Bacillus anthracis is a gram-positive and rod-shaped bacterium that causes anthrax, a deadly disease to livestock and, occasionally, to humans. It is the only permanent (obligate) pathogen within the genus Bacillus. Its infection is a type of zoonosis, as it is transmitted from animals to humans. It was discovered by a German physician Robert Koch in 1876, and became the first bacterium to be experimentally shown as a pathogen. The discovery was also the first scientific evidence for the germ theory of diseases.
B. anthracis measures about 3 to 5 μm long and 1 to 1.2 μm wide. The reference genome consists of a 5,227,419 bp circular chromosome and two extrachromosomal DNA plasmids, pXO1 and pXO2, of 181,677 and 94,830 bp respectively, which are responsible for the pathogenicity. It forms a protective layer called endospore by which it can remain inactive for many years and suddenly becomes infective under suitable environmental conditions. Because of the resilience of the endospore, the bacterium is one of the most popular biological weapons. The protein capsule (poly-D-gamma-glutamic acid) is key to evasion of the immune response. It feeds on the heme of blood protein haemoglobin using two secretory siderophore proteins, IsdX1 and IsdX2.
Untreated B. anthracis infection is usually deadly. Infection is indicated by inflammatory, black, necrotic lesions (eschars). The sores usually appear on the face, neck, arms, or hands. Fatal symptoms include a flu-like fever, chest discomfort, diaphoresis (excessive sweating), and body aches. The first animal vaccine against anthrax was developed by French chemist Louis Pasteur in 1881. Different animal and human vaccines are now available. The infection can be treated with common antibiotics such as penicillins, quinolones, and tetracyclines.
Description
B. anthracis are rod-shaped bacteria, approximately 3 to 5 μm long and 1 to 1.2 μm wide. When grown in culture, they tend to form long chains of bacteria. On agar plates, they form large colonies several millimeters across that are generally white or cream colored. Most B. anthracis strains produce a capsule that gives colonies a slimy mucus-like appearance.
It is one of few bacteria known to synthesize a weakly immunogenic and antiphagocytic protein capsule (poly-D-gamma-glutamic acid) that disguises the vegetative bacterium from the host immune system. Most bacteria are surrounded by a polysaccharide capsule rather than poly-g-D-glutamic acid which provides an evolutionary advantage to B. anthracis. Polysaccharides are associated with adhesion of neutrophil-secreted defensins that inactivate and degrade the bacteria. By not containing this macromolecule in the capsule, B. anthracis can evade a neutrophilic attack and continue to propagate infection. The difference in capsule composition is also significant because poly-g-D-glutamic acid has been hypothesized to create a negative charge which protects the vegetative phase of the bacteria from phagocytosis by macrophages. The capsule is degraded to a lower molecular mass and released from the bacterial cell surface to act as a decoy to protect the bacteria from complement.
Like Bordetella pertussis, it forms a calmodulin-dependent adenylate cyclase exotoxin known as anthrax edema factor, along with anthrax lethal factor. It bears close genotypic and phenotypic resemblance to Bacillus cereus and Bacillus thuringiensis. All three species share cellular dimensions and morphology. All form oval spores located centrally in an unswollen sporangium. B. anthracis endospores, in particular, are highly resilient, surviving extremes of temperature, low-nutrient environments, and harsh chemical treatment over decades or centuries.