Core Framework & Diagram How Do Bacteria Evade the Immune System?

Primarily because Mycobacterium tuberculosis survives inside macrophages and grows extremely slowly (doubling time approximately 20 hours — far slower than ordinary bacteria's 20 minutes). Most antibiotics are most effective against rapidly dividing bacteria (many antibiotics target bacterial cell wall synthesis or protein synthesis, only effective against actively dividing bacteria); against slowly growing or metabolically inactive Mycobacterium tuberculosis, longer drug exposure is needed to ensure complete clearance. Also, Mycobacterium tuberculosis can survive in a 'dormant state' (non-replicating state) insensitive to many drugs, requiring specific drugs (like rifampicin, pyrazinamide) to cover this state.

Useful, but depends on serotype matching. Streptococcus pneumoniae has over 90 serotypes, each with different capsular polysaccharide structures. Antibodies against one serotype typically have very limited cross-protection against other serotypes. Pneumococcal conjugate vaccines (PCV13, PCV20, PCV23) cover the most common disease-causing serotypes. If the infection is a serotype covered by the vaccine, vaccine-produced IgG can effectively opsonize that bacterium. If it's an uncovered serotype, vaccine protection is limited — also why getting a pneumonia vaccine doesn't prevent all pneumonia types.

Intracellular bacterial clearance has two levels. First, infected cells (primarily macrophages) present bacterial antigen fragments via MHC-II to CD4+ T cells. After CD4+ T cells activate, they secrete IFN-γ to activate macrophages into M1 highly efficient bactericidal mode, attempting to force phagosome maturation and destroy intracellular bacteria. Second, infected cells similarly present some intracellular bacterial protein fragments via MHC-I (through cross-presentation mechanism), allowing CD8+ T cells to recognize and kill infected cells — after infected cell death, intracellular bacteria are released extracellularly and can then be handled by antibodies and neutrophils.

Listeria's special characteristic is its ability to survive and slowly grow at refrigerator temperature (4°C), making refrigerated foods potential risk sources. High-risk foods: unpasteurized soft cheeses (like Brie, Camembert), deli meats (particularly sliced cold-stored ones), insufficiently heated ready-to-eat foods, smoked seafood. Prevention: don't store ready-to-eat foods in refrigerator beyond recommended time; keep refrigerator temperature below 4°C; pregnant women, older adults, and immunocompromised individuals should avoid high-risk foods; ensure food internal temperature exceeds 75°C when heating.