Slender rods that sometimes show branching
filamentous forms resembling fungal mycelium.
In liquid cultures they form a mold-like pellicle~ hence
the name ‘mycobacterium’ = Fungus like bacteria.
They are acid-fast, aerobic, non-motile, non-capsulated
and non-sporing, obligate parasites, opportunistic
pathogens and saprophytes.
Robert Koch in 1882, isolated the mammalian strain and
proved its causative role in Tuberculosis.
Mycobacterium has many bacilli, most common of them
are M. tuberculosis and M. lepra.
Straight or slightly curved rod of about 3µm x 0.3µm
Occurs either in pairs or small clumps.
Gram Positive; but they resist decolorisation after being
stained with basic dyes ~ but they resist decolorisation
with 20% Sulphuric acid and absolute alcohol when
treated with acid fast stains.
O Colonies appear in about 2
weeks & sometimes may
take up to 8 weeks.
TEMPERATURE is 37º C;
Growth doesn’t occur
above 40ºC or below
O OPTIMUM pH is 6.4 to 7
O M. tuberculosis are
O The organisms are highly
susceptible even to traces
of toxic substances
present in the media.
O Koch originally grew the
bacilli on heat coagulated
O SOLID MEDIA:
O LIQUID MEDIA:
* Proskauer & Beck’s
* Sula & Sauton’s
Cultural Differences of Liquid & Solid Media
Growth in Liquid
Media Growth in Solid Media
O Growth begins at the
bottom, without dispersing
O The organisms creeps up
the sides and forms a
prominent surface pellicle
which extends along the
side of the medium.
O Diffuse growth is observed
in Dubos’ medium.
O Virulent strains form long
serpentine cords while a-
virulent strains grow in a
dry, rough, raised, irregu
lar colonies with a
O They are creamy-
yellowish or buff colored
on further incubation.
O They are tenacious and
not easily emulsified.
M. Tuberculosis as seen on Lowenstein-Jensen
Lifecycle of M. tuberculosis in Human
Tuberculosis spreads through the air when a person with untreated
pulmonary TB coughs or sneezes. Once in the body, the tuberculosis
bacilli has about 5 stages in its life cycle:
Stage 1: Onset
Bacteria is inhaled through the air and typically engulfed by alveolar
macrophages. At this instant, disease progression depends on the virulence
of the inhaled strain and the anti mycobacterial capabilities of the
macrophage in question. In some cases, the bacteria are able to reproduce
and initiate the infection. Tuberculosis begins when the inhaled
mycobacterial nuclei reach aveolar machrophages
Stage 2: Symbiosis
If the initial macrophage does not succeed in killing the bacteria, the
bacteria will replicate until the macrophage bursts. The bacteria are now
engulfed by other alveolar macrophages and non activated macrophages.
The macrophages that arrive from the bloodstream engulf the exposed
bacteria in a symbiotic manner—neither the host nor the bacteria is harmed
Stage 3: Initial Caseous Necrosis
The next stage of disease development begins when bacterial reproduction
slows. Growth slows because as the bacteria reproduce, they kill all the
surrounding non activated macrophages and run out of cells to divide within.
In addition, the increased number of bacteria produces anoxic conditions
and reduces the local pH The bacteria can no longer reproduce in this
tubercule, but can remain alive for long periods of time at this state. The
host kills its own tissues to prevent the spread of the bacteria. Also at this
stage, the host will test positive for tuberculin.
Stage 4: Interplay of Tissue-Damaging and Macrophage Activating
Macrohpages surround the tubercule, some of which may be inactivate. M.
tuberculosis uses the inactive macrophages to reproduce, causing the
tubercule to grow. The tubercule may break off or spread into the
bronchus, and then other parts of the lung. If the tubercules reach the blood
stream, the patient can develop tuberculosis outside of the lungs, which is
known as milliary tuberculosis. Secondary lesion can develop almost
anywhere within the body, but are commonly found in the
bones, joints, lymph nodes, and genitourinary system.
Stage 5: Liquefaction and Cavity Formation
At some point the centres of the tubercles may liquefy, which produces a
very conducive environment for the bacteria and rapid spread of the
disease. Only a very small % of infected individuals will progress to this
Resistance of M. tuberculosis
Heat Labile ~ Killed at 60ºC in 15 – 20 minutes.
Cultures are killed by exposure to sunlight for 2 hours .
Cultures remain viable at RT for 6 – 8 months and can stored
up to 2 years at -20ºC.
Bacilli in sputum remain alive for 20 to 30 hours
in droplet nuclei are viable for 8 to 10 days.
Bacilli are resistant to ~ Chemical disinfectants, 5%
phenols, 15% Sulphuric acid, 3% nitric acid, 5% Oxalic acid.
But they are very sensitive to ~ Formaldehyde and
Gluteraldehyde. They are destroyed by tincture of Iodine in 5
minutes & by 80% ethanol in 2 to 10 minutes.
o Demonstrating the bacilli in the lesion by microscopy
o Isolating the bacilli in the culture
o Transmitting the infection to experimental animals
o Demonstrating hypersensitivity to tuberculoprotein
PROPHYLAXIS: EARLY DETECTION AND
TREATMENT ~ IMMUNOPROPHYLAXIS
* By Intradermal Injections of live attenuated vaccine developed by
Calmette and Guerin in 1921, called BCG - Bacille Calmette Guerin .
* Injection of BCG in animals induces self limited infection with
production of small tubercles, thereby giving rise to delayed
hypersensitivity and immunity.
* BCG also has many complications :
Local: Abscess, Indolent ulcer, Tuberculoids, Confluent and lupoid
Regional: Enlargement and Suppulation of draining Lymph nodes.
General: Fever, Mediastinal adenitis, Erythema nodosum, tendency
to keloid formation and rarely non-fatal meningitis.
In TB endemic countries such as India, BCG vaccines are
administered to babies by intradermal injections immediately after
birth or as early as possible after that , but before age of 12 months.
Babies born to AFB +ve mothers should be given BCG vaccine
ONLY AFTER a course of preventive chemotherapy.