Multidrug efflux pumps of Gram-positive bacteria.

Gram-positive organisms are responsible for some of the most serious of human infections. Resistance to front-line antimicrobial agents can complicate otherwise curative therapy. These organisms possess multiple drug resistance mechanisms, with drug efflux being a significant contributing factor. Efflux proteins belonging to all five transporter families are involved, and frequently can transport multiple structurally unrelated compounds resulting in a multidrug resistance (MDR) phenotype. In addition to clinically relevant antimicrobial agents, MDR efflux proteins can transport environmental biocides and disinfectants which may allow persistence in the healthcare environment and subsequent acquisition by patients or staff. Intensive research on MDR efflux proteins and the regulation of expression of their genes is ongoing, providing some insight into the mechanisms of multidrug recognition and transport. Inhibitors of many of these proteins have been identified, including drugs currently being used for other indications. Structural modifications guided by structure-activity studies have resulted in the identification of potent compounds. However, lack of broad-spectrum pump inhibition combined with potential toxicity has hampered progress. Further work is required to gain a detailed understanding of the multidrug recognition process, followed by application of this knowledge in the design of safer and more highly potent inhibitors.

Structural mechanism of transcription regulation of the Staphylococcus aureus multidrug efflux operon mepRA by the MarR family repressor MepR.

The multidrug efflux pump MepA is a major contributor to multidrug resistance in Staphylococcus aureus. MepR, a member of the multiple antibiotic resistance regulator (MarR) family, represses mepA and its own gene. Here, we report the structure of a MepR-mepR operator complex. Structural comparison of DNA-bound MepR with 'induced' apoMepR reveals the large conformational changes needed to allow the DNA-binding winged helix-turn-helix motifs to interact with the consecutive major and minor grooves of the GTTAG signature sequence. Intriguingly, MepR makes no hydrogen bonds to major groove nucleobases. Rather, recognition-helix residues Thr60, Gly61, Pro62 and Thr63 make sequence-specifying van der Waals contacts with the TTAG bases. Removing these contacts dramatically affects MepR-DNA binding activity. The wings insert into the flanking minor grooves, whereby residue Arg87, buttressed by Asp85, interacts with the O2 of T4 and O4' ribosyl oxygens of A23 and T4. Mutating Asp85 and Arg87, both conserved throughout the MarR family, markedly affects MepR repressor activity. The His14':Arg59 and Arg10':His35:Phe108 interaction networks stabilize the DNA-binding conformation of MepR thereby contributing significantly to its high affinity binding. A structure-guided model of the MepR-mepA operator complex suggests that MepR dimers do not interact directly and cooperative binding is likely achieved by DNA-mediated allosteric effects.

Mutations within the mepA operator affect binding of the MepR regulatory protein and its induction by MepA substrates in Staphylococcus aureus.

The expression of mepA, encoding the Staphylococcus aureus MepA multidrug efflux protein, is repressed by the MarR homologue MepR. Repression occurs through binding of two MepR dimers to an operator with two homologous and closely approximated pseudopalindromic binding sites (site 1 [S1] and site 2 [S2]). MepR binding is impeded in the presence of pentamidine, a MepA substrate. The effects of various mepA operator mutations on MepR binding were determined using electrophoretic mobility shift assays and isothermal titration calorimetry, and an in vivo confirmation of the effects observed was established for a fully palindromic operator mutant. Altering the S1-S2 spacing by 1 to 4 bp severely impaired S2 binding, likely due to a physical collision between adjacent MepR dimers. Extension of the spacing to 9 bp eliminated the S1 binding-mediated DNA allostery required for efficient S2 binding, consistent with positive cooperative binding of MepR dimers. Binding of a single dimer to S1 was maintained when S2 was disrupted, whereas disruption of S1 eliminated any significant binding to S2, also consistent with positive cooperativity. Palindromization of binding sites, especially S2, enhanced MepR affinity for the mepA operator and reduced MepA substrate-mediated MepR induction. As a result, the on-off equilibrium between MepR and its binding sites was shifted toward the on state, resulting in less free MepR being available for interaction with inducing ligand. The selective pressure(s) under which mepA expression is advantageous likely contributed to the accumulation of mutations in the mepA operator, resulting in the current sequence from which MepR is readily induced by MepA substrates.

Development of a markerless knockout method for Actinobacillus succinogenes.

