New targets and strategies for the development of antibacterial agents

Microtiter plate with built-in oxygen sensors: a novel approach to investigate the dynamics of Pseudomonas aeruginosa growth suppression in the presence of divalent cations and antibiotics

The depletion of dissolved oxygen in a defined synthetic medium can be measured in real time, using a micro-well plate format, associated with a fluorescent plate reader. This technology is appropriate for investigating the effect of antibiotics on cell kinetics because there is a direct correlation between the latter and the amount of dissolved oxygen in the medium of an assay. In this study, the metabolic activity of the opportunistic human pathogen Pseudomonas aeruginosa PA01 was investigated using the OxoPlate OP96U optical sensor technology. The response of P. aeruginosa to aminoglycoside antibiotics when Ca²⁺and Mg²⁺ ions are present in the Evans defined synthetic medium was measured. The results revealed that the effect of antibiotics on P. aeruginosa is influenced by the concentration of divalent cations present in the test medium, although the efficiency of Ca²⁺ in supressing antibiotic activity was found to be greater than that of Mg²⁺. By comparison to tobramycin, the effect of amikacin is largely inhibited by the Ca²⁺and Mg²⁺concentrations. The study results underscore that the reliability of the observation of growth inhibitors is enhanced by the oxygen consumption measurements. Thus, the OxoPlate OP96U system is proven to be an accurate method to test the effectiveness of antibiotic treatments against P. aeruginosa.

The Signaling Molecule Indole Inhibits Induction of the AR2 Acid Resistance System in Escherichia coli

Induction of the AR2 acid response system of Escherichia coli occurs at a moderately low pH (pH 5.5) and leads to high levels of resistance to pH levels below 2.5 in the presence of glutamate. Induction is mediated in part by the EvgAS two component system. Here, we show that the bacterial signaling molecule indole inhibits the induction of key promoters in the AR2 system and blocks the development of glutamate-dependent acid resistance. The addition of tryptophan, the precursor for indole biosynthesis, had the same effects, and this block was relieved in a tnaA mutant, which is unable to synthesize indole. Expression of a constitutively active EvgS protein was able to relieve the inhibition caused by indole, consistent with EvgS being inhibited directly or indirectly by indole. Indole had no effect on autophosphorylation of the isolated cytoplasmic domain of EvgS. This is consistent with a model where indole directly or indirectly affects the ability of EvgS to detect its inducing signal or to transduce this information across the cytoplasmic membrane. The inhibitory activity of indole on the AR2 system is not related to its ability to act as an ionophore, and, conversely, the ionophore CCCP had no effect on acid-induced AR2 promoter activity, showing that the proton motive force is unlikely to be a signal for induction of the AR2 system.

Nucleoside Derived Antibiotics to Fight Microbial Drug Resistance: New Utilities for an Established Class of Drugs?

Novel antibiotics are urgently needed to combat the rise of infections due to drug-resistant microorganisms. Numerous natural nucleosides and their synthetically modified analogues have been reported to have moderate to good antibiotic activity against different bacterial and fungal strains. Nucleoside-based compounds target several crucial processes of bacterial and fungal cells such as nucleoside metabolism and cell wall, nucleic acid, and protein biosynthesis. Nucleoside analogues have also been shown to target many other bacterial and fungal cellular processes although these are not well characterized and may therefore represent opportunities to discover new drugs with unique mechanisms of action. In this Perspective, we demonstrate that nucleoside analogues, cornerstones of anticancer and antiviral treatments, also have great potential to be repurposed as antibiotics so that an old drug can learn new tricks.

