Mycobacterium tuberculosis

Staining means coloring of the bacteria for proper viewing under the microscope. It is very important part of identifying TB bacteria. Mycobacterium tuberculosis (MTB) is a slow growing aerobic bacterium that divides in every 16 to 20 hours; this is extremely slow compared to other bacteria that have division times measured in minutes. In contrast, one of the fastest growing bacteria is a strain of E. coli that can divide roughly every 20 minutes. As MTB only has one phosphorlipid outer membrane, it is classified as Gram-positive bacteria. But if a Gram stain is performed, MTB either stains very weakly Gram-positive or does not retain dye, due to high lipid content of its cell wall. MTB is a small rod shaped bacteria (Bacillus), which can withstand weak disinfectants and can survive in a dry state for weeks. Normally the bacteria can only grow inside a host, so in vitro (outside host) culture of M. tuberculosis took a long time to develop, but is now a routine laboratory procedure. MTB is identified microscopically by its staining characteristics; it retains certain stains often being treated with acid solution & thus is classified as an “acid-fast bacillus” or AFB. This can also be visualized by fluorescent microscopy and by an auramine-rhodomine stain.