Mycobacterium tuberculosis

Mycobacterium tuberculosis

Introduction

The genus Mycobacterium included in the family Mycobac-teriaceae consists of nonmotile, non–spore-forming, and aero-bic bacilli. They are slightly curved rods, sometimes showing filamentous and branching forms. In liquid medium, they form mould-like pellicle; hence, the name Mycobacterium is derived from the word “mould” meaning fungus-like bacterium.

      The cell wall is thick, complex, and rich in lipids, which makes the surface hydrophobic. The lipid-rich cell wall also makes the bacteria resistant to commonly used disinfectants and lab-oratory staining reagents. Because of their lipid-rich waxy cell wall, mycobacteria are difficult to stain. But once stained, they resist decolorization with acid solution and alcohol. They are, therefore, called acid-fast bacilli (AFB). Although they are Gram positive, they are difficult to stain, and they stain poorly with Gram stain.

      Most mycobacteria are slow growers, the generation time being 2–24 hours. These bacteria require 3–8 weeks of incuba-tion to produce demonstrable colonies on the solid media.

Classification

The genus Mycobacterium contains more than 70 species, many of which cause disease in humans. Mycobacterium species causing most human infections are summarized in Table 41-1. Mycobacterium leprae was the first species of the genus Mycobacterium to be identified by Hansen in 1874. Furthermore, Robert Koch in 1882 isolated the mammalian tubercle bacillus and established its role as a causative agent of tuberculosis, as it satisfied all the criteria of Koch’s postulates. Subsequently, Mycobacterium tuberculosis was demonstrated as the causative agent of tuberculosis.

    Mycobacterium tuberculosis complex is the term used cur-rently, which includes four species that can cause tuberculosis in humans and other mammals. These are:

·           M. tuberculosis, the human type;

·           Mycobacterium bovis, bovine type;

·           Mycobacterium africanum, a species intermediate betweenhuman and bovine type and causing human tuberculosis mostly in tropical Africa; and

·           Mycobacterium microti, another species which is pathogenicfor goats and other small animals but does not cause any human infection.

Saprophytic mycobacteria are the third group of mycobacterial species isolated from different types of sources. This species has been isolated from cold- and warm-blooded animals, from skin ulcers, and from environmental sources, such as soil and water. Originally, these saprophytic mycobacteria were called atypi-cal environmental or opportunistic mycobacteria, but these species are now designated as mycobacteria other than typical tubercle (MOTT ).