Actinobacillus succinogenes is one of the best natural succinate-producing organisms, but it still needs engineering to further increase succinate yield and productivity. In this study, we developed a markerless knockout method for A. succinogenes using natural transformation or electroporation. The Escherichia coli isocitrate dehydrogenase gene with flanking flippase recognition target sites was used as the positive selection marker, making use of A. succinogenes's auxotrophy for glutamate to select for growth on isocitrate. The Saccharomyces cerevisiae flippase recombinase (Flp) was used to remove the selection marker, allowing its reuse. Finally, the plasmid expressing flp was cured using acridine orange. We demonstrate that at least two consecutive deletions can be introduced into the same strain using this approach, that no more than a total of 1 kb of DNA is needed on each side of the selection cassette to protect from exonuclease activity during transformation, and that no more than 200 bp of homologous DNA is needed on each side for efficient recombination. We also demonstrate that electroporation can be used as an alternative transformation method to obtain knockout mutants and that an enriched defined medium can be used for direct selection of knockout mutants on agar plates with high efficiency. Single-knockout mutants of the fumarate reductase and of the pyruvate formate lyase-encoding genes were obtained using this knockout strategy. Double-knockout mutants were also obtained by deleting the citrate lyase-, ß-galactosidase-, and aconitase-encoding genes in the pyruvate formate lyase knockout mutant strain.

Respiratory glycerol metabolism of Actinobacillus succinogenes 130Z for succinate production.

Actinobacillus succinogenes 130Z naturally produces among the highest levels of succinate from a variety of inexpensive carbon substrates. A few studies have demonstrated that A. succinogenes can anaerobically metabolize glycerol, a waste product of biodiesel manufacture and an inexpensive feedstock, to produce high yields of succinate. However, all these studies were performed in the presence of yeast extract, which largely removes the redox constraints associated with fermenting glycerol, a highly reduced molecule. We demonstrated that A. succinogenes cannot ferment glycerol in minimal medium, but that it can metabolize glycerol by aerobic or anaerobic respiration. These results were expected based on the A. succinogenes genome, which encodes respiratory enzymes, but no pathway for 1,3-propanediol production. We investigated A. succinogenes's glycerol metabolism in minimal medium in a variety of respiratory conditions by comparing growth, metabolite production, and in vitro activity of terminal oxidoreductases. Nitrate inhibited succinate production by inhibiting fumarate reductase expression. In contrast, growth in the presence of dimethylsulfoxide and in microaerobic conditions allowed high succinate yields. The highest succinate yield was 0.75 mol/mol glycerol (75 % of the maximum theoretical yield) in continuous microaerobic cultures. A. succinogenes could also grow and produce succinate on partially refined glycerols obtained directly from biodiesel manufacture. Finally, by expressing a heterologous 1,3-propanediol synthesis pathway in A. succinogenes, we provide the first proof of concept that A. succinogenes can be engineered to grow fermentatively on glycerol.

Mutagenesis and modeling to predict structural and functional characteristics of the Staphylococcus aureus MepA multidrug efflux pump.

MepA is a multidrug and toxin extrusion (MATE) family protein and the only MATE protein encoded within the Staphylococcus aureus genome. Structural data for MATE proteins are limited to a single high-resolution example, NorM of Vibrio cholerae. Substitution mutations were created in MepA using gradient plates containing both a substrate and reserpine as an efflux pump inhibitor. Site-directed mutagenesis of plasmid-based mepA was used to reproduce these mutations, as well as unique or low-frequency mutations identified in mepA-overexpressing clinical strains, and to mutagenize conserved acidic residues. The effect of these changes on protein function was quantitated in a norA-disrupted host strain by susceptibility testing with and without inhibitors and by determining the proficiency of ethidium efflux. Up-function substitutions clustered in the carboxy half of MepA, near the cytoplasmic face of the protein. Repeated application of the same gradient plate conditions frequently reproduced identical substitution mutations, suggesting that individual residues are required for interaction with specific substrates. Acidic residues critical to protein function were identified in helices 4 and 5. In silico modeling revealed an outward-facing molecule, with helices 1, 2, 4, 7, 8, and 10 having contact with a central cavity that may represent a substrate translocation pathway. Functionally important residues within this cavity included S81, A161, M291, and A302. These data provide a critical starting point for understanding how MATE multidrug efflux proteins function and will be useful in refining crystallographic data when they are available.

Functional consequences of substitution mutations in MepR, a repressor of the Staphylococcus aureus MepA multidrug efflux pump gene.

