition of drug efflux pumps would (i) significantly decrease the level of intrinsic resistance, (ii) reverse acquired resistance, and (iii) result in a decreased frequency of emergence of P. aeruginosa strains highly resistant to fluoroquinolones in clinical settings.

Energy-dependent efflux from Leishmania promastigotes of substrates of the mammalian multidrug resistance pumps

We demonstrated the existence of three transport activities in promastigotes of Leishmania braziliensis, Leishmania guyanensis, and Leishmania mexicana. The first activity, an energy-dependent efflux of pirarubicin, was observed in all Leishmania species and inhibited by verapamil, by 2-[4-(diphenylmethyl)-1-piperazinyl]ethyl-5-(trans-4,6-dimethyl-1, 3,2-dioxaphosphorinan-2-yl)-2,6-dimethyl-4-(3-nitrophenyl)-3-py ridinecarboxylate P oxide (PAK104P) and by the phenothiazine derivatives: thioridazine, prochlorperazine, trifluoperazine, chlorpromazine and trifluoropromazine. The second activity, an energy-dependent efflux of calcein acetoxymethylester, was observed in all Leishmania species and inhibited by PAK104P and the same phenothiazine derivatives, but not by verapamil. The third activity, an energy-dependent efflux of calcein, was clearly detected in L. braziliensis and guyanensis and inhibited only by prochlorperazine and trifluoperazine. The fact that prochlorperazine and trifluoperazine inhibited the energy-dependent efflux of the three substrates suggests that these activities are mediated by the same transport system. It is noteworthy that the transport system identified in this study shares several properties with the mammalian multidrug resistance pump, MRP1. Pirarubicin, calcein acetoxymethylester and calcein are well known substrates of the MRP. Furthermore, the three types of inhibitors are also inhibitors of the MRP function.

A single membrane-embedded negative charge is critical for recognizing positively charged drugs by the Escherichia coli multidrug resistance protein MdfA

The nature of the broad substrate specificity phenomenon, as manifested by multidrug resistance proteins, is not yet understood. In the Escherichia coli multidrug transporter, MdfA, the hydrophobicity profile and PhoA fusion analysis have so far identified only one membrane-embedded charged amino acid residue (E26). In order to determine whether this negatively charged residue may play a role in multidrug recognition, we evaluated the expression and function of MdfA constructs mutated at this position. Replacing E26 with the positively charged residue lysine abolished the multidrug resistance activity against positively charged drugs, but retained chloramphenicol efflux and resistance. In contrast, when the negative charge was preserved in a mutant with aspartate instead of E26, chloramphenicol recognition and transport were drastically inhibited; however, the mutant exhibited almost wild-type multidrug resistance activity against lipophilic cations. These results suggest that although the negative charge at position 26 is not essential for active transport, it dictates the multidrug resistance character of MdfA. We show that such a negative charge is also found in other drug resistance transporters, and its possible significance regarding multidrug resistance is discussed.

Active efflux and diffusion are involved in transport of Pseudomonas aeruginosa cell-to-cell signals

Many gram-negative bacteria communicate by N-acyl homoserine lactone signals called autoinducers (AIs). In Pseudomonas aeruginosa, cell-to-cell signaling controls expression of extracellular virulence factors, the type II secretion apparatus, a stationary-phase sigma factor (sigmas), and biofilm differentiation. The fact that a similar signal, N-(3-oxohexanoyl) homoserine lactone, freely diffuses through Vibrio fischeri and Escherichia coli cells has led to the assumption that all AIs are freely diffusible. In this work, transport of the two P. aeruginosa AIs, N-(3-oxododecanoyl) homoserine lactone (3OC12-HSL) (formerly called PAI-1) and N-butyryl homoserine lactone (C4-HSL) (formerly called PAI-2), was studied by using tritium-labeled signals. When [3H]C4-HSL was added to cell suspensions of P. aeruginosa, the cellular concentration reached a steady state in less than 30 s and was nearly equal to the external concentration, as expected for a freely diffusible compound. In contrast, [3H]3OC12-HSL required about 5 min to reach a steady state, and the cellular concentration was 3 times higher than the external level. Addition of inhibitors of the cytoplasmic membrane proton gradient, such as azide, led to a strong increase in cellular accumulation of [3H]3OC12-HSL, suggesting the involvement of active efflux. A defined mutant lacking the mexA-mexB-oprM-encoded active-efflux pump accumulated [3H]3OC12-HSL to levels similar to those in the azide-treated wild-type cells. Efflux experiments confirmed these observations. Our results show that in contrast to the case for C4-HSL, P. aeruginosa cells are not freely permeable to 3OC12-HSL. Instead, the mexA-mexB-oprM-encoded efflux pump is involved in active efflux of 3OC12-HSL. Apparently the length and/or degree of substitution of the N-acyl side chain determines whether an AI is freely diffusible or is subject to active efflux by P. aeruginosa.

