Streptococcus pyogenes or Group A Streptococcus (GAS) is a strictly human pathogenic bacterium, responsible for approximately 517,000 deaths per year worldwide. Its spectrum of infection is wide, ranging from superficial infections to severe, invasive infections. The first stages of severe GAS infections correspond to the bacterial adhesion to the tissue and their multiplication on its surface. The mechanisms involved in the early stages of infection have not yet been elucidated. Different studies have analyzed them, including one using an in vivo model of necrotizing fasciitis in non-human primates. More than 11.6 % of the strains collected at the point of inoculation have point mutations in the fabT gene, whereas clinical isolates from invasive infections do not. This highlights a counter-selection of fabT mutant strains in severe infections. Furthermore, inoculation of fabT-mutant GAS strains reveals that these strains have a strongly attenuated virulence. The goal of this thesis is to understand the link between FabT loss of function and this virulence defect. FabT is the transcriptional repressor of the genes involved in the fatty acid biosynthetic pathway, FASII, in Streptococci, Enterococci and Lactococci. FabT thus controls the fatty acid composition of the bacterial membrane. Our hypothesis is that these modifications, affecting membrane homeostasis, may impact the bacterial ability to adapt to its environment and thus account for the virulence defect.
After construction of a fabT mutant strain in GAS, the role of FabT was studied in a situation mimicking physiological conditions in vivo by adding, in the culture medium, fatty acids, which coupled to an acyl carrier protein, ACP, are FabT corepressors. Using a wide range of techniques from classical bacteriology to “omics”, we defined that a mutation in FabT controls, probably directly, the expression of the FASII genes and of a single other gene, degV. The deregulation of the FASII genes in the fabT mutant strain then influences the fatty acid and lipid composition of the GAS membrane, which may account for a lack of adaptation of the bacterium to its environment. To better characterize the relationship between FabT and virulence, we compared the behavior of wild-type and fabT mutant strains during the first stages of GAS infection. The wild-type strain multiplies in the presence of human cells, using lipids secreted by these cells. However, the fabT mutant strain exhibits defects in adhesion and in growth in the presence of these cells, and this limits colonization during infection. In fact, in an ex vivo human tissue infection model, the fabT mutant strain shows a colonization defect on the surface of this tissue due to the multiplication defect. The growth defect results in part from a higher mortality of the fabT mutant strain, but also from a higher energy consumption because of the absence of FASII repression.
Thus, the lack of FASII repression in a fabT-mutant strain leads to an unnecessary cycle of fatty acid synthesis that limits the bacterium's ability to multiply and thus restricts the virulence of GAS.