The capsule of Bacillus anthracis, a review

The capsule of Bacillus anthracis, composed of poly- d-glutamic acid, serves as one of the principal virulence factors during anthrax infection. By virtue of its negative charge, the capsule is purported to inhibit host defence through inhibition of phagocytosis of the vegetative cells by macrophages. In conjunction with lethal toxin and oedema toxin, whose target cells include macrophages and neutrophils, respectively, the capsule allows virulent anthrax bacilli to grow virtually unimpeded in the infected host. Spores germinating in the presence of serum and elevated CO2 release capsule through openings on the spore surface in the form of blebs which may coalesce before sloughing of the exosporium and outgrowth of the fully encapsulated vegetative cell. It has not been established that spore encapsulation plays a role in the early events of anthrax infection. The capsule appears exterior to the S-layer of the vegetative cell and does not require the S-layer for its attachment to the cell surface. The three membrane-associated enzymes required for synthesis of the capsule are encoded by the 60-MDa pX02 plasmid. The cistrons are arranged in the order capB, capC and capA and encode for proteins of 44, 16 and 46 kDa, respectively. The synthesis of capsule and toxin is, in part, under bicarbonate regulation by interaction of transacting proteins of the atxA gene on the 100-MDa pX01 toxin-encoding plasmid and the acpA gene on the pX02 plasmid. Therefore, capsule synthesis is enhanced in the presence of the atxA gene on the pX01 plasmid. An additional protein (with a predicted size of 51 kDa) is encoded by the dep gene located downstream from the cap region and appears to be a depolymerase that catalyses the hydrolysis of poly- d-glutamic acid into lower molecular weight polyglutamates. Although the biological function of the Dep protein is unknown, it has been proposed that the low molecular weight polyglutamates produced by the action of the enzyme may act to inhibit host defence mechanisms.