The species B. anthracis is a typical member of the B. cereus-Group of the genus Bacillus. Unlike B. cereus, B. anthracis is non-motile, otherwise they are facultatively anaerobic Gram-positive spore-bearing bacilli (1.0 µm wide or wider). The central- to sub-terminal spores do not distend the cells. They are not haemolytic and the only species of the group to be sensitive to penicillin. They form white colonies on bacteriological media. The cells form a poly-D-glutamic acid capsule. The chromosomal DNA of B. anthracis has a GC content of 33.2mol%.

Their liquifaction of gelatin is slow; they produce the typical "inverted fir-tree" growth in gelatin stabs. The lecithinase reaction is slow. They are not motile, ferment a few carbohydrates producing acid from glucose but not gas. Salicin is slowly fermented. They do not grow above 50°C but grow in up to 7% NaCl. They produce catalase, give a positive Voges-Proskauer reaction and reduce nitrate to nitrite. They hydrolyse casein but not starch.

Virulent strains of B. anthracis carry two large plasmids, both of which are required for full virulence. The plasmid pX01 comprising 174 kb encodes the three components of the antrax toxin, while plasmid pX02 comprising 95 kb encodes the poly-D-glutamic acid capsule.

Different serotypes of B. anthracis are not recognized. The toxins and their components as well as the capsule are antigenic.

Enterotoxins:

B. anthracis does not produce enterotoxins.

Anthrax Toxin:

Anthrax toxin consists of three separate components, none of which are individually biologically active. The parts are the protective antigen (PA), oedema factor (EF) and lethal factor (LF). Injection of PA into animals can cause protection from subsequent challenge by full toxin. EF is an adenylate cyclase that elevates cAMP levels up to 200-fold in some tissue culture cells. LF is believed the major cause of death due to anthrax toxin.

Other Toxins:

The poly-D-glutamic acid capsule inhibits phagocytosis.

Haemolysins:

B. anthracis strains are not haemolytic or only weakly so.

Adhesins & Fimbriae:

Apart from the poly-D-glutamic acid capsule there do not appear to be such structures involved in virulence.

The members of the B. anthracis spores are extremely resistant to heat, cold, dessication and chemical disinfection and can survive in soil for very long periods, which have been measured in decades. They may have to grow vegetatively to reach a certain level before being able to infect grazing animals. Earthworms, flies and vultures have all been incriminated in the dispersal of anthrax.

Enteric Infections:

Human intestinal infections by B. anthracis are very rare and are usually caused by ingestion of highly contaminated food. The case fatality rate is 25-50%.

Extraintestinal Infections:

Cutaneous anthrax is the most common form accounting for nearly 95% of human cases. This follows deposition of spores obtained by contact with contaminated materials underneath the epidermis either by means of a skin abrasion or an insect bite. After an incubation period of 2-7 days pustules develop, which later form an ulcer and become surrounded by oedema, which goes black. (The word anthrax comes from the Greek word anthrakos for coal.) Cutaneous anthrax is usually self-limiting with lesions healing spontaneously in over 80% of cases. If untreated death may occur in up to 20% of cases, especially if the lesions obstruct an airway.

Inhalation anthrax occurs from inhaling spores of B. anthracis. Following germination of the spores and growth of vegetative cells, oedema and haemorrhage sets in in the lungs. When the bacteria gain access to the lymphatic system a severe generally fatal infection and intoxication sets in.

Antrax menigitis and oropharyngeal anthrax have also been recorded but these are rare.

Animal Infections:

Herbivorous animals are the main victims of anthrax, but omnivores like swine and carnivores like dogs can get the disease although they appear more resistant than herbivores. The herbivores, like ruminants, develop cerebral anoxia, pulmonary oedema with death occurring suddenly. At autopsy in sheep and cattle, the spleen is enlarged with a "blackbery jam" appearance.

Horses, dogs and cattle may have chronic anthrax with oedema of the tongue and pharyngeal tissues. Asphyxia may cause death.

B. anthracis strains can be grown very easily on most microbiological media. Lack of haemolysis on sheep blood agar and typical colonial morphology can guide to identification. There are no specific enrichment media for B. anthracis. Subculture on nutrient (or brain heart infusion) agar containing bicarbonate (0.5%) and bovine serum albumin (0.7%) and incubated in an atmosphere of 5% CO₂ enhances growth of B. anthracis. The use of the M'Fadyean stain for the presence of the capsule or a specific fluorescent-antibody for the capsule can give a rapid diagnostic answer. Other diagnostic tests include testing for the production of toxin, susceptibility to penicillin, and a range of standard microbiological tests.

B. anthracis survives well in soil especially in the spore state for many years. It has even been suggested that it may survive more than a century.

There are no specific industrial uses of B. anthracis.

Human vaccines have been developed, but are of limited use and may cause significant side effects. There are none currently in general use.

There are animal vaccines in use, which can effectively protect animals from an affected herd.