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Mycobacterial diseases are among the oldest disease known to human beings. There are a lot of bitter memories of Mycobacterial invasion especially Tuberculosis and leprosy in human mind, even now after discovery of etiology, diagnostic tool, prevention, and treatment, still, the recalling the above diseases are frightening to human being. Although leprosy isn’t an serious disease nowadays but the TB after a long period of quietness, is going to be an serious risk factor for mankind, especially in third world countries, after cooperating with HIV. Some atypical Mycobacteria, which have weak pathogenicity now, adapt themselves to the new condition and present as a dangerous cause of disease. The researcher that have made themselves familiar with such Mycobacterial features, trying hard to discus carefully the specifities and weak points of these bacteria, to fight them back. The aim of this review article is to highlight one of Mycobacterial characteristic, which is bacteruosis slow growth rate.