Repurposing sunscreen as an antibiotic: zinc-activated avobenzone inhibits methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a major healthcare concern with associated healthcare costs reaching over ${\$}$1 billion in a single year in the USA. Antibiotic resistance in S. aureus is now observed against last line of defense antibiotics, such as vancomycin, linezolid, and daptomycin. Unfortunately, high throughput drug discovery approaches to identify new antibiotics effective against MRSA have not resulted in much tangible success over the last decades. Previously, we demonstrated the feasibility of an alternative drug discovery approach, the identification of metallo-antibiotics, compounds that gain antibacterial activity only after binding to a transition metal ion and as such are unlikely to be detected in standard drug screens. We now report that avobenzone, the primary active ingredient of most sunscreens, can be activated by zinc to become a potent antibacterial compound against MRSA. Zinc-activated avobenzone (AVB-Zn) potently inhibited a series of clinical MRSA isolates [minimal inhibitory concentration (MIC): 0.62-2.5 µM], without pre-existing resistance and activity without zinc (MIC: >10 µM). AVB-Zn was also active against clinical MRSA isolates that were resistant against the commonly used zinc-salt antibiotic bacitracin. We found AVB-Zn exerted no cytotoxicity on human cell lines and primary cells. Last, we demonstrate AVB-Zn can be deployed therapeutically as lotion preparations, which showed efficacy in a mouse wound model of MRSA infection. AVB-Zn thus demonstrates Zn-activated metallo-antibiotics are a promising avenue for future drug discovery.

Pyrazolyl Thioureas and Carbothioamides with an NNSN Motif against MSSA and MRSA

A novel series of copper-activatable drugs intended for use against methicillin-sensitive Staphylococcus aureus (MSSA) and methicillin-resistant S. aureus (MRSA) were synthesized, characterized, and tested against the MSSA strain Newman and the MRSA Lac strain (a USA300 strain), respectively. These drugs feature an NNSN structural motif, which enables the binding of copper. In the absence of copper, no activity against MSSA and MRSA at realistic drug concentrations was observed. Although none of the novel drug candidates exhibits a stereocenter, sub-micromolar activities against SA Newman and micromolar activities against SA Lac were observed in the presence, but not in the absence, of bioavailable copper. Copper influx is a component of cellular response to bacterial infections, which is often described as nutritional immunity.

A copper-dependent compound restores ampicillin sensitivity in multidrug-resistant Staphylococcus aureus

Multi-drug resistant Staphylococcus aureus, including methicillin-resistant S. aureus (MRSA), has become a worldwide, major health care problem. While initially restricted to clinical settings, drug resistant S. aureus is now one of the key causative agents of community-acquired infections. We have previously demonstrated that copper dependent inhibitors (CDIs), a class of antibiotics that are only active in the presence of copper ions, are effective bactericidal agents against MRSA. A second-generation CDI, APT-6K, exerted bactericidal activity at nanomolar concentrations. At sub-bactericidal concentrations, it effectively synergized with ampicillin to reverse drug resistance in multiple MRSA strains. APT-6K had a favorable therapeutic index when tested on eukaryotic cells (TI: > 30) and, unlike some previously reported CDIs, did not affect mitochondrial activity. These results further establish inhibitors that are activated by the binding of transition metal ions as a promising class of antibiotics, and for the first time, describe their ability to reverse existing drug resistance against clinically relevant antibiotics.

High-throughput screening and Bayesian machine learning for copper-dependent inhibitors of Staphylococcus aureus

