Inhibition of potassium transport and growth of mycobacteria exposed to clofazimine and B669 is associated with a calcium-independent increase in microbial phospholipase A2 activity

Investigation of ternary Zn-Co-Fe layered double hydroxide as a multifunctional 2D layered adsorbent for moxifloxacin and antifungal disinfection

Layered double hydroxides have recently gained wide interest as promising multifunctional nanomaterials. In this work, a multifunctional ternary Zn-Co-Fe LDH was prepared and characterized using XRD, FTIR, BET, TEM, SEM, and EDX. This LDH showed a typical XRD pattern with a crystallite size of 3.52 nm and a BET surface area of 155.9 m2/g. This LDH was investigated, for the first time, as an adsorbent for moxifloxacin, a common fluoroquinolones antibiotic, showing a maximum removal efficiency and equilibrium time of 217.81 mg/g and 60 min, respectively. Its antifungal activity, for the first time, was investigated against Penicillium notatum, Aspergillus flavus, Aspergillus fumigatus, Aspergillus niger, and Mucor fungi at various concentrations (1000-1.95 µg/mL). This LDH was found to be effective against a variety of fungal strains, particularly Penicillium and Mucor species and showed zones of inhibition of 19.3 and 21.6 mm for Penicillium and Mucor, respectively, with an inhibition of 85% for Penicillium species and 68.3% for Mucormycosis. The highest antifungal efficacy results were obtained at very low MIC concentrations (33.3 and 62 µg/ml) against Penicillium and Mucor, respectively. The results of this study suggest a promising multifunctional potential of this LDH for water and wastewater treatment and disinfection applications.

Clofazimine for the treatment of tuberculosis

Shorter (6-9 months), fully oral regimens containing new and repurposed drugs are now the first-choice option for the treatment of drug-resistant tuberculosis (DR-TB). Clofazimine, long used in the treatment of leprosy, is one such repurposed drug that has become a cornerstone of DR-TB treatment and ongoing trials are exploring novel, shorter clofazimine-containing regimens for drug-resistant as well as drug-susceptible tuberculosis. Clofazimine's repurposing was informed by evidence of potent activity against DR-TB strains in vitro and in mice and a treatment-shortening effect in DR-TB patients as part of a multidrug regimen. Clofazimine entered clinical use in the 1950s without the rigorous safety and pharmacokinetic evaluation which is part of modern drug development and current dosing is not evidence-based. Recent studies have begun to characterize clofazimine's exposure-response relationship for safety and efficacy in populations with TB. Despite being better tolerated than some other second-line TB drugs, the extent and impact of adverse effects including skin discolouration and cardiotoxicity are not well understood and together with emergent resistance, may undermine clofazimine use in DR-TB programmes. Furthermore, clofazimine's precise mechanism of action is not well established, as is the genetic basis of clofazimine resistance. In this narrative review, we present an overview of the evidence base underpinning the use and limitations of clofazimine as an antituberculosis drug and discuss advances in the understanding of clofazimine pharmacokinetics, toxicity, and resistance. The unusual pharmacokinetic properties of clofazimine and how these relate to its putative mechanism of action, antituberculosis activity, dosing considerations and adverse effects are highlighted. Finally, we discuss the development of novel riminophenazine analogues as antituberculosis drugs.

OXSR1 inhibits inflammasome activation by limiting potassium efflux during mycobacterial infection

Mycobacteria up-regulate host kinase OXSR1 preventing potassium efflux and inflammasome activation. Depletion or inhibition of OXSR1 potentiates inflammasome activation and decreases bacterial burden.

Pathogenic mycobacteria inhibit inflammasome activation to establish infection. Although it is known that potassium efflux is a trigger for inflammasome activation, the interaction between mycobacterial infection, potassium efflux, and inflammasome activation has not been investigated. Here, we use Mycobacterium marinum infection of zebrafish embryos and Mycobacterium tuberculosis infection of THP-1 cells to demonstrate that pathogenic mycobacteria up-regulate the host WNK signalling pathway kinases SPAK and OXSR1 which control intracellular potassium balance. We show that genetic depletion or inhibition of OXSR1 decreases bacterial burden and intracellular potassium levels. The protective effects of OXSR1 depletion are at least partially mediated by NLRP3 inflammasome activation, caspase-mediated release of IL-1β, and downstream activation of protective TNF-α. The elucidation of this druggable pathway to potentiate inflammasome activation provides a new avenue for the development of host-directed therapies against intracellular infections.

