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

Efflux pump effects on levofloxacin resistance in Mycobacterium abscessus

Mycobacterium abscessus (M. abscessus) inherently displays resistance to most antibiotics, with the underlying drug resistance mechanisms remaining largely unexplored. Efflux pump is believed to play an important role in mediating drug resistance. The current study examined the potential of efflux pump inhibitors to reverse levofloxacin (LFX) resistance in M. abscessus. The reference strain of M. abscessus (ATCC19977) and 60 clinical isolates, including 41 M. abscessus subsp. abscessus and 19 M. abscessus subsp. massilense, were investigated. The drug sensitivity of M. abscessus against LFX alone or in conjunction with efflux pump inhibitors, including verapamil (VP), reserpine (RSP), carbonyl cyanide 3-chlorophenylhydrazone (CCCP), or dicyclohexylcarbodiimide (DCC), were determined by AlarmarBlue microplate assay. Drug-resistant regions of the gyrA and gyrB genes from the drug-resistant strains were sequenced. The transcription level of the efflux pump genes was monitored using qRT-PCR. All the tested strains were resistant to LFX. The drug-resistant regions from the gyrA and gyrB genes showed no mutation associated with LFX resistance. CCCP, DCC, VP, and RSP increased the susceptibility of 93.3% (56/60), 91.7% (55/60), 85% (51/60), and 83.3% (50/60) isolates to LFX by 2 to 32-fold, respectively. Elevated transcription of seven efflux pump genes was observed in isolates with a high reduction in LFX MIC values in the presence of efflux pump inhibitors. Efflux pump inhibitors can improve the antibacterial activity of LFX against M. abscessus in vitro. The overexpression of efflux-related genes in LFX-resistant isolates suggests that efflux pumps are associated with the development of LFX resistance in M. abscessus.

A cluster of six respiratory cultures positive for Mycobacterium xenopi -Clinical characteristics and genomic characterization

Mycobacterium xenopi is a slow growing non-tuberculous mycobacterium (NTM) isolated from water systems and has been associated with pseudo-outbreaks and pulmonary infections in humans. We observed a cluster of six respiratory cultures positive for M. xenopi within a six-month period at our institution, approximately double our normal isolation rate of this organism. Only three of the six cases met clinical, radiographic, and microbiologic criteria for NTM infection. An investigation led by our hospital's Healthcare Epidemiology and Infection Program found no epidemiologic link between the six patients. Three isolates underwent whole-genome sequencing (WGS) and phylogenetic analysis confirmed they were non-clonal. In vitro susceptibility data found the isolates were sensitive to macrolides, moxifloxacin, and rifabutin. Our findings suggest that isolation of M. xenopi from pulmonary specimens may be increasing, further defines the genomic population structure of this potentially emerging infection, and establishes WGS as a useful tool for outbreak investigation strain typing.

A narrative review of nontuberculous mycobacterial pulmonary disease: microbiology, epidemiology, diagnosis, and management challenges

INTRODUCTION

Nontuberculous mycobacteria (NTM) are a diverse group of mycobacterial species that are ubiquitous in the environment. They are opportunistic pathogens that can cause a range of diseases, especially in individuals with underlying structural lung disease or compromised immune systems.

AREAS COVERED

This paper provides an in-depth analysis of NTM infections, including microbiology, environmental sources and transmission pathways, risk factors for disease, epidemiology, clinical manifestations and diagnostic approaches, guideline-based treatment recommendations, drugs under development, and management challenges.

EXPERT OPINION

Future approaches to the management of NTM pulmonary disease will require therapies that are well tolerated, can be taken for a shorter time period and perhaps less frequently, have few drug-drug interactions, and are active against the various strains of pathogens. As the numbers of infections increase, such therapies will be welcomed by clinicians and patients.

ABC superfamily transporter Rv1273c of Mycobacterium tuberculosis acts as a multidrug efflux pump

Efflux pump-mediated drug resistance in bacteria is a common occurrence effective for the general survival of the organism. The Mycobacterium tuberculosis genome has an abundance of adenosine triphosphate (ATP) dependent cassette transporter genes but only a handful of them are documented for their contribution to drug resistance. In this study, we inspected the potential of an ABC transporter Rv1273c from M. tuberculosis as a multidrug efflux pump and a contributor to intrinsic drug resistance. Expression of Rv1273c in Escherichia coli and M. smegmatis conferred tolerance to various structurally unrelated antibiotics. Lower accumulation of fluoroquinolones in intact E. coli and M. smegmatis cells expressing the transporter implied its active efflux activity. Energy-dependent efflux by Rv1273c was observed in real time using the lipophilic dye Nile Red. Expression of Rv1273c also resulted in an increase in biofilm formation by E. coli and M. smegmatis cells. Overall, the results indicate the possibility that Rv1273c might be a multidrug transporter with a wide substrate range and a probable contributor to biofilm formation.

The Extent of Antimicrobial Resistance Due to Efflux Pump Regulation

A key mechanism driving antimicrobial resistance (AMR) stems from the ability of bacteria to up-regulate efflux pumps upon exposure to drugs. The resistance gained by this up-regulation is pliable because of the tight regulation of efflux pump levels. This leads to temporary enhancement in survivability of bacteria due to higher efflux pump levels in the presence of antibiotics, which can be reversed when the cells are no longer exposed to the drug. Knowledge of the extent of resistance thus gained would inform intervention strategies aimed at mitigating AMR. Here, we combine mathematical modeling and experiments to quantify the maximum extent of resistance that efflux pump up-regulation can confer via phenotypic induction in the presence of drugs and genotypic abrogation of regulation. Our model describes the dynamics of drug transport in and out of cells coupled with the associated regulation of efflux pump levels and predicts the increase in the minimum inhibitory concentration (MIC) of drugs due to such regulation. To test the model, we measured the uptake and efflux as well as the MIC of the compound ethidium bromide (EtBr), a substrate of the efflux pump LfrA, in wild-type Mycobacterium smegmatis mc²155, as well as in two laboratory-generated strains. Our model captured the observed EtBr levels and MIC fold-changes quantitatively. Further, the model identified key parameters associated with the resulting resistance, variations in which could underlie the extent to which such resistance arises across different drug-bacteria combinations, potentially offering tunable handles to optimize interventions aimed at minimizing AMR.

The Putative Major Facilitator Superfamily (MFS) Protein Named Rv1877 in Mycobacterium tuberculosis Behaves as a Multidrug Efflux Pump

Efflux pumps are one of the major contributors in the intrinsic multidrug resistance of Mycobacterium tuberculosis. These active transporters, localized in the cytoplasmic membrane, often carry an array of unrelated substances, from toxic substances to metabolites and maintain cellular homeostasis. Rv1877, a putative Major Facilitator Superfamily efflux pump from M. tuberculosis, was investigated in this study. Expression of Rv1877 in Escherichia coli resulted in elevated resistance towards antibiotics of various families. A reversal of this resistance was observed in the presence of sub-inhibitory concentration of the uncoupler carbonyl cyanide-m-chlorophenylhydrazone, indicating its dependence on proton motive force (pmf). Lower intracellular accumulation of the fluoroquinolones ofloxacin and levofloxacin in E. coli cells harbouring Rv1877 implied an active efflux of the drugs. Interestingly, real time, energy-dependent efflux was demonstrated by cells expressing Rv1877 with a lipophilic dye Nile Red. In addition, expression of Rv1877 in trans increased the biofilm formation by the host E. coli cells. Moreover, in silico docking analysis of the molecular interactions between Rv1877 and antibiotics corroborated the experimental observations. Based on the in vivo analyses of Rv1877 in E. coli, it could be designated as a pmf-dependent multidrug transporter with the ability of extruding structurally unrelated antibiotics, preferably some of the fluoroquinolones, and a facilitator of biofilm formation.