One potential source of new antibacterials is through probing existing chemical libraries for copper-dependent inhibitors (CDIs), i.e., molecules with antibiotic activity only in the presence of copper. Recently, our group demonstrated that previously unknown staphylococcal CDIs were frequently present in a small pilot screen. Here, we report the outcome of a larger industrial anti-staphylococcal screen consisting of 40 771 compounds assayed in parallel, both in standard and in copper-supplemented media. Ultimately, 483 had confirmed copper-dependent IC50 values under 50 μM. Sphere-exclusion clustering revealed that these hits were largely dominated by sulfur-containing motifs, including benzimidazole-2-thiones, thiadiazines, thiazoline formamides, triazino-benzimidazoles, and pyridinyl thieno-pyrimidines. Structure-activity relationship analysis of the pyridinyl thieno-pyrimidines generated multiple improved CDIs, with activity likely dependent on ligand/ion coordination. Molecular fingerprint-based Bayesian classification models were built using Discovery Studio and Assay Central, a new platform for sharing and distributing cheminformatic models in a portable format, based on open-source tools. Finally, we used the latter model to evaluate a library of FDA-approved drugs for copper-dependent activity in silico. Two anti-helminths, albendazole and thiabendazole, scored highly and are known to coordinate copper ions, further validating the model's applicability.

Reduced bacterial contamination rates detected on silicone tourniquets compared to conventional tourniquets in clinical routine

Background

Tourniquets used for peripheral venous vascular access such as blood sampling are regularly contaminated in clinical routine. Although most contaminations are harmless, some pose a possible risk for infection. To improve peripheral venous access infection control standards, tourniquets with no or as few as possible bacterial burden should be used. Conventional tourniquets can be reprocessed by autoclaving or by incubating in disinfectants. However, both methods are time-consuming and not suitable for immediate use between patients. In contrast, silicone tourniquets can be quickly and simply reprocessed with wipe disinfection. In vitro studies from the manufacturer have demonstrated reduced bacterial contamination on silicone tourniquets after usage compared to conventional tourniquets. This study aims to independently investigate the bacterial load on both types of tourniquets in clinical routine.

Methods

In a first trial, new conventional and silicon tourniquets were used for blood sampling in one facility with strict guidelines for reprocessing (after each patient or not at all) for 1 day and tested for bacterial contamination. In a second trial, new tourniquets were used in four facilities while the mode and frequency of tourniquets’ reprocessing was defined individually by each facility. The number of treated patients, mode and frequency of reprocessing and other relevant handling measures were documented.

Results

Under controlled conditions, with strictly specified reprocessing, slightly fewer bacteria were found on silicone than on conventional tourniquets. In routine clinical practice the reprocessing frequency was not higher for silicone tourniquets in practice. Yet, in all four facilities, there were significantly fewer bacteria found on silicone than on conventional tourniquets.

Conclusion

Although tourniquets are classified as non-critical medical devices, results show – together with benefits of faster and easier reprocessing – that silicone tourniquets can improve infection control of venous vascular access.

Differential Susceptibility of Mycoplasma and Ureaplasma Species to Compound-Enhanced Copper Toxicity

Rationale

Mycoplasmas represent important etiologic agents of many human diseases. Due to increasing antimicrobial resistance and slow rate of novel discovery, unconventional methods of drug discovery are necessary. Copper ions are utilized in host microbial killing, and bacteria must regulate intracellular Cu concentrations to avoid toxicity. We hypothesized that human mollicutes may have susceptibility to Cu-induced toxicity, and compounds that augment copper-dependent killing.

Methods

Mycoplasma pneumoniae (Mpn), Ureaplasma parvum (Up), Ureaplasma urealyticum (Uu), and Mycoplasma hominis (Mh) were exposed to CuSO₄ to determine minimal inhibitory concentrations (MICs). Once inhibitory concentrations had been determined, bacteria were treated with an FDA-approved drug disulfiram (DSF), glyoxal bis(4-methyl-3-thiosemicarbazone) (GTSM), and 2,9-dimethyl-1,10-phenanthroline (neocuproine), with or without Cu²⁺, to determine compound MICs.

Results

Ureaplasma species and Mh were able to tolerate 30–60 μM CuSO₄, while Mpn tolerated over 10-fold higher concentrations (>1 mM). GTSM inhibited growth of all four organisms, but was unaffected by Cu²⁺ addition. Inhibition by GTSM was reduced by addition of the cell-impermeant Cu chelator, bathocuproine disulfonate (BCS). Neocuproine exhibited Cu-dependent growth inhibition of all organisms. DSF exhibited Cu-dependent growth inhibition against Mh at low micromolar concentrations, and at intermediate concentrations for Mpn.