Inactivation of a New Potassium Channel Increases Rifampicin Resistance and Induces Collateral Sensitivity to Hydrophilic Antibiotics in Mycobacterium smegmatis

Rifampicin is a critical first-line antibiotic for treating mycobacterial infections such as tuberculosis, one of the most serious infectious diseases worldwide. Rifampicin resistance in mycobacteria is mainly caused by mutations in the rpoB gene; however, some rifampicin-resistant strains showed no rpoB mutations. Therefore, alternative mechanisms must explain this resistance in mycobacteria. In this work, a library of 11,000 Mycobacterium smegmatis mc² 155 insertion mutants was explored to search and characterize new rifampicin-resistance determinants. A transposon insertion in the MSMEG_1945 gene modified the growth rate, pH homeostasis and membrane potential in M. smegmatis, producing rifampicin resistance and collateral susceptibility to other antitubercular drugs such as isoniazid, ethionamide and aminoglycosides. Our data suggest that the M. smegmatis MSMEG_1945 protein is an ion channel, dubbed MchK, essential for maintaining the cellular ionic balance and membrane potential, modulating susceptibility to antimycobacterial agents. The functions of this new gene point once again to potassium homeostasis impairment as a proxy to resistance to rifampicin. This study increases the known repertoire of mycobacterial ion channels involved in drug susceptibility/resistance to antimycobacterial drugs and suggests novel intervention opportunities, highlighting ion channels as druggable pathways.

Investigation of Clofazimine Resistance and Genetic Mutations in Drug-Resistant Mycobacterium tuberculosis Isolates

Recently, as clofazimine (CFZ) showed a good therapeutic effect in treating multi-drug-resistant tuberculosis (MDR-TB), the anti-tuberculosis activity and resistance were re-focused. Here, we investigated the CFZ resistance and genetic mutations of drug-resistant Mycobacterium tuberculosis (DR-Mtb) isolates to improve the diagnosis and treatment of drug-resistant TB patients. The minimal inhibitory concentration (MIC) of CFZ was examined by resazurin microtiter assay (REMA) with two reference strains and 122 clinical isolates from Korea. The cause of CFZ resistance was investigated in relation to the therapeutic history of patients. Mutations of Rv0678, Rv1979c and pepQ of CFZ resistant isolates were analyzed by PCR and DNA sequencing. The rate of CFZ resistance with MIC > 1 mg/L was 4.1% in drug-resistant Mtb isolates. The cause of CFZ resistance was not related to treatment with CFZ or bedaquiline. A CFZ susceptibility test should be conducted regardless of dugs use history. The four novel mutation sites were identified in the Rv0678 and pepQ genes related to CFZ resistance in this study.

Assessment of the efficacy of clofazimine alone and in combination with primary agents against Mycobacterium tuberculosis in vitro

OBJECTIVES

The chemotherapeutic regimens of drug-susceptible (DS)-tuberculosis (TB) patients comprise four primary anti-TB drugs; rifampicin (RMP), isoniazid (INH), ethambutol (EMB) and pyrazinamide (PZA), administered for six-to-nine months. These drug regimens target the various microbial populations that include actively-replicating (AR), slow-replicating (SR) and non-replicating (NR) organisms. Clofazimine (CFZ) has showed benefit in shortening DS-TB treatment in vivo from six to four months when used in combination with this regimen in murine models of experimental infection. However, its antimicrobial efficacy when used in combination with the primary drugs against the various microbial populations of Mycobacterium tuberculosis has not been demonstrated.

METHODS

In the current in vitro study, the inhibitory and bactericidal activities of CFZ in combination with the primary anti-TB drugs, RMP, INH and EMB against the AR and SR organisms in planktonic and biofilm-forming cultures, respectively, were evaluated by fractional inhibitory concentration index (FICI) and fractional bactericidal concentration index (FBCI) determinations, using the Loewe Additivity Model.

RESULTS

In planktonic cultures, CFZ demonstrated synergistic growth inhibitory activity in combination with RMP and INH individually and collectively. With respect to bactericidal activity, CFZ exhibited synergistic activity only in a two-drug combination with RMP. However, in biofilm-forming cultures, all CFZ-containing anti-TB drug combinations exhibited synergistic inhibitory and bactericidal effects, particularly in combination with RIF and INH.

CONCLUSION

Clofazimine exhibited synergistic effects in combination with primary anti-TB drugs against both planktonic and biofilm-forming cultures, showing potential benefit in augmenting treatment outcome when used during standard TB chemotherapy.

Functionalized Dioxonaphthoimidazoliums: A Redox Cycling Chemotype with Potent Bactericidal Activities against Mycobacterium tuberculosis

Disruption of redox homeostasis in mycobacteria causes irreversible stress induction and cell death. Here, we report the dioxonaphthoimidazolium scaffold as a novel redox cycling antituberculosis chemotype with potent bactericidal activity against growing and nutrient-starved phenotypically drug-resistant nongrowing bacteria. Maximal potency was dependent on the activation of the redox cycling quinone by the positively charged scaffold and accessibility to the mycobacterial cell membrane as directed by the lipophilicity and conformational characteristics of the N-substituted side chains. Evidence from microbiological, biochemical, and genetic investigations implicates a redox-driven mode of action that is reliant on the reduction of the quinone by type II NADH dehydrogenase (NDH2) for the generation of bactericidal levels of the reactive oxygen species (ROS). The bactericidal profile of a potent water-soluble analogue 32 revealed good activity against nutrient-starved organisms in the Loebel model of dormancy, low spontaneous resistance mutation frequency, and synergy with isoniazid in the checkerboard assay.