c-di-AMP signaling plays important role in determining antibiotic tolerance phenotypes of Mycobacterium smegmatis

In this study, we probe the role of secondary messenger c-di-AMP in drug tolerance, which includes both persister and resistant mutant characterization of Mycobacterium smegmatis. Specifically, with the use of c-di-AMP null and overproducing mutants, we showed how c-di-AMP plays a significant role in resistance mutagenesis against antibiotics with different mechanisms of action. We elucidated the specific molecular mechanism linking the elevated intracellular c-di-AMP level and high mutant generation and highlighted the significance of non-homology-based DNA repair. Further investigation enabled us to identify the unique mutational landscape of target and non-target mutation categories linked to intracellular c-di-AMP levels. Overall fitness cost of unique target mutations was estimated in different strain backgrounds, and then we showed the critical role of c-di-AMP in driving epistatic interactions between resistance genes, resulting in the evolution of multi-drug tolerance. Finally, we identified the role of c-di-AMP in persister cells regrowth and mutant enrichment upon cessation of antibiotic treatment.

A multidrug efflux protein in Mycobacterium tuberculosis; tap as a potential drug target for drug repurposing

Tuberculosis (TB) is a serious communicative disease caused by Mycobacterium tuberculosis. Although there are vaccines and drugs available to treat the disease, they are not efficient, moreover, multidrug-resistant TB (MDR-TB) become a major hurdle in its therapy. These MDR strains utilize the multidrug efflux pump as a decisive weapon to fight against antitubercular drugs. Tap membrane protein was observed as a crucial multidrug efflux pump in M. tuberculosis and its critical implication in MDR-MTB development makes it an effective drug target. In the present study, we have utilized various in silico approaches to predict the applicability of FDA-approved ion channel inhibitors and blockers as therapeutic leads against Tuberculosis by targeting multidrug efflux protein; Tap in MTB. Tap protein structure is predicted by Phyre2 server followed by model refinement, validation, physio-chemical catheterization and target prediction. Further, the interaction between Tap protein and ligands were analysed by molecular docking and MD simulation run of 100 ns. Based on implication and compatibility, 18 FDA-approved ion channel inhibitors and blockers are selected as a ligand against the Tap protein and eventually observed five ligands; Glimepiride, Flecainide, Flupiritine, Nimodipine and Amlodipine as promising compounds which have exhibited the significant stable interaction with Tap protein and are proposed to modulate or interfere with its activity. These compounds illustrated the substantial docking score and total binding enthalpy more than -7 kcal/mol and -42 kcal/mol respectively which implies that the selected FDA-approved compounds can spontaneously interact with the Tap protein to modulate its function. This study proposed Tap protein as a prominent drug target in MTB and investigated compounds that show considerable interaction with the Tap protein as potential therapeutic molecules. These interactions may lead to modulating or inhibit the activity of drug efflux protein thereby making MTB susceptible to antitubercular drugs.

Critical discussion on drug efflux in Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) can withstand months of antibiotic treatment. An important goal of tuberculosis research is to shorten the treatment to reduce the burden on patients, increase adherence to the drug regimen and thereby slow down the spread of drug resistance. Inhibition of drug efflux pumps by small molecules has been advocated as a promising strategy to attack persistent Mtb and shorten therapy. Although mycobacterial drug efflux pumps have been broadly investigated, mechanistic studies are scarce. In this critical review, we shed light on drug efflux in its larger mechanistic context by considering the intricate interplay between membrane transporters annotated as drug efflux pumps, membrane energetics, efflux inhibitors and cell wall biosynthesis processes. We conclude that a great wealth of data on mycobacterial transporters is insufficient to distinguish by what mechanism they contribute to drug resistance. Recent studies suggest that some drug efflux pumps transport structural lipids of the mycobacterial cell wall and that the action of certain drug efflux inhibitors involves dissipation of the proton motive force, thereby draining the energy source of all active membrane transporters. We propose recommendations on the generation and interpretation of drug efflux data to reduce ambiguities and promote assigning novel roles to mycobacterial membrane transporters.

Drug Resistance in Nontuberculous Mycobacteria: Mechanisms and Models

The genus Mycobacteria comprises a multitude of species known to cause serious disease in humans, including Mycobacterium tuberculosis and M. leprae, the responsible agents for tuberculosis and leprosy, respectively. In addition, there is a worldwide spike in the number of infections caused by a mixed group of species such as the M. avium, M. abscessus and M. ulcerans complexes, collectively called nontuberculous mycobacteria (NTMs). The situation is forecasted to worsen because, like tuberculosis, NTMs either naturally possess or are developing high resistance against conventional antibiotics. It is, therefore, important to implement and develop models that allow us to effectively examine the fundamental questions of NTM virulence, as well as to apply them for the discovery of new and improved therapies. This literature review will focus on the known molecular mechanisms behind drug resistance in NTM and the current models that may be used to test new effective antimicrobial therapies.

PNA-mediated efflux inhibition as a therapeutic strategy towards overcoming drug resistance in Mycobacterium smegmatis

The emergence of antibiotic-resistant strains of Mycobacterium tuberculosis and the decelerating development of new and effective antibiotics has impaired the treatment of tuberculosis (TB). Efflux pump inhibitors (EPIs) have the potential to improve the efficacy of existing anti-TB drugs although with toxicity limitations. Peptide nucleic acids (PNAs), oligonucleotide mimics, by virtue of their high nucleic acid binding specificity have the capability to overcome this drawback. We, therefore, investigated the efflux pump inhibitory properties of a PNA designed against an efflux pump of Mycobacterium smegmatis. LfrA, an efflux pump found in M. smegmatis, is majorly involved in conferring innate drug resistance to this strain and, therefore, was selected as a target for gene silencing via PNA. qRT-PCR and EtBr assays confirmed the EPI activity of the anti-lfrA PNA. On testing the effect of the anti-lfrA PNA on the bactericidal activity of a fluoroquinolone, norfloxacin, we observed that 5 μM of anti-lfrA PNA in combination with norfloxacin led to an enhanced killing of up to 2.5 log-fold against wild-type and a lab-generated multidrug resistant strain, exemplifying its potential in countering resistance. Improved efficacy was also observed against intra-macrophage mycobacteria, where the drug-PNA combination enhanced bacterial clearance by 1.3 log-fold. Further, no toxicity was observed with PNA concentrations up to 4 times higher than the efficacious anti-lfrA PNA concentration. Thus, PNA, as an adjuvant, presents a novel and viable approach to rejuvenate anti-TB therapeutics.

The ATP-Binding Cassette (ABC) Transport Systems in Mycobacterium tuberculosis: Structure, Function, and Possible Targets for Therapeutics

Simple Summary

Mycobacterium tuberculosis is a bacterium of great medical importance because it causes tuberculosis, a disease that affects millions of people worldwide. Two important features are related to this bacterium: its ability to infect and survive inside the host, minimizing the immune response, and the burden of clinical isolates that are highly resistant to antibiotics treatment. These two phenomena are directly affected by cell envelope proteins, such as proteins from the ATP-Binding Cassette (ABC transporters) superfamily. In this review, we have compiled information on all the M. tuberculosis ABC transporters described so far, both from a functional and structural point of view, and show their relevance for the bacillus and the potential targets for studies aiming to control the microorganism and structural features.

