Mycobacteria

They are aerobic, slow growing, filamentous or rod shaped, acid fast bacilli. Acid fastness is due to the presence of a waxy, lipid rich layer made of mycolic acid in the cell wall. They do not take the Gram stain. Cord factor is a glycolipid present in the cell wall and is associated with virulence. M.tuberculosis is the agent responsible for tuberculosis or TB.

TB is acquired by inhalation. M.tuberculosis is then ingested by alveolar macrophages where it survives within the phagosome by inhibiting phagosome-lysosome fusion. Primary lesion is seen in the lower lobes of the lung which along with enlarged draining lymph nodes form the “Ghon’s complex”. Reactivation lesions occur in the apical lobes of the lung. It causes a caseous granulomatous inflammation forming tuberculomas. It clinically presents as fever, night sweats, chronic cough with expectoration, hemoptysis, weight loss and loss of appetite. Pleural effusions may occur. M.tuberculosis or M.bovis may cause gastrointestinal tuberculosis presenting as abdominal pain, diarrhea, weight loss and fever. Other organs like kidneys, bones, meninges etc. may be involved. A disseminated form of tuberculosis with presence of multiple small tubercles in internal organs is seen in miliary tuberculosis.

This photomicrograph of a Ziehl-Neelsen-stained specimen, revealed the presence of magenta stained, rod-shaped, acid-fast, Mycobacterium tuberculosis bacilli.

Laboratory diagnosis is done by acid fast stain of sputum, gastric lavage fluid. Auramine staining showing yellow rods can be done for rapid diagnosis. Special media such as Lowenstein Jensen agar is used. Growth may take up to 8 weeks. BACTEC is a special radiometric liquid medium containing radiolabeled C14. Detection of CO2 14 is indicative of bacterial growth. M.tuberculosis is confirmed by biochemical tests being niacin positive, weakly catalase positive, nitrate reduction positive, sensitivity to pyrazinamide and resistance to TCH. Ribosomal RNA or DNA can be detected in tissue samples by nucleic acid amplification tests like PCR, ligase chain reaction, strand displacement amplification, DNA probes and microarray methods. Antibody detection can be done by ELISA, radioimmunoassay or latex agglutination tests. For diagnosis of latent infections, tuberculin/PPD skin test or Gamma interferon release assay or quantiferon gold assay is used. MODS or “microscopic observation drug susceptibility” assay is a good technique for rapid detection of M.tuberculosis and drug resistant TB. It’s a liquid culture medium to which INH or rifampin can be added to test for resistance. Growth is observed microscopically by sampling the liquid media rather than waiting for colonies to appear.

Atypical Mycobacteria

hey are present in the environment and can be differentiated from M.tuberculosis by being niacin negative, aryl sulfatase positive, resistant to antituberculosis drugs, some being rapid growers and some producing pigment.

Diseases caused by Atypical Mycobacteria