Role of the kdpDE Regulatory Operon of Mycobacterium tuberculosis in Modulating Bacterial Growth in vitro

Bacteria use K⁺-uptake transporters differentially for adaptation in varying growth conditions. In Mycobacterium tuberculosis, two K⁺-uptake systems, the Trk comprising the CeoB and CeoC proteins and the Kdp consisting of the two-component system (TCS), KdpDE and KdpFABC, have been characterized, but their selective utilization during bacterial growth has not been completely explored. In the current study, the roles of the M. tuberculosis KdpDE regulatory system alone and in association with the Trk transporters in bacterial growth were investigated by evaluating the growth of M. tuberculosis KdpDE-deletion and KdpDE/Trk (KT)-double knockout mutant strains in planktonic culture under standard growth conditions. The KT-double knockout mutant strain was first constructed using homologous recombination procedures and was evaluated together with the KdpDE-deletion mutant and the wild-type (WT) strains with respect to their rates of growth, K⁺-uptake efficiencies, and K⁺-transporter gene expression during planktonic growth. During growth at optimal K⁺ concentrations and pH levels, selective deletion of the TCS KdpDE (KdpDE-deletion mutant) led to attenuation of bacterial growth and an increase in bacterial K⁺-uptake efficiency, as well as dysregulated expression of the kdpFABC and trk genes. Deletion of both the KdpDE and the Trk systems (KT-double knockout) also led to severely attenuated bacterial growth, as well as an increase in bacterial K⁺-uptake efficiency. These results demonstrate that the KdpDE regulatory system plays a key role during bacterial growth by regulating K⁺ uptake via modulation of the expression and activities of both the KdpFABC and Trk systems and is important for bacterial growth possibly by preventing cytoplasmic K⁺ overload.

Repurposing Immunomodulatory Drugs to Combat Tuberculosis

Tuberculosis (TB) is an infectious disease caused by an obligate intracellular pathogen, Mycobacterium tuberculosis (M.tb) and is responsible for the maximum number of deaths due to a single infectious agent. Current therapy for TB, Directly Observed Treatment Short-course (DOTS) comprises multiple antibiotics administered in combination for 6 months, which eliminates the bacteria and prevents the emergence of drug-resistance in patients if followed as prescribed. However, due to various limitations viz., severe toxicity, low efficacy and long duration; patients struggle to comply with the prescribed therapy, which leads to the development of drug resistance (DR). The emergence of resistance to various front-line anti-TB drugs urgently require the introduction of new TB drugs, to cure DR patients and to shorten the treatment course for both drug-susceptible and resistant populations of bacteria. However, the development of a novel drug regimen involving 2-3 new and effective drugs will require approximately 20-30 years and huge expenditure, as seen during the discovery of bedaquiline and delamanid. These limitations make the field of drug-repurposing indispensable and repurposing of pre-existing drugs licensed for other diseases has tremendous scope in anti-DR-TB therapy. These repurposed drugs target multiple pathways, thus reducing the risk of development of drug resistance. In this review, we have discussed some of the repurposed drugs that have shown very promising results against TB. The list includes sulfonamides, sulfanilamide, sulfadiazine, clofazimine, linezolid, amoxicillin/clavulanic acid, carbapenems, metformin, verapamil, fluoroquinolones, statins and NSAIDs and their mechanism of action with special emphasis on their immunomodulatory effects on the host to attain both host-directed and pathogen-targeted therapy. We have also focused on the studies involving the synergistic effect of these drugs with existing TB drugs in order to translate their potential as adjunct therapies against TB.

A small molecule that mitigates bacterial infection disrupts Gram-negative cell membranes and is inhibited by cholesterol and neutral lipids