Abstract

Mycobacterium tuberculosis is the etiological agent of tuberculosis (TB), a disease that affects millions of people in the world and that is associated with several human diseases. The bacillus is highly adapted to infect and survive inside the host, mainly because of its cellular envelope plasticity, which can be modulated to adapt to an unfriendly host environment; to manipulate the host immune response; and to resist therapeutic treatment, increasing in this way the drug resistance of TB. The superfamily of ATP-Binding Cassette (ABC) transporters are integral membrane proteins that include both importers and exporters. Both types share a similar structural organization, yet only importers have a periplasmic substrate-binding domain, which is essential for substrate uptake and transport. ABC transporter-type importers play an important role in the bacillus physiology through the transport of several substrates that will interfere with nutrition, pathogenesis, and virulence. Equally relevant, exporters have been involved in cell detoxification, nutrient recycling, and antibiotics and drug efflux, largely affecting the survival and development of multiple drug-resistant strains. Here, we review known ABC transporters from M. tuberculosis, with particular focus on the diversity of their structural features and relevance in infection and drug resistance.

Efflux Pump Inhibitors Against Nontuberculous Mycobacteria

Over the last years, nontuberculous mycobacteria (NTM) have emerged as important human pathogens. Infections caused by NTM are often difficult to treat due to an intrinsic multidrug resistance for the presence of a lipid-rich outer membrane, thus encouraging an urgent need for the development of new drugs for the treatment of mycobacterial infections. Efflux pumps (EPs) are important elements that are involved in drug resistance by preventing intracellular accumulation of antibiotics. A promising strategy to decrease drug resistance is the inhibition of EP activity by EP inhibitors (EPIs), compounds that are able to increase the intracellular concentration of antimicrobials. Recently, attention has been focused on identifying EPIs in mycobacteria that could be used in combination with drugs. The aim of the present review is to provide an overview of the current knowledge on EPs and EPIs in NTM and also, the effect of potential EPIs as well as their combined use with antimycobacterial drugs in various NTM species are described.

Mutations in Efflux Pump Rv1258c (Tap) Cause Resistance to Pyrazinamide, Isoniazid, and Streptomycin in M. tuberculosis

Although drug resistance in Mycobacterium tuberculosis is mainly caused by mutations in drug activating enzymes or drug targets, there is increasing interest in the possible role of efflux in causing drug resistance. Previously, efflux genes have been shown to be upregulated upon drug exposure or implicated in drug resistance in overexpression studies, but the role of mutations in efflux pumps identified in clinical isolates in causing drug resistance is unknown. Here we investigated the role of mutations in efflux pump Rv1258c (Tap) from clinical isolates in causing drug resistance in M. tuberculosis. We constructed point mutations V219A and S292L in Rv1258c in the chromosome of M. tuberculosis and the point mutations were confirmed by DNA sequencing. The susceptibility of the constructed M. tuberculosis Rv1258c mutants to different tuberculosis drugs was assessed using conventional drug susceptibility testing in 7H11 agar in the presence and absence of efflux pump inhibitor piperine. A C14-labeled PZA uptake experiment was performed to demonstrate higher efflux activity in the M. tuberculosis Rv1258c mutants. Interestingly, the V219A and S292L point mutations caused clinically relevant drug resistance to pyrazinamide (PZA), isoniazid (INH), and streptomycin (SM), but not to other drugs in M. tuberculosis. While V219A point mutation conferred low-level drug resistance, the S292L mutation caused a higher level of resistance. Efflux inhibitor piperine inhibited INH and PZA resistance in the S292L mutant but not in the V219A mutant. The S292L mutant had higher efflux activity for pyrazinoic acid (the active form of PZA) than the parent strain. We conclude that point mutations in the efflux pump Rv1258c in clinical isolates can confer clinically relevant drug resistance, including PZA resistance, and could explain some previously unaccounted drug resistance in clinical strains. Future studies need to take efflux mutations into consideration for improved detection of drug resistance in M. tuberculosis and address their role in affecting treatment outcome in vivo.

The Anionic Phospholipids in the Plasma Membrane Play an Important Role in Regulating the Biochemical Properties and Biological Functions of RecA Proteins

Escherichia coli RecA (EcRecA) forms discrete foci that cluster at cell poles during normal growth, which are redistributed along the filamented cell axis upon induction of the SOS response. The plasma membrane is thought to act as a scaffold for EcRecA foci, thereby playing an important role in RecA-dependent homologous recombination. In addition, in vivo and in vitro studies demonstrate that EcRecA binds strongly to the anionic phospholipids. However, there have been almost no data on the association of mycobacterial RecA proteins with the plasma membrane and the effects of membrane components on their function. Here, we show that mycobacterial RecA proteins specifically interact with phosphatidylinositol and cardiolipin among other anionic phospholipids; however, they had no effect on the ability of RecA proteins to bind single-stranded DNA. Interestingly, phosphatidylinositol and cardiolipin impede the DNA-dependent ATPase activity of RecA proteins, although ATP binding is not affected. Furthermore, the ability of RecA proteins to promote DNA strand exchange is not affected by anionic phospholipids. Strikingly, anionic phospholipids suppress the RecA-stimulated autocatalytic cleavage of the LexA repressor. The Mycobacterium smegmatis RecA foci localize to the cell poles during normal growth, and these structures disassemble and reassemble into several foci along the cell after the induction of DNA damage. Taken together, these data support the notion that the interaction of RecA with cardiolipin and phosphatidylinositol, the major anionic phospholipids of the mycobacterial plasma membrane, may be physiologically relevant, as they provide a scaffold for RecA storage and may regulate recombinational DNA repair and the SOS response.

Overexpression of eis without a mutation in promoter region of amikacin- and kanamycin-resistant Mycobacterium tuberculosis clinical strain

BACKGROUND

Aminoglycosides such as amikacin and kanamycin are effective injectable second-line drugs for treatment of multidrug-resistant tuberculosis. Molecular mechanisms underlying aminoglycoside resistance are not well understood. We have previously identified the amikacin- and kanamycin-resistant M. tuberculosis MT433 clinical strain, of which all known mutations related to resistance have not been found. Drug efflux pump is one of reported resistance mechanisms that might play a role in aminoglycoside resistance.

METHODS

The expression levels of sixteen putative efflux pump genes, including eis and one regulator gene, whiB7, of MT433 in the presence of kanamycin were determined using the reverse transcription-quantitative PCR method. The effects of upregulated genes on amikacin and kanamycin resistance were investigated by overexpression in M. tuberculosis H37Ra strain.

RESULTS

Upon kanamycin exposure, other than whiB7 and eis that were found extremely overexpressed, two drug efflux pump genes, namely Rv1877 and Rv2846c, showed specifically high-level of expression in M. tuberculosis MT433 strain. However, direct effect of overexpressed Rv1877 and Rv2846c on amikacin and kanamycin resistance could not be demonstrated in M. tuberculosis H37Ra overexpressed strain.

CONCLUSIONS

Our finding demonstrated that overexpression of eis could occur without any mutations in the promoter region and be detectable in clinical isolate. This might be a consequence of overexpressed whiB7, resulting in amikacin and kanamycin resistance in M. tuberculosis MT433 strain.