AcrA is a highly asymmetric protein capable of spanning the periplasm

AcrA protein is a component of the multi-drug efflux complex AcrAB-TolC of Escherichia coli. Judged by the hypersusceptibility phenotype of acrA mutants, the AcrAB-TolC system pumps out an extraordinarily wide variety of antibiotics, chemotherapeutic agents, detergents and dyes. This complex traverses both the inner and outer membranes of E. coli and catalyzes efflux of the drugs directly into the medium. The coordinated operation of the inner membrane transporter AcrB and outer membrane channel TolC is thought to be mediated by AcrA. The latter is a lipoprotein located in the periplasmic space. We show here that a lipid-deficient derivative of AcrA is functionally active as demonstrated by the complementation of the hypersusceptibility phenotype of the acrA mutant. Purified non-lipidated and intact forms of AcrA were able to restore, with similar efficiency, the activity of AcrA-dependent efflux of erythromycin in Ca2+-sucrose-treated E. coli cells. Using analytical ultracentrifugation and dynamic light scattering techniques we determined hydrodynamic properties of the non-lipidated AcrA and found that AcrA exists in solution as a highly asymmetric monomeric molecule with an axial ratio of 8. This elongated shape of AcrA is compatible with the hypothesis that this protein spans the periplasmic space coordinating the concerted operation of inner and outer membrane components of the complex.

Antiseptics and disinfectants: activity, action, and resistance

Antiseptics and disinfectants are extensively used in hospitals and other health care settings for a variety of topical and hard-surface applications. A wide variety of active chemical agents (biocides) are found in these products, many of which have been used for hundreds of years, including alcohols, phenols, iodine, and chlorine. Most of these active agents demonstrate broad-spectrum antimicrobial activity; however, little is known about the mode of action of these agents in comparison to antibiotics. This review considers what is known about the mode of action and spectrum of activity of antiseptics and disinfectants. The widespread use of these products has prompted some speculation on the development of microbial resistance, in particular whether antibiotic resistance is induced by antiseptics or disinfectants. Known mechanisms of microbial resistance (both intrinsic and acquired) to biocides are reviewed, with emphasis on the clinical implications of these reports.

Site-directed chemical modification of cysteine-scanning mutants as to transmembrane segment II and its flanking regions of the Tn10-encoded metal-tetracycline/H+ antiporter reveals a transmembrane water-filled channel

Cysteine-scanning mutants, E32C to G62C, of the metal-tetracycline/H+ antiporter were constructed in order to precisely determine the membrane topology around putative transmembrane segment II. None of the mutants lost the ability to confer tetracycline resistance, indicating that the cysteine mutation in each mutant did not alter the protein conformation. [14C]N-Ethylmaleimide (NEM) binding to these cysteine mutants in isolated membranes was then investigated. The peptide chain of this region passes through the membrane at least once because residues 36 and 65 are exposed on the outside and inside surfaces of the membrane, respectively (Kimura, T., Ohnuma, M., Sawai, T., and Yamaguchi, A. (1997) J. Biol. Chem. 272, 580-585). However, there was no continuous segment in which all of the introduced cysteine residues showed almost no reactivity with [14C]NEM. The proportion of the unbound positions in the second half downstream from position 45 was 55% (10/18), which was clearly higher than that in the first half (21%; 3/14), suggesting that the second half is a transmembrane segment. Positions reactive to NEM appear periodically in the second half. They are located on one side of the helical wheel, suggesting that this side of the transmembrane helix faces a water-filled channel. The cysteine mutants as to the reactive positions in the second half were severely inactivated by NEM except for the P59C mutant, whereas the A40C mutant was the only one inactivated by NEM in the first half. These results suggest that the water-filled channel along this helical region may be a substrate translocation pathway.

Ofloxacin resistance mechanism in PA150 and PA300-clinical isolates of Pseudomonas aeruginosa in Korea

Five hundred and seventy clinical strains of Pseudomonas aeruginosa were isolated from August 1993 to August 1994 in Korea and screened for their resistance to ciprofloxacin, norfloxacin, and ofloxacin. Among these, two P. aeruginosa strains (PA150 and PA300) were selected based on their strong resistance (MICs > 50 micrograms/ml) to all three quinolones. The susceptible strain as well as two resistant strains had proton gradient-dependent efflux system. Efflux system in PA300 showed different specificities to ofloxacin and ciprofloxacin while PA 150 had less permeability for ofloxacin. Ofloxacin had a less inhibitory action on DNA synthesis in permeabilized cells of PA150 and PA300 than 1771M. When quinolone resistance determining region (QRDR) in gyrA was sequenced, PA300 had one missense mutation, Asn 116Tyr, which was newly reported in this work. The results showed that PA150 became ofloxacin resistant by reduced ofloxacin accumulation due to the existence of efflux system and low permeability, while resistance of PA300 was due to the efflux system and a mutation in QRDR of gyrA-the target site of quinolone.

Rapid emergence of high-level resistance to quinolones in Campylobacter jejuni associated with mutational changes in gyrA and parC

Quinolone resistance in clinical isolates of Campylobacter jejuni in Sweden increased more than 20-fold at the beginning of the 1990s. Resistance to 125 microgram of ciprofloxacin per ml in clinical isolates was associated with chromosomal mutations in C. jejuni leading to a Thr-86-Ile substitution in the gyrA product and a Arg-139-Gln substitution in the parC product.