Infections caused by Gram-negative bacteria are difficult to fight because these pathogens exclude or expel many clinical antibiotics and host defense molecules. However, mammals have evolved a substantial immune arsenal that weakens pathogen defenses, suggesting the feasibility of developing therapies that work in concert with innate immunity to kill Gram-negative bacteria. Using chemical genetics, we recently identified a small molecule, JD1, that kills Salmonella enterica serovar Typhimurium (S. Typhimurium) residing within macrophages. JD1 is not antibacterial in standard microbiological media, but rapidly inhibits growth and curtails bacterial survival under broth conditions that compromise the outer membrane or reduce efflux pump activity. Using a combination of cellular indicators and super resolution microscopy, we found that JD1 damaged bacterial cytoplasmic membranes by increasing fluidity, disrupting barrier function, and causing the formation of membrane distortions. We quantified macrophage cell membrane integrity and mitochondrial membrane potential and found that disruption of eukaryotic cell membranes required approximately 30-fold more JD1 than was needed to kill bacteria in macrophages. Moreover, JD1 preferentially damaged liposomes with compositions similar to E. coli inner membranes versus mammalian cell membranes. Cholesterol, a component of mammalian cell membranes, was protective in the presence of neutral lipids. In mice, intraperitoneal administration of JD1 reduced tissue colonization by S. Typhimurium. These observations indicate that during infection, JD1 gains access to and disrupts the cytoplasmic membrane of Gram-negative bacteria, and that neutral lipids and cholesterol protect mammalian membranes from JD1-mediated damage. Thus, it may be possible to develop therapeutics that exploit host innate immunity to gain access to Gram-negative bacteria and then preferentially damage the bacterial cell membrane over host membranes.

Drug targets exploited in Mycobacterium tuberculosis: Pitfalls and promises on the horizon

Tuberculosis is an ever evolving infectious disease that still claims about 1.8 million human lives each year around the globe. Although modern chemotherapy has played a pivotal role in combating TB, the increasing emergence of drug-resistant TB aligned with HIV pandemic threaten its control. This highlights both the need to understand how our current drugs work and the need to develop new and more effective drugs. TB drug discovery is revisiting the clinically validated drug targets in Mycobacterium tuberculosis using whole-cell phenotypic assays in search of better therapeutic scaffolds. Herein, we review the promises of current TB drug regimens, major pitfalls faced, key drug targets exploited so far in M. tuberculosis along with the status of newly discovered drugs against drug resistant forms of TB. New antituberculosis regimens that use lesser number of drugs, require shorter duration of treatment, are equally effective against susceptible and resistant forms of disease, have acceptable toxicity profiles and behave friendly with anti-HIV regimens remains top most priority in TB drug discovery.

Repurposing and Revival of the Drugs: A New Approach to Combat the Drug Resistant Tuberculosis

Emergence of drug resistant tuberculosis like multi drug resistant tuberculosis (MDR-TB), extensively drug-resistant tuberculosis (XDR-TB) and totally drug resistant tuberculosis (TDR-TB) has created a new challenge to fight against these bad bugs of Mycobacterium tuberculosis. Repurposing and revival of the drugs are the new trends/options to combat these worsen situations of tuberculosis in the antibiotics resistance era or in the situation of global emergency. Bactericidal and synergistic effect of repurposed/revived drugs along with the latest drugs bedaquiline and delamanid used in the treatment of MDR-TB, XDR-TB, and TDR-TB might be the choice for future promising combinatorial chemotherapy against these bad bugs.

Susceptibilities of MDR Mycobacterium tuberculosis isolates to unconventional drugs compared with their reported pharmacokinetic/pharmacodynamic parameters

Background

The second-line drugs recommended to treat drug-resistant TB are toxic, expensive and difficult to procure. Given increasing resistance, the need for additional anti-TB drugs has become more urgent. But new drugs take time to develop and are expensive. Some commercially available drugs have reported anti-mycobacterial activity but are not routinely used because supporting laboratory and clinical evidence is sparse.

Methods

We analysed 217 MDR M. tuberculosis isolates including 153 initial isolates from unique patients and 64 isolates from follow-up specimens during the course of treatment. The resazurin microdilution assay was performed to determine MICs of trimethoprim/sulfamethoxazole, mefloquine, thioridazine, clofazimine, amoxicillin/clavulanate, meropenem/clavulanate, nitazoxanide, linezolid and oxyphenbutazone. Isoniazid was used for validation. We calculated the MIC 50 and MIC 90 as the MICs at which growth of 50% and 90% of isolates was inhibited, respectively.

Results

The MIC 50 s, in mg/L, for initial isolates were as follows: trimethoprim/sulfamethoxazole, 0.2/4; mefloquine, 8; thioridazine, 4; clofazimine, 0.25; amoxicillin/clavulanate, 16/8; meropenem/clavulanate, 1/2.5; nitazoxanide, 16; linezolid, 0.25; and oxyphenbutazone, 40. The MIC 90 s, in mg/L, for initial isolates were as follows: trimethoprim/sulfamethoxazole, 0.4/8; mefloquine, 8; thioridazine, 8; clofazimine, 0.5; amoxicillin/clavulanate, 32/16; meropenem/clavulanate, 8/2.5; nitazoxanide, 16; linezolid, 0.25; and oxyphenbutazone, 60. By comparison, the MIC 90 of isoniazid was >4 mg/L, as expected. There was no evidence that previous treatment affected susceptibility to any drug.

