4.1 Classification of mycobacteria

The genus Mycobacterium is the only genus in the family Mycobacteraceae in the order Actinomycetales, which includes other mycolic acid-containing genera, namely Nocadia, Rhodococcus, Gordonia, and Tsukamurlla. Currently, the genus comprises of over 150 species and 13 subspecies [15, 16]. Within the genus, classification is based on several factors including growth rate and pathogenicity. Based on pathogenicity, it can be classified into two groups: tuberculous and nontuberculous mycobacteria, the latter also referred to as mycobacteria other than tuberculosis (MOTTs). A refined classification on this basis groups the genus into obligatory pathogens, potentially pathogenic (opportunistic) and saprophytic or ubiquitous microorganisms [17]. Obligatory pathogens belong to Mycobacterium tuberculosis complex (MTBC) group that comprise Mycobacterium bovis subsp. bovis, M. bovis subsp. caprae, M. tuberculosis, M. africanum, M. bovis BCG, M. canetti, M. microtti, M. pinnipedii, and M. leprae [18]. All MTBC species have identical 16S rRNA sequences and a 99.9% similarity at nucleotide level, and may even be considered subspecies, but differ significantly in their host range [19]. The potentially pathogenic mycobacteria, represented by the Mycobacterium avium complex (MAC), consists of closely related species and subspecies, which include, among others, M. avium subsp. avium, M. avium subsp. paratuberculosis, and M. avium subsp. hominissuis. The potentially pathogenic species are found in the environment as well as in the susceptible hosts and cause disease mainly in hosts with compromised immunity [20]. Saprophytic mycobacteria are the largest group found in the environment. Some, such as M. kansasii, M. asiaticum, M. interjectum, M. szulgai, M. fortuitum, M. celatum, M. ulcerans, M. smegmatis, and M. septicum have been associated with diseases in humans and animals [21].

On the basis of growth rate, the genus is classified into slow and rapidly growing species, with rapid-growers being those that produce grossly visible colonies in less than 7 days and slow-growers taking over 7 days. Slow-growing species are more commonly associated with pathogenicity than the fast-growing group [22].

In cattle and other ruminants, tuberculosis is caused mainly by the obligate pathogen Mycobacterium bovis subsp. bovis but infections by Mycobacterium bovis subsp. caprae, Mycobacterium tuberculosis and Mycobacterium africanum also occur [2–4]. In Central Europe, M. bovis subsp. caprae is the major cause of tuberculosis in cattle [23, 24]. The disease caused by M. bovis subsp. bovis and M. bovis subsp. caprae is commonly referred to as bovine or zoonotic tuberculosis. Although not as widely as zoonotic tuberculosis, MOTTs infections have been reported in cattle exhibiting granulomatous lesions identical to those caused by the MTBC complex [6, 15, 16].
