Quorum sensing and virulence in Gram positive pathogens: structure, function and inhibition of the agr system

The emergence, rapid spread and persistence of multi-antibiotic resistant bacteria is considered by the WHO as one of the three greatest global threats to human health. Antimicrobial resistance (AMR) threatens the treatment and outcome of even simple infections and common medical interventions (surgery, dentistry, obstetrics) that until recently were considered low-risk. Against this backdrop, the development of new classes of antibiotics has lagged far behind the urgent requirement for new drugs. This is in part because discovering safe and effective new antibiotics is scientifically challenging and because many major pharmaceutical companies withdrew from developing expensive new drugs likely to become rapidly obsolete through resistance. Consequently, novel antibacterial drugs with that do not succumb to conventional antibiotic resistance mechanisms, nor select for new forms of resistance, nor damage the host microflora, are desperately needed. The discovery of such drugs depends on a thorough understanding of the physiology and molecular biology of pathogenic bacteria and their strategies for colonizing host tissues and combatting host immune defences. These include the deployment of multiple virulence factors such as enzymes and toxins that cause host tissue damage and disease. Conventional antibiotics mostly act by killing bacteria and so exert enormous selective pressures leading to the emergence of resistant strains. If however, instead of killing bacteria, we simply prevent them deploying their virulence factors, infection should be attenuated with less pressure for resistance to emerge. Research on bacterial virulence factors and their control systems enables us to identify molecular target 'weak points' in bacteria so that methods for screening drug-like compounds active against such targets can be designed and new antibacterial drugs discovered. One promising target for anti-virulence agents is quorum sensing (QS). Although bacteria are single cell organisms, they can synchronize the activities of all the cells in a population through cell-to-cell communication. This is achieved through the production and sensing of signal molecules that inform the infecting bacteria that they are present in sufficient numerical strength to deploy their virulence factors and mount an attack. QS systems offer multiple molecular targets for anti-infective agents that include the production, export and response to QS signal molecules. In problematic multi-antibiotic resistant pathogens such as Staphylococcus aureus (including MRSA) and Clostridium difficile, virulence factors including many major exotoxins are controlled by the agr QS system that employs autoinducing peptide signal (AIP) molecules. In this research project we are seeking to understand in depth the way in which S. aureus produces and exports AIP signal molecules via two transmembrane proteins called AgrB and 1984 since these are key to QS and hence virulence. We propose to use a multidisciplinary approach combining microbiology with chemistry, structural biology to elucidate the functions and 3D structures of the key enzymes involved in AIP generation. We also plan to discover how AIPs are exported out of the bacterial cell and to develop new drug-like molecules that block AIP production generation in staphylococci. These will be tested for efficacy alone and in combination with conventional antibiotics in laboratory media and by using novel infection imaging tools that will provide information on when and where agr-dependent QS is switched on or off. The work will focus primarily on S. aureus but promising compounds will also be tested against enterococci, clostridia and listeria and other. other staphylococcal species.