Bacillus anthracis
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 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 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.
Image:B anthracis diagram en.png|thumb|right|Structure of B. anthracisUntreated B. anthracis infection is usually deadly. Infection is indicated by inflammatory, black, necrotic lesions. The sores usually appear on the face, neck, arms, or hands. Fatal symptoms include a flu-like fever, chest discomfort, diaphoresis, 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 that disguises the vegetative bacterium from the host immune system. 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. 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.
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. Because of these attributes, B. anthracis endospores are extraordinarily well-suited to use 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.
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 | 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 | 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 contains the genes that encode for the anthrax toxin components: pag, lef, and cya. These factors are contained within a 44.8-kb pathogenicity island 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 which synthesizes a poly-γ-D-glutamic acid 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. The AtxA from pXO1 controls expression of both AcpA and AcpB.Strains
The 89 known strains of B. anthracis include:- Sterne 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
- * Ames Ancestor
- * Ames Florida
- H9401, isolated from human patient in Korea; used in investigational anthrax vaccines
Evolution
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 analysis revealed that the B. anthracis species also includes strains annotated as B. cereus and ''B. thuringiensis.''