Conclusion

MICs for CuSO₄ differ widely among human mollicutes, with higher MICs for Mpn compared to Mh, Uu, and Up. DSF and Neocuproine exhibit Cu-dependent inhibition of mollicutes with copper concentrations between 25 and 50 μM. GTSM has copper-dependent anti-microbial activity at low levels of copper. Drug enhanced copper toxicity is a promising avenue for novel therapeutic development research with Mycoplasma and Ureaplasma species.

Efficacy of Intraoperative Antiseptic Techniques in the Prevention of Periprosthetic Joint Infection: Superiority of Betadine

BACKGROUND

Povidone-iodine (PI), chlorhexidine gluconate (CHG), and vancomycin (VANC) powder are common intrawound prophylactic agents to prevent periprosthetic joint infection during primary total joint arthroplasty. The aims of this study are (1) to determine the minimal inhibitory concentration (MIC) and time to death for PI, CHG, and VANC against multiple bacteria and (2) to determine time to death against bacteria dried on titanium discs.

METHODS

A standard quantitative suspension assay was performed to determine the MIC for PI, CHG, and VANC against methicillin-resistant Staphylococcus aureus (MRSA), Staphylococcus epidermidis, Haemophilus influenzae, Pseudomonas aeruginosa, Burkholderia cepacia, and Escherichia coli. Time to death assay was performed with time points of 0, 3, 30, and 60 minutes. Concentrations of antiseptic agents for time to death assay were 1% PI, 0.05% CHG, and 5 μg/mL VANC. Dry-phase bacteria on titanium discs were treated in a similar fashion.

RESULTS

The MIC of PI was 0.63%, CHG was 0.0031%, and VANC was 1.56 μg/mL. All 7 bacterial isolates were completely killed by PI at all times tested. CHG failed to kill MRSA and B cepacia at 0- and 3-minute exposures. Vancomycin completely killed MRSA and S epidermidis isolates between 18-20 hours of exposure. All bacterial isolates dried on titanium discs were eliminated by PI exposure on contact. E coli and S epidermidis were incompletely eliminated by CHG at 0 minutes, with all isolates eliminated at 3, 10, and 30 minutes.

CONCLUSION

Our study suggests that PI kills all bacteria tested immediately on contact and that the exposure time is not the key factor.

Assaying for Inorganic Polyphosphate in Bacteria

Inorganic polyphosphate (polyP) is a biological polymer found in cells from all domains of life, and is required for virulence and stress response in many bacteria. There are a variety of methods for quantifying polyP in biological materials, many of which are either labor-intensive or insensitive, limiting their usefulness. We present here a streamlined method for polyP quantification in bacteria, using a silica membrane column extraction optimized for rapid processing of multiple samples, digestion of polyP with the polyP-specific exopolyphosphatase ScPPX, and detection of the resulting free phosphate with a sensitive ascorbic acid-based colorimetric assay. This procedure is straightforward, inexpensive, and allows reliable polyP quantification in diverse bacterial species. We present representative polyP quantification from the Gram-negative bacterium (Escherichia coli), the Gram-positive lactic acid bacterium (Lactobacillus reuteri), and the mycobacterial species (Mycobacterium smegmatis). We also include a simple protocol for nickel affinity purification of mg quantities of ScPPX, which is not currently commercially available.

Pyrazolopyrimidinones, a novel class of copper-dependent bactericidal antibiotics against multi-drug resistant S. aureus

Pyrazolopyrimidinones traffic copper into S. aureus, depleting ATP and altering essential ion concentrations, resulting in the death of the bacteria.

