As compared to other Schizomycetes, mycobacteria (from the Greek muxes:mycetes and bacterion: rod) present some atypical characteristics that cause them to resemble mycetes. Nevertheless, there is no doubt as to their status as bacteria: they belong to the order of the Actinomycetaceae, and the family of Mycobacteriaceae. This family includes only the Mycobacterium genus, that accounts for more than 170 species, many of which are of no clinical interest . Despite a few peculiar differences observed in some species, the morphology of mycobacteria tends to be homogeneous. They are long, slender bacilli ranging from 1 to 5 μm in length and 0.2–0.6 μm in diameter. Coccobacillary forms, although rarely found in pathological samples, are common in colony-forming preparations. Mycobacteria have no locomotor organules and are strictly aerobic or microaerophylic, as well as being Gram-positive and pleomorphic. Their pleomorphism is due to the fact that they normally grow in flaments, sometimes branched (hence the name of this genus, to indicate their resemblance to mycetes, that being moulds, have a flament-like branched morphology), that fragment into bacilli or coccobacilli. Mycobacteria are not mobile nor sporogenic, and except for a few species, show fairly slow reproductive rhythms. In fact, while mean duplication time is about 20 min in Enterobacteriaceae, in the case of mycobacteria it exceeds 15 h. Except for M. leprae, all the species can be cultured, even if most of them require very complex culture media. The colonies show morphological differences among the species but generally feature a rough surface.