Conclusions

Most drugs demonstrated efficacy against M. tuberculosis . When these MICs are compared with the published pharmacokinetic/pharmacodynamic profiles of the respective drugs in humans, trimethoprim/sulfamethoxazole, meropenem/clavulanate, linezolid, clofazimine and nitazoxanide appear promising and warrant further clinical investigation.

Mechanisms of action and therapeutic efficacies of the lipophilic antimycobacterial agents clofazimine and bedaquiline

Drug-resistant (DR)-TB is the major challenge confronting the global TB control programme, necessitating treatment with second-line anti-TB drugs, often with limited therapeutic efficacy. This scenario has resulted in the inclusion of Group 5 antibiotics in various therapeutic regimens, two of which promise to impact significantly on the outcome of the therapy of DR-TB. These are the 're-purposed' riminophenazine, clofazimine, and the recently approved diarylquinoline, bedaquiline. Although they differ structurally, both of these lipophilic agents possess cationic amphiphilic properties that enable them to target and inactivate essential ion transporters in the outer membrane of Mycobacterium tuberculosis. In the case of bedaquiline, the primary target is the key respiratory chain enzyme F₁/F₀-ATPase, whereas clofazimine is less selective, apparently inhibiting several targets, which may underpin the extremely low level of resistance to this agent. This review is focused on similarities and differences between clofazimine and bedaquiline, specifically in respect of molecular mechanisms of antimycobacterial action, targeting of quiescent and metabolically active organisms, therapeutic efficacy in the clinical setting of DR-TB, resistance mechanisms, pharmacodynamics, pharmacokinetics and adverse events.

Targeting Kv1.3 channels to reduce white matter pathology after traumatic brain injury

Axonal injury is present in essentially all clinically significant cases of traumatic brain injury (TBI). While no effective treatment has been identified to date, experimental TBI models have shown promising axonal protection using immunosuppressants FK506 and Cyclosporine-A, with treatment benefits attributed to calcineurin inhibition or protection of mitochondrial function. However, growing evidence suggests neuroprotective efficacy of these compounds may also involve direct modulation of ion channels, and in particular Kv1.3. The present study tested whether blockade of Kv1.3 channels, using Clofazimine (CFZ), would alleviate TBI-induced white matter pathology in rodents. Postinjury CFZ administration prevented suppression of compound action potential (CAP) amplitude in the corpus callosum of adult rats following midline fluid percussion TBI, with injury and treatment effects primarily expressed in unmyelinated CAPs. Kv1.3 protein levels in callosal tissue extracts were significantly reduced postinjury, but this loss was prevented by CFZ treatment. In parallel, CFZ also attenuated the injury-induced elevation in pro-inflammatory cytokine IL1-β. The effects of CFZ on glial function were further studied using mixed microglia/astrocyte cell cultures derived from P3-5 mouse corpus callosum. Cultures of callosal glia challenged with lipopolysaccharide exhibited a dramatic increase in IL1-β levels, accompanied by reactive morphological changes in microglia, both of which were attenuated by CFZ treatment. These results support a cell specific role for Kv1.3 signaling in white matter pathology after TBI, and suggest a treatment approach based on the blockade of these channels. This therapeutic strategy may be especially efficacious for normalizing neuro-glial interactions affecting unmyelinated axons after TBI.

The anti-tuberculosis agents under development and the challenges ahead

Tuberculosis (TB) is a serious health problem causing 1.5 million deaths worldwide. After the discovery of first-line anti-TB drugs, the mortality rate declined sharply, however, the emergence of drug-resistant strains and HIV co-infection have led to increased incidence of this disease. A number of new potential antitubercular drug candidates with novel modes of action have entered clinical trials in recent years. Compounds such as gatifloxacin, moxifloxacin and linezolid, the already known antibiotics are currently being evaluated for their anti-TB activity. OPC-67683 and TMC207 have been approved for the treatment of MDR-TB patients recently, while PA-824, SQ109, PNU-100480, AZD5847, LL3858, SQ609, SQ641, BTZ043, DC-159a, CPZEN-45, Q-203, DNB1, TBA-354 are in various phases of clinical and preclinical developments. This review evaluates the current status of TB drug development and future aspects.

Expression of the Genes Encoding the Trk and Kdp Potassium Transport Systems of Mycobacterium tuberculosis during Growth In Vitro

Two potassium (K(+))-uptake systems, Trk and Kdp, are operative in Mycobacterium tuberculosis (Mtb), but the environmental factors triggering their expression have not been determined. The current study has evaluated the expression of these genes in the Mtb wild-type and a trk-gene knockout strain at various stages of logarithmic growth in relation to extracellular K(+) concentrations and pH. In both strains, mRNA levels of the K(+)-uptake encoding genes were relatively low compared to those of the housekeeping gene, sigA, at the early- and mid-log phases, increasing during late-log. Increased gene expression coincided with decreased K(+) uptake in the context of a drop in extracellular pH and sustained high extracellular K(+) concentrations. In an additional series of experiments, the pH of the growth medium was manipulated by the addition of 1N HCl/NaOH. Decreasing the pH resulted in reductions in both membrane potential and K(+) uptake in the setting of significant induction of genes encoding both K(+) transporters. These observations are consistent with induction of the genes encoding the active K(+) transporters of Mtb as a strategy to compensate for loss of membrane potential-driven uptake of K(+) at low extracellular pH. Induction of these genes may promote survival in the acidic environments of the intracellular vacuole and granuloma.

