Preclinical evaluation of novel antibacterial agents by microbiological and molecular techniques

Silver resistance in Gram-negative bacteria: a dissection of endogenous and exogenous mechanisms

Objectives

To gain a more detailed understanding of endogenous (mutational) and exogenous (horizontally acquired) resistance to silver in Gram-negative pathogens, with an emphasis on clarifying the genetic bases for resistance.

Methods

A suite of microbiological and molecular genetic techniques was employed to select and characterize endogenous and exogenous silver resistance in several Gram-negative species.

Results

In Escherichia coli, endogenous resistance arose after 6 days of exposure to silver, a consequence of two point mutations that were both necessary and sufficient for the phenotype. These mutations, in ompR and cusS, respectively conferred loss of the OmpC/F porins and derepression of the CusCFBA efflux transporter, both phenotypic changes previously linked to reduced intracellular accumulation of silver. Exogenous resistance involved derepression of the SilCFBA efflux transporter as a consequence of mutation in silS, but was additionally contingent on expression of the periplasmic silver-sequestration protein SilE. Silver resistance could be selected at high frequency (>10⁻⁹) from Enterobacteriaceae lacking OmpC/F porins or harbouring the sil operon and both endogenous and exogenous resistance were associated with modest fitness costs in vitro.

Conclusions

Both endogenous and exogenous silver resistance are dependent on the derepressed expression of closely related efflux transporters and are therefore mechanistically similar phenotypes. The ease with which silver resistance can become selected in some bacterial pathogens in vitro suggests that there would be benefit in improved surveillance for silver-resistant isolates in the clinic, along with greater control over use of silver-containing products, in order to best preserve the clinical utility of silver.

Redox-active compounds with a history of human use: antistaphylococcal action and potential for repurposing as topical antibiofilm agents

Objectives

To investigate the antistaphylococcal/antibiofilm activity and mode of action (MOA) of a panel of redox-active (RA) compounds with a history of human use and to provide a preliminary preclinical assessment of their potential for topical treatment of staphylococcal infections, including those involving a biofilm component.

Methods

Antistaphylococcal activity was evaluated by broth microdilution and by time–kill studies with growing and slow- or non-growing cells. The antibiofilm activity of RA compounds, alone and in combination with established antibacterial agents, was assessed using the Calgary Biofilm Device. Established assays were used to examine the membrane-perturbing effects of RA compounds, to measure penetration into biofilms and physical disruption of biofilms and to assess resistance potential. A living skin equivalent model was used to assess the effects of RA compounds on human skin.

Results

All 15 RA compounds tested displayed antistaphylococcal activity against planktonic cultures (MIC 0.25–128 mg/L) and 7 eradicated staphylococcal biofilms (minimum biofilm eradication concentration 4–256 mg/L). The MOA of all compounds involved perturbation of the bacterial membrane, whilst selected compounds with antibiofilm activity caused destructuring of the biofilm matrix. The two most promising agents [celastrol and nordihydroguaiaretic acid (NDGA)] in respect of antibacterial potency and selective toxicity against bacterial membranes acted synergistically with gentamicin against biofilms, did not damage artificial skin following topical application and exhibited low resistance potential.

Conclusions

In contrast to established antibacterial drugs, some RA compounds are capable of eradicating staphylococcal biofilms. Of these, celastrol and NDGA represent particularly attractive candidates for development as topical antistaphylococcal biofilm treatments.

Mutations in the proteolipid subunits of the vacuolar H+-ATPase provide resistance to indolotryptoline natural products

Indolotryptoline natural products represent a small family of structurally unique chromopyrrolic acid-derived antiproliferative agents. Like many prospective anticancer agents before them, the exploration of their potential clinical utility has been hindered by the limited information known about their mechanism of action. To study the mode of action of two closely related indolotryptolines (BE-54017, cladoniamide A), we selected for drug resistant mutants using a multidrug resistance-suppressed (MDR-sup) Schizosaccharomyces pombe strain. As fission yeast maintains many of the basic cancer-relevant cellular processes present in human cells, it represents an appealing model to use in determining the potential molecular target of antiproliferative natural products through resistant mutant screening. Full genome sequencing of resistant mutants identified mutations in the c and c′ subunits of the proteolipid substructure of the vacuolar H⁺-ATPase complex (V-ATPase). This collection of resistance-conferring mutations maps to a site that is distant from the nucleotide-binding sites of V-ATPase and distinct from sites found to confer resistance to known V-ATPase inhibitors. Acid vacuole staining, cross-resistance studies, and direct c/c′ subunit mutagenesis all suggest that indolotryptolines are likely a structurally novel class of V-ATPase inhibitors. This work demonstrates the general utility of resistant mutant selection using MDR-sup S. pombe as a rapid and potentially systematic approach for studying the modes of action of cytotoxic natural products.