The expression of mepA, encoding the Staphylococcus aureus MepA multidrug efflux protein, is repressed by the MarR homologue MepR. MepR dimers bind differently to operators upstream of mepR and mepA, with affinity being greatest at the mepA operator. MepR substitution mutations may result in mepA overexpression, with A103V most common in clinical strains. Evaluation of the functional consequences of this and other MepR substitutions using a lacZ reporter gene assay revealed markedly reduced repressor activity in the presence of Q18P, F27L, G97E, and A103V substitutions. Reporter data were generally supported by susceptibility and efflux assays, and electrophoretic mobility shift assays (EMSAs) confirmed compromised affinities of MepR F27L and A103V for the mepR and mepA operators. One mutant protein contained two substitutions (T94P and T132M); T132M compensated for the functional defect incurred by T94P and also rescued that of A103V but not F27L, establishing it as a limited-range suppressor. The function of another derivative with 10 substitutions was minimally affected, and this may be an extreme example of suppression involving interactions among several residues. Structural correlations for the observed functional effects were ascertained by modeling mutations onto apo-MepR. It is likely that F27L and A103V affect the protein-DNA interaction by repositioning of DNA recognition helices. Negative functional consequences of MepR substitution mutations may result from interference with structural plasticity, alteration of helical arrangements, reduced protein-cognate DNA affinity, or possibly association of MepR protomers. Structural determinations will provide further insight into the consequences of these and other mutations that affect MepR function, especially the T132M suppressor.

A genomic perspective on the potential of Actinobacillus succinogenes for industrial succinate production.

BACKGROUND: Succinate is produced petrochemically from maleic anhydride to satisfy a small specialty chemical market. If succinate could be produced fermentatively at a price competitive with that of maleic anhydride, though, it could replace maleic anhydride as the precursor of many bulk chemicals, transforming a multi-billion dollar petrochemical market into one based on renewable resources. Actinobacillus succinogenes naturally converts sugars and CO2 into high concentrations of succinic acid as part of a mixed-acid fermentation. Efforts are ongoing to maximize carbon flux to succinate to achieve an industrial process. RESULTS: Described here is the 2.3 Mb A. succinogenes genome sequence with emphasis on A. succinogenes's potential for genetic engineering, its metabolic attributes and capabilities, and its lack of pathogenicity. The genome sequence contains 1,690 DNA uptake signal sequence repeats and a nearly complete set of natural competence proteins, suggesting that A. succinogenes is capable of natural transformation. A. succinogenes lacks a complete tricarboxylic acid cycle as well as a glyoxylate pathway, and it appears to be able to transport and degrade about twenty different carbohydrates. The genomes of A. succinogenes and its closest known relative, Mannheimia succiniciproducens, were compared for the presence of known Pasteurellaceae virulence factors. Both species appear to lack the virulence traits of toxin production, sialic acid and choline incorporation into lipopolysaccharide, and utilization of hemoglobin and transferrin as iron sources. Perspectives are also given on the conservation of A. succinogenes genomic features in other sequenced Pasteurellaceae. CONCLUSIONS: Both A. succinogenes and M. succiniciproducens genome sequences lack many of the virulence genes used by their pathogenic Pasteurellaceae relatives. The lack of pathogenicity of these two succinogens is an exciting prospect, because comparisons with pathogenic Pasteurellaceae could lead to a better understanding of Pasteurellaceae virulence. The fact that the A. succinogenes genome encodes uptake and degradation pathways for a variety of carbohydrates reflects the variety of carbohydrate substrates available in the rumen, A. succinogenes's natural habitat. It also suggests that many different carbon sources can be used as feedstock for succinate production by A. succinogenes.

Studies on 2-phenylquinoline Staphylococcus aureus NorA efflux pump inhibitors: New insights on the C-6 position.

The alarming and rapid spread of antimicrobial resistance among bacteria represents a high risk for global health. Targeting factors involved in resistance to restore the activity of failing antibiotics is a promising strategy to overcome this urgent medical need. Efflux pump inhibitors are able to increase antibiotic concentrations in bacteria, thus they can be considered true antimicrobial resistance breakers. In this work, continuing our studies on inhibitors of the Staphylococcus aureus NorA pump, we designed, synthesized and biologically evaluated novel 2-phenylquinoline derivatives starting from our hits 1 and 2. Two of the synthesized compounds (6 and 7) bearing a C-6 benzyloxy group showed the best NorA inhibition activity, thereby providing an excellent starting point to direct future chemical optimizations.

