The mycobacteria: an introduction to nomenclature and pathogenesis

Tuberculosis, caused by Mycobacterium tuberculosis, and leprosy, caused by M. leprae, are diseases known since antiquity. In developing countries, tub erculosis is still the leading cause of mortality due to an infectious dise ase. Taxonomically, mycobacteria belong to the genus Mycobacterium, which i s the single genus within the family of Mycobacteriaceae, in the order Acti nomycetales. Actinomycetales include diverse micro-organisms, but mycobacte ria and allied taxa are easily distinguished on the basis of the ability to synthesise mycolic acids. Mycobacterial species are traditionally differen tiated on the basis of phenotypic characteristics, a nd the authors provide an updated list of the biochemical tests currently employed and the cultur e properties that help to discriminate among various species of mycobacteri a. However, as the phenotypic characteristics do not allow precise identifi cation of all species, recent molecular taxonomical approaches for mycobact erial classification and phylogeny are also described. Mycobacteria are! al so a leading cause of infection in various domesticated anima Is a nd wild life. The authors briefly describe the mycobacteria involved in animal infe ctions, the wildlife reservoirs and strategies to control bovine tuberculos is, and the use of molecular tools for diagnostics and epidemiology of myco bacterial infections in animals. The characteristic of intracellular parasi tism is discussed, in addition to the fate of pathogenic mycobacteria that have the ability to grow inside phagosomes and phagolysosomes of infected h ost macrophages. The mycobacteria I cell envelope, which is a complex tripa rtite structure containing a high proportion of lipids (approximately 30% t o 40% of the total weight) could play a crucial role in the adaptation of m ycobacteria to intracellular growth and survival, immune modulation and dru g resistance.