Exploitation of quorum sensing for the discovery of novel agents against staphylococci

The emergence, rapid spread and persistence of multi-antibiotic resistant bacteria constitutes a global health threat. The World Health Organization has stated that ?no population is more vulnerable to multi-antibiotic resistance than those admitted to hospital wards?. In the UK, healthcare associated infections account for over 5000 deaths annually and are associated with enormous personal and financial costs to the individual, their family and to the NHS (estimated at over #1 billion p.a.). In this context, both hospital-acquired methicillin-resistant Staphylococcus aureus (HA-MRSA) and Clostridium difficile are particularly problematic. This is not only a consequence of their ability to cause disease but also because antibiotic usage increases resistance and infection rates. Recently new ?community acquired? MRSA strains (CA-MRSA) have emerged which cause invasive infections in healthy young people. Against this backdrop, the development of new classes of antibiotics has lagged far behind the urgent requirement for new drugs, in part because of the reluctance of major pharmaceutical companies to develop expensive new drugs likely to become rapidly obsolete through resistance. Consequently we need to gain better insights into the infection-specific lifestyle of bacteria if we are to discover new ways of preventing and treating infection and reducing the selection of resistant strains. In this project we are seeking to understand how MRSA bacteria use chemical signals to communicate with each to make decisions about when to deploy the armoury of toxins they need to fight off human host defences, grow inside human cells and damage tissues. By understanding the chemical nature of these signals, the way in which they are produced by the bacterial cell and sensed by receptor proteins on the bacterial cell surface we will develop new drug molecules capable of controlling infection by blocking this signalling system. So far we have discovered three different families of small molecules which either inhibit signalling or growth or both. We will therefore use medicinal chemistry and biological approaches to understand how these compounds work at the molecular level and use this information to design agents with increased potency in antibacterial assays in the laboratory. The most promising compounds will be tested for efficacy in experimental animal infection models. The work with focus on primarily MRSA but promising compounds will also be tested against C. difficile and related pathogens.

Staphylococcal infections are of major clinical importance. MRSA strains which exhibit multi-antibiotic resistance and cause invasive infections in healthy individuals constitute a serious health threat. While resistance has evolved against virtually every antibiotic deployed, the development of new classes of antibiotics has lagged far behind the urgent requirement for new drugs. Many major pharmaceutical companies have withdrawn from the antibiotic discovery field mainly because of the huge economic cost of developing drugs likely to become rapidly obsolete through resistance. Thus, there is an urgent need to identify novel antibacterial targets and develop new agents effective against multi-resistant bacteria. This in turn depends on a thorough understanding of the basic molecular biology and physiology of bacterial pathogens. In this context, the attenuation of virulence through the blockade of sophisticated global regulatory systems such as quorum sensing (QS) offers an attractive target. In S. aureus, the agr QS system, which is also shared by Gram positive pathogens including Listeria, Enterococcus and Clostridium, is central to virulence gene regulation, intracellular survival and biofilm development and dispersal. Here we propose to build on our previous work by employing a multidisciplinary research strategy combining chemical biology with bacterial physiology and in vivo studies to obtain detailed molecular insights into (a) the recognition of auto-inducing peptide (AIP) signal molecules by the histidine sensor kinase receptor, AgrC, and (b) the generation and export of AIPs via the transmembrane enzyme, AgrB. Unexpectedly, both growth and agr-dependent QS in S. aureus can be inhibited by Gram negative bacterial QS signal molecules and we have discovered analogues (tetramic and tetronic acids) which exhibit potent activity against staphylococci and other Gram positive bacteria including Clostridium difficile. In addition we have identified a class of thiol-reactive aryl quinones which inhibit agr potentially through the inactivation of SarA family proteins in S. aureus which regulate virulence via agr-dependent and -independent mechanisms. We will therefore undertake extensive structure activity relationship, mechanistic and translational studies on the each compound class to elucidate their mechanisms of action and to develop agents with increased potency. The most promising compounds including novel AIP antagonists will be investigated for their activity against planktonic, biofilm and intracellular staphylococci in laboratory culture and for in vivo efficacy in experimental animal infection models. The work has broad implications for antibacterial agent discovery beyond S. aureus given the conservation of agr systems in Gram positive pathogens and two component systems in bacteria generally