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, often described as having a “boxcar” morphology due to their rectangular, blunt-ended appearance. 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.Choo, M. K., Sano, Y., Kim, C., Yasuda, K., Li, X. D., Lin, X., … Park, J. M. (2017). TLR sensing of bacterial spore-associated RNA triggers host immune responses with detrimental effects. Journal of Experimental Medicine, 214(5), 1297–1311. Most other bacterial species 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 host macrophages.Choudhury, B., Leoff, C., Saile, E., Wilkins, P., Quinn, C. P., Kannenberg, E. L., & Carlson, R. W. (2006). The Structure of the Major Cell Wall Polysaccharide of Bacillus anthracis is Species-specific. Journal of Biological Chemistry, 281(38), 27932–27941. 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 the host's complement system.Makino, S., M. Watarai, H. I. Cheun, T. Shirahata, and I. Uchida. 2002. Effect of the lower molecular capsule released from the cell surface of Bacillus anthracis on the pathogenesis of anthrax. J. Infect. Dis. 186:227–233.
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.
The endospore is a dehydrated cell with thick walls and additional layers that form inside the cell membrane. It can remain inactive for many years, but if it comes into a favorable environment, it begins to grow again. It initially develops inside the rod-shaped form. Features such as the location within the rod, the size and shape of the endospore, and whether or not it causes the wall of the rod to bulge out are characteristic of particular species of Bacillus. Depending upon the species, the endospores are round, oval, or occasionally cylindrical. They are highly refractile and contain dipicolinic acid. Electron micrograph sections show they have a thin outer endospore coat, a thick spore cortex, and an inner spore membrane surrounding the endospore contents. The endospores resist heat, drying, and many disinfectants (including 95% ethanol).Bergey's Manual of Systematic Bacteriology, vol. 2, p. 1105, 1986, Sneath, P.H.A.; Mair, N.S.; Sharpe, M.E.; Holt, J.G. (eds.); Williams & Wilkins, Baltimore, Maryland, USA Because of these attributes, B. anthracis endospores are extraordinarily well-suited to use (in powdered and aerosol form) as biological weapons. Such weaponization has been accomplished in the past by at least five state bioweapons programs—those of the United Kingdom, Japan, the United States, Russia, and Iraq—and has been attempted by several others.Zilinskas, Raymond A. (1999), "Iraq's Biological Warfare Program: The Past as Future?", Chapter 8 in: Lederberg, Joshua (editor), Biological Weapons: Limiting the Threat (1999), The MIT Press, pp 137-158.
Genome structure B. anthracis has a single chromosome which is a circular DNA molecule consisting lf 5,227,293-bp. It also has two circular, extrachromosomal, double-stranded DNA plasmids, pXO1 and pXO2. Both the pXO1 and pXO2 plasmids are required for full virulence and represent two distinct plasmid families.
Feature Chromosome pXO1 pXO2 Size (bp) 5,227,293 181,677 94,829 Number of genes 5,508 217 113 Replicon coding (%) 84.3 77.1 76.2 Average gene length (nt) 800 645 639 G+C content (%) 35.4 32.5 33.0 rRNA operons 11 0 0 tRNAs 95 0 0 sRNAs 3 2 0 Phage genes 62 0 0 Transposon genes 18 15 6 Disrupted reading frame 37 5 7 Genes with assigned function 2,762 65 38 Conserved hypothetical genes 1,212 22 19 Genes of unknown function 657 8 5 Hypothetical genes 877 122 51
pXO1 plasmid The pXO1 plasmid (182 kb) contains the genes that encode for the anthrax toxin components: pag (protective antigen, PA), lef (lethal factor, LF), and cya (edema factor, EF). These factors are contained within a 44.8-kb pathogenicity island (PAI) on the plasmid. The lethal factor toxin is a combination of PA with LF, and the edema factor toxin is a combination of PA with EF. The PAI also contains genes which encode a transcriptional activator AtxA and the repressor PagR, both of which regulate the expression of the anthrax toxin genes.
pXO2 plasmid pXO2 encodes a five-gene operon (capBCADE) which synthesizes a poly-γ-D-glutamic acid (polyglutamate) capsule. This capsule allows B. anthracis to evade the host immune system by protecting itself from phagocytosis. Expression of the capsule operon is activated by the transcriptional regulators AcpA and AcpB, located in the pXO2 pathogenicity island (35 kb). The AtxA from pXO1 controls expression of both AcpA and AcpB.