A complete high-quality MinION nanopore assembly of an extensively drug-resistant Mycobacterium tuberculosis Beijing lineage strain identifies novel variation in repetitive PE/PPE gene regions

A better understanding of the genomic changes that facilitate the emergence and spread of drug-resistant Mycobacterium tuberculosis strains is currently required. Here, we report the use of the MinION nanopore sequencer (Oxford Nanopore Technologies) to sequence and assemble an extensively drug-resistant (XDR) isolate, which is part of a modern Beijing sub-lineage strain, prevalent in Western Province, Papua New Guinea. Using 238-fold coverage obtained from a single flow-cell, de novo assembly of nanopore reads resulted into one contiguous assembly with 99.92 % assembly accuracy. Incorporation of complementary short read sequences (Illumina) as part of consensus error correction resulted in a 4 404 064 bp genome with 99.98 % assembly accuracy. This assembly had an average nucleotide identity of 99.7 % relative to the reference genome, H37Rv. We assembled nearly all GC-rich repetitive PE/PPE family genes (166/168) and identified variants within these genes. With an estimated genotypic error rate of 5.3 % from MinION data, we demonstrated identification of variants to include the conventional drug resistance mutations, and those that contribute to the resistance phenotype (efflux pumps/transporter) and virulence. Reference-based alignment of the assembly allowed detection of deletions and insertions. MinION sequencing provided a fully annotated assembly of a transmissible XDR strain from an endemic setting and showed its utility to provide further understanding of genomic processes within Mycobacterium tuberculosis.

Overcoming problems of poor drug penetration into bacteria: challenges and strategies for medicinal chemists

INTRODUCTION

Bacterial cell walls and membranes provide essential protection for bacteria against environmental influences. Different bacteria possess different cell envelopes and understanding each of these structures is crucial for the design of effective antibacterial drugs whose targets are intracellular. Optimal properties of drugs that are required for their entry into bacteria are still hard to predict. The guidelines that are suitable and well established for the penetration of a drug into eukaryotic cells are poorly adaptable to the complex world of pathogens. Areas covered: The factors that govern the penetration of anti-infection drugs into bacteria are examined and the available strategies to overcome this therapeutically very important barrier are reviewed. The areas covered include optimization of the physicochemical properties of compounds, utilization of iron-chelating compounds, i.e. siderophores, the use of efflux pump inhibitors, and of carriers such as liposomes. Expert opinion: Although several rules governing permeation have recently been proposed for effective antibacterial drugs, none of them has been so far established as the 'golden' rule. Thus, new research is needed to find a more general approach on how to increase the concentration of antibacterial compounds in bacterial cells.

Single nucleotide polymorphisms in efflux pumps genes in extensively drug resistant Mycobacterium tuberculosis isolates from Pakistan

It is challenging to understand mechanisms of drug resistance in Mycobacterium tuberculosis (MTB) due to the large variability in resistance associated genes. Efflux pump genes contribute to drug resistance and thus add to this complexity. Efflux pump gene protein superfamilies have been characterized by genome analysis of drug resistant strains and through in vitro transcriptional studies. However, there is limited information regarding efflux pump genes in extensively drug resistant (XDR) tuberculosis (TB) isolates. Whole genome sequencing (WGS) based analysis of 37 extensively drug resistant (XDR) and five drug sensitive (DS) MTB clinical isolates was performed. Single nucleotide polymorphisms (SNPs) in efflux pump genes Rv0194, Rv1217, Rv1218, drrA, drrB, Rv1258, Rv1634, Rv2688, Rv1273, Rv1819, Rv1458, Rv1877 and Rv1250 were determined in the clinical isolates as compared with the H37Rv reference strain. Allele frequencies of SNPs identified in XDR strains were compared with DS strains. Gene expression of Rv0194, Rv2688, Rv1634, drrA and drrB was determined in XDR -TB isolates (n = 9), DS-TB strains (n = 4) and H37Rv. We identified SNPs in XDR-TB isolates which were either unique or present at very low frequencies in DS strains; Rv0194 G170V; Rv1217 L151R; Rv1258 P369T and G391R; Rv1273 S118G and I175T; Rv1877 I534T; Rv1250 V318X/A and S333A, and Rv2688 P156T. The expression of Rv2688 and drrB was found to be raised in XDR-TB as compared with DS-TB strains. We identified unique SNPs in efflux pump genes which may be associated with increased drug resistance in the isolates. Increased levels of Rv2688 and drrB efflux pump gene expression observed in XDR strains even in the absence of antibiotics suggests that these clinical isolates may be more refractory to treatment. Further studies are required to directly associate these mutations with increased resistance in MTB.

New Insights in to the Intrinsic and Acquired Drug Resistance Mechanisms in Mycobacteria

Infectious diseases caused by clinically important Mycobacteria continue to be an important public health problem worldwide primarily due to emergence of drug resistance crisis. In recent years, the control of tuberculosis (TB), the disease caused by Mycobacterium tuberculosis (MTB), is hampered by the emergence of multidrug resistance (MDR), defined as resistance to at least isoniazid (INH) and rifampicin (RIF), two key drugs in the treatment of the disease. Despite the availability of curative anti-TB therapy, inappropriate and inadequate treatment has allowed MTB to acquire resistance to the most important anti-TB drugs. Likewise, for most mycobacteria other than MTB, the outcome of drug treatment is poor and is likely related to the high levels of antibiotic resistance. Thus, a better knowledge of the underlying mechanisms of drug resistance in mycobacteria could aid not only to select the best therapeutic options but also to develop novel drugs that can overwhelm the existing resistance mechanisms. In this article, we review the distinctive mechanisms of antibiotic resistance in mycobacteria.

Use of green fluorescent protein labeled non-tuberculous mycobacteria to evaluate the activity quaternary ammonium compound disinfectants and antibiotics

Although infections with NonTuberculous Mycobacteria have become less common in AIDS patients, they are important opportunistic infections after surgical procedures, likely because they are ubiquitous and not efficiently killed by many commonly used disinfectants. In Venezuela there have recently been many non-tuberculous mycobacteria soft tissue infections after minor surgical procedures, some apparently related to the use of a commercial disinfectant based on a Quaternary Ammonium Compound. We studied the activity of this and other quaternary ammonium compounds on different non-tuberculous mycobacteria by transforming the mycobacteria with a dnaA-gfp fusion and then monitoring fluorescence to gauge the capacity of different quaternary ammonium compounds to inhibit bacterial growth. The minimum inhibitory concentration varied for the different quaternary ammonium compounds, but M. chelonae and M. abscessus were consistently more resistant than M. smegmatis, and M. terrae more resistant than M. bovis BCG.

Novel TetR family transcriptional factor regulates expression of multiple transport-related genes and affects rifampicin resistance in Mycobacterium smegmatis

Transport-related genes significantly affect bacterial antibiotic resistance. However, the effects of these genes and their regulation of bacterial drug resistance in several mycobacterial species, including the fast-growing Mycobacterium smegmatis, the pathogen M. tuberculosis and M. avium have not been clearly characterized. We identified Ms4022 (MSMEG_4022) as a novel TetR family regulator that activates the expression of seven transport-related genes and affects drug resistance in M. smegmatis. Overexpression of Ms4022 inhibited M. smegmatis growth and enhanced mycobacterial resistance to the anti-tuberculosis drug rifampicin (RIF). By contrast, the Ms4022-deleted mycobacterial strain has shown sensitive to RIF. Ms4022 recognized three 19 bp non-palindromic motifs containing a 9 bp conserved region at their 5' end and it directly regulated seven transport-related genes, which affects mycobacterial resistance to RIF. Overexpression of three of seven transport-related genes (Ms1448, Ms1613, and Ms5278) inhibited the growth of M. smegmatis. This study improves our understanding of the function of mycobacterial transport-related genes and their regulation of bacterial drug resistance.

