Structural basis of multidrug recognition by BmrR, a transcription activator of a multidrug transporter

Rhodamine 19 Alkyl Esters as Effective Antibacterial Agents

Mitochondria-targeted antioxidants (MTAs) have been studied quite intensively in recent years as potential therapeutic agents and vectors for the delivery of other active substances to mitochondria and bacteria. Their most studied representatives are MitoQ and SkQ1, with its fluorescent rhodamine analog SkQR1, a decyl ester of rhodamine 19 carrying plastoquinone. In the present work, we observed a pronounced antibacterial action of SkQR1 against Gram-positive bacteria, but virtually no effect on Gram-negative bacteria. The MDR pump AcrAB-TolC, known to expel SkQ1, did not recognize and did not pump out SkQR1 and dodecyl ester of rhodamine 19 (C12R1). Rhodamine 19 butyl (C4R1) and ethyl (C2R1) esters more effectively suppressed the growth of ΔtolC Escherichia coli, but lost their potency with the wild-type E. coli pumping them out. The mechanism of the antibacterial action of SkQR1 may differ from that of SkQ1. The rhodamine derivatives also proved to be effective antibacterial agents against various Gram-positive species, including Staphylococcus aureus and Mycobacterium smegmatis. By using fluorescence correlation spectroscopy and fluorescence microscopy, SkQR1 was shown to accumulate in the bacterial membrane. Thus, the presentation of SkQR1 as a fluorescent analogue of SkQ1 and its use for visualization should be performed with caution.

Inhibition of the norA gene expression and the NorA efflux pump by the tannic acid

The NorA efflux pump of Staphylococcus aureus is known to play a major role in the development of resistance against quinolone drugs by reducing their concentration inside target pathogens. The objective of this study was to evaluate the ability of tannic acid to inhibit the gene expression of the NorA efflux pump in Staphylococcus aureus and to evaluate the in silico effect on the pump. Efflux pump inhibition was evaluated by fluorimetry. The checkerboard method evaluates the effect of the test substance in combination with an antimicrobial at different concentrations. To gene expression evaluation NorA the assay was performed using: a sub-inhibitory concentration preparation (MIC/4) of the antibiotic; a sub-inhibitory concentration preparation (MIC/4) of the antibiotic associated with tannic acid at a sub-inhibitory concentration (MIC/4). In this study, docking simulations were performed by the SWISSDOCK webserver. The ability of tannic acid to inhibit the NorA efflux pump can be related to both the ability to inhibit the gene expression of this protein, acting on signaling pathways involving the ArlRS membrane sensor. As well as acting directly through direct interaction with the NorA protein, as seen in the approach and in silico and in vitro per checkerboard method and fluorimetry of bromide accumulated in the cell.

A Specialized Polythioamide-Binding Protein Confers Antibiotic Self-Resistance in Anaerobic Bacteria

Understanding antibiotic resistance mechanisms is central to the development of anti-infective therapies and genomics-based drug discovery. Yet, many knowledge gaps remain regarding the resistance strategies employed against novel types of antibiotics from less-explored producers such as anaerobic bacteria, among them the Clostridia. Through the use of genome editing and functional assays, we found that CtaZ confers self-resistance against the copper chelator and gyrase inhibitor closthioamide (CTA) in Ruminiclostridium cellulolyticum. Bioinformatics, biochemical analyses, and X-ray crystallography revealed CtaZ as a founding member of a new group of GyrI-like proteins. CtaZ is unique in binding a polythioamide scaffold in a ligand-optimized hydrophobic pocket, thereby confining CTA. By genome mining using CtaZ as a handle, we discovered previously overlooked homologs encoded by diverse members of the phylum Firmicutes, including many pathogens. In addition to characterizing both a new role for a GyrI-like domain in self-resistance and unprecedented thioamide binding, this work aids in uncovering related drug-resistance mechanisms.

The bacterial yjdF riboswitch regulates translation through its tRNA-like fold

Unlike most riboswitches, which have one cognate effector, the bacterial yjdF riboswitch binds to diverse azaaromatic compounds, only a subset of which cause it to activate translation. We examined the yjdF aptamer domain by small-angle X-ray scattering, and found that in the presence of activating ligands, the RNA adopts an overall shape similar to that of tRNA. Sequence analyses suggested that the yjdF aptamer is a homolog of tRNA^Lys, and that two of the conserved loops of the riboswitch are equivalent to the D- and T-loops of tRNA, associating to form an elbow-like tertiary interaction. Chemical probing indicated that this association is promoted by activating ligands such as chelerythrine and harmine. In its native mRNA context, activator ligands stabilize the tRNA-like fold of the yjdF aptamer, outcompeting the attenuated state in which its T-loop base-pairs to the Shine-Dalgarno element of the mRNA. Moreover, we demonstrate that the liganded aptamer itself activates translation, as authentic tRNAs, when grafted into mRNA, can potently activate translation. Taken together, our data demonstrate the ability of tRNA to function as a small-molecule responsive cis regulatory element.

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Physiological Functions of Bacterial "Multidrug" Efflux Pumps

Bacterial multidrug efflux pumps have come to prominence in human and veterinary pathogenesis because they help bacteria protect themselves against the antimicrobials used to overcome their infections. However, it is increasingly realized that many, probably most, such pumps have physiological roles that are distinct from protection of bacteria against antimicrobials administered by humans. Here we undertake a broad survey of the proteins involved, allied to detailed examples of their evolution, energetics, structures, chemical recognition, and molecular mechanisms, together with the experimental strategies that enable rapid and economical progress in understanding their true physiological roles. Once these roles are established, the knowledge can be harnessed to design more effective drugs, improve existing microbial production of drugs for clinical practice and of feedstocks for commercial exploitation, and even develop more sustainable biological processes that avoid, for example, utilization of petroleum.