Antimicrobial activity of PVP from an Antarctic bacterium, Janthinobacterium sp. Ant5-2, on multi-drug and methicillin resistant Staphylococcus aureus

Multiple drug resistant (MDR) and methicillin-resistant Staphylococcus aureus (MRSA) have become increasingly prevalent as a community acquired infection. As a result limited treatment options are available with conventional synthetic antibiotics. Bioprospecting natural products with potent antimicrobial activity show promise for developing new drugs against this pathogen. In this study, we have investigated the antimicrobial activity of a purple violet pigment (PVP) from an Antarctic bacterium, Janthinobacterium sp. Ant5-2 on 15 clinical MDR and MRSA strains. The colorimetric resazurin assay was employed to determine the minimum inhibitory concentration (MIC₉₀) of PVP against MDR and MRSA. The MIC₉₀ ranged between 1.57 µg/mL and 3.13 µg/mL, which are significantly lower than many antimicrobials tested from natural sources against this pathogen. The spectrophotometrically determined growth analysis and total microscopic counts using Live/dead® BacLight™ fluorescent stain exhibited a steady decrease in viability of both MDR and MRSA cultures following treatment with PVP at the MIC levels. In silico predictive molecular docking study revealed that PVP could be a DNA-targeting minor groove binding antimicrobial compound. The continued development of novel antimicrobials derived from natural sources with the combination of a suite of conventional antibiotics could stem the rising pandemic of MDR and MRSA along with other deadly microbial pathogens.

A novel antibacterial gene transfer treatment for multidrug-resistant Acinetobacter baumannii-induced burn sepsis

Sepsis caused by multidrug-resistant bacterial infections in critically injured patients has become a major clinical problem. Recently, Acinetobacter baumannii (AB) wound infections, especially in our critically injured soldiers fighting in Iraq and Afghanistan, is posing a major clinical problem and an economic burden. ConjuGon, Inc., has developed a novel antibacterial therapeutic technology using bacterial conjugation. The donor cells are attenuated Escherichia coli carrying a conjugative plasmid. The expression of bactericidal genes cloned on the plasmid is tightly repressed in the donor cells but becomes de-repressed once mobilized into a pathogen and disrupts protein synthesis. Here, we tested the efficacy of this novel conjugation technology to control and eradicate a drug-resistant clinical isolate of AB wound infection both in vitro and in a murine burn sepsis model. C57Blk/6J mice were divided into burn (B) and burn sepsis (BS) groups. All animals received a 12% TBSA dorsal scald full-thickness burn. The BS group was inoculated with multidrug-resistant AB (1 x 10(5) colony-forming units [CFU]) at the burn wound site. BS animals were either untreated or treated with increasing concentrations (10(3) - 19(10) CFU) of attenuated donor E. coli encoding bactericidal proteins. The survival rate was monitored for 10 days. The ability of donor cells to significantly diminish AB levels in the burn wound 24 hours after injury was determined by quantitative cultures. Donor cells were highly effective in killing AB in vitro. In the burn sepsis model, 90% B group animals survived, and 40% to 50% BS animals survived with no treatment in 5 to 6 days. Treatment with donor cells at 10(10) to 10(6) provided significant survival advantage (P < .05). Quantitative cultures of burn wounds revealed that AB numbers increased from 3 x 10(4) CFU to 7.8 +/- 4.4 x 10(9) CFU in 24 hours in the untreated group. Single treatment with donor cells (10(10) CFU) significantly reduced AB in the burn wound to less than the levels seeded into the wound (1.23 +/- 0.5 x 10(4) CFU; P < .05). Taken together, these results indicate that this novel technology is an efficient method to control drug-resistant AB burn wound infections and prevent their systemic spread.

Design, synthesis, and evaluation of efflux substrate–metal chelator conjugates as potential antimicrobial agents

Maintaining a proper balance of metal concentrations is critical to the survival of bacteria. We have designed and synthesized a series of conjugates of metal chelators and efflux transporter substrates aimed at disrupting bacterial metal homeostasis to achieve bacterial killing. Biological studies showed that two of the compounds had very significant antimicrobial effect with an MIC value of 7.8 microg/mL against Gram-positive Bacillus subtilis.

Efflux pumps in bacteria: overview, clinical relevance, and potential pharmaceutical target

Trends in microbial resistance suggest a dramatic increase in the frequency of reports of multi-drug efflux pumps in bacteria and fungi. Although it is difficult to determine whether this increase is due to the increased attention given to this resistance mechanism, or an increase in frequency, efflux pumps are becoming an important consideration in resistance emergence. These efflux pumps comprise at least four different classes in Gram-positive and Gram-negative bacteria, as well as in Streptomyces and fungi. As more efflux pumps are characterised and studied, both biochemically and structurally, the opportunity for intervention may arise.