Molecular cloning and characterization of Tap, a putative multidrug efflux pump present in Mycobacterium fortuitum and Mycobacterium tuberculosis

A recombinant plasmid isolated from a Mycobacterium fortuitum genomic library by selection for gentamicin and 2-N'-ethylnetilmicin resistance conferred low-level aminoglycoside and tetracycline resistance when introduced into M. smegmatis. Further characterization of this plasmid allowed the identification of the M. fortuitum tap gene. A homologous gene in the M. tuberculosis H37Rv genome has been identified. The M. tuberculosis tap gene (Rv1258 in the annotated sequence of the M. tuberculosis genome) was cloned and conferred low-level resistance to tetracycline when introduced into M. smegmatis. The sequences of the putative Tap proteins showed 20 to 30% amino acid identity to membrane efflux pumps of the major facilitator superfamily (MFS), mainly tetracycline and macrolide efflux pumps, and to other proteins of unknown function but with similar antibiotic resistance patterns. Approximately 12 transmembrane regions and different sequence motifs characteristic of the MFS proteins also were detected. In the presence of the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP), the levels of resistance to antibiotics conferred by plasmids containing the tap genes were decreased. When tetracycline accumulation experiments were carried out with the M. fortuitum tap gene, the level of tetracycline accumulation was lower than that in control cells but was independent of the presence of CCCP. We conclude that the Tap proteins of the opportunistic organism M. fortuitum and the important pathogen M. tuberculosis are probably proton-dependent efflux pumps, although we cannot exclude the possibility that they act as regulatory proteins.

Multiple antibiotic resistance and efflux

Multiple antibiotic resistance in bacteria was at first thought to be caused exclusively by the combination of several resistance genes, each coding for resistance to a single drug. More recently, it became clear that such phenotypes are often achieved by the activity of drug efflux pumps. Some of these efflux pumps exhibit an extremely wide specificity covering practically all antibiotics, chemotherapeutic agents, detergents, dyes, and other inhibitors, the exception perhaps being very hydrophilic compounds. Such efflux pumps work with exceptional efficiency in Gram-negative bacteria through their synergistic interaction with the outer membrane barrier. It is disturbing that the antibacterial agents of the most advanced type, which are unaffected by common resistance mechanisms, are precisely the compounds whose use appears to select for multidrug-resistant mutants that overproduce these efflux pumps of wide specificity.

Clinical applications of quinolones

The quinolone antimicrobials are the class of inhibitors of bacterial topoisomerases that has been developed most fully for clinical use in human medicine. Initial members of the class had their greatest potency against Gram-negative bacteria, but newly developed members have exhibited increased potency against Gram-positive bacteria and soon agents will be available with additional activity against anaerobic bacteria, providing a broad spectrum of potency. After nalidixic acid, the earliest member of the class which was used for treatment of urinary tract infections, the later fluoroquinolone congeners have had sufficient potency, absorption, and distribution into tissue for additional uses in treatment of sexually transmitted diseases, infections of the gastrointestinal tract, respiratory tract, skin, and bones and joints. Tolerability of these agents in usual doses has been good. Acquired bacterial resistance resulting from clinical uses has occurred in particular among staphylococci and Pseudomonas aeruginosa. Intense drug use and ability of resistant pathogens to spread have also contributed to development of resistance in initially more susceptible pathogens such as Escherichia coli and Neisseria gonorrhoeae in certain settings. Preservation of the considerable clinical utility of the quinolone class for the long term will be affected by the extent to which their use is judicious.

Antibiotic resistance in community-acquired respiratory tract infections: current issues

OBJECTIVE

In recent years, antibiotic resistance has emerged as an important global problem. The major goal of this review is to update important issues pertaining to antibiotic resistance, with an emphasis on antibiotic resistance involving community-acquired respiratory pathogens. In addition, this review examines potential reasons why antibiotic resistance has increased in recent years, how clinicians can better understand commonly used laboratory antibiotic resistance tests, and possible solutions to the increasing problem of antibiotic resistance. The article emphasizes the diagnosis, therapy, and prevention of antibiotic-resistant infections.

DATA SOURCES

We identified relevant English-language articles through MEDLINE search (1966 to March 1998). All articles related to antibiotic resistance and the scope of the articles included original investigative articles, reviews, letters, and editorials. In addition, we selected additional references from the bibliographies of the identified articles.

STUDY SELECTION

We selected articles for detailed review if they provided direct insight into the cause of antibiotic resistance, testing for antibiotic resistance, or the treatment of antibiotic resistance. Most, but not all, of the articles selected pertained to antibiotic resistance and respiratory tract infections. We performed a detailed review on approximately 40% of the originally selected articles.

RESULTS

Multiple factors that play a significant role in the development of antibiotic resistance include the overuse of antibiotics in both humans and animals, situations such as day care that enhance transmission via frequent close personal contact, and widespread dissemination of resistant strains via global travel. Most respiratory pathogens have developed resistance to commonly used antibiotics either by producing beta-lactamase or by altering binding site proteins.

CONCLUSIONS

In many regions of the United States, the level of antibiotic resistance has impacted the clinical management of common respiratory pathogens. Future efforts to curtail antibiotic resistance will require a concerted effort in multiple areas, particularly enhanced epidemiologic surveillance to better detect resistance trends, judicious use of antibiotics, and new drug development.