Antibiotic binding releases autoinhibition of the TipA multidrug-resistance transcriptional regulator

Investigations of bacterial resistance strategies can aid in the development of new antimicrobial drugs as a countermeasure to the increasing worldwide prevalence of bacterial antibiotic resistance. One such strategy involves the TipA class of transcription factors, which constitute minimal autoregulated multidrug resistance (MDR) systems against diverse antibiotics. However, we have insufficient information regarding how antibiotic binding induces transcriptional activation to design molecules that could interfere with this process. To learn more, we determined the crystal structure of SkgA from Caulobacter crescentus as a representative TipA protein. We identified an unexpected spatial orientation and location of the antibiotic-binding TipAS effector domain in the apo state. We observed that the α6–α7 region of the TipAS domain, which is canonically responsible for forming the lid of antibiotic-binding cleft to tightly enclose the bound antibiotic, is involved in the dimeric interface and stabilized via interaction with the DNA-binding domain in the apo state. Further structural and biochemical analyses demonstrated that the unliganded TipAS domain sterically hinders promoter DNA binding but undergoes a remarkable conformational shift upon antibiotic binding to release this autoinhibition via a switch of its α6–α7 region. Hence, the promoters for MDR genes including tipA and RNA polymerases become available for transcription, enabling efficient antibiotic resistance. These insights into the molecular mechanism of activation of TipA proteins advance our understanding of TipA proteins, as well as bacterial MDR systems, and may provide important clues to block bacterial resistance.

The bacterial multidrug resistance regulator BmrR distorts promoter DNA to activate transcription

The MerR-family proteins represent a unique family of bacteria transcription factors (TFs), which activate transcription in a manner distinct from canonical ones. Here, we report a cryo-EM structure of a B. subtilis transcription activation complex comprising B. subtilis six-subunit (2αββ'ωε) RNA Polymerase (RNAP) core enzyme, σA, a promoter DNA, and the ligand-bound B. subtilis BmrR, a prototype of MerR-family TFs. The structure reveals that RNAP and BmrR recognize the upstream promoter DNA from opposite faces and induce four significant kinks from the -35 element to the -10 element of the promoter DNA in a cooperative manner, which restores otherwise inactive promoter activity by shortening the length of promoter non-optimal -35/-10 spacer. Our structure supports a DNA-distortion and RNAP-non-contact paradigm of transcriptional activation by MerR TFs.

The ABC of Biofilm Drug Tolerance: the MerR-Like Regulator BrlR Is an Activator of ABC Transport Systems, with PA1874-77 Contributing to the Tolerance of Pseudomonas aeruginosa Biofilms to Tobramycin

A hallmark of biofilms is their tolerance to killing by antimicrobial agents. In Pseudomonas aeruginosa, biofilm drug tolerance requires the c-di-GMP-responsive MerR transcriptional regulator BrlR. However, the mechanism by which BrlR mediates biofilm drug tolerance has not been elucidated. Here, we demonstrate that BrlR activates the expression of at least 7 ABC transport systems, including the PA1874-PA1875-PA1876-PA1877 (PA1874-77) operon, with chromatin immunoprecipitation and DNA binding assays confirming BrlR binding to the promoter region of PA1874-77. Insertional inactivation of the 7 ABC transport systems rendered P. aeruginosa PAO1 biofilms susceptible to tobramycin or norfloxacin. Susceptibility was linked to drug accumulation, with BrlR contributing to norfloxacin accumulation in a manner dependent on multidrug efflux pumps and the PA1874-77 ABC transport system. Inactivation of the respective ABC transport system, furthermore, eliminated the recalcitrance of biofilms to killing by tobramycin but not norfloxacin, indicating that drug accumulation is not linked to biofilm drug tolerance. Our findings indicate for the first time that BrlR, a MerR-type transcriptional activator, activates genes encoding several ABC transport systems, in addition to multiple multidrug efflux pump genes. Moreover, our data confirm a BrlR target contributing to drug tolerance, likely countering the prevailing dogma that biofilm tolerance arises from a multiplicity of factors.

GyrI-like proteins catalyze cyclopropanoid hydrolysis to confer cellular protection

GyrI-like proteins are widely distributed in prokaryotes and eukaryotes, and recognized as small-molecule binding proteins. Here, we identify a subfamily of these proteins as cyclopropanoid cyclopropyl hydrolases (CCHs) that can catalyze the hydrolysis of the potent DNA-alkylating agents yatakemycin (YTM) and CC-1065. Co-crystallography and molecular dynamics simulation analyses reveal that these CCHs share a conserved aromatic cage for the hydrolytic activity. Subsequent cytotoxic assays confirm that CCHs are able to protect cells against YTM. Therefore, our findings suggest that the evolutionarily conserved GyrI-like proteins confer cellular protection against diverse xenobiotics via not only binding, but also catalysis.

Role of Aromatic and Negatively Charged Residues of DrrB in Multisubstrate Specificity Conferred by the DrrAB System of Streptomyces peucetius

Resistance to the anticancer antibiotics, doxorubicin and daunorubicin, in the producer organism Streptomyces peucetius is conferred by an ABC transporter made of two proteins, DrrA and DrrB, which together form a dedicated exporter for these two antibiotics. Surprisingly, however, the DrrAB system exhibits broad substrate specificity overlapping with well-studied multidrug resistance transporters, including P-glycoprotein. Therefore, it provides an excellent model for studying the molecular basis of multispecificity in a prototype efflux system with the potential to unravel the origin and evolution of multidrug resistance. It has been suggested that multispecificity in multidrug exporters may be generally determined by the number and location of aromatic residues. Strategically placed negatively charged residues may also be critical for binding of cationic lipophilic drugs. We selected 13 aromatic and four negatively charged residues on the basis of their location in and/or near the predicted drug-binding pocket of DrrB for analysis. Indeed, mutations of most tested residues drastically inhibited doxorubicin efflux. Interestingly, several mutants lost resistance to doxorubicin and verapamil simultaneously but retained resistance to Hoechst 33342 and/or ethidium bromide, suggesting the presence of overlapping as well as independent drug-binding sites in a common drug-binding pocket of DrrB. This study provides the first comprehensive analysis of residues involved in drug binding in a bacterial multidrug resistance protein of the ABC superfamily, and it shows strong similarity in the molecular mechanism of polyspecific drug recognition between DrrAB and Pgp. Altogether, we conclude that aromatic residue-based multidrug specificity is conserved across domains and over long evolutionary periods. The significance of these findings is discussed.

