MYCOBACTERIUM

1. General

   1. Slim, rod shaped

   2. Acid fast- affects permeability

   3. Resists drying, chemicals, and germicides.

   4. Strictly aerobic

   5. .2-.4 X 10 m

   6. non-motile

   7. non-spore forming

   8. cell wall of N-glycolymuranic acid with a high % of mycoic acid on peptide and polysaccharides

   9. 10% CO₂, pH of 6.5-6.8, Lowenstein-Jensen (60% hom egg) with malachite to inhibit other bacteria

   10. Semisythetic Middlebrook 7H-11

   11. slow growth, mean generation time of 12-24 hrs

2. Virulenc factors

   1. Grow inside macrophages and monocytes

   2. Probably prevent phagosome-lysosome fusion or escapes the phagosome- MHC I associated cytotoxic T cell response

   3. LLO homologue (hemolysin)- escape from phagosome

   4. produces NH₄ to counteract acid in phagosome

   5. mycolic acids are toxic- immune response

   6. TNF -a causes lung damage

3. Epidemiology

   1. Inhalation of droplet nuclei, sometimes infx trough GI or skin. 1 cough has 3000 droplet nuclei, <10>

   2. Decline of mortality. 1985 steady decline stopped, and mortality increase 20% from economic conditions, IV drug use, and AIDS

4. Pathogenesis

   1. Usually pulmonary at the periphery of the midzone of the lung. Those that reach the small bronchi or alveoli are engulfed by macrophages. Some survive and multiply in the macrophages and are carried to the hilar lymph nodes. Inflammatory- minor and non-specific. Bacilli may be deposited in the liver, spleen, kidney, bone, brain, meninges and apices, and other parts of the lung. Flu-like symptoms and large lymph nodes at primary site, seen with x-ray

   2. Cell-mediated immunity to Mtb and hypersensitivity to tuberculoprotein develop in 2-6 weeks with formation of histologic tubercles at bacillary multiplication. T-cells recognize Ia antigen on infected macrophages

   3. Macrophages secrete Il-1 activating IFNand TNF activate local macrophages and recruit monocytes. NO mediate destruction of mycobacteria. Other cytokines stimulate accumulation of fibroblasts and deposition of collagen to wall off the bacteria. This leads to a tubercle- caseous granuloma.

   4. Primary infx handled well by host. M. tb multiplication ceases. Organisms die and lesions fibrose and calcify to produce Ghon complex seen on x-ray. Sometimes, bacilli remain alive and may a potential source of reactivation. In about 5% of patients, the infx is not contained and disseminates to many organs to produce military tuberculosis. This may happen when a necrotic tubercle erodes into a blood vessel.

   5. About 10% recovering from a Primary infx develop a disease later. Associated in older men, malnutrition, alcholism, diabetes, old age, or ’s lifestyle. Reactivation occurs in area of high oxygenation and low lymphatic drainage such as the apices. Small blood vessels may be involved in destruction of tissue, so patients cough up bloody sputum- highly infectious.

   6. Innate immunity is high. Races with a history f urbanization and exposure in 18^th and 19^th centuries have greater resistance. Native Americans and Eskimos low tolerance.

   7. Acquired immunity is cell-mediated, but incomplete.

5. Diagnosis

   1. Careful Hx

   2. Chest x-ray

   3. Tuberculin test- delayed-type hypersensitivity. Purified protein derivative (PPD) Intradermal injection of .1 ml of PPD of 5 TU. Read 48-72 hrs later. Induration, hardness, or 10 mm or more with erythema- positive reaction. Positive reaction- infx at some time or strong x-reactivitity mycobacterium species. No implication of activity

   4. Lab Dx-

      1. acid-fast visualized by Ziehl-Neelson on sputum samples. Not specific for M. tuberculosis. Sputum collected in a container not subject to contamination. Inevitably contaminated with normal flora. Carbolfuchasin->rinse-> alcohol-> methylene blue-> red cells on blue background.

      2. Cultures on solid media take up to 3 weeks or longer. Radioactive ¹⁴C- labeled palmitic acid. ¹⁴CO₂ liberated monitors growth.

   5. Cultivation in 3-8 weeks for PCR

6. Treatment and prevention

   1. Isoniazid and rifampin for extracellular and intracellular organisms

   2. Streptomycin for foe extracellular

   3. Ethambutol

   4. Two or three drug course for 6-9 months. One drug-relapse from resistant strain

   5. 10^-7 – 10^-10 mutation rate

   6. Prophylactic chemotherapy- isoniazid

      1. Radiologic evidence primary complex

      2. PPD + close contact with infx

      3. Child with close contact

      4. Known PPD converter

      5. Immunosuppressed

   7. Immunoprophylaxis- BCG (Bacillus Calmette and Guerin) vaccine

      1. Attenuated M. bovis strain. Not used in US since PPD test is diagnositic here

Category: Microbiology Notes