Synthesis of 3-acyltetramates by side chain manipulation and their antibacterial activity

An efficient approach for the introduction of 3-acyl side chain groups onto a core tetramate system, which are suitable for further manipulation by nucleophilic displacement or Horner-Wadsworth-Emmons coupling, provides access to a diverse library of substituted tetramates related to two distinct classes of natural products, equisetin and pramanicin. Assessment against S. aureus and E. coli indicated that some compounds exhibit significant antibacterial activity, providing unusual leads for further optimisation in the drug discovery process.

Screening for a diamond in the rough

Contrary to the standard of eliminating antimicrobial hits that collapse bacterial proton motive force (PMF), in this issue of Chemistry and Biology, Farha and colleagues describe the value of screens to identify molecules that dissipate PMF, yet are nonbacteriolytic and selectively toxic.

Biological evaluation of benzothiazole ethyl urea inhibitors of bacterial type II topoisomerases

The type II topoisomerases DNA gyrase (GyrA/GyrB) and topoisomerase IV (ParC/ParE) are well-validated targets for antibacterial drug discovery. Because of their structural and functional homology, these enzymes are amenable to dual targeting by a single ligand. In this study, two novel benzothiazole ethyl urea-based small molecules, designated compound A and compound B, were evaluated for their biochemical, antibacterial, and pharmacokinetic properties. The two compounds inhibited the ATPase activity of GyrB and ParE with 50% inhibitory concentrations of <0.1 μg/ml. Prevention of DNA supercoiling by DNA gyrase was also observed. Both compounds potently inhibited the growth of a range of bacterial organisms, including staphylococci, streptococci, enterococci, Clostridium difficile, and selected Gram-negative respiratory pathogens. MIC90s against clinical isolates ranged from 0.015 μg/ml for Streptococcus pneumoniae to 0.25 μg/ml for Staphylococcus aureus. No cross-resistance with common drug resistance phenotypes was observed. In addition, no synergistic or antagonistic interactions between compound A or compound B and other antibiotics, including the topoisomerase inhibitors novobiocin and levofloxacin, were detected in checkerboard experiments. The frequencies of spontaneous resistance for S. aureus were <2.3 × 10(-10) with compound A and <5.8 × 10(-11) with compound B at concentrations equivalent to 8× the MICs. These values indicate a multitargeting mechanism of action. The pharmacokinetic properties of both compounds were profiled in rats. Following intravenous administration, compound B showed approximately 3-fold improvement over compound A in terms of both clearance and the area under the concentration-time curve. The measured oral bioavailability of compound B was 47.7%.

Antibacterial effect of Manuka honey on Clostridium difficile

BACKGROUND

Manuka honey originates from the manuka tree (Leptospermum scoparium) and its antimicrobial effect has been attributed to a property referred to as Unique Manuka Factor that is absent in other types of honey. Antibacterial activity of Manuka honey has been documented for several bacterial pathogens, however there is no information on Clostridium difficile, an important nosocomial pathogen. In this study we investigated susceptibility of C. difficile to Manuka honey and whether the activity is bactericidal or bacteriostatic.

METHODS

Three C. difficile strains were subjected to the broth dilution method to determine minimum inhibitory concentrations (MIC) and minimum bactericidal concentrations (MBC) for Manuka honey. The agar well diffusion method was also used to investigate sensitivity of the C. difficile strains to Manuka honey.

RESULTS

The MIC values of the three C. difficile strains were the same (6.25% v/v). Similarly, MBC values of the three C. difficile strains were the same (6.25% v/v). The activity of Manuka honey against all three C. difficile strains was bactericidal. A dose-response relationship was observed between the concentrations of Manuka honey and zones of inhibition formed by the C. difficile strains, in which increasing concentrations of Manuka honey resulted in increasing size of zone of inhibition formed. Maximum zone of inhibition was observed at 50% (v/v) Manuka honey and the growth inhibition persisted over 7 days.

CONCLUSION

C. difficile is appreciably susceptible to Manuka honey and this may offer an effective way of treating infections caused by the organism.

Antibacterial activity and mode of action of tert-butylhydroquinone (TBHQ) and its oxidation product, tert-butylbenzoquinone (TBBQ)

OBJECTIVES

The antioxidant tert-butylhydroquinone (TBHQ) is a food additive reported to have antibacterial activity, and may therefore have application in the healthcare setting. This study sought to characterize the antibacterial activity and mode of action of TBHQ and its oxidation product, tert-butylbenzoquinone (TBBQ).