Are we facing a 'post-antibiotic era'?--a review of the literature regarding antimicrobial drug resistance

Since the introduction of antibiotics in the 1940s, antibiotic resistance has become an increasing problem. Today, multiple-antibiotic resistance is commonly associated with a number of clinically important pathogens and is therefore an important issue in clinical nursing practice. Epidemiological studies identify a number of important factors associated with increases in antimicrobial resistance. These include patterns of antimicrobial use, changes in medical and veterinary care and social practices affecting the transmission of microbes. Bacterial mechanisms of antibiotic resistance and the genetics of resistance-gene transfer are explored, with the intention of developing nurses' knowledge and understanding of control measures.

Functional characterization of ORCTL2--an organic cation transporter expressed in the renal proximal tubules

Chromosome 11p15.5 harbors a gene or genes involved in Beckwith-Wiedemann syndrome that confer(s) susceptibility to Wilms' tumor, rhabdomyosarcoma, and hepatoblastoma. We have previously identified a transcript at 11p15.5 which encodes a putative membrane transport protein, designated organic cation transporter-like 2 (ORCTL2), that shares homology with tetracycline resistance proteins and bacterial multidrug resistance proteins. In this report, we have investigated the transport properties of ORCTL2 and show that this protein can confer resistance to chloroquine and quinidine when overexpressed in bacteria. Immunohistochemistry analyses performed with anti-ORCTL2 polyclonal antibodies on human renal sections indicate that ORCTL2 is localized on the apical membrane surface of the proximal tubules. These results suggest that ORCTL2 may play a role in the transport of chloroquine and quinidine related compounds in the kidney.

Molecular cloning and functional analysis of a novel tetracycline resistance determinant, tet(V), from Mycobacterium smegmatis

The nucleotide sequence and mechanism of action of a tetracycline resistance gene from Mycobacterium smegmatis were determined. Analysis of a 2.2-kb sequence fragment showed the presence of one open reading frame, designated tet(V), encoding a 419-amino-acid protein (molecular weight, 44,610) with at least 10 transmembrane domains. A database search showed that the gene is homologous to membrane-associated antibiotic efflux pump proteins but not to any known tetracycline efflux pumps. The steady-state accumulation level of tetracycline by M. smegmatis harboring a plasmid carrying the tet(V) gene was about fourfold lower than that of the parental strain. Furthermore, the energy uncoupler carbonyl cyanide m-chlorophenylhydrazone blocked tetracycline efflux in deenergized cells. These results suggest that the tet(V) gene codes for a drug antiporter which uses the proton motive force for the active efflux of tetracycline. By primer-specific amplification the gene appears to be restricted to M. smegmatis and M. fortuitum.

Induced expression of the Candida albicans multidrug resistance gene CDR1 in response to fluconazole and other antifungals

The Candida albicans CDR1 gene encodes a member of the ABC-type family of multidrug transporters which has been shown to be involved in azole resistance. Using an in-frame gene fusion between the CDR1 open reading frame and the green fluorescent protein allele yEGFP3, an optimized derivative for its use in C. albicans, we show here how the CDR1-yEGFP3 gene expression is induced in response to azoles as well as to other structurally unrelated drugs like cycloheximide. Moderate increases were observed for calcofluor, canavanine, 5'-fluorcytosine, cilofungin and caffeine, while no induction was found for the antifungals benomyl and amphotericin B or hydrogen peroxide at subinhibitory concentrations. The use of confocal microscopy enabled us to localize the Cdr1p fusion protein at the cell periphery, thus suggesting a cytoplasmic membrane localization. These results suggest deregulation of CDR1 gene as a putative mechanism for the generation of azole resistance in this clinically important pathogenic fungus.

Structure of grepafloxacin relative to activity and safety profile

A comparison of the structure of ciprofloxacin and grepafloxacin shows that the two compounds are similar, with two exceptions: grepafloxacin has a methyl group at the 5 position and a methyl group attached to the 7-piperazinyl substituent. At the 1 position, both compounds have a cyclopropyl group, which is important for potency, but limits anaerobic activity. The methylpiperazine at position 7 in grepafloxacin is associated with its enhanced Gram-positive activity and long half-life. The methyl group at R5 is also thought to enhance Gram-positive activity. Ciprofloxacin's piperazine group at the 7 position is associated with good Gram-negative activity. Grepafloxacin's Gram-negative activity is comparable to that of ciprofloxacin's against Haemophilus influenzae, Moraxella catarrhalis and enteric Gram-negative bacilli. Studies of resistance development to fluoroquinolones suggest that grepafloxacin is associated with a reduced selection of resistance in Staphylococcus aureus, which is possibly related to the inhibition or avoidance of efflux transport by NorA.

