The identification and characterization of proteins of the Bacillus anthracis exosporium

Bacillus anthracis, the causative agent of anthrax, is a Gram-positive spore forming bacteria. The outermost layer of the spore is a poorly understood structure called the exosporium, and is composed of a basal layer and a hair-like nap. As the first part of the spore that comes into contact with host cells or the soil it is imperative that we gain a full understanding of the exosporium in order to assess its potential role in virulence and survival. The first step towards achieving this lies in identifying what proteins the exosporium is composed of and how they interact to form this structure. To this end we have identified twenty proteins found in or closely associated with the B. anthracis exosporium. Several of these proteins were unique and of great interest for determining a role of the exosporium. Among them is the collagen-like glycoprotein BclA that is the immunodominant protein on the surface of B. anthracis spores and the main structural component of the hair-like nap. BxpB, a basal layer protein, appears to interact with BclA and anchor it to the basal layer of the exosporium. ExsY also appears to interact with BclA and its absence due to deletion of the exsY gene results in aberrant exosporium formation. Alanine racemase found in purified exosporium is discontinuously localized on the spore surface and may act in conversion of the germinant L-alanine into the germination inhibitor D-alanine. We were able to use the localization of alanine racemase as a marker to show an absolute bias of the vegetative cell to always outgrow from the same end of every spore. Although much progress has been made in the last few years, further research on the Bacillus anthracis exosporium is necessary for a complete understanding of the interactions among its protein components and their role in the bacteria's survival.