METHODS

The stability of TBHQ/TBBQ was studied in buffer. Susceptibility testing was performed by broth microdilution, and killing and lytic activity were evaluated by viable counting and culture turbidity measurements. Mode of action studies included following the incorporation of radiolabelled precursors into macromolecules. The effect of TBHQ/TBBQ upon bacterial and mammalian membranes was assessed using the BacLight(TM) assay and by monitoring the haemolysis of equine erythrocytes.

RESULTS

TBHQ underwent oxidation in solution to form TBBQ. When oxidation was prevented, TBHQ lacked useful antibacterial activity, indicating that TBBQ is responsible for the antibacterial activity attributed to TBHQ. TBBQ demonstrated activity against Staphylococcus aureus SH1000 (MIC 8 mg/L) and against a panel of clinical S. aureus isolates (MIC90 16 mg/L). TBBQ at 4× MIC caused a >4 log10 drop in cell viability within 6 h without lysis, and eradicated staphylococcal biofilms at 8× MIC. TBBQ did not display preferential inhibition of any single macromolecular synthetic pathway, but caused loss of staphylococcal membrane integrity without haemolytic activity.

CONCLUSIONS

TBBQ is responsible for the antibacterial activity previously ascribed to TBHQ. TBBQ prompts loss of staphylococcal membrane integrity; it is rapidly and extensively bactericidal, but is non-lytic. In view of the potent and selective bactericidal activity of TBBQ, this compound warrants further investigation as a candidate antistaphylococcal agent.

Viable screening targets related to the bacterial cell wall

The synthesis of the bacterial peptidoglycan has been recognized for over 50 years as fertile ground for antibacterial discovery. Initially, empirical screening of natural products for inhibition of bacterial growth detected many chemical classes of antibiotics whose specific mechanisms of action were eventually dissected and defined. Of the nontoxic antibiotics discovered, most were found to be inhibitors of either protein synthesis or cell wall synthesis, which led to more directed screening for inhibitors of these pathways. Directed screening and design programs for cell wall inhibitors have been undertaken since the 1960s. In that time it has become clear that, while certain steps and intermediates have yielded selective inhibitors and are established targets, other potential targets have not yielded inhibitors whose antibacterial activity is proven to be solely due to that inhibition. Why has this search been so problematic? Are the established targets still worth pursuing? This review will attempt to answer these and other questions and evaluate the viability of targets related to peptidoglycan synthesis.

In vivo validation of thymidylate kinase (TMK) with a rationally designed, selective antibacterial compound

There is an urgent need for new antibacterials that pinpoint novel targets and thereby avoid existing resistance mechanisms. We have created novel synthetic antibacterials through structure-based drug design that specifically target bacterial thymidylate kinase (TMK), a nucleotide kinase essential in the DNA synthesis pathway. A high-resolution structure shows compound TK-666 binding partly in the thymidine monophosphate substrate site, but also forming new induced-fit interactions that give picomolar affinity. TK-666 has potent, broad-spectrum Gram-positive microbiological activity (including activity against methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus), bactericidal action with rapid killing kinetics, excellent target selectivity over the human ortholog, and low resistance rates. We demonstrate in vivo efficacy against S. aureus in a murine infected-thigh model. This work presents the first validation of TMK as a compelling antibacterial target and provides a rationale for pursuing novel clinical candidates for treating Gram-positive infections through TMK.

The 2012 Garrod lecture: discovery of antibacterial drugs in the 21st century

The discovery and development of antibacterial drugs in the twentieth century were major scientific and medical achievements that have had profound benefits for human society. However, in the twenty-first century the widespread global occurrence of bacteria resistant to the antibiotics and synthetic drugs discovered in the previous century threatens to reverse our ability to treat infectious diseases. Although some new drugs are in development they do not adequately cover growing medical needs. Furthermore, these drugs are mostly derivatives of older classes already in use and therefore prone to existing bacterial resistance mechanisms. Thus, new drug classes are urgently needed. Despite investment in antibacterial drug discovery, no new drug class has been discovered in the past 20 years. In this review, based upon my career as a research scientist in the field of antibacterial drug discovery, I consider some of the technical reasons for the recent failure and look to the future developments that may help to reverse the poor current success rate. Diversification of screening libraries to include new natural products will be important as well as ensuring that the promising drug hits arising from structure-based drug design can achieve effective concentrations at their target sites within the bacterial cell.

