Obligate in the context of Obligate intracellular parasite

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.

Thermithiobacillus is a genus of nonsporeforming, rod-shaped, Gram-negative bacteria. The name derives from the Latin thermae, for warm baths, and the Classical Greek θείος, theios for sulfur. The type species of this genus was previously assigned to the genus Thiobacillus, but it was reclassified on the basis of 16S rRNA analysis in 2000, creating this genus.

A phylogenetic analysis, using 98 protein families confirmed this reassignment. This chemolithoautotrophic genus is obligately aerobic and moderately thermophilic (43-45 °C). The type species is Thermithiobacillus tepidarius. A second species, Thermithiobacillus plumbiphilus, was published in 2016.