2-Phenylquinoline S. aureus NorA Efflux Pump Inhibitors: Evaluation of the Importance of Methoxy Group Introduction.

Antimicrobial resistance (AMR) represents a hot topic in drug discovery. Besides the identification of new antibiotics, the use of nonantibiotic molecules to block resistance mechanisms is a powerful alternative. Bacterial efflux pumps exert an early step in AMR development by allowing bacteria to grow at subinhibitorial drug concentrations. Thus, efflux pump inhibitors (EPIs) offer a great opportunity to fight AMR. Given our experience in developing Staphylococcus aureus NorA EPIs, in this work, starting from the 2-phenylquinoline hit 1, we planned the introduction of methoxy groups on the basis of their presence in known NorA EPIs. Among the 35 different synthesized derivatives, compounds 3b and 7d exhibited the best NorA inhibition activity by restoring at very low concentrations ciprofloxacin MICs against resistant S. aureus strains. Interestingly, both compounds displayed EPI activities at nontoxic concentrations for human cells as well as highlighted promising results by preliminary pharmacokinetic studies.

Improved Potency of Indole-Based NorA Efflux Pump Inhibitors: From Serendipity toward Rational Design and Development.

The NorA efflux pump is a potential drug target for reversal of resistance to selected antibacterial agents, and recently we described indole-based inhibitor candidates. Herein we report a second class of inhibitors derived from them but with significant differences in shape and size. In particular, compounds 13 and 14 are very potent inhibitors in that they demonstrated the lowest IC50 values (2 µM) ever observed among all indole-based compounds we have evaluated.

Searching for Novel Inhibitors of the S.†aureus NorA Efflux Pump: Synthesis and Biological Evaluation of the 3-Phenyl-1,4-benzothiazine Analogues.

Bacterial resistance to antimicrobial agents has become an increasingly serious health problem in recent years. Among the strategies by which resistance can be achieved, overexpression of efflux pumps such as NorA of Staphylococcus aureus leads to a sub-lethal concentration of the antibacterial agent at the active site that in turn may predispose the organism to the development of high-level target-based resistance. With an aim to improve both the chemical stability and potency of our previously reported 3-phenyl-1,4-benzothiazine NorA inhibitors, we replaced the benzothiazine core with different nuclei. None of the new synthesized compounds showed any appreciable intrinsic antibacterial activity, and, in particular, 2-(3,4-dimethoxyphenyl)quinoline (6 c) was able to decrease, in a concentration-dependent manner, the ciprofloxacin MIC against the norA-overexpressing strains S. aureus SA-K2378 (norA++) and SA-1199B (norA+/A116E GrlA).

Genetic Identification of a PilT Motor in Geobacter sulfurreducens Reveals a Role for Pilus Retraction in Extracellular Electron Transfer.

The metal-reducing bacterium Geobacter sulfurreducens requires the expression of conductive pili to reduce iron oxides and to wire electroactive biofilms, but the role of pilus retraction in these functions has remained elusive. Here we show that of the four PilT proteins encoded in the genome of G. sulfurreducens, PilT3 powered pilus retraction in planktonic cells of a PilT-deficient strain of P. aeruginosa and restored the dense mutant biofilms to wild-type levels. Furthermore, PilT3 and PilT4 rescued the twitching motility defect of the PilT-deficient mutant. However, PilT4 was the only paralog whose inactivation in G. sulfurreducens lead to phenotypes associated with the hyperpiliation of non-retractile mutants such as enhanced adhesion and biofilm-forming abilities. In addition, PilT4 was required to reduce iron oxides. Taken together, the results indicate that PilT4 is the motor ATPase of G. sulfurreducens pili and reveal a previously unrecognized role for pilus retraction in extracellular electron transfer, a strategy that confers on Geobacter spp. an adaptive advantage for metal reduction in the natural environment.

Indole Based Weapons to Fight Antibiotic Resistance: A Structure-Activity Relationship Study.