Metal Ion Binding at the Catalytic Site Induces Widely Distributed Changes in a Sequence Specific Protein-DNA Complex

Metal ion cofactors can alter the energetics and specificity of sequence specific protein–DNA interactions, but it is unknown if the underlying effects on structure and dynamics are local or dispersed throughout the protein–DNA complex. This work uses EcoRV endonuclease as a model, and catalytically inactive lanthanide ions, which replace the Mg²⁺ cofactor. Nuclear magnetic resonance (NMR) titrations indicate that four Lu³⁺ or two La³⁺ cations bind, and two new crystal structures confirm that Lu³⁺ binding is confined to the active sites. NMR spectra show that the metal-free EcoRV complex with cognate (GATATC) DNA is structurally distinct from the nonspecific complex, and that metal ion binding sites are not assembled in the nonspecific complex. NMR chemical shift perturbations were determined for ¹H–¹⁵N amide resonances, for ¹H–¹³C Ile-δ-CH₃ resonances, and for stereospecifically assigned Leu-δ-CH₃ and Val-γ-CH₃ resonances. Many chemical shifts throughout the cognate complex are unperturbed, so metal binding does not induce major conformational changes. However, some large perturbations of amide and side chain methyl resonances occur as far as 34 Å from the metal ions. Concerted changes in specific residues imply that local effects of metal binding are propagated via a β-sheet and an α-helix. Both amide and methyl resonance perturbations indicate changes in the interface between subunits of the EcoRV homodimer. Bound metal ions also affect amide hydrogen exchange rates for distant residues, including a distant subdomain that contacts DNA phosphates and promotes DNA bending, showing that metal ions in the active sites, which relieve electrostatic repulsion between protein and DNA, cause changes in slow dynamics throughout the complex.

Solution Binding and Structural Analyses Reveal Potential Multidrug Resistance Functions for SAV2435 and CTR107 and Other GyrI-like Proteins

Multidrug resistance (MDR) refers to the acquired ability of cells to tolerate a broad range of toxic compounds. One mechanism cells employ is to increase the level of expression of efflux pumps for the expulsion of xenobiotics. A key feature uniting efflux-related mechanisms is multidrug (MD) recognition, either by efflux pumps themselves or by their transcriptional regulators. However, models describing MD binding by MDR effectors are incomplete, underscoring the importance of studies focused on the recognition elements and key motifs that dictate polyspecific binding. One such motif is the GyrI-like domain, which is found in several MDR proteins and is postulated to have been adapted for small-molecule binding and signaling. Here we report the solution binding properties and crystal structures of two proteins containing GyrI-like domains, SAV2435 and CTR107, bound to various ligands. Furthermore, we provide a comparison with deposited crystal structures of GyrI-like proteins, revealing key features of GyrI-like domains that not only support polyspecific binding but also are conserved among GyrI-like domains. Together, our studies suggest that GyrI-like domains perform evolutionarily conserved functions connected to multidrug binding and highlight the utility of these types of studies for elucidating mechanisms of MDR.

Drug resistance mechanisms and their regulation in non-albicans Candida species

Fungal pathogens use various mechanisms to survive exposure to drugs. Prolonged treatment very often leads to the stepwise acquisition of resistance. The limited number of antifungal therapeutics and their mostly fungistatic rather than fungicidal character facilitates selection of resistant strains. These are able to cope with cytotoxic molecules by acquisition of appropriate mutations, re-wiring gene expression and metabolic adjustments. Recent evidence points to the paramount importance of the permeability barrier and cell wall integrity in the process of adaptation to high drug concentrations. Molecular details of basal and acquired drug resistance are best characterized in the most frequent human fungal pathogen, Candida albicans Effector genes directly related to the acquisition of elevated tolerance of this species to azole and echinocandin drugs are well described. The emergence of high-level drug resistance against intrinsically lower susceptibility to azoles in yeast species other than C. albicans is, however, of particular concern. This is due to their steadily increasing contribution to high mortality rates associated with disseminated infections. Recent findings concerning underlying mechanisms associated with elevated drug resistance suggest a link to cell wall and plasma membrane metabolism in non-albicans Candida species.

Control of Biofilms with the Fatty Acid Signaling Molecule cis-2-Decenoic Acid

Biofilms are complex communities of microorganisms in organized structures attached to surfaces. Importantly, biofilms are a major cause of bacterial infections in humans, and remain one of the most significant challenges to modern medical practice. Unfortunately, conventional therapies have shown to be inadequate in the treatment of most chronic biofilm infections based on the extraordinary innate tolerance of biofilms to antibiotics. Antagonists of quorum sensing signaling molecules have been used as means to control biofilms. QS and other cell-cell communication molecules are able to revert biofilm tolerance, prevent biofilm formation and disrupt fully developed biofilms, albeit with restricted effectiveness. Recently however, it has been demonstrated that Pseudomonas aeruginosa produces a small messenger molecule cis-2-decenoic acid (cis-DA) that shows significant promise as an effective adjunctive to antimicrobial treatment of biofilms. This molecule is responsible for induction of the native biofilm dispersion response in a range of Gram-negative and Gram-positive bacteria and in yeast, and has been shown to reverse persistence, increase microbial metabolic activity and significantly enhance the cidal effects of conventional antimicrobial agents. In this manuscript, the use of cis-2-decenoic acid as a novel agent for biofilm control is discussed. Stimulating the biofilm dispersion response as a novel antimicrobial strategy holds significant promise for enhanced treatment of infections and in the prevention of biofilm formation.

Architecture and roles of periplasmic adaptor proteins in tripartite efflux assemblies

Recent years have seen major advances in the structural understanding of the different components of tripartite efflux assemblies, which encompass the multidrug efflux (MDR) pumps and type I secretion systems. The majority of these investigations have focused on the role played by the inner membrane transporters and the outer membrane factor (OMF), leaving the third component of the system – the Periplasmic Adaptor Proteins (PAPs) – relatively understudied. Here we review the current state of knowledge of these versatile proteins which, far from being passive linkers between the OMF and the transporter, emerge as active architects of tripartite assemblies, and play diverse roles in the transport process. Recognition between the PAPs and OMFs is essential for pump assembly and function, and targeting this interaction may provide a novel avenue for combating multidrug resistance. With the recent advances elucidating the drug efflux and energetics of the tripartite assemblies, the understanding of the interaction between the OMFs and PAPs is the last piece remaining in the complete structure of the tripartite pump assembly puzzle.