Protein phosphatase, Mg2+/Mn2+-dependent 1A controls the innate antiviral and antibacterial response of macrophages during HIV-1 and Mycobacterium tuberculosis infection

Co-infection with HIV-1 and Mycobacterium tuberculosis (Mtb) is a major public health issue. While some research has described how each pathogen accelerates the course of infection of the other pathogen by compromising the immune system, very little is known about the molecular biology of HIV-1/Mtb co-infection at the host cell level. This is somewhat surprising, as both pathogens are known to replicate and persist in macrophages. We here identify Protein Phosphatase, Mg²⁺/Mn²⁺-dependent 1A (PPM1A) as a molecular link between Mtb infection and increased HIV-1 susceptibility of macrophages. We demonstrate that both Mtb and HIV-1 infection induce the expression of PPM1A in primary human monocyte/macrophages and THP-1 cells. Genetic manipulation studies revealed that increased PPMA1 expression rendered THP-1 cells highly susceptible to HIV-1 infection, while depletion of PPM1A rendered them relatively resistant to HIV-1 infection. At the same time, increased PPM1A expression abrogated the ability of THP-1 cells to respond to relevant bacterial stimuli with a proper cytokine/chemokine secretion response, blocked their chemotactic response and impaired their ability to phagocytose bacteria. These data suggest that PPM1A, which had previously been shown to play a role in the antiviral response to Herpes Simplex virus infection, also governs the antibacterial response of macrophages to bacteria, or at least to Mtb infection. PPM1A thus seems to play a central role in the innate immune response of macrophages, implying that host directed therapies targeting PPM1A could be highly beneficial, in particular for HIV/Mtb co-infected patients.

Copper and Antibiotics: Discovery, Modes of Action, and Opportunities for Medicinal Applications

Copper is a ubiquitous element in the environment as well as living organisms, with its redox capabilities and complexation potential making it indispensable for many cellular functions. However, these same properties can be highly detrimental to prokaryotes and eukaryotes when not properly controlled, damaging many biomolecules including DNA, lipids, and proteins. To restrict free copper concentrations, all bacteria have developed mechanisms of resistance, sequestering and effluxing labile copper to minimize its deleterious effects. This weakness is actively exploited by phagocytes, which utilize a copper burst to destroy pathogens. Though administration of free copper is an unreasonable therapeutic antimicrobial itself, due to insufficient selectivity between host and pathogen, small-molecule ligands may provide an opportunity for therapeutic mimicry of the immune system. By modulating cellular entry, complex stability, resistance evasion, and target selectivity, ligand/metal coordination complexes can synergistically result in high levels of antibacterial activity. Several established therapeutic drugs, such as disulfiram and pyrithione, display remarkable copper-dependent inhibitory activity. These findings have led to development of new drug discovery techniques, using copper ions as the focal point. High-throughput screens for copper-dependent inhibitors against Mycobacterium tuberculosis and Staphylococcus aureus uncovered several new compounds, including a new class of inhibitors, the NNSNs. In this review, we highlight the microbial biology of copper, its antibacterial activities, and mechanisms to discover new inhibitors that synergize with copper.

A Macrophage Infection Model to Predict Drug Efficacy Against Mycobacterium Tuberculosis

In the last 40 years, only a single new antituberculosis drug was FDA approved. New tools that improve the drug development process will be essential to accelerate the development of next-generation antituberculosis drugs. The drug development process seems to be hampered by the inefficient transition of initially promising hits to candidate compounds that are effective in vivo. In this study, we introduce an inexpensive, rapid, and BSL-2 compatible infection model using macrophage-passaged Mycobacterium tuberculosis (Mtb) that forms densely packed Mtb/macrophage aggregate structures suitable for drug efficacy testing. Susceptibility to antituberculosis drugs determined with this Mtb/macrophage aggregate model differed from commonly used in vitro broth-grown single-cell Mtb cultures. Importantly, altered drug susceptibility correlated well with the reported ability of the respective drugs to generate high tissue and cerebrospinal fluid concentrations relative to their serum concentrations, which seems to be the best predictors of in vivo efficacy. Production of these Mtb/macrophage aggregates could be easily scaled up to support throughput efforts. Overall, its simplicity and scalability should make this Mtb/macrophage aggregate model a valuable addition to the currently available Mtb drug discovery tools.

