Antimicrobials for bacterial bioterrorism agents

Inhibition of Inosine-5'-monophosphate Dehydrogenase from Bacillus anthracis: Mechanism Revealed by Pre-Steady-State Kinetics

Inosine-5'-monophosphate dehydrogenase (IMPDH) catalyzes the conversion of inosine 5'-monophosphate (IMP) to xanthosine 5'-monophosphate (XMP). The enzyme is an emerging target for antimicrobial therapy. The small molecule inhibitor A110 has been identified as a potent and selective inhibitor of IMPDHs from a variety of pathogenic microorganisms. A recent X-ray crystallographic study reported that the inhibitor binds to the NAD(+) cofactor site and forms a ternary complex with IMP. Here we report a pre-steady-state stopped-flow kinetic investigation of IMPDH from Bacillus anthracis designed to assess the kinetic significance of the crystallographic results. Stopped-flow kinetic experiments defined nine microscopic rate constants and two equilibrium constants that characterize both the catalytic cycle and details of the inhibition mechanism. In combination with steady-state initial rate studies, the results show that the inhibitor binds with high affinity (Kd ≈ 50 nM) predominantly to the covalent intermediate on the reaction pathway. Only a weak binding interaction (Kd ≈ 1 μM) is observed between the inhibitor and E·IMP. Thus, the E·IMP·A110 ternary complex, observed by X-ray crystallography, is largely kinetically irrelevant.

Targeting virulence not viability in the search for future antibacterials

New antibacterials need new approaches to overcome the problem of rapid antibiotic resistance. Here we review the development of potential new antibacterial drugs that do not kill bacteria or inhibit their growth, but combat disease instead by targeting bacterial virulence.

Treatment and prophylaxis of melioidosis

Melioidosis, infection with Burkholderia pseudomallei, is being recognised with increasing frequency and is probably more common than currently appreciated. Treatment recommendations are based on a series of clinical trials conducted in Thailand over the past 25 years. Treatment is usually divided into two phases: in the first, or acute phase, parenteral drugs are given for ≥10 days with the aim of preventing death from overwhelming sepsis; in the second, or eradication phase, oral drugs are given, usually to complete a total of 20 weeks, with the aim of preventing relapse. Specific treatment for individual patients needs to be tailored according to clinical manifestations and response, and there remain many unanswered questions. Some patients with very mild infections can probably be cured by oral agents alone. Ceftazidime is the mainstay of acute-phase treatment, with carbapenems reserved for severe infections or treatment failures and amoxicillin/clavulanic acid (co-amoxiclav) as second-line therapy. Trimethoprim/sulfamethoxazole (co-trimoxazole) is preferred for the eradication phase, with the alternative of co-amoxiclav. In addition, the best available supportive care is needed, along with drainage of abscesses whenever possible. Treatment for melioidosis is unaffordable for many in endemic areas of the developing world, but the relative costs have reduced over the past decade. Unfortunately there is no likelihood of any new or cheaper options becoming available in the immediate future. Recommendations for prophylaxis following exposure to B. pseudomallei have been made, but the evidence suggests that they would probably only delay rather than prevent the development of infection.

Interplay between three RND efflux pumps in doxycycline-selected strains of Burkholderia thailandensis

Background

Efflux systems are involved in multidrug resistance in most Gram-negative non-fermentative bacteria. We have chosen Burkholderia thailandensis to dissect the development of multidrug resistance phenotypes under antibiotic pressure.

Methodology/Principal Findings

We used doxycycline selection to obtain several resistant B. thailandensis variants. The minimal inhibitory concentrations of a large panel of structurally unrelated antibiotics were determined ± the efflux pump inhibitor phenylalanine-arginine ß-naphthylamide (PAßN). Membrane proteins were identified by proteomic method and the expressions of major efflux pumps in the doxycycline selected variants were compared to those of the parental strains by a quantitative RT-PCR analysis. Doxycycline selected variants showed a multidrug resistance in two major levels corresponding to the overproduction of two efflux pumps depending on its concentration: AmrAB-OprA and BpeEF-OprC. The study of two mutants, each lacking one of these pumps, indicated that a third pump, BpeAB-OprB, could substitute for the defective pump. Surprisingly, we observed antagonistic effects between PAßN and aminoglycosides or some ß-lactams. PAßN induced the overexpression of AmrAB-OprA and BpeAB-OprB pump genes, generating this unexpected effect.

Conclusions/Significance

These results may account for the weak activity of PAßN in some Gram-negative species. We clearly demonstrated two antagonistic effects of this molecule on bacterial cells: the blocking of antibiotic efflux and an increase in efflux pump gene expression. Thus, doxycycline is a very efficient RND efflux pump inducer and PAßN may promote the production of some efflux pumps. These results should be taken into account when considering antibiotic treatments and in future studies on efflux pump inhibitors.

Repurposing screens identify rifamycins as potential broad-spectrum therapy for multidrug-resistant Acinetobacter baumannii and select agent microorganisms

Aims: Estimates suggest that the drug discovery and development processes take between 10 and 15 years, with costs ranging between US$500 million and $2 billion. A growing number of bacteria have become resistant to approved antimicrobials. For example, the Gram-negative bacterium Acinetobacter baumannii has become multidrug resistant (MDR) and is now an important pathogen to the US military in terms of wound infections. Industry experts have called for a ‘disruptive’ transformation of the drug discovery process to find new chemical entities for treating drug-resistant infections. One such attempt is drug ‘repurposing’ or ‘repositioning’ – that is, identification and development of new uses for existing or abandoned pharmacotherapies. Materials & methods: Using a novel combination of screening technologies based on cell growth and cellular respiration, we screened 450 US FDA-approved drugs from the NIH National Clinical Collection against a dozen clinical MDR A. baumannii (MDRAb) isolates from US soldiers and Marines. We also screened the collection against a diverse set of select agent surrogate pathogens. Results: Seventeen drugs showed promising antimicrobial activity against all MDRAb isolates and select agent surrogates; three of these compounds – all rifamycins – were found to be effective at preventing growth and preventing cellular respiration of MDRAb and select agent surrogate bacteria when evaluated in growth prevention assays, highlighting the potential for repurposing. Conclusion: We report the discovery of a class of known compounds whose repurposing may be useful in solving the current problem with MDRAb and may lead to the discovery of broad-spectrum antimicrobials.

Extracytoplasmic stress responses induced by antimicrobial cationic polyethylenimines

The ability of an antimicrobial, cationic polyethylenimine (PEI+) to induce the three known extracytoplasmic stress responses of Escherichia coli was quantified. Exposure of E. coli to PEI+ in solution revealed specific, concentration-dependent induction of the Cpx extracytoplasmic cellular stress response, ~2.0-2.5-fold at 320 μg/mL after 1.5 h without significant induction of the σ(E) or Bae stress responses. In comparison, exposure of E. coli to a non-antimicrobial polymer, poly(ethylene oxide) (PEO), resulted in no induction of the three stress responses. The antimicrobial small molecule vanillin, a known membrane pore-forming compound, was observed to cause specific, concentration-dependent induction of the σ(E) stress response, ~6-fold at 640 μg/mL after 1.5 h, without significant induction of the Cpx or Bae stress responses. The different stress response induction profiles of PEI+ and vanillin suggest that although both are antimicrobial compounds, they interact with the bacterial membrane and extracytoplasmic area by unique mechanisms. EPR studies of liposomes containing spin-labeled lipids exposed to PEI+, vanillin, and PEO reveal that PEI+ and PEO increased membrane stability, whereas vanillin was found to have no effect.