Structural basis and dynamics of multidrug recognition in a minimal bacterial multidrug resistance system

TipA is a transcriptional regulator found in diverse bacteria. It constitutes a minimal autoregulated multidrug resistance system against numerous thiopeptide antibiotics. Here we report the structures of its drug-binding domain TipAS in complexes with promothiocin A and nosiheptide, and a model of the thiostrepton complex. Drug binding induces a large transition from a partially unfolded to a globin-like structure. The structures rationalize the mechanism of promiscuous, yet specific, drug recognition: (i) a four-ring motif present in all known TipA-inducing antibiotics is recognized specifically by conserved TipAS amino acids; and (ii) the variable part of the antibiotic is accommodated within a flexible cleft that rigidifies upon drug binding. Remarkably, the identified four-ring motif is also the major interacting part of the antibiotic with the ribosome. Hence the TipA multidrug resistance mechanism is directed against the same chemical motif that inhibits protein synthesis. The observed identity of chemical motifs responsible for antibiotic function and resistance may be a general principle and could help to better define new leads for antibiotics.

Bacterial multidrug efflux pumps: mechanisms, physiology and pharmacological exploitations

Multidrug resistance (MDR) refers to the capability of bacterial pathogens to withstand lethal doses of structurally diverse drugs which are capable of eradicating non-resistant strains. MDR has been identified as a major threat to the public health of human being by the World Health Organization (WHO). Among the four general mechanisms that cause antibiotic resistance including target alteration, drug inactivation, decreased permeability and increased efflux, drug extrusion by the multidrug efflux pumps serves as an important mechanism of MDR. Efflux pumps not only can expel a broad range of antibiotics owing to their poly-substrate specificity, but also drive the acquisition of additional resistance mechanisms by lowering intracellular antibiotic concentration and promoting mutation accumulation. Over-expression of multidrug efflux pumps have been increasingly found to be associated with clinically relevant drug resistance. On the other hand, accumulating evidence has suggested that efflux pumps also have physiological functions in bacteria and their expression is subject tight regulation in response to various of environmental and physiological signals. A comprehensive understanding of the mechanisms of drug extrusion, and regulation and physiological functions of efflux pumps is essential for the development of anti-resistance interventions. In this review, we summarize the development of these research areas in the recent decades and present the pharmacological exploitation of efflux pump inhibitors as a promising anti-drug resistance intervention.

BrlR from Pseudomonas aeruginosa is a c-di-GMP-responsive transcription factor

The transcriptional regulator BrlR is a member of the MerR family of multidrug transport activators that contributes to the high-level drug tolerance of Pseudomonas aeruginosa biofilms. While MerR regulators are known to activate both the expression of multidrug efflux pump genes and their own transcription upon inducer binding, little is known about BrlR activation. We demonstrate using promoter reporter strains, in vivo and in vitro DNA-binding assays combined with 5'RACE, that BrlR binds to its own promoter, likely via a MerR-like palindromic sequence. Unlike known MerR multidrug transport activators, BrlR and brlR expression are not activated by multidrug transporter substrates. Instead, BrlR-DNA binding was enhanced by the secondary messenger c-di-GMP. In addition to enhanced BrlR-DNA binding, c-di-GMP levels contributed to PbrlR promoter activity in initial attached cells with elevated c-di-GMP levels correlating with increased expression of brlR. While not harbouring amino acid motifs resembling previously defined c-di-GMP-binding domains, BrlR was found to bind c-di-GMP in vitro at a ratio of one c-di-GMP per two BrlR. Cross-linking assays confirmed dimer formation to be enhanced in the presence of elevated c-di-GMP levels. Our findings demonstrate BrlR to be an unusual MerR-family member in that BrlR function and expression require the secondary messenger c-di-GMP.

β-Lactam selectivity of multidrug transporters AcrB and AcrD resides in the proximal binding pocket

β-Lactams are mainstream antibiotics that are indicated for the prophylaxis and treatment of bacterial infections. The AcrA-AcrD-TolC multidrug efflux system confers much stronger resistance on Escherichia coli to clinically relevant anionic β-lactam antibiotics than the homologous AcrA-AcrB-TolC system. Using an extensive combination of chimeric analysis and site-directed mutagenesis, we searched for residues that determine the difference in β-lactam specificity between AcrB and AcrD. We identified three crucial residues at the "proximal" (or access) substrate binding pocket. The simultaneous replacement of these residues in AcrB by those in AcrD (Q569R, I626R, and E673G) transferred the β-lactam specificity of AcrD to AcrB. Our findings indicate for the first time that the difference in β-lactam specificity between AcrB and AcrD relates to interactions of the antibiotic with residues in the proximal binding pocket.

The MerR-like regulator BrlR confers biofilm tolerance by activating multidrug efflux pumps in Pseudomonas aeruginosa biofilms

A defining characteristic of biofilms is antibiotic tolerance that can be up to 1,000-fold greater than that of planktonic cells. In Pseudomonas aeruginosa, biofilm tolerance to antimicrobial agents requires the biofilm-specific MerR-type transcriptional regulator BrlR. However, the mechanism by which BrlR mediates biofilm tolerance has not been elucidated. Genome-wide transcriptional profiling indicated that brlR was required for maximal expression of genes associated with antibiotic resistance, in particular those encoding the multidrug efflux pumps MexAB-OprM and MexEF-OprN. Chromatin immunoprecipitation (ChIP) analysis revealed a direct regulation of these genes by BrlR, with DNA binding assays confirming BrlR binding to the promoter regions of the mexAB-oprM and mexEF-oprN operons. Quantitative reverse transcriptase PCR (qRT-PCR) analysis further indicated BrlR to be an activator of mexAB-oprM and mexEF-oprN gene expression. Moreover, immunoblot analysis confirmed increased MexA abundance in cells overexpressing brlR. Inactivation of both efflux pumps rendered biofilms significantly more susceptible to five different classes of antibiotics by affecting MIC but not the recalcitrance of biofilms to killing by bactericidal agents. Overexpression of either efflux pump in a ΔbrlR strain partly restored tolerance of ΔbrlR biofilms to antibiotics. Expression of brlR in mutant biofilms lacking both efflux pumps partly restored antimicrobial tolerance of biofilms to wild-type levels. Our results indicate that BrlR acts as an activator of multidrug efflux pumps to confer tolerance to P. aeruginosa biofilms and to resist the action of antimicrobial agents.