Cysteine-scanning mutagenesis around transmembrane segment III of Tn10-encoded metal-tetracycline/H+ antiporter

Each amino acid in the putative transmembrane helix III and its flanking regions (from Gly-62 to Tyr-98) of the Tn10-encoded metal-tetracycline/H+ antiporter (Tet(B)) was individually replaced with Cys. Out of these 37 cysteine-scanning mutants, the mutants from G62C to R70C and from S92C to Y98C showed high or intermediate reactivity with [14C]N-ethylmaleimide (NEM) except for the M64C mutant. On the other hand, the mutants from R71C to S91C showed almost no reactivity with NEM except for the P72C mutant. These results confirm that the transmembrane helix III is composed of 21 residues from Arg-71 to Ser-91. The majority of Cys replacement mutants retained high or moderate tetracycline transport activity. Cys replacements for Gly-62, Asp-66, Ser-77, Gly-80, and Asp-84 resulted in almost inactive Tet(B) (less than 3% of the wild-type activity). The Arg-70 --> Cys mutant retained very low activity due to a mercaptide between Co2+ and a SH group (Someya, Y., and Yamaguchi, A. (1996) Biochemistry 35, 9385-9391). Three of these six important residues (Ser-77, Gly-80, and Asp-84) are located in the transmembrane helix III and one (Arg-70) is located in the flanking region. These four functionally important residues are located on one side of the helical wheel. Only two of the residual 31 Cys mutants were inactivated by NEM (S65C and L97C). Ser-65 and Leu-97 are located on the cytoplasmic and periplasmic loops, respectively, in the topology of Tet(B). The degree of inactivation of these Cys mutants with SH reagents was dependent on the volume of substituents. In the presence of tetracycline, the reactivity of the S65C mutant with NEM was significantly increased, in contrast, the reactivity of L97C was greatly reduced, indicating that the cytoplasmic and periplasmic loop regions undergo substrate-induced conformational change in the mutually opposite direction.

Involvement of an efflux system in high-level fluoroquinolone resistance of Shigella dysenteriae

Shigella dysenteriae represent one of the growing list of antibiotic-resistant bacteria. Quinolones are widely employed to treat shigellosis. However, quinolone resistance has already been reported, necessitating an understanding of the mechanisms of development of resistance. We demonstrate that high-level fluoroquinolone resistance of S. dysenteriae exposed to these antibiotics may occur in the absence of gyrA mutations and involve a proton motive force(pmf)-dependent efflux system.

Isolation and characterization of a putative multidrug resistance pump from Vibrio cholerae

Multidrug-resistant strains of Vibrio cholerae (the causative agent of the diarrhoeal disease cholera) have recently been described. In an attempt to identify a homologue of the Escherichia coli TolC in V. cholerae, we isolated a DNA fragment (pVC) that enabled an E. coli tolC mutant to grow in the presence of 0.05% deoxycholate (DOC). However, other TolC defects were not complemented. Nucleotide sequence analysis of this fragment revealed the presence of two open reading frames (ORF1 and ORF2) separated by 9 bp and encoding 42.4 and 55.8 kDa proteins respectively. The translational products of these two ORFs correlated closely with the molecular weights of the predicted proteins. The deduced amino acid sequences of ORF1 and ORF2 showed a high degree of similarity with conserved regions of the E. coli efflux pump proteins, EmrA and EmrB. The presence of pVC2 within the E. coli efflux pump mutants defective in either the emrAB or the acrAB genes provided the mutants with resistance against several antibiotics. A V. cholerae isogenic mutant defective in ORF2 was constructed by gene replacement. Characterization of this mutant has shown it to be more sensitive to CCCP, PMA, PCP, nalidixic acid and DOC than the parent strain. These results suggest that ORF1 and ORF2 constitute an operon encoding two components of a putative multidrug resistance pump in V. cholerae. In addition, the presence of both structural and functional similarities between VceAB and EmrAB suggests that VceAB is a homologue of EmrAB.

Bacterial energetics and antimicrobial resistance

Defining resistance to antimicrobics with specific terms is very useful as the precise mechanisms often lead to better susceptibility testing and improved treatment of bacterial infections. When one can characterize a class of phenotypic resistant organisms as electron transport variants, this has value in that it provides information about the basis for resistance and removes them from the non-specific categorization as phenotypically resistant. Studies of bacterial small colony variants (SCVs) has provided new insights into antimicrobial resistance as well as the connection between expression of virulence factors and energy metabolism. It also provides a framework for further studies which link antibiotic resistance to phase of growth, availability of nutrients, other environmental conditions, and adaptations to stress into a single model wherein the bacterial cell uses the products of energy metabolism to detect its sense of well being and to alter its response to antibiotics (Fig. I). Levels of NADH and ATP are able to act as signaling molecules in both gram positive an gram negative bacteria to activate stress sigma factors and two component systems. In the future, a full understanding of the signaling pathways in bacterial pathogens may provide new targets for the development of drugs to reduce bacterial virulence and to enhance the activity of existing antibiotics.

Roles of acidic residues in the hydrophilic loop regions of metal-tetracycline/H+ antiporter Tet(K) of Staphylococcus aureus

Three transmembrane glutamic acid residues play essential roles in the metal-tetracycline/H+ antiporter Tet(K) of Staphylococcus aureus [Fujihira et al., FEBS Lett. 391 (1996) 243-246]. In the putative hydrophilic loop region of the Tet(K) and Tet(L) proteins, six acidic residues are conserved. Asp74, Asp200, Asp318 and Glu381 are located on the putative cytoplasmic side, and Asp39 and Glu345 on the putative periplasmic side. These residues were replaced by a neutral amino acid residue or a charge-conserved one. In contrast to the transmembrane glutamic acid residues, the replacement of the two glutamic acid residues (Glu345 and Glu381) did not affect the tetracycline resistance level. Out of the other four aspartic acid residues, the only essential residue is Asp318, any replacement of which resulted in complete loss of the tetracycline resistance and transport activity. Asp318 is located in cytoplasmic loop 10-11 in the putative 14-transmembrane-segment topology of Tet(K). In the case of the tetracycline exporters of Gram-negative bacteria, the only essential acidic residue in the cytoplasmic loop region is located in loop 2-3 [Yamaguchi et al., Biochemistry 31 (1992) 8344-8348]. It may be a general role for tetracycline efflux proteins that three transmembrane and one cytoplasmic acidic residues are mandatory for the tetracycline transport function.