An improved small-molecule inhibitor of FtsZ with superior in vitro potency, drug-like properties, and in vivo efficacy

The bacterial cell division protein FtsZ is an attractive target for small-molecule antibacterial drug discovery. Derivatives of 3-methoxybenzamide, including compound PC190723, have been reported to be potent and selective antistaphylococcal agents which exert their effects through the disruption of intracellular FtsZ function. Here, we report the further optimization of 3-methoxybenzamide derivatives towards a drug candidate. The in vitro and in vivo characterization of a more advanced lead compound, designated compound 1, is described. Compound 1 was potently antibacterial, with an average MIC of 0.12 μg/ml against all staphylococcal species, including methicillin- and multidrug-resistant Staphylococcus aureus and Staphylococcus epidermidis. Compound 1 inhibited an S. aureus strain carrying the G196A mutation in FtsZ, which confers resistance to PC190723. Like PC190723, compound 1 acted on whole bacterial cells by blocking cytokinesis. No interactions between compound 1 and a diverse panel of antibiotics were measured in checkerboard experiments. Compound 1 displayed suitable in vitro pharmaceutical properties and a favorable in vivo pharmacokinetic profile following intravenous and oral administration, with a calculated bioavailability of 82.0% in mice. Compound 1 demonstrated efficacy in a murine model of systemic S. aureus infection and caused a significant decrease in the bacterial load in the thigh infection model. A greater reduction in the number of S. aureus cells recovered from infected thighs, equivalent to 3.68 log units, than in those recovered from controls was achieved using a succinate prodrug of compound 1, which was designated compound 2. In summary, optimized derivatives of 3-methoxybenzamide may yield a first-in-class FtsZ inhibitor for the treatment of antibiotic-resistant staphylococcal infections.

Prospective screening of novel antibacterial inhibitors of dihydrofolate reductase for mutational resistance

Resistance to trimethoprim (TMP) resulting from point mutations in the enzyme drug target dihydrofolate reductase (DHFR) drives the development of new antifolate inhibitors effective against methicillin-resistant Staphylococcus aureus (MRSA). For the past several years we have used structure-based design to create propargyl-linked antifolates that are highly potent antibacterial agents. In order to focus priority on the development of lead compounds with a low propensity to induce resistance, we prospectively evaluated resistance profiles for two of these inhibitors in an MRSA strain. By selection with the lead inhibitors, we generated resistant strains that contain single point mutations F98Y and H30N associated with TMP resistance and one novel mutation, F98I, in DHFR. Encouragingly, the pyridyl propargyl-linked inhibitor selects mutants at low frequency (6.85 × 10(-10) to 1.65 × 10(-9)) and maintains a low MIC (2.5 μg/ml) and a low mutant prevention concentration (1.25 μg/ml), strongly supporting its position as a lead compound. Results from this prospective screening method inform the continued design of antifolates effective against mutations at the Phe 98 position. Furthermore, the method can be used broadly to incorporate ideas for overcoming resistance early in the development process.

Macromolecular synthesis and membrane perturbation assays for mechanisms of action studies of antimicrobial agents

The definition and confirmation of the mechanism of action of an NCE is central to antimicrobial drug discovery. Most antibiotics currently in clinical use selectively target and block one or more bacterial macromolecular synthesis processes, e.g., DNA replication, RNA synthesis (transcription), protein synthesis (translation), cell wall (peptidoglycan) synthesis, and fatty acid (lipid) biosynthesis. This unit includes two protocols for determining the effect of test compounds on macromolecular synthesis, one in test tube format and the other in 96-well plate format. A membrane potential depolarization protocol is also provided. Disruption of cell membrane integrity may be a legitimate mechanism of action for antibacterials, but it also may be the result of nonspecific cell membrane activity, an effect that must be ruled out for mammalian cells. These assays provide useful means for verifying inhibition of an intended target pathway with investigational antimicrobial compounds. They can also be used as valuable secondary assays for lead optimization to eliminate inhibitors that display nonselective toxicity.

Identification of a novel protein synthesis inhibitor active against gram-positive bacteria

In an effort to identify novel antibacterial chemotypes, we performed a whole-cell screen for inhibitors of Staphylococcus aureus growth and pursued those compounds with previously uncharacterized antibacterial activity. This process resulted in the identification of a benzothiazolium salt, ABTZ-1, that displayed potent antibacterial activity against Gram-positive pathogens. Several clinically desirable qualities were demonstrated for ABTZ-1 including potent activity against multidrug-resistant clinical isolates of methicillin-resistant S. aureus (MRSA) and vancomycin-resistant enterococci (VRE), retention of this activity in human serum, and low hemolytic activity. The antibacterial activity of ABTZ-1 was attributed to its inhibition of bacterial translation, as this compound prevented the incorporation of [³⁵S]methionine into S. aureus proteins, and ABTZ-1-resistant strains were cross-resistant to known inhibitors of bacterial translation. ABTZ-1 represents a promising new class of antibacterial agents.

