Taxonomy
There are seven major lineages in the Mycobacterium tuberculosis complex (MTBC), with lineages 5 and 6 classified as ''Mycobacterium africanum''. MTBC lineage 5 is ''M. africanum'' type 1, West African 1 (MAF1), and is classified based on a characteristic deletion of Region of Differentiation (RD) 711. MAF1 is commonly found around the Gulf of Guinea. MTBC lineage 6 is also known as ''M. africanum'' type 1, West African 2 (MAF2), and is classified based on a deletion of RD702. MAF2 is prevalent in Western Africa. ''M. africanum'' type 2, East African, was previously recognized as a strain of ''Mycobacterium africanum''; it was recently reclassified as ''Mycobacterium tuberculosis'' genotype “Uganda” in a sublineage of MTBC lineage 4.History
''M. africanum'' was first described as a subspecies within the MTBC, with phenotypic characteristics intermediate between ''M. tuberculosis'' and ''M. bovis'', based on biochemical testing by Castets in 1968. Early genetic analysis showed that it was distinct from ''M. tuberculosis'' due to a genomic RD9 deletion and distinct ''GyrB'' nucleotide sequence, and distinct from M. bovis due to an intact RD12 and RD4.Microbiology
''M. africanum'' is grown in pyruvate-containing media under low oxygen conditions, and forms characteristic “dysgonic” colonies. Unlike ''M. tuberculosis'', ''M. africanum'' shows catalase activity, is nitrate negative, and is susceptible to thiopene-2-carboxylic acid hydrazide (TCH) and pyrazinamide (PZA). ''M. africanum'' is also slower growing than ''M. tuberculosis'', typically taking 10 weeks to develop colonies rather than 3 to 4 for ''M. tuberculosis''.Epidemiology
''M. africanum'' is most commonly found in West African countries. It is an infection of humans only and is spread by an airborne route from individuals with open cases of disease. It is not fully understood why the distribution of ''M. africanum'' is limited to West Africa, with only sporadic cases found in other regions. Phylogenetic evidence shows that ''M. africanum'' branched at an early stage from modern Mtb lineages in America, Europe and Asia. Some research suggests that ''M. africanum'' is adapted to west African populations. ''M. africanum'' may be being outcompeted by other Mtb lineages in other regions; however, genetic studies have found no difference in the number of virulence genes or genetic diversity between ''M. tuberculosis'' and ''M. africanum''. No animal reservoir has been identified for ''Mycobacterium africanum'' despite having been found various wild animals. It has a similar degree of infectivity to the regular ''M. tuberculosis'' organism but is less likely to progress to clinical disease in an immunocompetent individual. However, ''M. africanum'' is more likely to progress from infection to causing disease in an HIV positive patient. In countries where ''M. africanum'' is endemic, it represents an important