Corynebacterium () is a genus of Gram-positive bacteria and most are aerobic. They are bacilli (rod-shaped), and in some phases of life they are, more specifically, club-shaped, which inspired the genus name (coryneform means "club-shaped"). They are widely distributed in nature in the microbiota of animals (including the human microbiota) and are mostly innocuous, most commonly existing in commensal relationships with their hosts. Some, such as C. glutamicum, are commercially useful. Others can cause human disease, including, most notably, diphtheria, which is caused by C. diphtheriae. As with various species of amicrobiota (including their relatives in the genera Arcanobacterium and Trueperella), they usually are not pathogenic, but can occasionally opportunistically capitalize on atypical access to tissues (via wounds) or weakened host defenses. Taxonomy The genus Corynebacterium was created by Lehmann and Neumann in 1896 as a taxonomic group to contain the bacterial rods responsible for causing diphtheria. The genus was defined based on morphological characteristics. Based on studies of 16S-rRNA, they have been grouped into the subdivision of Gram-positive Eubacteria with high G:C content, with close phylogenetic relationship to Arthrobacter, Mycobacterium, Nocardia, and Streptomyces. The term comes from Greek κορύνη, 'club, mace, staff, knobby plant bud or shoot' and βακτήριον, 'little rod'. The term "diphtheroids" is used to represent corynebacteria that are nonpathogenic; for example, C. diphtheriae would be excluded. The term diphtheroid comes from Greek διφθέρα, 'prepared hide, leather'. Genomics Comparative analysis of corynebacterial genomes has led to the identification of several conserved signature indels (CSIs) that are unique to the genus. Two examples of CSIs are a two-amino-acid insertion in a conserved region of the enzyme phosphoribose diphosphate:decaprenyl-phosphate phosphoribosyltransferase and a three-amino-acid insertion in acetate kinase, both of which are found only in Corynebacterium species. Both of these indels serve as molecular markers for species of the genus Corynebacterium. Additionally, 16 conserved signature proteins, which are uniquely found in Corynebacterium species, have been identified. Three of these have homologs found in the genus Dietzia, which is believed to be the closest related genus to Corynebacterium. In phylogenetic trees based on concatenated protein sequences or 16S rRNA, the genus Corynebacterium forms a distinct clade, within which is a distinct subclade, cluster I. The cluster is made up of the species C. diphtheriae, C. pseudotuberculosis, C. ulcerans, C. aurimucosum, C. glutamicum, and C. efficiens. This cluster is distinguished by several conserved signature indels, such as a two-amino-acid insertion in LepA