The membrane as a target for controlling hypervirulent Clostridium difficile infections

OBJECTIVES

The stationary phase of Clostridium difficile, which is primarily responsible for diarrhoeal symptoms, is refractory to antibiotic killing. We investigated whether disrupting the functions of the clostridial membrane is an approach to control C. difficile infections by promptly removing growing and non-growing cells.

METHODS

The bactericidal activities of various membrane-active agents were determined against C. difficile logarithmic-phase and stationary-phase cultures and compared with known antibiotics. Their effects on the synthesis of ATP, toxins A/B and sporulation were also determined. The effect of rodent caecal contents on anti-difficile activities was examined using two reutericyclin lead compounds, clofazimine, daptomycin and other comparator antibiotics.

RESULTS

Most membrane-active agents and partially daptomycin showed concentration-dependent killing of both logarithmic-phase and stationary-phase cultures. The exposure of cells to compounds at their MBC resulted in a rapid loss of viability with concomitant reductions in cellular ATP, toxins A/B and spore numbers. With the exception of nisin, these effects were not due to membrane pore formation. Interestingly, the activity of the proton ionophore nigericin significantly increased as the growth of C. difficile decreased, suggesting the importance of the proton gradient to the survival of non-growing cells. The activities of the lipophilic antimicrobials reutericyclins and clofazimine were reduced by caecal contents.

CONCLUSIONS

These findings indicate that C. difficile is uniquely susceptible to killing by molecules affecting its membrane function and bioenergetics, indicating that the clostridial membrane is a novel antimicrobial target for agents to alleviate the burden of C. difficile infections.

Streptomycin-starved Mycobacterium tuberculosis 18b, a drug discovery tool for latent tuberculosis

Mycobacterium tuberculosis 18b, a streptomycin (STR)-dependent mutant that enters a viable but nonreplicating state in the absence of STR, has been developed as a simple model for drug testing against dormant bacilli. Here, we further evaluated the STR-starved 18b (SS18b) model both in vitro and in vivo by comparing the behavior of 22 approved and experimental tuberculosis drugs. Using the resazurin reduction microplate assay (REMA), rifampin (RIF), rifapentine (RPT), TMC207, clofazimine (CFM), and linezolid (LIN) were found to be active against SS18b in vitro, and their bactericidal activity was confirmed by determining the number of CFU. A latent 18b infection was established in mice, and some of the above-mentioned drugs were used for treatment, either alone or in combination with RIF. RIF, RPT, TMC207, CFM, and pyrazinamide (PZA) were all active in vivo, while cell wall inhibitors were not. A comparative kinetic study of rifamycin efficacy was then undertaken, and the results indicated that RPT clears latent 18b infection in mice faster than RIF. Intrigued by the opposing responses of live and dormant 18b cells to cell wall inhibitors, we conducted a systematic analysis of 14 such inhibitors using REMA. This uncovered an SS18b signature (CWPRED) that accurately predicted the activities of cell wall inhibitors and performed well in a blind study. CWPRED will be useful for establishing the mode of action of compounds with unknown targets, while the SS18b system should facilitate the discovery of drugs for treating latent tuberculosis.

Modulated phases of phospholipid bilayers induced by tocopherols

The influence of α-, γ- and δ-tocopherols on the structure and phase behavior of dipalmitoyl phosphatidylcholine (DPPC) bilayers has been determined from X-ray diffraction studies on oriented multilayers. In all the three cases the main-transition temperature (T(m)) of DPPC was found to decrease with increasing tocopherol concentration up to around 25 mol%. Beyond this the main transition is suppressed in the case of γ-tocopherol, whereas T(m) becomes insensitive to composition in the other two cases. The pre-transition is found to be suppressed over a narrow tocopherol concentration range between 7.5 and 10 mol% in DPPC-γ-tocopherol and DPPC-δ-tocopherol bilayers, and the ripple phase occurs down to the lowest temperature studied. In all the three cases a modulated phase is observed above a tocopherol concentration of about 10 mol%, which is similar to the P(β) phase reported in DPPC-cholesterol bilayers. This phase is found to occur even in excess water conditions at lower tocopherol concentrations, and consists of bilayers with periodic height modulation. These results indicate the ability of tocopherols to induce local curvature in membranes, which could be important for some of their biological functions.

