Candidatus Arthromitus
Candidatus Arthromitus is a group of morphologically distinct filamentous bacteria found almost exclusively in terrestrial arthropods. They were first discovered in the guts of insects by Joseph Leidy. Despite their morphological similarity to Candidatus Savagella or segmented filamentous bacteria, they form a distinct lineage within the Lachnospiraceae. History References to Arthromitus date back to 1849, when they were first observed by renowned physician and naturalist Joseph Leidy, who named the organisms Arthromitus (from Greek Arthro-, joint + mitos, thread). The filamentous morphology of the original Arthromitus was thought to be so unique that similar filaments observed in the vertebrate guts were erroneously placed under the division Candidatus Arthromitus. However, 16S rRNA-based phylogeny revealed the Arthromitus filaments, first observed by Leidy in arthropod guts, to belong to a group within the Lachnospiraceae, making them quite distant from previously observed filamentous organisms from vertebrate guts. These isomorphic bacteria from vertebrates were hence moved to a new division designated Candidatus Savagella. A poorly designed set of experiments aimed at isolating Arthromitus from termite guts led to the conclusion that Arthromitus is in fact a growth stage of Bacillus cereus. This mistaken finding was later disproved when the true identity of Arthromitus was confirmed.