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Catabolite Sensing Regulatory Pathways in Group A Streptococcus Control Virulence and Host-Pathogen Interactions
Date of Award
Doctor of Philosophy (PhD)
As a pathogen Streptococcus pyogenes (Group A streptococcus) is remarkable for its ability to cause a wide range of diseases and infect hundreds of millions of people yearly. Group A streptococcus faces a unique challenge in its pathogenesis cycle in that humans are its only known reservoir, and colonization and infection commonly occur in the same tissue. Thus to persist within a specific host niche Group A streptococcus has developed a sizable arsenal of effectors and control mechanisms allowing it to combat and adapt to the host immune response. In most cases alteration of regulatory pathways controlling transcription of virulence factor genes leads to significant changes in the ability of Group A streptococcus to cause disease. Several environmental signals and regulators effecting expression of virulence factors have been identified. One of the most pervasive signals effecting streptococcal virulence factor transcription is growth phase. In virtually all described incidences, virulence factor transcript abundance is influenced by growth phase, yet specific signals and mechanisms of sensing growth phase have remained elusive. These studies were undertaken to study the mechanisms of growth phase dependent transcriptional regulation of Group A streptococcal virulence factors, to elucidate global changes in transcriptional control due to altered growth phase regulators, and to examine the role of growth phase regulation in host- pathogen interactions during streptococcal infection. In pursuit of these goals, we initially analyzed a growth phase controlled physiologic trait, the production of H2O2, linked to virulence in related pathogens. We identified the gene responsible for H2O2 production, lactate oxidase (lctO), and through transcriptional and biochemical analyses identified the carbon catabolite control protein (CcpA) as a central transcriptional regulator of growth phase in response to glucose availability. Inactivation of CcpA revealed global changes in virulence factor transcription and overlap with another recently identified carbohydrate regulator, LacD.1. Studies on these regulators indicated that while they respond to a common signal their effects on co- regulated genes are restricted to discrete times during streptococcal infection. Disruption of CcpA in particular led to attenuation of virulence and mis-regulation of a large contingent of virulence genes in infected tissues. Misregulation of target genes in the CcpA mutant correlated with an increase in host TNFα transcription, and occurred at times when wild type streptococci were causing maximal disease. Ablation of TNFα in the host led to an increase in local virulence in the CcpA deficient strain, and a switch from local to invasive disease during infection by wild type streptococci. In total, our studies have shown that growth phase regulation is comprised of multiple overlapping and organized pathways. Furthermore, functional growth phase regulation is necessary for Group A streptococcus to cause fully virulent disease, perhaps due to the interaction of growth phase regulated factors with host immune signaling pathways. Finally we have shown that TNFα is necessary to control localized infection and prevent dissemination to deeper tissues during infections by a streptococcal strain that normally causes localized non- spreading lesions.
Understanding the mechanisms by which Group A streptococcus senses the host environment and interacts with the immune system is especially critical in studies of streptococcal pathogenesis and development of anti-streptococcal therapies. A broad spectrum streptococcal protective vaccine is unavailable and without treatment relatively minor streptococcal infections can progress to life threatening invasive disease or lead to highly disabling nonsuppurative sequelae. Our studies suggest that targeting growth phase sensitive regulatory pathways is a good strategy in combating streptococcal disease. In addition, in light of the use of many immunosuppressive therapies as treatments for various ailments, our results indicate the need for further investigations into the role TNFα in controlling human streptococcal diseases; patients receiving antiTNFα treatments may be at an increased risk for serious infections by Group A streptococcus. Further analysis and identification of specific growth phase controlled factors contributing to attenuation of CcpA deficient strains, identification of factors involved in alteration of host-pathogen interactions resulting in disparate TNFα responses, and elucidation of the mechanisms by which TNFα production leads to control of localized infection will provide insights into Gram positive bacterial infection, and the molecular mechanisms determining the outcome of Group A streptococcal disease.
Chair and Committee
John Atkinson, Peter T. Chivers, David Haslam, L. David Sibley, Joseph Vogel
Kietzman, Colin Christopher, "Catabolite Sensing Regulatory Pathways in Group A Streptococcus Control Virulence and Host-Pathogen Interactions" (2010). Arts & Sciences Electronic Theses and Dissertations. 127.