The acrAB homolog of Haemophilus influenzae codes for a functional multidrug efflux pump

Disruption of gene HI0894 or HI0895 in Haemophilus influenzae Rd, homologs of Escherichia coli acrAB multidrug efflux genes, caused hypersusceptibility to erythromycin, rifampin, novobiocin, and dyes such as ethidium bromide and crystal violet and increased accumulation of radioactive erythromycin, showing that these genes are expressed and contribute to the baseline level resistance of this organism through active drug efflux. The gene disruption did not produce detectable changes in susceptibility to several other antibiotics, possibly because rapid influx of small antibiotic molecules through the large H. influenzae porin channels counterbalances their efflux.

Glutamate residues located within putative transmembrane helices are essential for TetA(P)-mediated tetracycline efflux

The tetA(P) gene from Clostridium perfringens encodes a unique membrane protein that is responsible for the active efflux of tetracycline from resistant cells. The novel TetA(P) protein has neither the typical structure nor the conserved motifs that are found in tetracycline efflux proteins from classes A through H or classes K and L. Site-directed mutagenesis of selected residues within TetA(P) was performed to elucidate their role in tetracycline efflux. Glutamate residues 52 and 59, negatively charged residues located within putative transmembrane helix 2, could not be replaced by either glutamine or aspartate and so were essential for tetracycline efflux. Replacement of Glu89, which was located at the end of helix 3, by aspartate but not by glutamine allowed TetA(P) function, indicating the importance of a carboxyl group at this position. After mutation of the Asp67 residue, located within cytoplasmic loop 1, no immunoreactive protein was detected. It is concluded that negatively charged residues that appear to be located within or near the membrane are important for the function of TetA(P).

Mechanisms of multidrug transporters

Drug resistance, mediated by various mechanisms, plays a crucial role in the failure of the drug-based treatment of various infectious diseases. As a result, these infectious diseases re-emerge rapidly and cause many victims every year. Another serious threat is imposed by the development of multidrug resistance (MDR) in eukaryotic (tumor) cells, where many different drugs fail to perform their therapeutic function. One of the causes of the occurrence of MDR in these cells is the action of transmembrane transport proteins that catalyze the active extrusion of a large number of structurally and functionally unrelated compounds out of the cell. The mode of action of these MDR transporters and their apparent lack of substrate specificity is poorly understood and has been subject to many speculations. In this review we will summarize our current knowledge about the occurrence, mechanism and molecular basis of (multi-)drug resistance especially as found in bacteria.

Protein folding coupled to disulphide bond formation

Protein folding that is coupled to disulphide bond formation has many experimental advantages. In particular, the kinetic roles and importance of all the disulphide intermediates can be determined, usually unambiguously. This contrasts with other types of protein folding, where the roles of any intermediates detected are usually not established. Nevertheless, there is considerable confusion in the literature about even the best-characterized disulphide folding pathways. This article attempts to set the record straight.

A novel compound, 1,1-dimethyl-5(1-hydroxypropyl)-4,6,7-trimethylindan, is an effective inhibitor of the tet(K) gene-encoded metal-tetracycline/H+ antiporter of Staphylococcus aureus

A novel indan derivative, 1,1-dimethyl-5-(1-hydroxypropyl)-4,6,7-trimethylindan (Ro 07-3149), was found to be a strong inhibitor of the tet(K) gene-encoded tetracycline/H+ antiporter of Staphylococcus aureus. One micromole of this compound per mg membrane protein was enough for complete inhibition of the Tet(K)-mediated tetracycline transport and tetracycline-coupled proton transport, without the energy state of the membrane being affected. The mode of inhibition was non-competitive. Although this compound caused membrane de-energization at a high concentration, the IC50 value for de-energization (7.3 micromol/mg membrane protein) was about 17 times and 33 times higher than the values for Tet(K)-mediated proton/tetracycline antiport and [3H]tetracycline transport, respectively, indicating that the inhibitory action of Ro 07-3149 is not due to the uncoupling effect of the inhibitor.

Regulation of the "tetCD" genes of transposon Tn10

In addition to the genes involved in tetracycline resistance, the loop region of the composite transposon Tn10 contains two other known genes, tetC and tetD, whose functions are unclear. Using primarily a genetic approach, we examined tetCD gene expression and regulation. The tetC gene product, TetC, is a diffusible repressor of both tetC and tetD transcription. Despite an earlier claim by others, we do not detect induction of either tetC or tetD by tetracycline (Tc) or several of its analogs. Although the 5' ends of the tetC and tetD messages overlap due to transcription from convergent promoters, we find no evidence for anti-sense RNA control. The operator for the TetC repressor has been localized. We also demonstrate that transcription from the tetD promoter probably terminates within IS10-Right and does not apparently interfere with Tn10 or IS10-Right transposition or its regulation.