Targeting Biofilm Associated Staphylococcus aureus Using Resazurin Based Drug-susceptibility Assay

Most pathogenic bacteria are able to form biofilms during infection, but due to the difficulty of manipulating and assessing biofilms, the vast majority of laboratory work is conducted with planktonic cells. Here, we describe a peg plate biofilm assay as performed with Staphylococcus aureus. Bacterial biofilms are grown on pegs attached to a 96-well microtiter plate lid, washed through gentle submersion in buffer, and placed in a drug challenge plate. After subsequent incubation they are again washed and moved to a final recovery plate, in which the fluorescent dye resazurin serves as a viability indicator. This assay offers greatly increased ease-of-use, reliability, and reproducibility, as well as a wealth of data when conducted as a kinetic read. Moreover, this assay can be adapted to a medium-throughput drug screening approach by which an endpoint fluorescent readout is taken instead, offering a path for drug discovery efforts.

Development of a web-based tool for automated processing and cataloging of a unique combinatorial drug screen

Facing totally resistant bacteria, traditional drug discovery efforts have proven to be of limited use in replenishing our depleted arsenal of therapeutic antibiotics. Recently, the natural anti-bacterial properties of metal ions in synergy with metal-coordinating ligands have shown potential for generating new molecule candidates with potential therapeutic downstream applications. We recently developed a novel combinatorial screening approach to identify compounds with copper-dependent anti-bacterial properties. Through a parallel screening technique, the assay distinguishes between copper-dependent and independent activities against Mycobacterium tuberculosis with hits being defined as compounds with copper-dependent activities. These activities must then be linked to a compound master list to process and analyze the data and to identify the hit molecules, a labor intensive and mistake-prone analysis. Here, we describe a software program built to automate this analysis in order to streamline our workflow significantly. We conducted a small, 1440 compound screen against M. tuberculosis and used it as an example framework to build and optimize the software. Though specifically adapted to our own needs, it can be readily expanded for any small- to medium-throughput screening effort, parallel or conventional. Further, by virtue of the underlying Linux server, it can be easily adapted for chemoinformatic analysis of screens through packages such as OpenBabel. Overall, this setup represents an easy-to-use solution for streamlining processing and analysis of biological screening data, as well as offering a scaffold for ready functionality expansion.

Combinatorial phenotypic screen uncovers unrecognized family of extended thiourea inhibitors with copper-dependent anti-staphylococcal activity

The continuous rise of multi-drug resistant pathogenic bacteria has become a significant challenge for the health care system. In particular, novel drugs to treat infections of methicillin-resistant Staphylococcus aureus strains (MRSA) are needed, but traditional drug discovery campaigns have largely failed to deliver clinically suitable antibiotics. More than simply new drugs, new drug discovery approaches are needed to combat bacterial resistance. The recently described phenomenon of copper-dependent inhibitors has galvanized research exploring the use of metal-coordinating molecules to harness copper's natural antibacterial properties for therapeutic purposes. Here, we describe the results of the first concerted screening effort to identify copper-dependent inhibitors of Staphylococcus aureus. A standard library of 10 000 compounds was assayed for anti-staphylococcal activity, with hits defined as those compounds with a strict copper-dependent inhibitory activity. A total of 53 copper-dependent hit molecules were uncovered, similar to the copper independent hit rate of a traditionally executed campaign conducted in parallel on the same library. Most prominent was a hit family with an extended thiourea core structure, termed the NNSN motif. This motif resulted in copper-dependent and copper-specific S. aureus inhibition, while simultaneously being well tolerated by eukaryotic cells. Importantly, we could demonstrate that copper binding by the NNSN motif is highly unusual and likely responsible for the promising biological qualities of these compounds. A subsequent chemoinformatic meta-analysis of the ChEMBL chemical database confirmed the NNSNs as an unrecognized staphylococcal inhibitor, despite the family's presence in many chemical screening libraries. Thus, our copper-biased screen has proven able to discover inhibitors within previously screened libraries, offering a mechanism to reinvigorate exhausted molecular collections.