Application of combinatorial chemistry to antimicrobial drug discovery

The emergence of pathogens resistant to currently available treatments is seen as a public health crisis. Since few new classes of antimicrobial drugs have been developed in the last two decades, it is becoming increasingly probable that healthcare providers will be faced with infections for which no chemotherapeutic agent is available. A renewed emphasis is being placed on employing the most advanced drug discovery technologies in the development of new antimicrobials. The recently introduced technologies of combinatorial chemistry offer new sources of chemical diversity, as well as methods with which to produce and rapidly test them. In the last few years, many groups have adopted a number of approaches in order to apply combinatorial chemistry to antimicrobial drug discovery. These combinatorial strategies, and the manner in which they are used to develop new screening formats or to identify new chemical leads are, reviewed.

Interactions among strategies associated with bacterial infection: pathogenicity, epidemicity, and antibiotic resistance

Infections have been the major cause of disease throughout the history of human populations. With the introduction of antibiotics, it was thought that this problem should disappear. However, bacteria have been able to evolve to become antibiotic resistant. Nowadays, a proficient pathogen must be virulent, epidemic, and resistant to antibiotics. Analysis of the interplay among these features of bacterial populations is needed to predict the future of infectious diseases. In this regard, we have reviewed the genetic linkage of antibiotic resistance and bacterial virulence in the same genetic determinants as well as the cross talk between antibiotic resistance and virulence regulatory circuits with the aim of understanding the effect of acquisition of resistance on bacterial virulence. We also discuss the possibility that antibiotic resistance and bacterial virulence might prevail as linked phenotypes in the future. The novel situation brought about by the worldwide use of antibiotics is undoubtedly changing bacterial populations. These changes might alter the properties of not only bacterial pathogens, but also the normal host microbiota. The evolutionary consequences of the release of antibiotics into the environment are largely unknown, but most probably restoration of the microbiota from the preantibiotic era is beyond our current abilities.

Peptide ligands in antibacterial drug discovery: use as inhibitors in target validation and target-based screening

There is an urgent need to develop novel classes of antibiotics to counter the inexorable rise of resistant bacterial pathogens. Modern antibacterial drug discovery is focused on the identification and validation of novel protein targets that may have a suitable therapeutic index. In combination with assays for function, the advent of microbial genomics has been invaluable in identifying novel antibacterial drug targets. The major challenge in this field is the implementation of methods that validate protein targets leading to the discovery of new chemical entities. Ligand-directed drug discovery has the distinct advantage of having a concurrent analysis of both the importance of a target in the disease process and its amenability to functional modulation by small molecules. VITA is a process that enables a target-based paradigm by using peptide ligands for direct in vitro and in vivo validation of antibacterial targets and the implementation of high-throughput assays to identify novel inhibitory molecules. This process can establish sufficient levels of confidence indicating that the target is relevant to the disease process and inhibition of the target will lead to effective disease treatment.

Digging for innate immunity since Darwin and Metchnikoff

Immune systems are, increasingly, being studied from comparative perspectives. The analysis of the immune-defense systems of invertebrates, such as fruit flies and earthworms, is an important part of this effort. These systems are innate, natural non-specific, non-anticipatory and non-clonal. This is in contrast to the macrophage T and B systems that characterize vertebrate adaptive immunity whose properties can be categorized as adaptive, induced, specific, anticipatory, and clonal. In this review, we will focus on the earthworm system. Earthworms, like other complex invertebrates, possess several leukocyte types and synthesize and secrete a variety of immunoprotective molecules. The system as a whole effects phagocytosis, encapsulation, agglutination, opsonization, clotting and lysis of foreign components. At least two major leukocytes, small coelomocytes, and large coelomocytes mediate lytic reactions against several targets. Destruction of tumor cells in vitro shows that phagocytosis and natural killer cell responses are distinct properties of coelomocytes. A third type, the chlorogogen cell, synthesizes and sheds effector lytic molecules. Among the lytic molecules, three have been identified and sequenced (fetidins, CCF-1, lysenin) and another has been discovered (eiseniapore), while three other molecules, H(1) H(2) H(3), share agglutinating and lysing functions. In contrast to these, Lumbricin I is the only known molecule of the earthworm system that is antimicrobial but non-lytic. Altogether the cellular and humoral components of the earthworm system function to distinguish between self and not self, dispose of internal (cancer?), damaged components and external antigens (microbes). The evolutionary context of the earthworm innate immune system is discussed at the end of this article.