Antibiotic resistance represents a worldwide concern, especially regarding the outbreak of methicillin-resistant Staphylococcus aureus, a common cause for serious skin and soft tissues infections. A major contributor to Staphylococcus aureus antibiotic resistance is the NorA efflux pump, which is able to extrude selected antibacterial drugs and biocides from the membrane, lowering their effective concentrations. Thus, the inhibition of NorA represents a promising and challenging strategy that would allow recycling of substrate antimicrobial agents. Among NorA inhibitors, the indole scaffold proved particularly effective and suitable for further optimization. In this study, some unexplored modifications on the indole scaffold are proposed. In particular, for the first time, substitutions at the C5 and N1 positions have been designed to give 48 compounds, which were synthesized and tested against norA-overexpressing S. aureus. Among them, 4 compounds have NorA IC50 values lower than 5.0 µM proving to be good efflux pump inhibitor (EPI) candidates. In addition, preliminary data on their ADME (absorption, distribution, metabolism, and excretion) profile is reported.

Clonal relatedness is a predictor of spontaneous multidrug efflux pump gene overexpression in Staphylococcus aureus.

Increased expression of genes encoding multidrug resistance efflux pumps (MDR-EPs) contributes to antimicrobial agent and biocide resistance in Staphylococcus aureus. Previously identified associations between norA overexpression and spa type t002 meticillin-resistant S. aureus (MRSA), and a similar yet weaker association between mepA overexpression and type t008 meticillin-susceptible S. aureus (MSSA), in clinical isolates are suggestive of clonal dissemination. It is also possible that related strains are prone to mutations resulting in overexpression of specific MDR-EP genes. Exposure of non-MDR-EP-overexpressing clinical isolates to biocides and dyes can select for MDR-EP-overexpressing mutants. spa types t002 and t008 isolates are predominated by multilocus sequencing typing sequence types (STs) 5 and 8, respectively. In this study, non-MDR-EP gene-overexpressing clinical isolates (MRSA and MSSA) representing ST5 and ST8 were subjected to single exposures of ethidium bromide (EtBr) to select for EtBr-resistant mutants. Measurements of active EtBr transport among mutants were used to demonstrate an efflux-proficient phenotype. Using quantitative reverse-transcription PCR, it was found that EtBr-resistant mutants of ST5 and ST8 parental strains predominantly overexpressed mepA (100%) and mdeA (83%), respectively, regardless of meticillin sensitivity. Associations between clonal lineage and MDR-EP gene overexpression differed from those previously observed and suggest the latter is due to clonal spread of efflux-proficient strains. The predilection of in vitro-selected mutants of related strains to overexpress the same MDR-EP gene indicates the presence of a consistent mutational process.

A mass spectrometry-based assay for improved quantitative measurements of efflux pump inhibition.

Bacterial efflux pumps are active transport proteins responsible for resistance to selected biocides and antibiotics. It has been shown that production of efflux pumps is up-regulated in a number of highly pathogenic bacteria, including methicillin resistant Staphylococcus aureus. Thus, the identification of new bacterial efflux pump inhibitors is a topic of great interest. Existing assays to evaluate efflux pump inhibitory activity rely on fluorescence by an efflux pump substrate. When employing these assays to evaluate efflux pump inhibitory activity of plant extracts and some purified compounds, we observed severe optical interference that gave rise to false negative results. To circumvent this problem, a new mass spectrometry-based method was developed for the quantitative measurement of bacterial efflux pump inhibition. The assay was employed to evaluate efflux pump inhibitory activity of a crude extract of the botanical Hydrastis Canadensis, and to compare the efflux pump inhibitory activity of several pure flavonoids. The flavonoid quercetin, which appeared to be completely inactive with a fluorescence-based method, showed an IC50 value of 75 µg/mL with the new method. The other flavonoids evaluated (apigenin, kaempferol, rhamnetin, luteolin, myricetin), were also active, with IC50 values ranging from 19 µg/mL to 75 µg/mL. The assay described herein could be useful in future screening efforts to identify efflux pump inhibitors, particularly in situations where optical interference precludes the application of methods that rely on fluorescence.

Inhibition of drug efflux pumps in Staphylococcus aureus: current status of potentiating existing antibiotics.

The emergence of multidrug-resistant Staphylococcus aureus coupled with a declining output of new antibiotic treatment options from the pharmaceutical industry is a growing worldwide healthcare problem. Multidrug efflux pumps are known to play a role in antibiotic and biocide resistance in S. aureus. These membrane transporters are capable of extruding drugs and other structurally unrelated compounds, hence decreasing intracellular concentration and increasing survival. Coadministration of efflux pump inhibitors (EPIs) with antibiotics that are pump substrates could increase intracellular drug levels, thus bringing renewed efficacy to existing antistaphylococcal agents. Numerous EPIs have been identified or synthesized over the past two decades; these include existing pharmacologic drugs, naturally occurring compounds, and synthetic derivatives thereof. This review describes the current progress in EPI development for use against S. aureus.