The MerR-like transcriptional regulator BrlR contributes to Pseudomonas aeruginosa biofilm tolerance

Biofilms are composed of surface-attached microbial communities. A hallmark of biofilms is their profound tolerance of antimicrobial agents. While biofilm drug tolerance has been considered to be multifactorial, our findings indicate, instead, that bacteria within biofilms employ a classical regulatory mechanism to resist the action of antimicrobial agents. Here we report that the transcriptional regulator BrlR, a member of the MerR family of multidrug transport activators, plays a role in the high-level drug tolerance of biofilms formed by Pseudomonas aeruginosa. Expression of brlR was found to be biofilm specific, with brlR inactivation not affecting biofilm formation, motility, or pslA expression but increasing ndvB expression. Inactivation of brlR rendered biofilms but not planktonic cells grown to exponential or stationary phase significantly more susceptible to hydrogen peroxide and five different classes of antibiotics by affecting the MICs and the recalcitrance of biofilms to killing by microbicidal antimicrobial agents. In contrast, overexpression of brlR rendered both biofilms and planktonic cells more tolerant to the same compounds. brlR expression in three cystic fibrosis (CF) isolates was elevated regardless of the mode of growth, suggesting a selection for constitutive brlR expression upon in vivo biofilm formation associated with chronic infections. Despite increased brlR expression, however, isolate CF1-8 was as susceptible to tobramycin as was a ΔbrlR mutant because of a nonsense mutation in brlR. Our results indicate for the first time that biofilms employ a specific regulatory mechanism to resist the action of antimicrobial agents in a BrlR-dependent manner which affects MIC and recalcitrance to killing by microbicidal antimicrobial agents.

A glutamate switch controls voltage-sensitive phosphatase function

Ciona intestinalis voltage sensing phosphatase Ci-VSP couples a voltage-sensing domain (VSD) to a lipid phosphatase similar to the tumor suppressor PTEN. How the VSD controls enzyme function has been unclear. Here, we present high-resolution crystal structures of the Ci-VSP enzymatic domain that reveal conformational changes in a key loop, termed the “gating loop”, that controls access to the active site by a mechanism in which residue Glu411 directly competes with substrate. Structure-based mutations that restrict gating loop conformation impair catalytic function and demonstrate that Glu411 also contributes to substrate selectivity. Structure-guided mutations further define an interaction between the gating loop and linker that connects the phosphatase to the VSD for voltage control of enzyme activity. Together, the data suggest that functional coupling between the gating loop and the linker forms the heart of the regulatory mechanism that controls voltage-dependent enzyme activation.

Simulations of substrate transport in the multidrug transporter EmrD

EmrD is a multidrug resistance (MDR) transporter from Escherichia coli, which is involved in the efflux of amphipathic compounds from the cytoplasm, and the first MDR member of the major facilitator superfamily to be crystallized. Molecular dynamics simulation of EmrD in a phospholipid bilayer was used to characterize the conformational dynamics of the protein. Motions that support a previously proposed lateral diffusion pathway for substrate from the cytoplasmic membrane leaflet into the EmrD central cavity were observed. In addition, the translocation pathway of meta-chloro carbonylcyanide phenylhydrazone (CCCP) was probed using both standard and steered molecular dynamics simulation. In particular, interactions of a few specific residues with CCCP have been identified. Finally, a large motion of two residues, Val 45 and Leu 233, was observed with the passage of CCCP into the periplasmic space, placing a lower bound on the extent of opening required at this end of the protein for substrate transport. Overall, our simulations probe details of the transport pathway, motions of EmrD at an atomic level of detail, and offer new insights into the functioning of MDR transporters.

Novel bacterial MerR-like regulators their role in the response to carbonyl and nitrosative stress

Recognition of the diversity of transcriptional regulators of the MerR family has increased considerably over the last decade and it has been established that not all MerR-like regulators are involved in metal ion recognition. A new type of MerR-like regulator was identified in Neisseria gonorrhoeae that is distinct from metal-binding MerR proteins. This novel transcription factor, the Neisseria merR-like regulator (NmlR) is related to a large and diverse group of MerR-like regulators. A common feature of the majority of the genes encoding the nmlR-related genes is that they predicted to control the expression of adhC, which encodes a glutathione-dependent alcohol dehydrogenase. The function of the NmlR regulon appears to be to defend the bacterial cell against carbonyl stress and in some cases nitrosative stress. A potential role for NmlR in bacterial pathogenesis has been identified in Neisseria gonorrhoeae and Streptococcus pneumoniae. Although it is not known how NmlR is activated it is suggested that conserved cysteine residues may be involved in thiol-based signaling.

Crystal structure of an integron gene cassette-associated protein from Vibrio cholerae identifies a cationic drug-binding module

BACKGROUND

The direct isolation of integron gene cassettes from cultivated and environmental microbial sources allows an assessment of the impact of the integron/gene cassette system on the emergence of new phenotypes, such as drug resistance or virulence. A structural approach is being exploited to investigate the modularity and function of novel integron gene cassettes.

METHODOLOGY/PRINCIPAL FINDINGS

We report the 1.8 Å crystal structure of Cass2, an integron-associated protein derived from an environmental V. cholerae. The structure defines a monomeric beta-barrel protein with a fold related to the effector-binding portion of AraC/XylS transcription activators. The closest homologs of Cass2 are multi-drug binding proteins, such as BmrR. Consistent with this, a binding pocket made up of hydrophobic residues and a single glutamate side chain is evident in Cass2, occupied in the crystal form by polyethylene glycol. Fluorescence assays demonstrate that Cass2 is capable of binding cationic drug compounds with submicromolar affinity. The Cass2 module possesses a protein interaction surface proximal to its drug-binding cavity with features homologous to those seen in multi-domain transcriptional regulators.

CONCLUSIONS/SIGNIFICANCE

Genetic analysis identifies Cass2 to be representative of a larger family of independent effector-binding proteins associated with lateral gene transfer within Vibrio and closely-related species. We propose that the Cass2 family not only has capacity to form functional transcription regulator complexes, but represents possible evolutionary precursors to multi-domain regulators associated with cationic drug compounds.