Strains The 89 known strains of B. anthracis include: Sterne strain (34F2; aka the "Weybridge strain"), used by Max Sterne in his 1930s vaccines Vollum strain, formerly weaponized by the US, UK, and Iraq; isolated from a cow in Oxfordshire, UK, in 1935 Vollum M-36, virulent British research strain; passed through macaques 36 times Vollum 1B, weaponized by the US and UK in the 1940s-60s Vollum-14578, used in UK bio-weapons trials which severely contaminated Gruinard Island in 1942 V770-NP1-R, the avirulent, nonencapsulated strain used in the BioThrax vaccine Anthrax 836, highly virulent strain weaponized by the USSR; discovered in Kirov in 1953 Ames strain, isolated from a cow in Texas in 1981; famously used in AMERITHRAX letter attacks (2001) Ames Ancestor Ames Florida H9401, isolated from human patient in Korea; used in investigational anthrax vaccines
Evolution Whole genome sequencing has made reconstruction of the B. anthracis phylogeny extremely accurate. A contributing factor to the reconstruction is B. anthracis being monomorphic, meaning it has low genetic diversity, including the absence of any measurable lateral DNA transfer since its derivation as a species. The lack of diversity is due to a short evolutionary history that has precluded mutational saturation in single nucleotide polymorphisms.
A short evolutionary time does not necessarily mean a short chronological time. When DNA is replicated, mistakes occur which become genetic mutations. The buildup of these mutations over time leads to the evolution of a species. During the B. anthracis lifecycle, it spends a significant amount of time in the soil spore reservoir stage, in which DNA replication does not occur. These prolonged periods of dormancy have greatly reduced the evolutionary rate of the organism.
Related strains B. anthracis belongs to the B. cereus group consisting of the strains: B. cereus, B. anthracis, B. thuringiensis, B. mycoides, and B. pseudomycoides. The first three strains are pathogenic or opportunistic to insects or mammals, while the last three are not considered pathogenic. The strains of this group are genetically and phenotypically heterogeneous overall, but some of the strains are more closely related and phylogenetically intermixed at the chromosome level. The B. cereus group generally exhibits complex genomes and most carry varying numbers of plasmids.
B. cereus is a soil-dwelling bacterium which can colonize the gut of invertebrates as a symbiont and is a frequent cause of food poisoning It produces an emetic toxin, enterotoxins, and other virulence factors. The enterotoxins and virulence factors are encoded on the chromosome, while the emetic toxin is encoded on a 270-kb plasmid, pCER270.
B. thuringiensis is a microorganism pathogen and is characterized by production of parasporal crystals of insecticidal toxins Cry and Cyt. The genes encoding these proteins are commonly located on plasmids which can be lost from the organism, making it indistinguishable from B. cereus.
A phylogenomic analysis of the Cereus clade combined with average nucleotide identity (ANI) analysis revealed that the B. anthracis species also includes strains annotated as B. cereus and B. thuringiensis.
Pseudogene PlcR is a global transcriptional regulator which controls most of the secreted virulence factors in B. cereus and B. thuringiensis. It is chromosomally encoded and is ubiquitous throughout the cell. In B. anthracis, however, the plcR gene contains a single base change at position 640, a nonsense mutation, which creates a dysfunctional protein. While 1% of the B. cereus group carries an inactivated plcR gene, none of them carries the specific mutation found only in B. anthracis.