Bacillus anthracis physiology and genetics

Abstract

Bacillus anthracis is a member of the Bacillus cereus group species (also known as the "group 1 bacilli"), a collection of Gram-positive spore-forming soil bacteria that are non-fastidious facultative anaerobes with very similar growth characteristics and natural genetic exchange systems. Despite their close physiology and genetics, the B. cereus group species exhibit certain species-specific phenotypes, some of which are related to pathogenicity. B. anthracis is the etiologic agent of anthrax. Vegetative cells of B. anthracis produce anthrax toxin proteins and a poly-d-glutamic acid capsule during infection of mammalian hosts and when cultured in conditions considered to mimic the host environment. The genes associated with toxin and capsule synthesis are located on the B. anthracis plasmids, pXO1 and pXO2, respectively. Although plasmid content is considered a defining feature of the species, pXO1- and pXO2-like plasmids have been identified in strains that more closely resemble other members of the B. cereus group. The developmental nature of B. anthracis and its pathogenic (mammalian host) and environmental (soil) lifestyles of make it an interesting model for study of niche-specific bacterial gene expression and physiology.

Figure 1

B. anthracis cell morphology. (A) B. anthracis cells cultured on NBY medium containing 0.8% sodium bicarbonate and incubated at 37°C in at atmosphere containing 5% CO₂. Inserts show cells viewed in India Ink. Top, parent strain. Bottom, capBCADE-null mutant. (B) Fescue seedling in soil inoculated with B. anthracis spores and incubated at 30°C. Insert shows vegetative cells of B. anthracis in the rhizosphere of plant roots visualized using fluorescent microscopy as described (Saile and Koehler, 2006). (C) Macaca fascicularis (Cynomolgus macaque) animals. Insert shows blood smear taken from an infected animal with prominent bacteremia.

Figure 2

AtxA and virulence factor synthesis. Motifs apparent in the AtxA amino acid sequence are: WH, winged-helix; HTH, helix-turn-helix; PRD1 and PRD2, Phosphotransferase system Regulated Domains. PDGA, poly-D-glutamic acid. EIIB, Enzyme IIB similarity.