Coping with antibiotic resistance: combining nanoparticles with antibiotics and other antimicrobial agents

The worldwide escalation of bacterial resistance to conventional medical antibiotics is a serious concern for modern medicine. High prevalence of multidrug-resistant bacteria among bacteria-based infections decreases effectiveness of current treatments and causes thousands of deaths. New improvements in present methods and novel strategies are urgently needed to cope with this problem. Owing to their antibacterial activities, metallic nanoparticles represent an effective solution for overcoming bacterial resistance. However, metallic nanoparticles are toxic, which causes restrictions in their use. Recent studies have shown that combining nanoparticles with antibiotics not only reduces the toxicity of both agents towards human cells by decreasing the requirement for high dosages but also enhances their bactericidal properties. Combining antibiotics with nanoparticles also restores their ability to destroy bacteria that have acquired resistance to them. Furthermore, nanoparticles tagged with antibiotics have been shown to increase the concentration of antibiotics at the site of bacterium-antibiotic interaction, and to facilitate binding of antibiotics to bacteria. Likewise, combining nanoparticles with antimicrobial peptides and essential oils generates genuine synergy against bacterial resistance. In this article, we aim to summarize recent studies on interactions between nanoparticles and antibiotics, as well as other antibacterial agents to formulate new prospects for future studies. Based on the promising data that demonstrated the synergistic effects of antimicrobial agents with nanoparticles, we believe that this combination is a potential candidate for more research into treatments for antibiotic-resistant bacteria.

Toward new therapeutics for skin and soft tissue infections: propargyl-linked antifolates are potent inhibitors of MRSA and Streptococcus pyogenes

Hospital- and community-acquired, complicated skin and soft tissue infections, often attributed to Staphylococcus aureus and Streptococcus pyogenes, present a significant health burden that is associated with increased health care costs and mortality. As these two species are difficult to discern on diagnosis and are associated with differential profiles of drug resistance, the development of an efficacious antibacterial agent that targets both organisms is a high priority. Herein we describe a structure-based drug development effort that has produced highly potent inhibitors of dihydrofolate reductase from both species. Optimized propargyl-linked antifolates containing a key pyridyl substituent display antibacterial activity against both methicillin-resistant S. aureus and S. pyogenes at MIC values below 0.1 µg/mL and minimal cytotoxicity against mammalian cells. Further evaluation against a panel of clinical isolates shows good efficacy against a range of important phenotypes such as hospital- and community-acquired strains as well as strains resistant to vancomycin.

Antimicrobial active clothes display no adverse effects on the ecological balance of the healthy human skin microflora

The progressive public use of antimicrobial clothes has raised issues concerning skin health. A placebo-controlled side-to-side study was run with antimicrobial clothes versus fabrics of similar structure but minus the antimicrobial activity, to evaluate possible adverse effects on the healthy skin microflora. Sixty volunteers were enrolled. Each participant received a set of form-fitting T-shirts constructed in 2 halves: an antibacterial half, displaying activities of 3–5 log-step reductions due to silver-finishes or silver-loaded fibres and a nonantibacterial control side. The microflora of the scapular skin was analyzed weekly for opportunistic and pathogenic microorganisms over six weeks. The antibacterial halves did not disturb the microflora in number or composition, whereas a silver-containing deodorant displayed a short-term disturbance. Furthermore, parameters of skin morphology and function (TEWL, pH, moisture) did not show any significant shifts. In summary, antimicrobial clothes did not show adverse effects on the ecological balance of the healthy skin microflora.

Activity of and development of resistance to corallopyronin A, an inhibitor of RNA polymerase

We explored the properties of corallopyronin A (CorA), a poorly characterized inhibitor of bacterial RNA polymerase (RNAP). It displayed a 50% inhibitory concentration of 0.73 μM against RNAP, compared with 11.5 nM for rifampin. The antibacterial activity of CorA was also inferior to rifampin, and resistant mutants of Staphylococcus aureus were easily selected. The mutations conferring resistance resided in the rpoB and rpoC subunits of RNAP. We conclude that CorA is not a promising antibacterial drug candidate.

