Microbiology and Immunology 3820A Lecture Notes - Lecture 6: Immunosuppression, Tuberculosis, Cell-Mediated Immunity
9 Dec 2016
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Slow growth, poor staining and acid fastness (i. e once stained, resistant to decolorization by acid) Cell walls that are 60% lipid, uncluding unique long chain fatt acids called mycolic acids. Classification: tubercle bacilli complex, mott (mycobacteria other than tubercle bacilli, mycobacterium leprae (causal agent of leprosy, tuberculosis complex. Induration is measured in mm 48-72 hours later. Interpretation depends on pretest probability in accordance with bayes theorem. Contact cases, immunosuppressed or suspicious chest x-ray (high pretest probability) are positive at > 5 mm. Those from high prevalence countries, healthcare workers, in prison or institutions or other medical risk factors are positive at > 10 mm. No risk factors are positive at > 15 mm. Sensitivity increases with 2 tests a few weeks apart; repeated testing does not lead to false positives. It takes 4-6 weeks after exposure to develop a positive. Paradoxically, those with advanced tb may become anergic and have a negative test.