Structure of a cation-bound multidrug and toxic compound extrusion transporter

Transporter proteins from the MATE (multidrug and toxic compound extrusion) family are vital in metabolite transport in plants, directly affecting crop yields worldwide. MATE transporters also mediate multiple-drug resistance (MDR) in bacteria and mammals, modulating the efficacy of many pharmaceutical drugs used in the treatment of a variety of diseases. MATE transporters couple substrate transport to electrochemical gradients and are the only remaining class of MDR transporters whose structure has not been determined. Here we report the X-ray structure of the MATE transporter NorM from Vibrio cholerae determined to 3.65 Å, revealing an outward-facing conformation with two portals open to the outer leaflet of the membrane and a unique topology of the predicted 12 transmembrane helices distinct from any other known MDR transporter. We also report a cation-binding site in close proximity to residues previously deemed critical for transport. This conformation probably represents a stage of the transport cycle with high affinity for monovalent cations and low affinity for substrates.

Topologically random insertion of EmrE supports a pathway for evolution of inverted repeats in ion-coupled transporters

Inverted repeats in ion-coupled transporters have evolved independently in many unrelated families. It has been suggested that this inverted symmetry is an essential element of the mechanism that allows for the conformational transitions in transporters. We show here that small multidrug transporters offer a model for the evolution of such repeats. This family includes both homodimers and closely related heterodimers. In the former, the topology determinants, evidently identical in each protomer, are weak, and we show that for EmrE, an homodimer from Escherichia coli, the insertion into the membrane is random, and dimers are functional whether they insert into the cytoplasmic membrane with the N- and C-terminal domains facing the inside or the outside of the cell. Also, mutants designed to insert with biased topology are functional regardless of the topology. In the case of EbrAB, a heterodimer homologue supposed to interact antiparallel, we show that one of the subunits, EbrB, can also function as a homodimer, most likely in a parallel mode. In addition, the EmrE homodimer can be forced to an antiparallel topology by fusion of an additional transmembrane segment. The simplicity of the mechanism of coupling ion and substrate transport and the few requirements for substrate recognition provide the robustness necessary to tolerate such a unique and unprecedented ambiguity in the interaction of the subunits and in the dimer topology relative to the membrane. The results suggest that the small multidrug transporters are at an evolutionary junction and provide a model for the evolution of structure of transport proteins.

Conformational plasticity of the coiled-coil domain of BmrR is required for bmr operator binding: the structure of unliganded BmrR

The multidrug-binding transcription regulator BmrR from Bacillus subtilis is a MerR family member that binds to a wide array of cationic lipophilic toxins to activate the transcription of the multidrug efflux pump gene bmr. Transcription activation from the sigma(A)-dependent bmr operator requires BmrR to remodel the nonoptimal 19-bp spacer between the -10 promoter element and the -35 promoter element in order to facilitate productive RNA polymerase binding. Despite the availability of several structures of BmrR bound to DNA and drugs, the lack of a BmrR structure in its unliganded or apo (DNA free and drug free) state hinders our full understanding of the structural transitions required for DNA binding and transcription activation. Here, we report the crystal structure of the constitutively active, unliganded BmrR mutant BmrR(E253Q/R275E). Superposition of the ligand-free (apo BmrR(E253Q/R275E)) and DNA-bound BmrR structures reveals that apo BmrR must undergo significant rearrangement in order to assume the DNA-bound conformation, including an outward rotation of minor groove binding wings, an inward movement of helix-turn-helix motifs, and a downward relocation of pliable coiled-coil helices. Computational analysis of the DNA-free and DNA-bound structures reveals a flexible joint that is located at the center of the coiled-coil helices. This region, which is composed of residues 94 through 98, overlaps the helical bulge that is observed only in the apo BmrR structure. This conformational hinge is likely common to other MerR family members with large effector-binding domains, but appears to be missing from the smaller metal-binding MerR family members. Interestingly, the center-to-center distance of the recognition helices of apo BmrR is 34 A and suggests that the conformational change from the apo BmrR structure to the bmr operator-bound BmrR structure is initiated by the binding of this transcription activator to a more B-DNA-like conformation.

Biochemical characterization of the multidrug regulator QacR distinguishes residues that are crucial to multidrug binding and induction of qacA transcription

Staphylococcus aureus transcription factor QacR regulates expression of the qacA multidrug efflux determinant. In response to binding cationic lipophilic compounds, including ethidium and rhodamine 6G, QacR dissociates from the qacA operator alleviating repression. Such ligand binding uniformly induces a coil-to-helix transition of residues Thr(89)-Tyr(93) revealing an asymmetric binding pocket in QacR containing two distinct subpockets. Here, the functional significance of hydrophobic, aromatic, and polar residues characteristic of the rhodamine 6G pocket and the proximal Tyr(92), proposed to facilitate the transcriptionally active conformation, was examined. Notably, the presence of Tyr(92) was not essential for QacR structural changes between DNA-bound and induced conformations. Furthermore, although mutation of the majority of residues contacting rhodamine 6G exerted moderate effects on QacR-rhodamine 6G binding, mutation of Leu(54) and Gln(96), and cumulative mutations involving these with Tyr(93) and Tyr(123), imparted a dramatic decrease in QacR-rhodamine 6G binding affinity. This equated with impaired dissociation of QacR from its operator DNA in the presence of this ligand in S. aureus, delineating the important role of these residues in the QacR-rhodamine 6G interaction. Additionally, despite maintaining a high affinity for ethidium, QacR mutants involving Leu(54), Tyr(93), Gln(96), and Tyr(123), which denote the interface between the rhodamine 6G and ethidium subpockets, were unable to be induced from operator DNA in the presence of ethidium in S. aureus. This highlights the significant contribution of these residues to QacR-mediated derepression of qacA transcription following ligand binding in the distal subpocket and may be important for the general mechanism irrespective of the ligand bound.

