**5. Etiological data on the mycobacterial strains in HIV/TB co-infection**

HIV patients are frequently infected by virulent strains of M.tbc. The virulence of a particular strain depends on the genetic composition of M.tbc. Thus the Beijing genotype of M.tbc mostly found in Asia is considered the most aggressive genotype and has been associated with CSF dissemination and multidrug resistance to antituberculous agents in HIV patients [56]. Infections with M. bovis are rare and occur mostly in HIV Hispanic patients. Despite the high environmental exposure to nontuberculous mycobacteria CNS involvement is rare even in AIDS patients and usually occurs at a CD4+ count under 10 cells/mm3. The pathogenic mechanisms behind the interactions established between the host and virulent mycobacteria are less documented. The infection with Mycobacterium avium complex (MAC) remains the most studied and most frequent nontuberculous mycobacteria accounting for the atypical tuberculous manifestations in the advanced stages of AIDS infection [57]. The Mycobacterium avium intracellulare (MAI) serotypes 4 and 8 are the most prevalent in AIDS patients [58].

Sporadic cases of NTB with other mycobacteria have also been recorded in AIDS patients following disseminated infection [59]. MAC is an ubiquitary environmental mycobacteria which colonizes the gastrointestinal and respiratory tract but is also able to invade the epithelial cells and the intestinal wall [60]. Virulent strains isolated from AIDS patients are able to penetrate the mucosal barriers and resist intracellular killing by macrophages resulting in a disseminated infection. Further studies on the interaction between M. avium and the HIVinfected cells confirmed the inhibition of several cytokines secreted by the Th1 CD4+cells, natural killer cells and macrophages.These ultimately favour the intracellular survival of M. avium and even accelerate its growth rate [61,62]. The neurologic involvement due to MAC in advanced stages of AIDS generally presents as TBM following a disseminated infection with prolonged bacteremia [63-66]. The comparative aspects of the CNS invasions with M.tbc and nontuberculous mycobacteria in HIV hosts are presented in table 1