Clofazimine: current status and future prospects

Clofazimine, a lipophilic riminophenazine antibiotic, possesses both antimycobacterial and anti-inflammatory activities. However, its efficacy has been demonstrated only in the treatment of leprosy, not in human tuberculosis, despite the fact that this agent is impressively active in vitro against multidrug-resistant strains of Mycobacterium tuberculosis. Recent insights into novel targets and mechanisms of antimicrobial and anti-inflammatory activity coupled with the acquisition of innovative drug delivery technologies have, however, rekindled interest in clofazimine as a potential therapy for multidrug- and extensively multidrug-resistant tuberculosis in particular, as well as several autoimmune diseases. The primary objective of this review is to critically evaluate these recent developments and to assess their potential impact on improving the therapeutic efficacy and versatility of clofazimine.

Targeting bacterial membrane function: an underexploited mechanism for treating persistent infections

Persistent infections involving slow-growing or non-growing bacteria are hard to treat with antibiotics that target biosynthetic processes in growing cells. Consequently, there is a need for antimicrobials that can treat infections containing dormant bacteria. In this Review, we discuss the emerging concept that disrupting the bacterial membrane bilayer or proteins that are integral to membrane function (including membrane potential and energy metabolism) in dormant bacteria is a strategy for treating persistent infections. The clinical applicability of these approaches is exemplified by the efficacy of lipoglycopeptides that damage bacterial membranes and of the diarylquinoline TMC207, which inhibits membrane-bound ATP synthase. Despite some drawbacks, membrane-active agents form an important new means of eradicating recalcitrant, non-growing bacteria.

Tocopherols and tocotrienols in membranes: a critical review

The familiar role of tocols (tocopherols and tocotrienols) as lipid-soluble chain-terminating inhibitors of lipid peroxidation is currently in the midst of a reinterpretation. New biological activities have been described for tocols that apparently are not dependent on their well-established antioxidant behaviour. These activities could well be real, but there remain large gaps in our understanding of the behaviour of tocols in membranes, especially when it comes to the alpha-, beta-, gamma-, delta-chroman methylation patterns and the seemingly special nature of tocotrienols. It is inappropriate to make conclusions and develop models based on in vivo (or cell culture) results with reference to in vitro measurements of antioxidant activity. When present in biological membranes, tocols will experience a large variation in the local composition of phospholipids and the presence of neutral lipids such as cholesterol, both of which would be expected to change the efficiency of antioxidant action. It is likely that tocols are not homogeneously dispersed in a membrane, but it is still not known whether any specific combination of lipid head group and acyl chains are conferred special protection from peroxidation, nor do we currently appreciate the structural role that tocols play in membranes. Tocols may enhance curvature stress or counteract similar stresses generated by other lipids such as lysolipids. This review will outline what is known about the location and behaviour of tocols in phospholipid bilayers. We will draw mainly from the biophysical literature, but will attempt to extend the discussion to biologically relevant phenomena when appropriate. We hope that it will assist researchers when designing new experiments and when critically assessing the results, in turn providing a more thorough understanding of the biochemistry of tocols.

Purification and characterization of mycobacterial phospholipase A: an activity associated with mycobacterial cutinase

We describe mycobacterial phospholipase A activity (MPLA) and, using reverse genetics, have associated this activity with putative mycobacterial cutinase. PLAs, which hydrolyze fatty acids on phospholipids, play a significant role in human inflammatory states and disease pathogenesis. In prokaryotes, the recognition of their role in virulence is more recent. Cutinases are serine esterases whose primary substrate is cutin, the waxy exterior layer of plants. Mycobacterium tuberculosis has maintained seven putative cutinases, though it should not encounter cutin; we demonstrate that known cutinases and MPLA cleave phospholipids in a PLA-type manner and also hydrolyze Tween. We analyzed cutinase motifs in mycobacteria and found the motif very prevalent. All mycobacteria tested had MPLA activity. These studies suggest an alternative use for putative cutinases by the M. tuberculosis group that is likely related to MPLA activity and lipid metabolism.

Antituberculosis drugs: ten years of research

Tuberculosis is today amongst the worldwide health threats. As resistant strains of Mycobacterium tuberculosis have slowly emerged, treatment failure is too often a fact, especially in countries lacking the necessary health care organisation to provide the long and costly treatment adapted to patients. Because of lack of treatment or lack of adapted treatment, at least two million people will die of tuberculosis this year. Due to this concern, this infectious disease was the focus of renewed scientific interest in the last decade. Regimens were optimized and much was learnt on the mechanisms of action of the antituberculosis drugs used. Moreover, the quest for original drugs overcoming some of the problems of current regimens also became the focus of research programmes and many new series of M. tuberculosis growth inhibitors were reported. This review presents the drugs currently used in antituberculosis treatments and the most advanced compounds undergoing clinical trials. We then provide a description of their mechanism of action along with other series of inhibitors known to act on related biochemical targets. This is followed by other inhibitors of M. tuberculosis growth, including recently reported compounds devoid of a reported mechanism of action.