Better control of antibiotic resistance

This text summarizes the conclusions of the French Working Party to Promote Research to Control Bacterial Resistance, initiated by the French Institute for Public Health Surveillance. The goal was to identify and prioritize the research areas most pertinent to the evolution of antibiotic resistance. The working group was part of a nationwide consultation of experts in the field of bacterial resistance and was coordinated with 2 other groups addressing (1) the use and surveillance of resistance to antibiotics and (2) the control and prevention of resistance to antibiotics. The proposals were discussed at a meeting held on 13 January 1999 by a large group of French microbiologists and clinicians who specialize in infectious diseases. The expert panel stressed that the determinants of evolution of antimicrobial resistance and the possibility of reversing this evolution are not completely known or understood. It emphasized the need for efforts to anticipate emergence of new resistances, to analyze the consequences of bacterial resistance, to develop rapid tests for determination of susceptibility to antibiotics, and to develop new antibiotics.

Rapid antibiotic susceptibility testing via electrochemical measurement of ferricyanide reduction by Escherichia coli and Clostridium sporogenes

Electrochemical measurement of respiratory chain activity allows rapid and reliable screening for antibiotic susceptibility in microorganisms. Chronoamperometry and chronocoulometry of suspensions of aerobically cultivated E. coli combined with the non-native oxidant potassium hexacyanoferrate(III) (ferricyanide) yield signals for reoxidation of the reduction product ferrocyanide that are much smaller if the E. coli has been incubated briefly with an effective antibiotic compound. Chronocoulometric results, obtained following 20-min incubation with antibiotic and 2-min measurement in assay buffer containing 50 mM ferricyanide and 10 mM succinate, at +0.50 V vs Ag/AgCl at a Pt working electrode, were compared with traditional disk diffusion susceptibility testing, which requires overnight incubation on agar plates; the results show significantly lower accumulation of ferrocyanide in all cases in which growth inhibition was observed in the disk diffusion assay. A range of antibiotic compounds (13) were examined that possess different mechanisms of action. Quantitative determination of IC50 values for penicillin G and chloramphenicol yielded values that were 100-fold higher than those obtained by standard turbidity methods after 10-h incubation; this is likely a result of the very brief (10 min) exposure time to the antibiotics. Addition of 5 microM 2,6-dichlorophenolindophenol, a hydrophobic electron-transfer mediator, to the assay mixture allowed susceptibility testing of a Gram-positive obligate anaerobe, Clostridium sporogenes. This rapid new assay will facilitate clinical susceptibility testing, allowing appropriate treatment virtually as soon as a clinical isolate can be obtained.

Novel fluorescent technology platform for high throughput cytotoxicity and proliferation assays

We have developed a novel fluorescent Oxygen BioSensor technology platform adaptable to many applications in the area of drug discovery and development, particularly cell-based assays. This biosensor technology requires no additional reagents or incubations, and affords continuous real-time readout of dissolved oxygen concentrations. Since the level of oxygen dissolved in an assay's medium correlates to the number and viability of the cells in the medium, this technology is ideally suited for monitoring cell viability, proliferation, or death. The technology is particularly well suited to investigating cells' kinetic responses to proliferative or toxic stimuli, such as drugs. When incorporated into a 96- or 384-well microplate format, it is compatible with standard laboratory automation systems. Here we present data illustrating the application of the Oxygen BioSensor technology for rapid, homogeneous detection and evaluation of metabolic activity of a variety of eukaryotic and prokaryotic cells, including mammalian cells, insect cells, yeast, and bacteria. In the absence of toxic substances, we find a good correlation between cell number and signal over a wide range of cell concentrations and growth times. To evaluate the usefulness of the Oxygen BioSensor for cytotoxicity assays, we have performed a series of experiments using a range of toxic agents and cell types, including both bacteria and mammalian cell lines. In a side-by-side comparison to standard MTT assays using HL60 cells, comparable IC(50) values were found with the Oxygen BioSensor for five different toxins or drugs. This assay method does not have the need for additional reagents, handling steps, or incubation periods required by the MTT assays.

