Mycobacterium tuberculosis (MTB) is the primary etiological agent of
tuberculosis in humans (since the disease may be due to other mycobacteria of the
MTB complex such as M. bovis). It belongs to the order of the Actinomycetales and the
Mycobacteriaceae family. It is a bacillus that lacks capsule or flagella and does not
produce spores or toxins; it measures 0.5 by four microns. Its generation time is
prolonged (up to 24 hours). It is an aerobic bacillus that, if necessary, can persist under
anaerobic conditions.
It has a cell wall of extremely complex composition, with great strength and thickness,
constituted up to 60% by lipids, generally known as mycolic acids that form complexes
with polysaccharides such as arabinogalactan and peptidoglycan; these lipids determine
their resistance to discoloration by alcohol-acid after they have been stained with
carbol fuchsin (hence the term acid-fast bacilli acid or AFB). A distinctive feature of
the MTB cell wall is its content of N-glycolimuranic acid instead of N-acetylmuramic
acid found in most bacteria.
The unusual cell wall of MTB also allows it to survive initially in the macrophage. The
cell wall also constitutes a robust and highly impermeable barrier to harmful
compounds and drugs. MTB can sense when the local tissue conditions are inadequate
for survival (low oxygen tension and nutrient depletion), as in the macrophages and
granulomas, responding by the activation of a dormant state, in which the bacilli stop
multiplying, down-regulates its metabolism and activates anaerobic metabolism.
Keywords: Acid-fast bacilli, Granulomas, Mycobacterium tuberculosis, Mycolic
acids.