Structural and functional aspects of the multidrug efflux pump AcrB

The tripartite efflux system AcrA/AcrB/TolC is the main pump in Escherichia coli for the efflux of multiple antibiotics, dyes, bile salts and detergents. The inner membrane component AcrB is central to substrate recognition and energy transduction and acts as a proton/drug antiporter. Recent structural studies show that homotrimeric AcrB can adopt different monomer conformations representing consecutive states in an allosteric functional rotation transport cycle. The conformational changes create an alternate access drug transport tunnel including a hydrophobic substrate binding pocket in one of the cycle intermediates.

ST1710-DNA complex crystal structure reveals the DNA binding mechanism of the MarR family of regulators

ST1710, a member of the multiple antibiotic resistance regulator (MarR) family of regulatory proteins in bacteria and archaea, plays important roles in development of antibiotic resistance, a global health problem. Here, we present the crystal structure of ST1710 from Sulfolobus tokodaii strain 7 complexed with salicylate, a well-known inhibitor of MarR proteins and the ST1710 complex with its promoter DNA, refined to 1.8 and 2.10 A resolutions, respectively. The ST1710-DNA complex shares the topology of apo-ST1710 and MarR proteins, with each subunit containing a winged helix-turn-helix (wHtH) DNA binding motif. Significantly large conformational changes occurred upon DNA binding and in each of the dimeric monomers in the asymmetric unit of the ST1710-DNA complex. Conserved wHtH loop residues interacting with the bound DNA and mutagenic analysis indicated that R89, R90 and K91 were important for DNA recognition. Significantly, the bound DNA exhibited a new binding mechanism.

Structure of P-glycoprotein reveals a molecular basis for poly-specific drug binding

P-glycoprotein (P-gp) detoxifies cells by exporting hundreds of chemically unrelated toxins but has been implicated in multidrug resistance (MDR) in the treatment of cancers. Substrate promiscuity is a hallmark of P-gp activity, thus a structural description of poly-specific drug-binding is important for the rational design of anticancer drugs and MDR inhibitors. The x-ray structure of apo P-gp at 3.8 angstroms reveals an internal cavity of approximately 6000 angstroms cubed with a 30 angstrom separation of the two nucleotide-binding domains. Two additional P-gp structures with cyclic peptide inhibitors demonstrate distinct drug-binding sites in the internal cavity capable of stereoselectivity that is based on hydrophobic and aromatic interactions. Apo and drug-bound P-gp structures have portals open to the cytoplasm and the inner leaflet of the lipid bilayer for drug entry. The inward-facing conformation represents an initial stage of the transport cycle that is competent for drug binding.

Structure of the transcriptional regulator LmrR and its mechanism of multidrug recognition

LmrR is a PadR-related transcriptional repressor that regulates the production of LmrCD, a major multidrug ABC transporter in Lactococcus lactis. Transcriptional regulation is presumed to follow a drug-sensitive induction mechanism involving the direct binding of transporter ligands to LmrR. Here, we present crystal structures of LmrR in an apo state and in two drug-bound states complexed with Hoechst 33342 and daunomycin. LmrR shows a common topology containing a typical beta-winged helix-turn-helix domain with an additional C-terminal helix involved in dimerization. Its dimeric organization is highly unusual with a flat-shaped hydrophobic pore at the dimer centre serving as a multidrug-binding site. The drugs bind in a similar manner with their aromatic rings sandwiched in between the indole groups of two dimer-related tryptophan residues. Multidrug recognition is facilitated by conformational plasticity and the absence of drug-specific hydrogen bonds. Combined analyses using site-directed mutagenesis, fluorescence-based drug binding and protein-DNA gel shift assays reveal an allosteric coupling between the multidrug- and DNA-binding sites of LmrR that most likely has a function in the induction mechanism.

Bacterial multidrug transport through the lens of the major facilitator superfamily

Multidrug transporters are membrane proteins that expel a wide spectrum of cytotoxic compounds from the cell. Through this function, they render cells resistant to multiple drugs. These transporters are found in many different families of transport proteins, of which the largest is the major facilitator superfamily. Multidrug transporters from this family are highly represented in bacteria and studies of them have provided important insight into the mechanism underlying multidrug transport. This review summarizes the work carried out on these interesting proteins and underscores the differences and similarities to other transport systems.

Transport proteins of the ABC family and multidrug resistance of tumor cells

Some new data concerning the role of transport proteins of the ABC family in multidrug resistance (MDR) of human tumor cells, and problems connected with regulation of these proteins are considered. MDR is a complex phenomenon that may be caused simultaneously by several mechanisms functioning in one and the same cell. Among them there may be the alterations of activity of several transport proteins. Activation of these proteins may be associated with alterations of activities of different cell protective systems and of the signal transduction pathways involved in regulation of proliferation, differentiation, and apoptosis. Clinical significance of multifactor MDR is discussed.

Structures of BmrR-drug complexes reveal a rigid multidrug binding pocket and transcription activation through tyrosine expulsion

BmrR is a member of the MerR family and a multidrug binding transcription factor that up-regulates the expression of the bmr multidrug efflux transporter gene in response to myriad lipophilic cationic compounds. The structural mechanism by which BmrR binds these chemically and structurally different drugs and subsequently activates transcription is poorly understood. Here, we describe the crystal structures of BmrR bound to rhodamine 6G (R6G) or berberine (Ber) and cognate DNA. These structures reveal each drug stacks against multiple aromatic residues with their positive charges most proximal to the carboxylate group of Glu-253 and that, unlike other multidrug binding pockets, that of BmrR is rigid. Substitution of Glu-253 with either alanine (E253A) or glutamine (E253Q) results in unpredictable binding affinities for R6G, Ber, and tetraphenylphosphonium. Moreover, these drug binding studies reveal that the negative charge of Glu-253 is not important for high affinity binding to Ber and tetraphenylphosphonium but plays a more significant, but unpredictable, role in R6G binding. In vitro transcription data show that E253A and E253Q are constitutively active, and structures of the drug-free E253A-DNA and E253Q-DNA complexes support a transcription activation mechanism requiring the expulsion of Tyr-152 from the multidrug binding pocket. In sum, these data delineate the mechanism by which BmrR binds lipophilic, monovalent cationic compounds and suggest the importance of the redundant negative electrostatic nature of this rigid drug binding pocket that can be used to discriminate against molecules that are not substrates of the Bmr multidrug efflux pump.