The Molecular Genetics of Fluoroquinolone Resistance in Mycobacterium tuberculosis

The fluoroquinolones (FQs) are synthetic antibiotics effectively used for curing patients with multidrug-resistant tuberculosis (TB). When a multidrug-resistant strain develops resistance to the FQs, as in extensively drug-resistant strains, obtaining a cure is much more difficult, and molecular methods can help by rapidly identifying resistance-causing mutations. The only mutations proven to confer FQ resistance in M. tuberculosis occur in the FQ target, the DNA gyrase, at critical amino acids from both the gyrase A and B subunits that form the FQ binding pocket. GyrA substitutions are much more common and generally confer higher levels of resistance than those in GyrB. Molecular techniques to detect resistance mutations have suboptimal sensitivity because gyrase mutations are not detected in a variable percentage of phenotypically resistant strains. The inability to find gyrase mutations may be explained by heteroresistance: bacilli with a resistance-conferring mutation are present only in a minority of the bacterial population (>1%) and are therefore detected by the proportion method, but not in a sufficient percentage to be reliably detected by molecular techniques. Alternative FQ resistance mechanisms in other bacteria--efflux pumps, pentapeptide proteins, or enzymes that inactivate the FQs--have not yet been demonstrated in FQ-resistant M. tuberculosis but may contribute to intrinsic levels of resistance to the FQs or induced tolerance leading to more frequent gyrase mutations. Moxifloxacin is currently the best anti-TB FQ and is being tested for use with other new drugs in shorter first-line regimens to cure drug-susceptible TB.

Proteasome Accessory Factor C (pafC) Is a novel gene Involved in Mycobacterium Intrinsic Resistance to broad-spectrum antibiotics--Fluoroquinolones

Antibiotics resistance poses catastrophic threat to global public health. Novel insights into the underlying mechanisms of action will inspire better measures to control drug resistance. Fluoroquinolones are potent and widely prescribed broad-spectrum antibiotics. Bacterial protein degradation pathways represent novel druggable target for the development of new classes of antibiotics. Mycobacteria proteasome accessory factor C (pafC), a component of bacterial proteasome, is involved in fluoroquinolones resistance. PafC deletion mutants are hypersensitive to fluoroquinolones, including moxifloxacin, norfloxacin, ofloxacin, ciprofloxacin, but not to other antibiotics such as isoniazid, rifampicin, spectinomycin, chloramphenicol, capreomycin. This phenotype can be restored by complementation. The pafC mutant is hypersensitive to H2O2 exposure. The iron chelator (bipyridyl) and a hydroxyl radical scavenger (thiourea) can abolish the difference. The finding that pafC is a novel intrinsic selective resistance gene provided new evidence for the bacterial protein degradation pathway as druggable target for the development of new class of antibiotics.

Polyamines inhibit porin-mediated fluoroquinolone uptake in mycobacteria

Polyamines decrease the permeability of the outer membrane of Escherichia coli to fluoroquinolones and β-lactams. In this study, we tested the effect of four polyamines (spermidine, spermine, cadaverine and putrescine) on fluoroquinolone uptake in Mycobacterium bovis BCG. Our results show that polyamines are also capable of reducing the permeability of the mycobacterial outer membrane to fluoroquinolones. Spermidine was most effective and demonstrated reversible dose- and pH-dependent inhibition of ciprofloxacin accumulation. The extent of this inhibition was demonstrated across the fluoroquinolone compound class to varying degrees. Furthermore, we have shown that the addition of spermidine increases the survival of M. bovis BCG after a 5-day exposure to ciprofloxacin by up to 25 times. The treatment of actively-replicating Mycobacterium tuberculosis with spermidine reduced ciprofloxacin accumulation by half while non-replicating nutrient-starved M. tuberculosis cultures lacked similar sensitivity to polyamines. Gene expression studies showed that several outer membrane proteins are significantly down-regulated during the shift to non-replication. Collectively, these characteristics of fluoroquinolone uptake in M. bovis BCG are consistent with facilitated transport by porin-like proteins and suggest that a reduction in intracellular uptake contributes to the phenotypic drug resistance demonstrated by M. tuberculosis in the non-replicating state.

Reduced drug uptake in phenotypically resistant nutrient-starved nonreplicating Mycobacterium tuberculosis

During active tuberculosis a spectrum of physiologically different Mycobacterium tuberculosis bacilli reside in human tissues. Subpopulations of the pathogen survive antibiotic treatment for a prolonged time in a dormant state of phenotypic drug resistance, a phenomenon independent of genetic mutations. Here, we used an established culture model of nutrient deprivation to shift down M. tuberculosis from growth to nonreplicating survival, which is characterized by a drastic loss of drug susceptibility. Liquid chromatography coupled with mass spectrometry techniques were employed to quantify drug penetration in replicating and nutrient-starved nonreplicating bacilli. We found that intracellular concentrations of fluoroquinolones, rifamycins, and linezolid were lower in nonreplicating M. tuberculosis. Studies with pump inhibitors suggest that the observed differences were independent of efflux processes. We conclude that decreased drug permeability contributes to phenotypic drug resistance of dormant M. tuberculosis.

Antimicrobial efflux pumps and Mycobacterium tuberculosis drug tolerance: evolutionary considerations

The need for lengthy treatment to cure tuberculosis stems from phenotypic drug resistance, also known as drug tolerance, which has been previously attributed to slowed bacterial growth in vivo. We discuss recent findings that challenge this model and instead implicate macrophage-induced mycobacterial efflux pumps in antimicrobial tolerance. Although mycobacterial efflux pumps may have originally served to protect against environmental toxins, in the pathogenic mycobacteria, they appear to have been repurposed for intracellular growth. In this light, we discuss the potential of efflux pump inhibitors such as verapamil to shorten tuberculosis treatment by their dual inhibition of tolerance and growth.

The role of transport mechanisms in mycobacterium tuberculosis drug resistance and tolerance

In the fight against tuberculosis, cell wall permeation of chemotherapeutic agents remains a critical but largely unsolved question. Here we review the major mechanisms of small molecule penetration into and efflux from Mycobacterium tuberculosis and other mycobacteria, and outline how these mechanisms may contribute to the development of phenotypic drug tolerance and induction of drug resistance. M. tuberculosis is intrinsically recalcitrant to small molecule permeation thanks to its thick lipid-rich cell wall. Passive diffusion appears to account for only a fraction of total drug permeation. As in other bacterial species, influx of hydrophilic compounds is facilitated by water-filled open channels, or porins, spanning the cell wall. However, the diversity and density of M. tuberculosis porins appears lower than in enterobacteria. Besides, physiological adaptations brought about by unfavorable conditions are thought to reduce the efficacy of porins. While intracellular accumulation of selected drug classes supports the existence of hypothesized active drug influx transporters, efflux pumps contribute to the drug resistant phenotype through their natural abundance and diversity, as well as their highly inducible expression. Modulation of efflux transporter expression has been observed in phagocytosed, non-replicating persistent and multi-drug resistant bacilli. Altogether, M. tuberculosis has evolved both intrinsic properties and acquired mechanisms to increase its level of tolerance towards xenobiotic substances, by preventing or minimizing their entry. Understanding these adaptation mechanisms is critical to counteract the natural mechanisms of defense against toxic compounds and develop new classes of chemotherapeutic agents that positively exploit the influx and efflux pathways of mycobacteria.