In vitro studies indicate a high resistance potential for the lantibiotic nisin in Staphylococcus aureus and define a genetic basis for nisin resistance

Lantibiotics such as nisin (NIS) are peptide antibiotics that may have a role in the chemotherapy of bacterial infections. A perceived benefit of lantibiotics for clinical use is their low propensity to select resistance, although detailed resistance studies with relevant bacterial pathogens are lacking. Here we examined the development of resistance to NIS in Staphylococcus aureus, establishing that mutants, including small-colony variants, exhibiting substantial (4- to 32-fold) reductions in NIS susceptibility could be selected readily. Comparative genome sequencing of a single NISr mutant exhibiting a 32-fold increase in NIS MIC revealed the presence of only two mutations, leading to the substitutions V229G in the purine operon repressor, PurR, and A208E in an uncharacterized protein encoded by SAOUHSC_02955. Independently selected NISr mutants also harbored mutations in the genes encoding these products. Reintroduction of these mutations into the S. aureus chromosome alone and in combination revealed that SAOUHSC_02955(A208E) made the primary contribution to the resistance phenotype, conferring up to a 16-fold decrease in NIS susceptibility. Bioinformatic analyses suggested that this gene encodes a sensor histidine kinase, leading us to designate it "nisin susceptibility-associated sensor (nsaS)." Doubling-time determinations and mixed-culture competition assays between NISr and NISs strains indicated that NIS resistance had little impact on bacterial fitness, and resistance was stable in the absence of selection. The apparent ease with which S. aureus can develop and maintain NIS resistance in vitro suggests that resistance to NIS and other lantibiotics with similar modes of action would arise in the clinic if these agents are employed as chemotherapeutic drugs.

Challenges of antibacterial discovery

The discovery of novel small-molecule antibacterial drugs has been stalled for many years. The purpose of this review is to underscore and illustrate those scientific problems unique to the discovery and optimization of novel antibacterial agents that have adversely affected the output of the effort. The major challenges fall into two areas: (i) proper target selection, particularly the necessity of pursuing molecular targets that are not prone to rapid resistance development, and (ii) improvement of chemical libraries to overcome limitations of diversity, especially that which is necessary to overcome barriers to bacterial entry and proclivity to be effluxed, especially in Gram-negative organisms. Failure to address these problems has led to a great deal of misdirected effort.

Antibacterials from the sea

The ocean contains a host of macroscopic life in a great microbial soup. Unlike the terrestrial environment, an aqueous environment provides perpetual propinquity and blurs spatial distinctions. Marine organisms are under a persistent threat of infection by resident pathogenic microbes including bacteria, and in response they have engineered complex organic compounds with antibacterial activity from a diverse set of biological precursors. The diluting effect of the ocean drives the construction of potent molecules that are stable to harsh salty conditions. Members of each class of metabolite-ribosomal and non-ribosomal peptides, alkaloids, polyketides, and terpenes-have been shown to exhibit antibacterial activity. The sophistication and diversity of these metabolites points to the ingenuity and flexibility of biosynthetic processes in Nature. Compared with their terrestrial counterparts, antibacterial marine natural products have received much less attention. Thus, a concerted effort to discover new antibacterials from marine sources has the potential to contribute significantly to the treatment of the ever increasing drug-resistant infectious diseases.

XF-73, a novel antistaphylococcal membrane-active agent with rapid bactericidal activity

OBJECTIVES

XF-73 is a novel porphyrin antibacterial agent previously reported to inhibit a range of gram-positive bacterial species, including Staphylococcus aureus. Its mode of action is unknown. Using S. aureus as a model organism we sought to examine the basis of its antibacterial activity.

METHODS

The effects of XF-73 on the growth and survival of S. aureus SH1000 were investigated by viable count and culture absorbance techniques. Inhibition of macromolecular synthesis and disruption of membrane integrity after exposure to XF-73 were examined by radiolabelling experiments, the BacLight fluorescent dye assay and measurement of K(+) and ATP leakage from the cell. The effect of XF-73 on a staphylococcal coupled transcription-translation system was also investigated.

RESULTS

XF-73 was rapidly bactericidal against S. aureus SH1000 and demonstrated more rapid killing kinetics than all other comparator agents when tested at an equivalent multiple (4x) of the MIC. Exposure of S. aureus to XF-73 for 10 min completely inhibited DNA, RNA and protein synthesis. XF-73 had no effect on transcription and translation in vitro. Cells exposed to XF-73 gave a positive response in the BacLight assay, which detects membrane damage. The drug also caused substantial loss of K(+) and ATP from the cell, but did not promote bacterial lysis.

CONCLUSIONS

XF-73 exhibited rapid membrane-perturbing activity, which is likely to be responsible for inhibition of macromolecular synthesis and the death of staphylococci exposed to the drug.

Transcriptional signature following inhibition of early-stage cell wall biosynthesis in Staphylococcus aureus

To facilitate mode of action studies on antibacterial inhibitors of early-stage cell wall biosynthesis (CWB), we determined the transcriptional response of Staphylococcus aureus to depletion/inhibition of enzymes in this pathway by DNA microarray analysis. We identified a transcriptional signature distinct from that previously observed following exposure to inhibitors of late-stage CWB.