The molecular mechanisms of allosteric mutations impairing MepR repressor function in multidrug-resistant strains of Staphylococcus aureus.

UNLABELLED: Overexpression of the Staphylococcus aureus multidrug efflux pump MepA confers resistance to a wide variety of antimicrobials. mepA expression is controlled by MarR family member MepR, which represses mepA and autorepresses its own production. Mutations in mepR are a primary cause of mepA overexpression in clinical isolates of multidrug-resistant S. aureus. Here, we report crystal structures of three multidrug-resistant MepR variants, which contain the single-amino-acid substitution A103V, F27L, or Q18P, and wild-type MepR in its DNA-bound conformation. Although each mutation impairs MepR function by decreasing its DNA binding affinity, none is located in the DNA binding domain. Rather, all are found in the linker region connecting the dimerization and DNA binding domains. Specifically, the A103V substitution impinges on F27, which resolves potential steric clashes via displacement of the DNA binding winged-helix-turn-helix motifs that lead to a 27-fold reduction in DNA binding affinity. The F27L substitution forces F104 into an alternative rotamer, which kinks helix 5, thereby interfering with the positioning of the DNA binding domains and decreasing mepR operator affinity by 35-fold. The Q18P mutation affects the MepR structure and function most significantly by either creating kinks in the middle of helix 1 or completely unfolding its C terminus. In addition, helix 5 of Q18P is either bent or completely dissected into two smaller helices. Consequently, DNA binding is diminished by 2,000-fold. Our structural studies reveal heretofore-unobserved allosteric mechanisms that affect repressor function of a MarR family member and result in multidrug-resistant Staphylococcus aureus. IMPORTANCE: Staphylococcus aureus is a major health threat to immunocompromised patients. S. aureus multidrug-resistant variants that overexpress the multidrug efflux pump mepA emerge frequently due to point mutations in MarR family member MepR, the mepA transcription repressor. Significantly, the majority of MepR mutations identified in these S. aureus clinical isolates are found not in the DNA binding domain but rather in a linker region, connecting the dimerization and DNA binding domains. The location of these mutants underscores the critical importance of a properly functioning allosteric mechanism that regulates MepR function. Understanding the dysregulation of such allosteric MepR mutants underlies this study. The high-resolution structures of three such allosteric MepR mutants reveal unpredictable conformational consequences, all of which preclude cognate DNA binding, while biochemical studies emphasize their debilitating effects on DNA binding affinity. Hence, mutations in the linker region of MepR and their structural consequences are key generators of multidrug-resistant Staphylococcus aureus.

A new plant-derived antibacterial is an inhibitor of efflux pumps in Staphylococcus aureus.

An in-depth evaluation was undertaken of a new antibacterial natural product (1) recently isolated and characterised from the plant Hypericum olympicum L. cf. uniflorum. Minimum inhibitory concentrations (MICs) were determined for a panel of bacteria, including: meticillin-resistant and -susceptible strains of Staphylococcus aureus, Staphylococcus epidermidis and Staphylococcus haemolyticus; vancomycin-resistant and -susceptible Enterococcus faecalis and Enterococcus faecium; penicillin-resistant and -susceptible Streptococcus pneumoniae; group A streptococci (Streptococcus pyogenes); and Clostridium difficile. MICs were 2-8 mg/L for most staphylococci and all enterococci, but were ≥16 mg/L for S. haemolyticus and were >32 mg/L for all species in the presence of blood. Compound 1 was also tested against Gram-negative bacteria, including Escherichia coli, Pseudomonas aeruginosa and Salmonella enterica serovar Typhimurium but was inactive. The MIC for Mycobacterium bovis BCG was 60 mg/L, and compound 1 inhibited the ATP-dependent Mycobacterium tuberculosis MurE ligase [50% inhibitory concentration (IC(50)) = 75 µM]. In a radiometric accumulation assay with a strain of S. aureus overexpressing the NorA multidrug efflux pump, the presence of compound 1 increased accumulation of (14)C-enoxacin in a concentration-dependent manner, implying inhibition of efflux. Only moderate cytotoxicity was observed, with IC50 values of 12.5, 10.5 and 8.9 µM against human breast, lung and fibroblast cell lines, respectively, highlighting the potential value of this chemotype as a new antibacterial agent and efflux pump inhibitor.