Mycobacteria, metals, and the macrophage

Mycobacterium tuberculosis is a facultative intracellular pathogen that thrives inside host macrophages. A key trait of M. tuberculosis is to exploit and manipulate metal cation trafficking inside infected macrophages to ensure survival and replication inside the phagosome. Here, we describe the recent fascinating discoveries that the mammalian immune system responds to infections with M. tuberculosis by overloading the phagosome with copper and zinc, two metals which are essential nutrients in small quantities but are toxic in excess. M. tuberculosis has developed multi-faceted resistance mechanisms to protect itself from metal toxicity including control of uptake, sequestration inside the cell, oxidation, and efflux. The host response to infections combines this metal poisoning strategy with nutritional immunity mechanisms that deprive M. tuberculosis from metals such as iron and manganese to prevent bacterial replication. Both immune mechanisms rely on the translocation of metal transporter proteins to the phagosomal membrane during the maturation process of the phagosome. This review summarizes these recent findings and discusses how metal-targeted approaches might complement existing TB chemotherapeutic regimens with novel anti-infective therapies.

RNAP II processivity is a limiting step for HIV-1 transcription independent of orientation to and activity of endogenous neighboring promoters

Since HIV-1 has a propensity to integrate into actively expressed genes, transcriptional interference from neighboring host promoters has been proposed to contribute to the establishment and maintenance HIV-1 latency. To gain insights into how endogenous promoters influence HIV-1 transcription we utilized a set of inducible T cell lines and characterized whether there were correlations between expression of endogenous genes, provirus and long terminal repeat architecture. We show that neighboring promoters are active but have minimal impact on HIV-1 transcription, in particular, expression of the endogenous gene did not prevent expression of HIV-1 following induction of latent provirus. We also demonstrate that releasing paused RNAP II by diminishing negative elongation factor (NELF) is sufficient to reactivate transcriptionally repressed HIV-1 provirus regardless of the integration site and orientation of the provirus suggesting that NELF-mediated RNAP II pausing is a common mechanism of maintaining HIV-1 latency.

An AP-1 binding site in the enhancer/core element of the HIV-1 promoter controls the ability of HIV-1 to establish latent infection

Following integration, HIV-1 in most cases produces active infection events; however, in some rare instances, latent infection events are established. The latter have major clinical implications, as latent infection allows the virus to persist despite antiretroviral therapy. Both the cellular factors and the viral elements that potentially determine whether HIV-1 establishes active or latent infection events remain largely elusive. We detail here the contribution of different long terminal repeat (LTR) sequences for the establishment of latent HIV-1 infection. Using a panel of full-length replication-competent virus constructs that reflect naturally occurring differences of HIV-1 subtype-specific LTRs and targeted LTR mutants, we found the primary ability of HIV-1 to establish latent infection in this system to be controlled by a four-nucleotide (nt) AP-1 element just upstream of the NF-κB element in the viral promoter. Deletion of this AP-1 site mostly deprived HIV-1 of the ability to establish latent HIV-1 infection. Extension of this site to a 7-nt AP-1 sequence massively promoted latency establishment, suggesting that this promoter region represents a latency establishment element (LEE). Given that these minimal changes in a transcription factor binding site affect latency establishment to such large extent, our data support the notion that HIV-1 latency is a transcription factor restriction phenomenon.

Facts and fiction: cellular models for high throughput screening for HIV-1 reactivating drugs

A curative therapy for HIV-1 infection will have to include measures to eliminate the reservoir of latently HIV- 1 infected cells that allow the virus to persist despite otherwise successful therapy. To date, all efforts to deplete the latent reservoir by triggering viral reactivation have used preexisting drugs that are believed to potentially target molecular mechanisms controlling HIV-1 infection. These therapeutic attempts were not clinically successful. Only in the last few years have cellular models of latent HIV-1 infection suitable for high throughput screening been developed and concerted drug discovery efforts were initiated to discover new HIV-1 reactivating drugs. We here provide a historic overview about the development of cell models with latent HIV-1 infection that lend themselves to drug discovery. We provide an overview from the first reported latently infected cell lines to current in vitro models of latent HIV-1 infection in primary T cells, and compare their potential to be used in future large-scale drug screening efforts.

