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  Mycobacterium
tuberculosis, the etiological agent of tuberculosis (TB), is an aerobic
rod microorganism, still and asporogenic. It is usually classified as
Gram +ve and it is characterized by resistance to acid-alcohol
decolorization ("acid-fast bacilli"), which can be shown by
Ziehl-Nielsen dyeing. Such microorganisms are very resistant to chemicals
and physical agents, they have an average sensitivity to heat and they
can endure desiccation very well.
Five varieties of TB bacillus have been identified (human, bovine, avian,
murine and that of cold-blooded animals), but only the first two kinds
are of importance in human pathology.
Transmission follows the respiratory route (through exposure to the
bacilli present in the droplets of the bronchial secretion of the
infected subject). Another route of infection, though rare, is
represented by the cutaneous-mucous route (through contact of cutaneous
lesions or mucous membranes with infected material). Indirect infection,
through contaminated objects, is extremely rare. Infection is possible
until the bacilli are present in the infected patient's secretions. Sometimes,
if the patient is not treated or is inadequately treated, the infective
period can last for years. The infective degree basically depends on the
number of emitted bacilli and on their virulence.
The bacillus usually enters the body through the respiratory system and,
therefore, the most frequent localization is pulmonary. An exudative
focus forms and this is usually followed by improvement and recovery with
consequent residual caseation (primary complex). Immunity rapidly
develops and, therefore, the subject becomes more resistant to
reinfection. However, in about 20% of cases the bacillus remains latent
in the affected nodes. Only very rarely a primary pulmonary lesion
evolves in pulmonary TB and only in some cases, through different
mechanisms (blood dissemination, intrabronchial dissemination), there can
be a diffused localization of the bacteria (miliary TB).
The primary infection can also be acquired through the alimentary route
with intestinal localization.
Post-primary TB arises following a new infection that can be exogenous
(penetration of new bacilli from outside) or endogenous (reactivation of
latent mycobacteria following any form of immunocmpromise).
Post-primary pulmonary TB, if not adequately treated, may progress until
cavern formation occurs in the lungs. From such lesions, the bacteria may
diffuse through the bronchopulmonary secretion to the GI tract. Furthermore,
lymphohaematic dissemination is possible with localization in different
organs, such as kidneys, meninges, brain, etc.
For additional information
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