Novel approaches to the discovery of antimicrobial agents

With the completion of numerous bacterial genome sequences, the discovery of antibacterial drugs has fully entered the genomic era. The strategies for effectively using genomic information for target identification, target characterization, screen development and compound evaluation are emerging, and have greatly increased the number of antibacterial targets available for screening. Fortunately, simultaneous efforts in improving miniaturization, robotics and database tools are underway so that the potential of genomics can be realized.

Bacteria lacking a multidrug pump: a sensitive tool for drug discovery

Microorganisms express multidrug resistance pumps (MDRs) that can confound antibiotic discovery. We propose the use of mutants deficient in MDRs to overcome this problem. Sensitivity to quinolones and to amphipathic cations (norfloxacin, benzalkonium chloride, cetrimide, pentamidine, etc.) was increased 5- to 30-fold in a Staphylococcus aureus mutant with a disrupted chromosomal copy of the NorA MDR. NorA was required both for increased sensitivity to drugs in the presence of an MDR inhibitor and for increased rate of cation efflux. This requirement suggests that NorA is the major MDR protecting S. aureus from the antimicrobials studied. A 15- to 60-fold increase in sensitivity to antimicrobials also was observed in wild-type cells at an alkaline pH that favors accumulation of cations and weak bases. This effect was synergistic with a norA mutation, resulting in an increase up to 1,000-fold in sensitivity to antimicrobials. The usefulness of applying MDR mutants for natural product screening was demonstrated further by increased sensitivity of the norA- strain to plant alkaloid antimicrobials, which might be natural MDR substrates.

Screening for novel antimicrobials from encoded combinatorial libraries by using a two-dimensional agar format

A sensitive lawn-based format has been developed to screen bead-tethered combinatorial chemical libraries for antimicrobial activity. This method has been validated with beads linked to penicillin V via a photocleavable chemical linker in several analyses including a spike-and-recover experiment. The lawn-based screen sensitivity was modified to detect antibacterial compounds of modest potency, and a demonstration experiment with a naive combinatorial library of over 46,000 individual triazines was evaluated for antibacterial activity. Numerous hits were identified, and both active and inactive compounds were resynthesized and confirmed in traditional broth assays. This demonstration experiment suggests that novel antimicrobial compounds can be easily identified from very large combinatorial libraries of small, nonpeptidic compounds.

Antibacterial agents that inhibit two-component signal transduction systems

A class of antibacterials has been discovered that inhibits the growth of Gram-positive pathogenic bacteria. RWJ-49815, a representative of a family of hydrophobic tyramines, in addition to being a potent bactericidal Gram-positive antibacterial, inhibits the autophosphorylation of kinase A of the KinA::Spo0F two-component signal transduction system in vitro. Analogs of RWJ-49815 vary greatly in their ability to inhibit growth of bacteria and this ability correlates directly with their activity as kinase A inhibitors. Compared with the potent quinolone, ciprofloxacin, RWJ-49815 exhibits reduced resistance emergence in a laboratory passage experiment. Inhibition of the histidine protein kinase::response regulator two-component signal transduction pathways may present an opportunity to depress chromosomal resistance emergence by targeting multiple proteins with a single inhibitor in a single bacterium. Such inhibitors may represent a class of antibacterials that potentially may represent a breakthrough in antibacterial therapy.

Bacterial protein secretion--a target for new antibiotics?

The heavy use of antibiotics over recent decades has resulted in widespread resistance of bacteria to many drugs. Overcoming resistance requires new approaches to antibiotic development, including the exploitation of new targets in the bacterial cell. Protein secretion is essential for bacterial cell growth and virulence, so it could be a suitable target for new therapeutic agents.