A microbiological assessment of novel nitrofuranylamides as anti-tuberculosis agents

OBJECTIVES

Nitrofuranylamides (NFAs) are nitroaromatic compounds that have recently been discovered and have potent anti-tuberculosis (TB) activity. A foundational study was performed to evaluate whether this class of agents possesses microbiological properties suitable for future antimycobacterial therapy.

METHODS

Five representative compounds of the NFA series were evaluated by standard microbiological assays to determine MICs, MBCs, activity against anaerobic non-replicating persistent Mycobacterium tuberculosis, post-antibiotic effects (PAEs), antibiotic synergy and the basis for resistance.

RESULTS

The antimicrobial activity of these compounds was restricted to bacteria of the M. tuberculosis complex, and all compounds were highly active against drug-susceptible and -resistant strains of M. tuberculosis, with MICs 0.0004-0.05 mg/L. Moreover, no antagonism was observed with front-line anti-TB drugs. Activity was also retained against dormant bacilli in two in vitro low-oxygen models for M. tuberculosis persistence. A long PAE was observed, which was comparable to that of rifampicin, but superior to isoniazid and ethambutol. Spontaneous NFA-resistant mutants arose at a frequency of 10(-5)-10(-7), comparable to that for isoniazid (10(-5)-10(-6)). Some of these mutants exhibited cross-resistance to one or both of the nitroimidazoles PA-824 and OPC-67683. Cross-resistance was associated with inactivation of the reduced F(420)-deazaflavin cofactor pathway and not with inactivation of the Rv3547, the nitroreductase for PA-824 and OPC-67683.

CONCLUSIONS

Based on these studies, NFAs have many useful antimycobacterial properties applicable to TB chemotherapy and probably possess a unique mode of action that results in good activity against active and dormant M. tuberculosis. Therefore, the further development of lead compounds in this series is warranted.

New antibacterial agents for treating infections caused by multi-drug resistant Gram-negative bacteria

BACKGROUND

Infections caused by multi-drug resistant (MDR) Gram-negative bacteria represent an ever-growing area of unmet medical need. To address this need, it is imperative that novel classes of antibiotics demonstrating activity against bacterial strains resistant to established antibiotics are introduced into the clinic.

OBJECTIVES

To examine the current status of the Gram-negative antibacterial pipeline, ranging from the more advanced preclinical candidates to drugs recently launched, and look to the future of anti-Gram-negative drug development.

METHODS

Information was compiled from scientific and patent literature, conference proceedings and company publications/websites.

RESULTS/CONCLUSIONS

None of the antibacterial agents currently in clinical trials that encompass Gram-negative bacteria in their spectrum of activity possess sufficiently novel modes of action to circumvent extant antibiotic resistance mechanisms. Furthermore, although some interesting anti-Gram-negative drug candidates are nearing the beginning of clinical trials, they are limited in number and, even in the best-case scenario, many years away from the clinic.

N-substituted 3-acetyltetramic acid derivatives as antibacterial agents

In order to expand the structure-activity relationship of tetramic acid molecules with structural similarity to the antibiotic reutericyclin, 22 compounds were synthesized and tested against a panel of clinically relevant bacteria. Key structural changes on the tetramic acid core affected antibacterial activity. Various compounds in the N-alkyl 3-acetyltetramic acid series exhibited good activity against Gram-positive bacterial pathogens including Bacillus anthracis, Propionibacterium acnes, Enterococcus faecalis, and both Methicillin-sensitive and -resistant Staphylococcus aureus.

The increasing use of silver-based products as antimicrobial agents: a useful development or a cause for concern?

Silver first gained regulatory approval for use as an antimicrobial agent in the early 20th century, but its usage diminished with the introduction of antibiotics in the 1940s. Recently, however, topical silver has gained popularity once again, principally in the management of open wounds. This has been largely due to the spread of methicillin-resistant Staphylococcus aureus and the resultant reduction in first-line antibiotic prescribing. The increase in the use of topical silver has raised issues concerning silver resistance, together with questions about the standardization of antimicrobial testing methods for silver. Issues related to silver product testing include a failure to establish standard procedures for determining MIC values, an absence of recognized breakpoints, a lack of conformity in the way different products release silver and variations in the effects of microbiological media on silver release and the measurement of inhibitory activity. The clinical incidence of silver resistance remains low, and emergence of resistance can be minimized if the level of silver ions released from products is high and the bactericidal activity rapid.

