Morphological Characterization of Mycobacterium tuberculosis

It is more than 100 years since the first Mycobacterium was isolated by Hansen (1874). That was leprosy bacillus, Mycobacterium leprae, which even today is still resisting all attempts to cultivate it in the laboratory. The tubercle bacillus, M. tuberculosis was discovered eight years later by Robert Koch (1882). The Koch discovery was confirmed by more efficient staining models of Ehrlich (1887) and ZiehlNeelsen (1883). Under Light microscope, the tubercle bacilli typically appear as straight or slightly curved rods. According to growth conditions and age of the culture, bacilli may vary in size and shape from coccobacilli to long rods. The dimensions of the bacilli have been reported to be 1-10 μm in length (usually 3-5 μm), and 0.2 -0.6 μm width. The possibility of morphological variations in tubercle bacilli was suggested by few investigators like Malassez and Vignal (1883), Nocard and Roux (1887), Metschnikoff (1888), Lubarsch(1899), Fischel(1893), and Vera and Rettger (1939). They showed under unfavorable conditions, i.e., a limited food supply, or oxygen deprivation, Mycobacterium assumed a swollen appearance without forming the vacuolar or globoid bodies (Vera and Rettger, 1939). These early reports were based on stained preparations and were subjected of severe criticism (Porter and Yegian, 1945). Today with advances in microscopic technique i.e., transmission electron microscope (TEM), scanning electron (SEM) and atomic force microcopy (AFM), almost all of investigators have been agreed that the Koch bacillus does not always manifest itself in the classical rod shape (figure 1). They become shorter in older cultures, filametous within macrophages and ovoid during starvation (Young et al., 2005; Farnia et al., 2010; Shleeva et al., 2011) and they may produce buds (Chauhan et al., 2006) and branches in extensively drug resistance strains (XDR-TB) (Velayati et al 2010; Farnia et al 2010). In the following parts the underlying mechanisms that may help the bacilli to change its morphology was highlighted.