Effects of clofazimine on potassium uptake by a Trk-deletion mutant of Mycobacterium tuberculosis

OBJECTIVES

This study was designed to investigate the effects of the membrane-active, anti-mycobacterial agent, clofazimine, on potassium (K+)-uptake by a mutant of Mycobacterium tuberculosis (MTB), in which the Trk system, the major K+ transporter of this microbial pathogen, had been selectively inactivated.

METHODS

The ceoB and ceoC genes of MTB, which encode the TrkA proteins, CeoB and CeoC, were deleted by homologous recombination, and the double-knockout mutant and wild-type strains compared with respect to K+ uptake and growth in the presence and absence of clofazimine (0.015-2.5 mg/L) using radioassay procedures.

RESULTS

Surprisingly, the magnitudes of K+ uptake and rate of growth of the ceoBC-knockout mutant were significantly (P < 0.05) greater than those of the wild-type strain, due, presumably, to induction of a back-up transporter. Exposure of both the wild-type strain and ceoBC-knockout mutant of MTB to clofazimine was accompanied by dose-related decreases in K+ uptake, as well as growth, which were of comparable magnitude for both strains.

CONCLUSIONS

These observations demonstrate that the major K+ transporter of MTB, Trk, as well as an uncharacterized inducible back-up system, is equally sensitive to the inhibitory actions of clofazimine.

The magic bullets and tuberculosis drug targets

Modern chemotherapy has played a major role in our control of tuberculosis. Yet tuberculosis still remains a leading infectious disease worldwide, largely owing to persistence of tubercle bacillus and inadequacy of the current chemotherapy. The increasing emergence of drug-resistant tuberculosis along with the HIV pandemic threatens disease control and highlights both the need to understand how our current drugs work and the need to develop new and more effective drugs. This review provides a brief historical account of tuberculosis drugs, examines the problem of current chemotherapy, discusses the targets of current tuberculosis drugs, focuses on some promising new drug candidates, and proposes a range of novel drug targets for intervention. Finally, this review addresses the problem of conventional drug screens based on inhibition of replicating bacilli and the challenge to develop drugs that target nonreplicating persistent bacilli. A new generation of drugs that target persistent bacilli is needed for more effective treatment of tuberculosis.

Antibiotic resistance in Staphylococcus aureus: concerns, causes and cures

Methicillin-resistant Staphylococcus aureus (MRSA) is currently a predominant and dangerous nosocomial pathogen. Unfortunately, infections caused by this organism are becoming more difficult to treat as further evolution of drug resistance occurs within the pathogen. Vancomycin has become the drug of choice for treating MRSA infections. However, treatment failures, adverse side effects and the emergence of vancomycin-resistant MRSA are leading to urgent requirements for alternative antiMRSA therapies. Linezolid is a new agent recently developed for Gram-positive infections, including MRSA. However, resistance to this drug is already developing. The need for superior antiMRSA drugs is therefore highly evident. This article explores some of the opportunities for the development of new treatments for MRSA. These arise principally from extensions of existing drug classes and the discovery of novel agents by genome-driven technologies. Other possibilities concern the re-evaluation of earlier pharmacophores that have not so far been developed as antiMRSA agents.

Identification of 1-lysophosphatidylethanolamine (C(16:1)) as an antimicrobial compound in the housefly, Musca domestica

We observed that a methanolic whole body extract of uninfected last instar larvae of the housefly, Musca domestica, displayed antifungal and antibacterial activity. We have further purified this extract to a single active fraction using reversed phase high performance liquid chromatography. The pure fraction inhibited growth of the Gram-positive bacteria Bacillus thuringiensis and the yeast Saccharomyces cerevisiae, but not the Gram-negative bacteria Escherichia coli. The active compound was determined to have a molecular mass of 451.2 Da. Further analysis by nuclear magnetic resonance identified the substance as mono-unsaturated 1-lysophosphatidylethanolamine (C(16:1)) (1-LPE). The structurally different and more common 2-LPE have been described as mediators of the antimicrobial activity of rimenophenazine antibiotic agents (Van Rensburg et al., 1992). Our results suggest that the isolated 1-LPE displays a higher activity in comparison, possibly based on structure-specific differences in activity.

Formation of iodinin by a strain of Acidithiobacillus ferrooxidans grown on elemental sulfur

The presence of the pigment iodinin, an Acidithiobacillus ferrooxidans culture metabolite, was demonstrated after growth of bacteria on elemental sulfur. The structure of iodinin was confirmed by X-ray structure analysis; its physiological role is discussed.