Construction of unmarked deletion mutants in mycobacteria

Site-specific recombinases such as the Saccharomyces cerevisiae Flp and the P1 phage Cre proteins have been increasingly used for the construction of unmarked deletions in bacteria. Both systems consist of an antibiotic resistance gene flanked by recognition sites in direct orientation and a curable plasmid for temporary expression of the respective recombinase gene. In this chapter, we describe strategies and methods of how to use sequence-specific recombination mediated by Flp and Cre to construct mutants of Mycobacterium smegmatis, Mycobacterium bovis BCG, and Mycobacterium tuberculosis.

Rv1698 of Mycobacterium tuberculosis represents a new class of channel-forming outer membrane proteins

Mycobacteria contain an outer membrane composed of mycolic acids and a large variety of other lipids. Its protective function is an essential virulence factor of Mycobacterium tuberculosis. Only OmpA, which has numerous homologs in Gram-negative bacteria, is known to form channels in the outer membrane of M. tuberculosis so far. Rv1698 was predicted to be an outer membrane protein of unknown function. Expression of rv1698 restored the sensitivity to ampicillin and chloramphenicol of a Mycobacterium smegmatis mutant lacking the main porin MspA. Uptake experiments showed that Rv1698 partially complemented the permeability defect of the M. smegmatis porin mutant for glucose. These results indicated that Rv1698 provides an unspecific pore that can partially substitute for MspA. Lipid bilayer experiments demonstrated that purified Rv1698 is an integral membrane protein that indeed produces channels. The main single channel conductance is 4.5 +/- 0.3 nanosiemens in 1 M KCl. Zero current potential measurements revealed a weak preference for cations. Whole cell digestion of recombinant M. smegmatis with proteinase K showed that Rv1698 is surface-accessible. Taken together, these experiments demonstrated that Rv1698 is a channel protein that is likely involved in transport processes across the outer membrane of M. tuberculosis. Rv1698 has single homologs of unknown functions in Corynebacterineae and thus represents the first member of a new class of channel proteins specific for mycolic acid-containing outer membranes.

Identification of two Mycobacterium smegmatis lipoproteins exported by a SecA2-dependent pathway

The SecA2 protein is part of a specialized protein export system of mycobacteria. We set out to identify proteins exported to the bacterial cell envelope by the mycobacterial SecA2 system. By comparing the protein profiles of cell wall and membrane fractions from wild-type and DeltasecA2 mutant Mycobacterium smegmatis, we identified the Msmeg1712 and Msmeg1704 proteins as SecA2-dependent cell envelope proteins. These are the first endogenous M. smegmatis proteins identified as dependent on SecA2 for export. Both proteins are homologous to periplasmic sugar-binding proteins of other bacteria, and both contain functional amino-terminal signal sequences with lipobox motifs. These two proteins appeared to be genuine lipoproteins as shown by Triton X-114 fractionation and sensitivity to globomycin, an inhibitor of lipoprotein signal peptidase. The role of SecA2 in the export of these proteins was specific; not all mycobacterial lipoproteins required SecA2 for efficient localization or processing. Finally, Msmeg1704 was recognized by the SecA2 pathway of Mycobacterium tuberculosis, as indicated by the appearance of an export intermediate when the protein was expressed in a DeltasecA2 mutant of M. tuberculosis. Taken together, these results indicate that a select subset of envelope proteins containing amino-terminal signal sequences can be substrates of the mycobacterial SecA2 pathway and that some determinants for SecA2-dependent export are conserved between M. smegmatis and M. tuberculosis.

