Mycobacteria & Atypical Bacteria

By completing this question set, you will apply the same core principle to every organism in this file: organism biology predicts diagnostic approach and treatment strategy. M. tuberculosis's mycolic acid wall explains both its acid-fast staining and its resistance to standard antibiotics. Its obligate aerobic metabolism explains why reactivation favors the lung apex. Its phagolysosomal evasion mechanisms explain why granuloma maintenance requires TNF-α, and why anti-TNF therapy is the most potent pharmacologic trigger of reactivation. The TST measures the CD4+ T-cell memory that this granuloma depends on, and IGRA improves specificity by using antigens absent from BCG. Chlamydia's loss of ATP-generating machinery makes it an obligate intracellular energy parasite that requires host ribosomes and host ATP, and simultaneously explains its lack of functional peptidoglycan (cannot construct peptidoglycan properly without energy) and thus its beta-lactam resistance. Mycoplasma's complete absence of cell wall explains both staining failure and intrinsic beta-lactam resistance by the same logic. Legionella's intracellular lifestyle in alveolar macrophages explains why beta-lactam penetration is poor and why the Dot/Icm T4SS — which hijacks host membrane trafficking to create an ER-like protective niche — is the central virulence mechanism. Throughout this file, the organizing thread is: what is unusual about this organism's structure or metabolism, and what are the clinical consequences of that unusual biology?