Mycobacterium tuberculosis-Epidemiology, Pathogenesis, and Treatment Introduction M. tuberculosis is gram-positive rods, aerobic, non-motile, non-sporeforming, intracellular bacteria. It has fastidious growth requirements, growth is enhanced by CO2 (5%-10%), divides slowly (up to 8 weeks), because of the complex cell wall. The common culture medium is Lowenstein- Jensen (composed with homogenized egg in the base nutrient), colonies appear after 3-6 weeks, they are rough, dry & light brownish yellow color colonies. Mycobacterium tuberculosis colonies on Lowenstein- Jensen medium Cell Structure – It is rich in lipids, with hydrophobic exteriors which provide resistance against various disinfectants, stains, acids & alkalis. It is different from a typical gram-positive cell wall structure : In the plasma membranes, phosphatidylinositol mannosidises, proteins, and lipoarabinomannan (LAM) are anchored. LAM has a functional association with the O-antigenic lipopolysaccharide that exists in bacteria. Porins and Transport protein span across the cell wall. The proteins have biological significant antigens, triggers the host’s cellular immune response, also used for diagnosis purposes as purified protein derivatives (PPDs). The cell wall of Mycobacterium tuberculosis Pathogenesis M. tuberculosis can cause lifelong infection, it affects the respiratory tract. When exposed, M. tuberculosis enters the respiratory airways & penetrates the alveoli, gets phagocytosed by alveolar macrophages. Bacteria prevents phagosome fusions with lysosomes (by blocking early endosomal autoantigen 1(EEA1). Instead phagosome fuse with intracellular vesicles (permit access to nutrients & replication). Macrophage secretes interleukin-12(IL-12) & tumor necrosis factor-α (TNF-α), in response to the infection. Cytokines increasing localized inflammation ( recruits T cells, NK cells, required interferons) People who have decreased productions of cytokines (IL-12 & TNF-α) or defects in cytokines receptors are at major risk of mycobacterial infections. Fused macrophages / Langhans giant cells/epithelial cells, with mycobacterium, form a necrotic mass (present in core) surrounded by a thick wall of macrophages & NK T cells, CD4 & CD8 which is completely called a granuloma. It prevents the further spread of the infection. Less antigenic burden leads to the formation of the small granuloma with minimal tissue damage. More bacteria are present, large necrotic granulomas are formed, encapsulated within fibrins which provide resistance against macrophage killing. (Bacteria can remain in a dormant stage and reinfect when the host immune system is weakened (old age or diseasecalled Reactivation). Epidemiology The natural reservoirs are humans and primates. The mode of transmission is by person-to-person contact (by inhaling infectious droplets). According to WHO, 1/3 rd of the population is infected by the disease tuberculosis. The mortality rate per year is 2 million / yr and new cases are 9 million worldwide. The highest incidence includes regions such as sub-Saharan Africa, Eastern Europe & Southeast Asia. People susceptible to disease caused by M.tuberculosis are drug & alcohol abusers, HIV patients, also health care workers. Sign & Symptoms The primary infection of tuberculosis is majorly asymptomatic or clinical symptoms like fever & malaise occur. The infiltrates in the lung (mid-zone) & enlarged lymph nodes can be observed by radiographs. It occurs in Pulmonary tuberculosis. The symptoms include dry cough (main), as a disease progresses there is sputum production, mixed with blood (known as hemoptysis), fever, sweating, malaise, fatigue, & weight loss comes along with the further disease progression The disease can also involve other organs such as bone, kidneys, brain, meninges & bowel. The untreated progressive disease usually takes 2-5 yrs to cause death but is rapid in HIV or immunocompromised patients. Diagnosis A very common diagnostic test to find out the exposure to the organism is the Tuberculin test (test for PPD) & Interferon -γ release, these are sensitive markers. Detection can be done by using microscopic observation of clinical specimens (sputum) smeared with Ziehl- Neelsen procedure. Rod shape mycobacterium can be observed by the acid-fast staining method Molecular probes are quite helpful in the diagnostic procedure. Treatment M. tuberculosis is susceptible to various effective antimicrobial agents as follows: First Line Drug Second Line Drug Isoniazid (Disrupts mycolic acid) Para-Aminosalicylic acid Ethambutol (affects LAM element in the cell wall) Ethionamide Rifampin Cycloserine Pyrazinamide Fluoroquinolones (e.g., ciprofloxacin & ofloxacin) Streptomycin (Inhibits cell wall synthesis) Kanamycin Second lines drugs are used in combinations with the first lines drugs if resistance or toxicity prevails. The oral chemotherapy treatment (Isoniazid & Ethambutol) of the disease, is usually continued for 18- 24 months. The time period of treatment is shortened by 6 months by using isoniazid, rifampin & pyrazinamide. For prevention, the BCG vaccine (Bacillus Calmette-Guerin) is available.
