Welcome
Genome structure
Mycobacterium tuberculosis has circular chromosomes
of about 4,200,000 nucleotides long. The G+C content is about 65%. (13)
The genome of M.
tuberculosis was studied generally using the strain M.
tuberculosis H37Rv. The genome contains about 4000 genes. Genes that code
for lipid metabolism are a very important part of the bacterial genome, and 8%
of the genome is involved in this activity. (7)
The different species of
the Mycobacterium tuberculosis complex show a 95-100% DNA
relatedness based on studies of DNA homology, and the sequence of the 16S rRNA gene
are exactly the same for all the species. So some scientists suggest that they
should be grouped as a single species while others argue that they should be
grouped as varieties or subspecies of M. tuberculosis. (2)
Plasmids in M.
tuberculosis are important in transferring virulence because genes on
the plasmids are more easily transferred than genes located on the chromosome.
One such 18kb plasmid in the M. tuberculosis H37Rv strain was
proven to conduct gene transfers.
Cell structure and metabolism
M. tuberculosis has a tough cell wall
that prevents passage of nutrients into and excreted from the cell, therefore
giving it the characteristic of slow growth rate. The cell wall of the pathogen
looks like a Gram-positive cell wall. The cell envelope contains a polypeptide
layer, a peptidoglycan layer, and free lipids. In addition, there is also a
complex structure of fatty acids such as mycolic acids that appear glossy. (8)
The M. tuberculosis cell wall contains three classes of
mycolic acids: alpha-, keto- and methoxymycolates. The cell wall also contains
lipid complexes including acyl glcolipids and other complex such as free lipids
and sulfolipids. There are porins in the membrane to facilitate transport.
Beneath the cell wall, there are layers of arabinogalactan and peptidoglycan
that lie just above the plasma membrane. (14)
The M. tuberculosis genome
encodes about 190 transcriptional regulators, including 13 sigma factors, 11
two-component system and more than 140 transcription regulators. Several regulators
have been found to respond to environmental distress, such as extreme cold or
heat, iron starvation, and oxidative stress. (11) To survive in these harsh
conditions for a prolonged period in the host, M. tuberculosis had
learned to adapt to the environment by allowing or inhibiting transcription
according to its surroundings.(3)