Multiparameter assessments to determine the effects of sugars and antimicrobials on a polymicrobial oral biofilm

Clinical studies indicate relationships between dental plaque, a naturally formed biofilm, and oral diseases. The crucial role of nonmicrobial biofilm constituents in maintaining biofilm structure and biofilm-specific attributes, such as resistance to shear and viscoelasticity, is increasingly recognized. Concurrent analyses of the diverse nonmicrobial biofilm components for multiparameter assessments formed the focus of this investigation. Comparable numbers of Actinomyces viscosus, Streptococcus sanguinis, Streptococcus mutans, Neisseria subflava, and Actinobacillus actinomycetemcomitans cells were seeded into multiple wells of 96-well polystyrene plates for biofilm formation. Quantitative fluorescence and confocal laser scanning microscopy (CLSM) examined the influences of dietary sugars, incubation conditions, ingredients in oral hygiene formulations, and antibiotics on biofilm components. Biofilm extracellular polymeric substances (EPS) were examined with an optimized mixture of fluorescent lectins, with biofilm proteins, lipids, and nucleic acids detected with specific fluorescent stains. Anaerobic incubation of biofilms resulted in significantly more biofilm EPS and extractable carbohydrates than those formed under aerobic conditions (P < 0.05). Sucrose significantly enhanced biofilm EPS in comparison to fructose, galactose, glucose, and lactose (P < 0.05). CLSM demonstrated thicker biofilms under sucrose-replete conditions, along with significant increases in biofilm EPS, proteins, lipids, and nucleic acids, than under conditions of sucrose deficiency (P < 0.05). Agents in oral hygiene formulations (chlorhexidine, ethanol, and sodium lauryl sulfate), a mucolytic agent (N-acetyl-L-cysteine), and antibiotics with different modes of action (amoxicillin, doxycycline, erythromycin, metronidazole, and vancomycin) inhibited biofilm components (P < 0.05). Multiparameter analysis indicated a dose-dependent inhibition of biofilm EPS and protein by chlorhexidine and sodium lauryl sulfate, along with distinctive inhibitory patterns for subinhibitory concentrations of antibiotics. Collectively, these results highlight multiparameter assessments as a broad platform for simultaneous assessment of diverse biofilm components.

Molecular genetic and structural modeling studies of Staphylococcus aureus RNA polymerase and the fitness of rifampin resistance genotypes in relation to clinical prevalence

The adaptive and further evolutionary responses of Staphylococcus aureus to selection pressure with the antibiotic rifampin have not been explored in detail. We now present a detailed analysis of these systems. The use of rifampin for the chemotherapy of infections caused by S. aureus has resulted in the selection of mutants with alterations within the beta subunit of the target enzyme, RNA polymerase. Using a new collection of strains, we have identified numerous novel mutations in the beta subunits of both clinical and in vitro-derived resistant strains and established that additional, undefined mechanisms contribute to expression of rifampin resistance in clinical isolates of S. aureus. The fitness costs associated with rifampin resistance genotypes were found to have a significant influence on their clinical prevalence, with the most common clinical genotype (H481N, S529L) exhibiting no fitness cost in vitro. Intragenic mutations which compensate for the fitness costs associated with rifampin resistance in clinical strains of S. aureus were identified for the first time. Structural explanations for rifampin resistance and the loss of fitness were obtained by molecular modeling of mutated RNA polymerase enzymes.

Comparison of assays for detection of agents causing membrane damage in Staphylococcus aureus

OBJECTIVES

To develop a novel beta-galactosidase leakage assay for Staphylococcus aureus and to evaluate this alongside other simple methods for detection of agents that cause membrane damage in staphylococci.

METHODS

Using a PCR-based approach, a derivative of S. aureus RN4220 was constructed carrying the Escherichia coli lacZ gene under the control of the strong staphylococcal promoter, cap1A. Leakage of beta-galactosidase (BG) from this strain was examined after exposure for 10 min to various membrane-damaging agents at 4 x MIC, using a fluorescence assay and the substrate 4-methylumbelliferyl-beta-d-galactoside. Other assays for membrane damage involving protoplast lysis (PL), leakage of material absorbing at 260 nm (OD) and ATP release as well as the BacLight (BL) assay were carried out using established methods.

RESULTS

All the assays, with the exception of the PL assay, detected membrane damage induced by cetyltrimethylammonium bromide, nisin, clofazimine and protegrin IB-367. However, the ability to detect membrane damage induced by these agents differed between the assay systems. The assays also varied considerably in their signal-to-noise ratio, with the ATP assay providing values for nisin approaching 100-fold that of the control.

CONCLUSIONS

The PL assay is unsuitable for detection of membrane-damaging agents in S. aureus. The other assays, including the BG assay, detect membrane damage. The OD assay should be sufficient for most purposes since it is effective, rapid and cheap to perform. Studies requiring maximum sensitivity and discrimination should employ the ATP assay.