Antimicrobial susceptibility testing, drug resistance mechanisms, and therapy of infections with nontuberculous mycobacteria

Within the past 10 years, treatment and diagnostic guidelines for nontuberculous mycobacteria have been recommended by the American Thoracic Society (ATS) and the Infectious Diseases Society of America (IDSA). Moreover, the Clinical and Laboratory Standards Institute (CLSI) has published and recently (in 2011) updated recommendations including suggested antimicrobial and susceptibility breakpoints. The CLSI has also recommended the broth microdilution method as the gold standard for laboratories performing antimicrobial susceptibility testing of nontuberculous mycobacteria. This article reviews the laboratory, diagnostic, and treatment guidelines together with established and probable drug resistance mechanisms of the nontuberculous mycobacteria.

Docking studies on novel analogues of 8 methoxy fluoroquinolones against GyrA mutants of Mycobacterium tuberculosis

BACKGROUND

Fluoroquinolone resistance is a serious threat in the battle against the treatment of multi drug resistant tuberculosis (MDR-TB) and extensively drug resistant tuberculosis (XDR-TB). Fluoroquinolone resistant isolates from India had shown to have evolved several mutants in the quinolone resistance determining region (QRDR) of DNA gyrase A subunit (GyrA), the target of fluoroquinolone. In view of high prevalence of mutations in the 'hot spot' region, a study on combinatorial drug design was carried out to identify better analogues for the treatment of MDR-TB. The gyrA subunit 'hot spot' region of codons 90, 94 and 95 were modeled into their corresponding protein folds and used as receptors for the docking studies. Further, invitro tests were carried using the parent compounds, namely gatifloxacin and moxifloxacin and correlated with the obtained docking scores.

RESULTS

Molecular docking and in vitro studies correlated well in demonstrating the enhanced activity of moxifloxacin, when compared to gatifloxacin, on ofloxacin sensitive and resistant strains comprising of clinical isolates of MDR-TB. The evolved lead structures targeting against mutant QRDR receptors were guanosine and cholesteryl esters of gatifloxacin and moxifloxacin. They showed consistently high binding affinity values of -10.3 and -10.1 kcal/mol respectively with the target receptors. Of these, the guanosine ester showed highest binding affinity score and its log P value lied within the Lipinski's range indicating that it could have better absorptivity when it is orally administered thereby having an enhanced activity against MTB.

CONCLUSIONS

The docking results showed that the addition of the cholesteryl and guanosine esters to the 'DNA gyrase binding' region of gatifloxacin and moxifloxacin enhanced the binding affinity of these parent molecules with the mutant DNA gyrase receptors. Viewing the positive correlation for the docking and in vitro results with the parent compounds, these lead structures could be further evaluated for their in vitro and in vivo activity against MDR-TB.

Pyrazinoic acid efflux rate in Mycobacterium tuberculosis is a better proxy of pyrazinamide resistance

Pyrazinamide is one of the most important drugs in the treatment of latent Mycobacterium tuberculosis infection. The emergence of strains resistant to pyrazinamide represents an important public health problem, as both first- and second-line treatment regimens include pyrazinamide. The accepted mechanism of action states that after the conversion of pyrazinamide into pyrazinoic acid by the bacterial pyrazinamidase enzyme, the drug is expelled from the bacteria by an efflux pump. The pyrazinoic acid is protonated in the extracellular environment and then re-enters the mycobacterium, releasing the proton and causing a lethal disruption of the membrane. Although it has been shown that mutations causing significant loss of pyrazinamidase activity significantly contribute to pyrazinamide resistance, the mechanism of resistance is not completely understood. The pyrazinoic acid efflux rate may depend on multiple factors, including pyrazinamidase activity, intracellular pyrazinamidase concentration, and the efficiency of the efflux pump. Whilst the importance of the pyrazinoic acid efflux rate to the susceptibility to pyrazinamide is recognized, its quantitative effect remains unknown. Thirty-four M. tuberculosis clinical isolates and a Mycobacterium smegmatis strain (naturally resistant to PZA) were selected based on their susceptibility to pyrazinamide, as measured by Bactec 460TB and the Wayne method. For each isolate, the initial velocity at which pyrazinoic acid is released from the bacteria and the initial velocity at which pyrazinamide enters the bacteria were estimated. The data indicated that pyrazinoic acid efflux rates for pyrazinamide-susceptible M. tuberculosis strains fell within a specific range, and M. tuberculosis strains with a pyrazinoic acid efflux rate below this range appeared to be resistant. This finding contrasts with the high pyrazinoic acid efflux rate for M. smegmatis, which is innately resistant to pyrazinamide: its pyrazinoic acid efflux rate was found to be 900 fold higher than the average efflux rate for M. tuberculosis strains. No significant variability was observed in the pyrazinamide flux rate. The pyrazinoic acid efflux rate explained 61% of the variability in Bactec pyrazinamide susceptibility, 24% of Wayne activity, and 51% of the Bactec 460TB growth index. In contrast, pyrazinamidase activity accounted for only 27% of the Bactec pyrazinamide susceptibility. This finding suggests that mechanisms other than pncA mutations (reduction of pyrazinamidase activity) are also implicated in pyrazinamide resistance, and that pyrazinoic acid efflux rate acts as a better proxy for pyrazinamide resistance than the presence of pncA mutations. This is relevant to the design of molecular diagnostics for pyrazinamide susceptibility, which currently rely on pncA gene mutation detection.

Phenotypic and molecular characterization of quinolone resistance in Mycobacterium abscessus subsp. bolletii recovered from postsurgical infections

Several outbreaks of infections caused by rapidly growing mycobacteria (RGM) were reported in many Brazilian states (2032 notified cases) from 2004 to 2010. Most of the confirmed cases were mainly associated with Mycobacterium massiliense (recently renamed as Mycobacterium abscessus subsp. bolletii) BRA100 clone, recovered from patients who had undergone invasive procedures in which medical instruments had not been properly sterilized and/or disinfected. Since quinolones have been an option for the treatment of general RGM infections and have been suggested for therapeutic schemes for these outbreaks, we evaluated the in vitro activities of all generations of quinolones for clinical and reference RGM by broth microdilution, and analysed the peptide sequences of the quinolone resistance determining regions (QRDRs) of GyrA and GyrB after DNA sequencing followed by amino acid translation. Fifty-four isolates of M. abscessus subsp. bolletii, including clone BRA100, recovered in different states of Brazil, and 19 reference strains of RGM species were characterized. All 54 M. abscessus subsp. bolletii isolates were resistant to all generations of quinolones and showed the same amino acids in the QRDRs, including the Ala-83 in GyrA, and Arg-447 and Asp-464 in GyrB, described as being responsible for an intrinsic low level of resistance to quinolones in mycobacteria. However, other RGM species showed distinct susceptibilities to this class of antimicrobials and patterns of mutations contrary to what has been traditionally defined, suggesting that other mechanisms of resistance, different from gyrA or gyrB mutations, may also be involved in resistance to high levels of quinolones.