The MtrD protein of Neisseria gonorrhoeae is a member of the resistance/nodulation/division protein family constituting part of an efflux system

The mtr (multiple transferable resistance) system of Neisseria gonorrhoeae mediates resistance of gonococci to structurally diverse hydrophobic agents (HAs) through an energy-dependent efflux process. Recently, complete or partial ORFs that encode membrane proteins (MtrC, MtrD, MtrE) forming an efflux pump responsible for removal of HAs from gonococci were identified and appeared to constitute a single transcriptional unit. In this study, the complete nucleotide sequence of the mtrD gene was determined, permitting the characterization of the MtrD protein. The full-length MtrD protein has a predicted molecular mass of nearly 114 kDa, putatively containing a 56 amino acid signal peptide. MtrD displays significant amino acid sequence similarity to a family of cytoplasmic membrane proteins, termed resistance/nodulation/division (RND) proteins, which function as energy-dependent transporters of antibacterial agents and secrete bacterial products to the extracellular fluid. The predicted topology of the MtrD transporter protein revealed 12 potential membrane-spanning domains, which were clustered within the central and C-terminal regions of the primary sequence. Loss of MtrD due to insertional inactivation of the mtrD gene rendered gonococci hypersusceptible to several structurally diverse HAs, including two fatty acids (capric acid and palmitic acid) and a bile salt (cholic acid), but not hydrophilic antibiotics such as ciprofloxacin and streptomycin. Since gonococci often infect mucosal sites rich in toxic fatty acids and bile salts, the expression of the mtr efflux system may promote growth of gonococci under hostile conditions encountered in vivo.

The MtrR repressor binds the DNA sequence between the mtrR and mtrC genes of Neisseria gonorrhoeae

Gonococcal resistance to antimicrobial hydrophobic agents (HAs) is due to energy-dependent removal of HAs from the bacterial cell by the MtrCDE membrane-associated efflux pump. The mtrR (multiple transferrable resistance Regulator) gene encodes a putative transcriptional repressor protein (MtrR) believed to be responsible for regulation of mtrCDE gene expression. Gel mobility shift and DNase I footprint assays that used a maltose-binding protein (MBP)-MtrR fusion protein demonstrated that the MtrR repressor is capable of specifically binding the DNA sequence between the mtrR and mtrC genes. This binding site was localized to a 26-nucleotide stretch that includes the promoter utilized for mtrCDE transcription and, on the complementary strand, a 22-nucleotide stretch that contains the -35 region of the mtrR promoter. A single transition mutation (A-->G) within the MtrR-binding site decreased the affinity of the target DNA for MtrR and enhanced gonococcal resistance to HAs when introduced into HA-susceptible strain FA19 by transformation. Since this mutation enhanced expression of the mtrCDE gene complex but decreased expression of the mtrR gene, the data are consistent with the notion that MtrR acts as a transcriptional repressor of the mtrCDE efflux pump protein genes.

Membrane topology of the metal-tetracycline/H+ antiporter TetA(K) from Staphylococcus aureus

A series of fusions to the reporter proteins alkaline phosphatase and beta-galactosidase have been constructed in the predicted periplasmic and cytoplasmic loops of TetA(K), a protein responsible for efflux-mediated tetracycline resistance in Staphylococcus aureus. The results support a topological model of 14 transmembrane segments for TetA(K).

Removal of high concentrations of nitrate from industrial wastewaters by bacteria

Klebsiella oxytoca isolate 15 was isolated from the grounds of a nitration factory and was found to be tolerant to nitrate at concentrations up to 0.5 to 1 M. Physicochemical parameters for optimal growth conditions for K. oxytoca isolate 15 were established. Growth took place when the nitrate concentration in the medium was less than 150 mM, and full nitrate consumption required about 14 g of C per g of N. This strain was able to remove nitrate without accumulating nitrite. The system was scaled up to a 40-liter pilot plant and was operated on-site satisfactorily.

MdfA, an Escherichia coli multidrug resistance protein with an extraordinarily broad spectrum of drug recognition

Multidrug resistance (MDR) translocators recently identified in bacteria constitute an excellent model system for studying the MDR phenomenon and its clinical relevance. Here we describe the identification and characterization of an unusual MDR gene (mdfA) from Escherichia coli. mdfA encodes a putative membrane protein (MdfA) of 410 amino acid residues which belongs to the major facilitator superfamily of transport proteins. Cells expressing MdfA from a multicopy plasmid are substantially more resistant to a diverse group of cationic or zwitterionic lipophilic compounds such as ethidium bromide, tetraphenylphosphonium, rhodamine, daunomycin, benzalkonium, rifampin, tetracycline, and puromycin. Surprisingly, however, MdfA also confers resistance to chemically unrelated, clinically important antibiotics such as chloramphenicol, erythromycin, and certain aminoglycosides and fluoroquinolones. Transport experiments with an E. coli strain lacking F1-F0 proton ATPase activity indicate that MdfA is a multidrug transporter that is driven by the proton electrochemical gradient.