Porins are required for uptake of phosphates by Mycobacterium smegmatis

Phosphorus is an essential nutrient, but how phosphates cross the mycobacterial cell wall is unknown. Phosphatase activity in whole cells of Mycobacterium smegmatis was significantly lower than that in lysed cells, indicating that access to the substrate was restricted. The loss of the outer membrane (OM) porin MspA also reduced the phosphatase activity in whole cells compared to that in lysed cells. A similar result was obtained for M. smegmatis that overexpressed endogenous alkaline phosphatase, indicating that PhoA is not a surface protein, contrary to a previous report. The uptake of phosphate by a mutant lacking the porins MspA and MspC was twofold lower than that by wild-type M. smegmatis. Strikingly, the loss of these porins resulted in a severe growth defect of M. smegmatis on low-phosphate plates. We concluded that the OM of M. smegmatis represents a permeability barrier for phosphates and that Msp porins are the only OM channels for the diffusion of phosphate in M. smegmatis. However, phosphate diffusion through Msp pores is rather inefficient as shown by the 10-fold lower permeability of M. smegmatis for phosphate compared to that for glucose. This is likely due to the negative charges in the constriction zone of Msp porins. The phosphatase activity in whole cells of Mycobacterium bovis BCG was significantly less than that in lysed cells, indicating a similar uptake pathway for phosphates in slow-growing mycobacteria. However, porins that could mediate the diffusion of phosphates across the OM of M. bovis BCG and Mycobacterium tuberculosis are unknown.

The growth rate of Mycobacterium smegmatis depends on sufficient porin-mediated influx of nutrients

Mycobacteria have a unique outer membrane (OM) that is thicker than any other known biological membrane. Nutrients cross this permeability barrier by diffusion through porins. MspA is the major porin of Mycobacterium smegmatis. In this study we showed that three paralogues of MspA, namely MspB, MspC and MspD are also porins. However, only the mspA and mspC genes were expressed in the wild-type strain. None of the single deletion mutants displayed a significant OM permeability defect except for the mspA mutant. Deletion of the mspA gene caused activation of transcription of mspB and/or mspD in three independent strains by unknown chromosomal mutations. It is concluded that mspB and mspD provide backup porins for M. smegmatis. This also indicated that a minimal porin-mediated OM permeability is essential for survival of M. smegmatis. Electron microscopy in combination with quantitative image analysis of protein gels revealed that the number of pores per cell dropped from 2400 to 800 and 150 for the DeltamspA and DeltamspA DeltamspC mutant (ML10) respectively. The very low number of pores correlated well with the at least 20-fold lower channel activity of detergent extracts of the ML10 strain and its 15- and 75-fold lower permeability to nutrient molecules such as serine and glucose respectively. The amount of Msp porin and the OM permeability of the triple porin mutant lacking mspA, mspC and mspD was not altered. The growth rate of M. smegmatis dropped drastically with its porin-mediated OM permeability in contrast to porin mutants of Escherichia coli. These results show that porin-mediated influx of nutrients is a major determinant of the growth rate of M. smegmatis.

4-Dihydromethyltrisporate dehydrogenase, an enzyme of the sex hormone pathway in Mucor mucedo, is constitutively transcribed but its activity is differently regulated in (+) and (−) mating types

4-Dihydromethyltrisporate dehydrogenase (TDH) converts the (+) mating type sex pheromone 4-dihydromethyltrisporate into methyltrisporate. In Mucor mucedo, this conversion is required only in the (-) mating type. Expression of the TDH encoding TSP1 gene was analyzed qualitatively using reverse-transcribed PCR. TSP1 is constitutively transcribed in the (+) and in the (-) mating type, irrespective of the mating situation. By immunodetection, the translation product is also formed constitutively. In contrast to gene expression, TDH enzyme activity depends on the sexual status of the mycelium. Activity is restricted to the sexually stimulated (-) mating type. Non-stimulated (-), as well as stimulated and non-stimulated (+) mycelia exhibit no activity and do not influence activity in stimulated (-) mycelia. Time course analysis shows strongly increased enzyme activity at 80 min after stimulation. Low activity exists from the onset of stimulation, indicating that additional regulation mechanisms are involved in TDH function.