Anthrax

Appearance

''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 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. . TLR sensing of bacterial spore-associated RNA triggers host immune responses with detrimental effects. ''Journal of Experimental Medicine'', ''214'', 1297–1311. https://doi.org/10.1084/jem.20161141 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 inactive 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.Choudhury, B., Leoff, C., Saile, E., Wilkins, P., Quinn, C. P., Kannenberg, E. L., & Carlson, R. W. . The Structure of the Major Cell Wall Polysaccharide of Bacillus anthracis is Species-specific. ''Journal of Biological Chemistry'', ''281'', 27932–27941. https://doi.org/10.1074/jbc.M605768200 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.

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.

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.