Mycobacterium tuberculosis efpA encodes an efflux protein of the QacA transporter family

The Mycobacterium tuberculosis H37Rv efpA gene encodes a putative efflux protein, EfpA, of 55,670 Da. The deduced EfpA protein was similar in secondary structure to Pur8, MmrA, TcmA, LfrA, EmrB, and other members of the QacA transporter family (QacA TF) which mediate antibiotic and chemical resistance in bacteria and yeast. The predicted EfpA sequence possessed all transporter motifs characteristic of the QacA TF, including those associated with proton-antiport function and the motif considered to be specific to exporters. The 1,590-bp efpA open reading frame was G+C rich (65%), whereas the 40-bp region immediately upstream had an A+T bias (35% G+C). Reverse transcriptase-PCR assays indicated that efpA was expressed in vitro and in situ. Putative promoter sequences were partially overlapped by the A+T-rich region and by a region capable of forming alternative secondary structures indicative of transcriptional regulation in analogous systems. PCR single-stranded conformational polymorphism analysis demonstrated that these upstream flanking sequences and the 231-bp, 5' coding region are highly conserved among both drug-sensitive and multiply-drug-resistant isolates of M. tuberculosis. The efpA gene was present in the slow-growing human pathogens M. tuberculosis, Mycobacterium leprae, and Mycobacterium bovis and in the opportunistic human pathogens Mycobacterium avium and Mycobacterium intracellular. However, efpA was not present in 17 other opportunistically pathogenic or nonpathogenic mycobacterial species.

Biotechnological aspects of membrane function

An overview is given of the biotechnological utilizability of various features of cell membranes. Techniques are given that describe how to make use of the barrier and transport functions of membranes for biotechnological purposes, ranging from cell permeabilization and construction of immobilized biocatalysts to manipulating excretion and uptake properties of the membranes by various methods. Glucose transporters, iron-transporting membrane systems, and pumps engaged in pleiotropic drug resistance are treated in more detail as particularly biotechnologically important membrane proteins.

The purified Bacillus subtilis tetracycline efflux protein TetA(L) reconstitutes both tetracycline-cobalt/H+ and Na+(K+)/H+ exchange

Recent work has suggested that the chromosomally encoded TetA(L) transporter of Bacillus subtilis, for which no physiological function had been shown earlier, not only confers resistance to low concentrations of tetracycline but is also a multifunctional antiporter protein that has dominant roles in both Na(+)- and K(+)-dependent pH homeostasis and in Na+ resistance during growth at alkaline pH. To rigorously test this hypothesis, TetA(L) has been purified with a hexahistidine tag at its C terminus and reconstituted into proteoliposomes. The TetA(L)-hexahistidine proteoliposomes exhibit high activities of tetracycline-cobalt/H+, Na+/ H+, and K+/H+ antiport in an assay in which an outwardly directed proton gradient is artificially imposed and solute uptake is monitored. Tetracycline uptake depends on the presence of cobalt and vice versa, with the cosubstrates being transported in a 1:1 ratio. Evidence for the electrogenicity of both tetracycline-cobalt/H+ and Na+/H+ antiports is presented. K+ and Li+ inhibit Na+ uptake, but there is little cross-inhibition between Na+ and tetracycline-cobalt uptake activities. The results strongly support the conclusion that TetA(L) is a multifunctional antiporter. They expand the roster of such porters to encompass one with a complex organic substrate and monovalent cation substrates that may have distinct binding domains, and provide the first functional reconstitution of a member of the 14-transmembrane segment transporter family.

Sequencing and expression of a gene encoding a bile acid transporter from Eubacterium sp. strain VPI 12708

Eubacterium sp. strain VPI 12708 expresses inducible bile acid 7alpha-dehydroxylation activity via a multistep pathway. The genes encoding several of the inducible proteins involved in the pathway have been previously mapped to a bile acid-inducible (bai) operon in Eubacterium sp. strain VPI 12708. We now report the cloning, sequencing, and characterization of the baiG gene, which is part of the bai operon. The predicted amino acid sequence of the BaiG polypeptide shows significant homology to several membrane transport proteins, including sugar and antibiotic resistance transporters, which are members of the major facilitator superfamily. Hydrophilicity plots of BaiG show a high degree of similarity to class K and L TetA proteins from gram-positive bacteria, and, like these classes of TetA proteins, BaiG has 14 proposed transmembrane domains. The baiG gene was cloned into Escherichia coli and shown to confer an energy-dependent bile acid uptake activity. Primary bile acids were preferentially transported into E. coli cells expressing this gene, with at least sevenfold and fourfold increases in the uptake of cholic acid and chenodeoxycholic acid, respectively, over control reactions. Less transport activity was observed with cholylglycine, 7-oxocholic acid, and deoxycholic acid. The transport activity was inhibited by the proton ionophores carbonyl cyanide m-chlorophenylhydrazone, 2,4-dinitrophenol, and nigericin but not by the potassium ionophore valinomycin, suggesting that the transport is driven by the proton motive force across the cell membrane. In summary, we have cloned, sequenced, and expressed a bile acid-inducible bile acid transporter from Eubacterium sp. strain VPI 12708. To our knowledge, this is the first report of the cloning and expression of a gene encoding a procaryotic bile acid transporter.