Future directions for drug transporter modelling

Since the late 1980s computational methods such as quantitative structure-activity relationship (QSAR) and pharmacophore approaches have become more widely applied to assess interactions between drug-like molecules and transporters, starting with P-glycoprotein (P-gp). Identifying molecules that interact with P-gp and other transporters is important for drug discovery, but it is normally ascertained using laborious in vitro and in vivo studies. Computational QSAR and pharmacophore models can be used to screen commercial databases of molecules rapidly and suggest those likely to bind as substrates or inhibitors for transporters. These predictions can then be readily verified in vitro, thus representing a more efficient route to screening. Recently, the application of this approach has seen the identification of new substrates and inhibitors for several transporters. The successful application of computational models and pharmacophore models in particular to predict transporter binding accurately represents a way to anticipate drug-drug interactions of novel molecules from molecular structure. These models may also see incorporation in future pharmacokinetic-pharmacodynamic models to improve predictions of in vivo drug effects in patients. The implications of early assessment of transporter activity, current advances in QSAR, and other computational methods for future development of these and systems-based approaches will be discussed.

Structural insight on the mechanism of regulation of the MarR family of proteins: high-resolution crystal structure of a transcriptional repressor from Methanobacterium thermoautotrophicum

Transcriptional regulators belonging to the MarR family are characterized by a winged-helix DNA binding domain. These transcriptional regulators regulate the efflux and influx of phenolic agents in bacteria and archaea. In Escherichia coli, MarR regulates the multiple antibiotic resistance operon and its inactivation produces a multiple antibiotic resistance phenotype. In some organisms, active efflux of drug compounds will produce a drug resistance phenotype, whereas in other organisms, active influx of chlorinated hydrocarbons results in their rapid degradation. Although proteins in the MarR family are regulators of important biological processes, their mechanism of action is not well understood and structural information about how phenolic agents regulate the activity of these proteins is lacking. This article presents the three-dimensional structure of a protein of the MarR family, MTH313, in its apo form and in complex with salicylate, a known inactivator. A comparison of these two structures indicates that the mechanism of regulation involves a large conformational change in the DNA binding lobe. Electrophoretic mobility shift assay and biophysical analyses further suggest that salicylate inactivates MTH313 and prevents it from binding to its promoter region.

Crystal structure of the transcriptional regulator AcrR from Escherichia coli

The AcrAB multidrug efflux pump, which belongs to the resistance nodulation division (RND) family, recognizes and extrudes a wide range of antibiotics and chemotherapeutic agents and causes the intrinsic antibiotic resistance in Escherichia coli. The expression of AcrAB is controlled by the transcriptional regulator AcrR, whose open reading frame is located 141 bp upstream of the acrAB operon. To understand the structural basis of AcrR regulation, we have determined the crystal structure of AcrR to 2.55-A resolution, revealing a dimeric two-domain molecule with an entirely helical architecture similar to members of the TetR family of transcriptional regulators. Each monomer of AcrR forms a multientrance pocket of 350 A(3) in the ligand-binding domain. The ligand-binding pocket is surrounded with mostly hydrophobic residues. In addition, a completely buried negatively charged glutamate, expected to be critical for drug binding, is located at the center of the binding pocket. The crystal structure provides novel insight into the mechanisms of ligand binding and AcrR regulation.

When biochemistry meets structural biology: the cautionary tale of EmrE

When biochemistry meets structural biology a more complete understanding of the mechanism of biological macromolecules is usually achieved. Several high-resolution structures of ion-coupled transporters have enriched the understanding of mechanisms of substrate recognition, translocation and coupling of substrate fluxes. However, two X-ray structures of EmrE, the smallest ion-coupled multi-drug transporter, raised questions over the veracity of the structural model and represented a cautionary tale about the difficulty of determining the 3D structures of membrane proteins and the dangers of ignoring biochemical results. The 3D structures of EmrE have since been retracted because of faulty software, but the suggestion that the protomers in the dimer are in an antiparallel topological orientation sparked controversy that is still ongoing.

Different Modes of Binding of Mono- and Biaromatic Effectors to the Transcriptional Regulator TTGV

Members of the IclR family of regulators exhibit a highly conserved effector recognition domain and interact with a limited number of effectors. In contrast with most IclR family members, TtgV, the transcriptional repressor of the TtgGHI efflux pump, exhibits multidrug recognition properties. A three-dimensional model of the effector domain of TtgV was generated based on the available three-dimensional structure of several IclR members, and a series of point mutants was created. Using isothermal titration calorimetry, we determined the binding parameters of the most efficient effectors for TtgV and its mutant variants. All mutants bound biaromatic compounds with higher affinity than the wild-type protein, whereas monoaromatic compounds were bound with lower affinity. This tendency was particularly pronounced for mutants F134A and H200A. TtgVF134A bound 4-nitrotoluene with an affinity 13-fold lower than that of TtgV (17.4+/-0.6 microM). This mutant bound 1-naphthol with an affinity of 5.7 microM, which is seven times as great as that of TtgV (40 microM). The TtgVV223A mutant bound to DNA with the same affinity as the wild-type TtgV protein, but it remained bound to the target operator in the presence of effectors, suggesting that Val-223 could be part of an intra-TtgV signal recognition pathway. Thermodynamic analyses of the binding of effectors to TtgV and to its mutants in complex with their target DNA revealed that the binding of biaromatic compounds resulted in a more efficient release of the repressor protein than the binding of monoaromatics. The physiological significance of these findings is discussed.

Multiple molecular mechanisms for multidrug resistance transporters

The acquisition of multidrug resistance is a serious impediment to improved healthcare. Multidrug resistance is most frequently due to active transporters that pump a broad spectrum of chemically distinct, cytotoxic molecules out of cells, including antibiotics, antimalarials, herbicides and cancer chemotherapeutics in humans. The paradigm multidrug transporter, mammalian P-glycoprotein, was identified 30 years ago. Nonetheless, success in overcoming or circumventing multidrug resistance in a clinical setting has been modest. Recent structural and biochemical data for several multidrug transporters now provide mechanistic insights into how they work. Organisms have evolved several elegant solutions to ridding the cell of such cytotoxic compounds. Answers are emerging to questions such as how multispecificity for different drugs is achieved, why multidrug resistance arises so readily, and what chance there is of devising a clinical solution.