Efflux as a mechanism for drug resistance in Mycobacterium tuberculosis

Tuberculosis remains an important global public health problem, with an estimated prevalence of 14 million individuals with tuberculosis worldwide in 2007. Because antibiotic treatment is one of the main tools for tuberculosis control, knowledge of Mycobacterium tuberculosis drug resistance is an important component for the disease control strategy. Although several gene mutations in specific loci of the M. tuberculosis genome have been reported as the basis for drug resistance, additional resistance mechanisms are now believed to exist. Efflux is a ubiquitous mechanism responsible for intrinsic and acquired drug resistance in prokaryotic and eukaryotic cells. Mycobacterium tuberculosis presents one of the largest numbers of putative drug efflux pumps compared with its genome size. Bioinformatics as well as direct and indirect evidence have established relationships among drug efflux with intrinsic or acquired resistance in M. tuberculosis. This minireview describes the current knowledge on drug efflux in M. tuberculosis.

Ethidium bromide transport across Mycobacterium smegmatis cell-wall: correlation with antibiotic resistance

Background

Active efflux systems and reduced cell-wall permeability are considered to be the main causes of mycobacterial intrinsic resistance to many antimicrobials. In this study, we have compared the Mycobacterium smegmatis wild-type strain mc²155 with knockout mutants for porins MspA (the main porin of M. smegmatis) and MspC, the efflux pump LfrA (the main efflux pump system of M. smegmatis) and its repressor LfrR for their ability to transport ethidium bromide (EtBr) on a real-time basis. This information was then correlated with minimum inhibitory concentrations (MICs) of several antibiotics in the presence or absence of the efflux inhibitors chlorpromazine, thioridazine and verapamil.

Results

In the absence of porins MspA and MspC, accumulation of ethidium bromide decreased and the cells became more resistant to several antibiotics, whereas the knockout mutant for the LfrA pump showed increased accumulation of EtBr and increased susceptibility to EtBr, rifampicin, ethambutol and ciprofloxacin. Moreover, the efflux inhibitors caused a reduction of the MICs of streptomycin, rifampicin, amikacin, ciprofloxacin, clarithromycin and erythromycin in most of the strains tested.

Conclusions

The methodology used in this study demonstrated that porin MspA plays an important role in the influx of quaternary ammonium compounds and antibiotics and that efflux via the LfrA pump is involved in low-level resistance to several antimicrobial drugs in M. smegmatis. The results obtained with this non-pathogenic mycobacterium will be used in future studies as a model for the evaluation of the activity of the same efflux inhibitors on the susceptibility of multidrug resistant strains of Mycobacterium tuberculosis to isoniazid and rifampicin.

Structural plasticity and distinct drug-binding modes of LfrR, a mycobacterial efflux pump regulator

The TetR-like transcriptional repressor LfrR controls the expression of the gene encoding the Mycobacterium smegmatis efflux pump LfrA, which actively extrudes fluoroquinolones, cationic dyes, and anthracyclines from the cell and promotes intrinsic antibiotic resistance. The crystal structure of the apoprotein form of the repressor reveals a structurally asymmetric homodimer exhibiting local unfolding and a blocked drug-binding site, emphasizing the significant conformational plasticity of the protein necessary for DNA and multidrug recognition. Crystallographic and calorimetric studies of LfrR-drug complexes further confirm the intrinsic flexibility of the homodimer, which provides a dynamic mechanism to broaden multidrug binding specificity and may be a general property of transcriptional repressors regulating microbial efflux pump expression.

Mycolic acid cyclopropanation is essential for viability, drug resistance, and cell wall integrity of Mycobacterium tuberculosis

Mycobacterium tuberculosis infection remains a major global health problem complicated by escalating rates of antibiotic resistance. Despite the established role of mycolic acid cyclopropane modification in pathogenesis, the feasibility of targeting this enzyme family for antibiotic development is unknown. We show through genetics and chemical biology that mycolic acid methyltransferases are essential for M. tuberculosis viability, cell wall structure, and intrinsic resistance to antibiotics. The tool compound dioctylamine, which we show acts as a substrate mimic, directly inhibits the function of multiple mycolic acid methyltransferases, resulting in loss of cyclopropanation, cell death, loss of acid fastness, and synergistic killing with isoniazid and ciprofloxacin. These results demonstrate that mycolic acid methyltransferases are a promising antibiotic target and that a family of virulence factors can be chemically inhibited with effects not anticipated from studies of each individual enzyme.

Efflux pump gene hefA of Helicobacter pylori plays an important role in multidrug resistance

AIM: To determine whether efflux systems contribute to multidrug resistance of H pylori.

METHODS: A chloramphenicol-induced multidrug resistance model of six susceptible H pylori strains (5 isolates and H pylori NCTC11637) was developed. Multidrug-resistant (MDR) strains were selected and the minimal inhibitory concentration (MIC) of erythromycin, metronidazole, penicillin G, tetracycline, and ciprofloxacin in multidrug resistant strains and their parent strains was determined by agar dilution tests. The level of mRNA expression of hefA was assessed by fluorescence real-time quantitative PCR. A H pylori LZ1026 knockout mutant (ΔH pylori LZ1026) for (putative) efflux protein was constructed by inserting the kanamycin resistance cassette from pEGFP-N2 into hefA, and its susceptibility profiles to 10 antibiotics were evaluated.

RESULTS: The MIC of six multidrug-resistant strains (including 5 clinical isolates and H pylori NCTC11637) increased significantly (≥ 4-fold) compared with their parent strains. The expression level of hefA gene was significantly higher in the MDR strains than in their parent strains (P = 0.033). A H pylori LZ1026 mutant was successfully constructed and the ΔH pylori LZ1026 was more susceptible to four of the 10 antibiotics. All the 20 strains displayed transcripts for hefA that confirmed the in vitro expression of these genes.

CONCLUSION: The efflux pump gene hefA plays an important role in multidrug resistance of H pylori.

The Mycobacterium tuberculosis virulence factor trehalose dimycolate imparts desiccation resistance to model mycobacterial membranes

Mycobacteria, including persistent pathogens like Mycobacterium tuberculosis, have an unusual membrane structure in which, outside the plasma membrane, a nonfluid hydrophobic fatty acid layer supports a fluid monolayer rich in glycolipids such as trehalose 6,6'-dimycolate (TDM; cord factor). Given the abilities of mycobacteria to survive desiccation and trehalose in solution to protect biomolecules and whole organisms during freezing, drying, and other stresses, we hypothesized that TDM alone may suffice to confer dehydration resistance to the membranes of which it is a constituent. We devised an experimental model that mimics the structure of mycobacterial envelopes in which an immobile hydrophobic layer supports a TDM-rich, two-dimensionally fluid leaflet. We have found that TDM monolayers, in stark contrast to phospholipid membranes, can be dehydrated and rehydrated without loss of integrity, as assessed by fluidity and protein binding. Strikingly, this protection from dehydration extends to TDM-phospholipid mixtures with as little as 25 mol % TDM. The dependence of the recovery of membrane mobility upon rehydration on TDM fraction shows a functional form indicative of spatial percolation, implying that the connectivity of TDM plays a crucial role in membrane preservation. Our observations are the first reported instance of dehydration resistance provided by a membrane glycolipid.

Synthesis and antibacterial activity of 4,5,6,7-tetrahydro-thieno[3,2-c]pyridine quinolones

Synthesis and antibacterial activity of a number of substituted 4,5,6,7-tetrahydro-thieno[3,2-c]pyridine quinolones is reported. The antibacterial activities were evaluated in standard in vitro MIC assay method. Some of the compounds showed in vitro (MIC) antibacterial activity comparable to those of Gatifloxacin, Ciprofloxacin, and Sparfloxacin.

