Bacillus anthracis is a Gram-positive spore-forming, rod-shaped bacterium, with a width of 1-1.2µm and a length of 3-5µm. It can be grown in an ordinary nutrient medium under aerobic or anaerobic conditions^[1].

Structure of Bacillus anthracis.

It is the only bacterium known to synthesize a protein capsule (D-glutamate), and the only pathogenic bacterium to carry its own adenylyl cyclase virulence factor (edema factor). It bears close genotypical and phenotypical resemblance to Bacillus cereus and Bacillus thuringiensis. All three species share cellular dimensions and morphology. All form oval spores located centrally in a non-swollen sporangium. Bacillus anthracis spores in particular are highly resilient, surviving extremes of temperature, low-nutrient environments, and harsh chemical treatment over decades or centuries.

Historical background

CapD protein crystal structure of Bacillus anthracis.

Casimir Davaine demonstrated that the symptoms of anthrax were invariably accompanied by the microbe B. anthracis.^[2] Aloys Pollender is also credited for this discovery. B. anthracis was the first bacterium conclusively demonstrated to cause disease, by Robert Koch in 1876.^[3] The species name anthracis is from the Greek anthrakis (ἄνθραξ), meaning coal and referring to the most common form of the disease, cutaneous anthrax, in which large black skin lesions are formed.

Pathogenesis

Three forms of anthrax disease are recognized based on their form of inoculation.

Cutaneous

the most common form (95%), causes a localized inflammatory black necrotic lesion (eschar)

Pulmonary

highly fatal and characterized by sudden massive chest edema followed by cardiovascular shock

Gastrointestinal

rare but also fatal (causes death to 25%) type results from ingestion of spores

Treatment

Infections with B. anthracis can be treated with β-lactam antibiotics such as penicillin, and others which are active against Gram-positive bacteria.^[4] Penicillin-resistant B. anthracis can be treated with fluoroquinolones such as ciprofloxacin or tetracycline antibiotics such as doxycycline.

Laboratory research

Components of tea, such as polyphenols, have the ability to inhibit the activity both of Bacillus anthracis and its toxin considerably; spores, however, are not affected. The addition of milk to the tea completely inhibits its antibacterial activity against anthrax^[5]. Activity against the B. athracis in the laboratory does not prove that drinking tea affects the course of an infection, since it is unknown how these polyphenols are absorbed and distributed within the body.

Host Interactions

As with other pathogenic bacteria, B. anthracis must acquire iron in order to grow and proliferate in its host environment. The most readily available iron sources for pathogenic bacteria are the heme groups used by the host in the transport of oxygen. In order to scavenge heme from host hemoglobin and myoglobin, B. anthracis uses two secretory siderophore proteins, IsdX1 and IsdX2. These proteins can separate heme from hemoglobin, allowing surface proteins of B. anthracis to transport it into the cell.^[6]