Picture 2: This is an acid-fast stain of MTB. Pink rods are
called acid-fast bacilli in terminology. MTB have a cell wall with lipids
including fatty acids, specifically mycolic acid, making them resistant to
phagocytic destruction, and acid-fast.
In the laboratory, it is possible to growth MTB with either
Middlebrook’s medium which is an agar-based medium or Lowenstein-Jensen medium
which is an egg-based medium. In these media, MTB will be shown small and buff
colored when grown. This process takes 4-6 weeks.
MTB’s cell wall is quite different when compared to other
bacteria. It has a unique cell wall, peptidoglycan structure, with its rich
with complex lipids. These complex lipids give important properties to MTB such
as; impermeability to stains and dyes, resistance to many antibiotics,
resistance to killing by acidic and alkaline compounds, resistance osmotic
lysis via complement deposition, resistance to lethal oxidations and survival
inside of macrophages. Three major components of MTB is mycolic acids, cord
factor, and Wax-D.
Mycolic acids are unique alfa-branched lipids found in the
cell wall. The %50 dry weight of the
bacterium is made up mycolic acids. This mycolic acid is a strong hydrophobic
molecule and it has an effect on permeability. Virulence of MTB is significant
due to mycolic acids.
Cord Factor is responsible fort he serpentine cording which
is chains of cell smears made from in vitro-grown colonies form. It is toxic
and most abundantly produced in virulent strains of MTB.
Wax-D in the cell envelope is the major component of Freund’s
of MTB 9
Genomic analysis of MTB was published in 1998. It has 4
million base pairs, with 3,959 genes; 40% of these genes have had their
function characterized.The genome contains 250 genes involved in fatty acid
metabolism, with 39 of those involved generating the waxy coat. Such large
numbers of conserved genes show the evolutionary importance of the waxy coat to
pathogen survival. Moreover, experimental studies have since validated the
importance of a cell wall lipid metabolism for M. tuberculosis, consisting
entirely of host-derived lipids such as fats and cholesterol. Bacteria were
isolated from the lungs of infected mice and were shown to use fatty acids over
carbohydrate substrates. M. tuberculosis can also grow on the cholesterol as a
sole source of carbon and is involved in the cholesterol use pathway. About 10%
of the coding capacity is taken up by the PE / PPE gene families that encode
acidic, glycine-rich proteins. These proteins have a conserved N-terminal
motif, deletion of which impairs growth in macrophages and granulomas. Nine
noncoding sRNAs have been characterized by M. tuberculosis, with a further 56 predicted in a
bioinformatic.In 2013, a study was conducted on the genome of several
sensitive, ultrasonic, and multiresistant strains of M. tuberculosis. Results
reveal new relationships and drug resistance genes associated with some of the
genes and intergenic regions associated with drug resistance.
Mechanisms and Virulence Factors10
What makes M. Tuberculosis virulent? This questions’ answer
is not simple, clear and complete yet. Even though the organism apparently does
not produce any toxins, it possesses a huge mechanism of structural and
physiological properties that have been recognized for their contribution to
mycobacterial virulence and to the pathology of tuberculosis. There is more
than one mechanism for this process. Here some properties of MTB that
contribute to its virulence:
*MTB has special mechanisms for cell entry which can bind to
mannose receptors on macrophages via the cell wall-associated mannosylated
*MTB has intracellular growth which is given the effective
evading the immune system -antibodies and complement are ineffective- When MTB
is phagocytosed, phagosome-lysosomal fusion is inhibited by secretion of a
protein that modifies the phagosome membrane.
*MTB has the mechanism that detoxification of oxygen radicals
which gives some of the important regulation, activation, production features
*Slow generation time is one of important feature MTB’s
virulence properties. Because of slow generation time, the immune system may not
readily recognize the bacteria or may not be triggered effectively to eliminate
*As we know, MTB has high lipid concentration in cell wall
which gives impermeability and resistance to antimicrobial agents, resistance
to killing by acidic and alkaline compounds and also resistance to osmotic
lysis via complement attack by lysozyme.
Furthermore, there are proteins, enzymes which have important
role virulence mechanisms on MTB. 19-kDa protein, Erp(which is surface-located
protein), Mas enzyme, OmpA(which is porin-like protein) and so on. All of them
are considered, MTB’s virulence factor gives important pieces of information to
researchers to understand MTBs’ mechanism in the human body.