Bacillus subtilis stress responses

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Project Summary/Abstract Bacteria and humans have a complex relationship: our abundant commensal organisms provide numerous benefits, whereas pathogenic bacteria impose a large burden of morbidity and mortality. The immune system restricts bacterial growth through nutritional immunity, antimicrobial peptides, lytic enzymes, and phagocytic cells. Potential pathogens respond to these threats by the activation of specific adaptive responses, many of which are critical for virulence. We study stress responses in Bacillus subtilis, a model Gram positive bacterium. One project addresses responses to the changing availability of the essential nutrient metal ions zinc, iron, and manganese. The immune system restricts the growth of pathogens by metal sequestration, both in tissues (e.g. by calprotectin) and after phagocytosis. In addition, phagocytic cells kill cells by metal intoxication. We have demonstrated that metal ion homeostasis relies on specific metal-sensing transcription factors that respond to limitation and excess of iron (Fur and PerR), manganese (MntR), and zinc (Zur and CzrA). We will characterize the genes regulated by these transcription factors, their roles in metal homeostasis, and identify the physiological effects that result from both metal ion limitation and intoxication. This work will build upon our recent identification of the major efflux systems for both iron and manganese. The insights from these studies will be directly relevant to the similar stress responses present in human pathogens. The immune system also restricts the growth of pathogens by production of antibacterial peptides and lytic enzymes, both of which affect the integrity of the cell envelope. The cell envelope is also a target for many of our most important antibiotics. In a second project, we have defined several distinct cell envelope stress responses in B. subtilis, with a focus on those regulated by alternative sigma factors. We have identified an array of mutants with alterations in stress response pathways and in key central metabolic pathways that have elevated sensitivity to cell wall antibiotics, including the critically important beta-lactams. In addition, we explore newly discovered antibiotic synergies with possible implications for clinical approaches. Selection of antibiotic resistant suppressors provides a powerful approach for delineating the basis of antibiotic synergies, and the roles of specific stress response pathways. These pathways are central to cell envelope homeostasis generally, in addition to their role in sensing and responding to antibiotic-induced stress, and are implicated in the emergence of antibiotic tolerance and resistance in pathogens.