Characterization of the superoxide dismutases of Bacillus anthracis: Global and local approaches to the study of bacterial oxidative stress and metal ion homeostasis.

摘要

Bacillus anthracis, the causative agent of the disease anthrax, is a gram-positive bacterium found in soil habitats worldwide and exists in two morphologies: infectious, inert spores and replicative vegetative bacilli. We hypothesize that protective antioxidant enzymes and metal uptake systems contribute to bacterial fitness during the establishment of disease. We undertook in vitro physiological studies of B. anthracis on both global and local scales to define the roles that superoxide dismutases (SODs) play in this microbe's biology and to better characterize the connection between bacterial oxidative stress and metal ion homeostasis.; The creation of multiple sod deletion (Deltasod ) mutants showed that of the four putative B. anthracis sods, sodA1 encodes the predominantly active enzyme responsible for protection from oxidative insults. The SODA1 and SODA2 paralogs form active homodimers and heterodimers, but only slight physiological redundancy exists between the two. A third paralog, SOD15, is differentially expressed upon entry into stationary/sporulation phase, and is a member of a four-gene operon that may be involved in bacterial morphology. However, the SODs are not essential for survival within a mouse model of inhalational anthrax.; The global transcriptional profiles of B. anthracis to physiological redox perturbations imposed by hydrogen peroxide and paraquat showed that whereas hydrogen peroxide results in a response mainly defined by DNA metabolism, endogenous superoxide elicits a response indicative of metal ion homeostasis imbalances. B. anthracis produces two iron-chelating siderophores: petrobactin and bacillibactin. We found that transcriptional control of the genes needed for synthesis of these two metabolites is unique, and that end-point metabolite yields vary depending on metal availability and oxygenation. Lastly, we found that manganese availability protects B. anthracis from oxidative damage to proteins, and that mutants lacking both sodA1 and sodA2 are sensitive to manganese and iron limitation, connecting antioxidant capability, metal homeostasis, and metabolic efficiency. These in vitro observations shed light on the multiple physiological capabilities of B. anthracis and reveal the many ways that this microorganism is able to efficiently establish disease.