Tuberculosis chemotherapy: the influence of bacillary stress and damage response pathways on drug efficacy

The global tuberculosis (TB) control effort is focused on interrupting transmission of the causative agent, Mycobacterium tuberculosis, through chemotherapeutic intervention in active infectious disease. The insufficiency of this approach is manifest in the inexorable annual increase in TB infection and mortality rates and the emergence of multidrug-resistant isolates. Critically, the limited efficacy of the current frontline anti-TB drug combination suggests that heterogeneity of host and bacillary physiologies might impair drug activity. This review explores the possibility that strategies enabling adaptation of M. tuberculosis to hostile in vivo conditions might contribute to the subversion of anti-TB chemotherapy. In particular, evidence that infecting bacilli are exposed to environmental and host immune-mediated DNA-damaging insults suggests a role for error-prone DNA repair synthesis in the generation of chromosomally encoded antibiotic resistance mutations. The failure of frontline anti-TB drugs to sterilize a population of susceptible bacilli is independent of genetic resistance, however, and instead implies the operation of alternative tolerance mechanisms. Specifically, it is proposed that the emergence of persister subpopulations might depend on the switch to an altered metabolic state mediated by the stringent response alarmone, (p)ppGpp, possibly involving some or all of the many toxin-antitoxin modules identified in the M. tuberculosis genome.

AsnB is involved in natural resistance of Mycobacterium smegmatis to multiple drugs

Mycobacteria are naturally resistant to most common antibiotics and chemotherapeutic agents. The underlying molecular mechanisms are not fully understood. In this paper, we describe a hypersensitive mutant of Mycobacterium smegmatis, MS 2-39, which was isolated by screening for transposon insertion mutants of M. smegmatis mc2155 that exhibit increased sensitivity to rifampin, erythromycin, or novobiocin. The mutant MS 2-39 exhibited increased sensitivity to all three of the above mentioned antibiotics as well as fusidic acid, but its sensitivity to other antibiotics, including isoniazid, ethambutol, streptomycin, chloramphenicol, norfloxacin, tetracycline, and beta-lactams, remained unchanged. Uptake experiment with hydrophobic agents and cell wall lipid analysis suggest that the mutant cell wall is normal. The transposon insertion was localized within the asnB gene, which is predicted to encode a glutamine-dependent asparagine synthetase. Transformation of the mutant with wild-type asnB of mc2155 or asnB of Mycobacterium tuberculosis complemented the drug sensitivity phenotype. These results suggest that AsnB plays a role in the natural resistance of mycobacteria.

Role of mycobacterial efflux transporters in drug resistance: an unresolved question

Two mechanisms are thought to be involved in the natural drug resistance of mycobacteria: the mycobacterial cell wall permeability barrier and active multidrug efflux pumps. Genes encoding drug efflux transporters have been isolated from several mycobacterial species. These proteins transport tetracycline, fluoroquinolones, aminoglycosides and other compounds. Recent reports have suggested that efflux pumps may also be involved in transporting isoniazid, one of the main drugs used to treat tuberculosis. This review highlights recent advances in our understanding of efflux-mediated drug resistance in mycobacteria, including the distribution of efflux systems in these organisms, their substrate profiles and their contribution to drug resistance. The balance between the drug transport into the cell and drug efflux is not yet clearly understood, and further studies are required in mycobacteria.

Evaluation of antimycobacterial and DNA gyrase inhibition of fluoroquinolone derivatives

The antimycobacterial activity (both in vitro and in vivo) and DNA gyrase inhibition of newly synthesized fluoroquinolone derivatives were tested against Mycobacterium tuberculosis H(37)Rv and Mycobacterium smegmatis, respectively. Among the synthesized compounds, compound F11 was found to exhibit the most potent in vitro antimycobacterial activity with a MIC value of 0.78 microg/ml, and a selectivity index of more than 80 while not being cytotoxic to the Vero cell line up to 62.5 microg/ml. When evaluated for in vivo antimycobacterial activity, compound F11 demonstrated a paramount decrease of bacterial load in lung and spleen tissues compared to the control and better than the standard drug ciprofloxacin.

Trends in fluoroquinolone resistance of Mycobacterium tuberculosis complex in a Taiwanese medical centre: 1995-2003

OBJECTIVES

Fluoroquinolones are being used more frequently for the treatment of multidrug-resistant (MDR) strains of Mycobacterium tuberculosis complex (MTB). This study was designed to determine the frequency of the emergence of fluoroquinolone-resistant strains in Taiwan and to assess whether this might be due to use of fluoroquinolones for treatment of patients with MDR or because of increased use of fluoroquinolones in the community for treatment of other infections. We also sought to determine whether there might be clonal spread of fluoroquinolone resistance.

METHODS

A total of 3497 clinical isolates of M. tuberculosis complex were obtained during 1995-2003, of which 141 were selected. They consisted of 62 isolates fully susceptible to four first-line drugs, 33 isolates resistant to rifampicin and isoniazid (MDR), and 46 isolates with a variety of any drug resistant patterns other than MDR (combination group). The MICs were determined for ciprofloxacin, ofloxacin and levofloxacin.

RESULTS

An increase in the MIC90 and rates of resistance to ciprofloxacin, ofloxacin and levofloxacin were noted only in the MDR group. The rates were higher among strains isolated between 1998-2003 compared with those obtained between 1995-1997 (rate of resistance, 20% versus 7.7%; MIC > or = 4 mg/L versus 1-2 mg/L). Among the 10 fluoroquinolone-resistant isolates, five (50%) possessed mutations other than S95T in the gyrA gene. No gyrB mutation was found in any of the clinical isolates.

CONCLUSIONS

These findings suggest that fluoroquinolone resistance is the result of treatment of patients with MDR strains rather than from use in the general community in Taiwan. The emergence of fluoroquinolone resistance among MDR strains reinforces the need for routine fluoroquinolone susceptibility testing whenever these drugs might be used.

The Mycobacterium tuberculosis iniA gene is essential for activity of an efflux pump that confers drug tolerance to both isoniazid and ethambutol

Little is known about the intracellular events that occur following the initial inhibition of Mycobacterium tuberculosis by the first-line antituberculosis drugs isoniazid (INH) and ethambutol (EMB). Understanding these pathways should provide significant insights into the adaptive strategies M. tuberculosis undertakes to survive antibiotics. We have discovered that the M. tuberculosis iniA gene (Rv 0342) participates in the development of tolerance to both INH and EMB. This gene is strongly induced along with iniB and iniC (Rv 0341 and Rv 0343) by treatment of Mycobacterium bovis BCG or M. tuberculosis with INH or EMB. BCG strains overexpressing M. tuberculosis iniA grew and survived longer than control strains upon exposure to inhibitory concentrations of either INH or EMB. An M. tuberculosis strain containing an iniA deletion showed increased susceptibility to INH. Additional studies showed that overexpression of M. tuberculosis iniA in BCG conferred resistance to ethidium bromide, and the deletion of iniA in M. tuberculosis resulted in increased accumulation of intracellular ethidium bromide. The pump inhibitor reserpine reversed both tolerance to INH and resistance to ethidium bromide in BCG. These results suggest that iniA functions through an MDR-pump like mechanism, although IniA does not appear to directly transport INH from the cell. Analysis of two-dimensional crystals of the IniA protein revealed that this predicted transmembrane protein forms multimeric structures containing a central pore, providing further evidence that iniA is a pump component. Our studies elucidate a potentially unique adaptive pathway in mycobacteria. Drugs designed to inhibit the iniA gene product may shorten the time required to treat tuberculosis and may help prevent the clinical emergence of drug resistance.