Bacteriology
Core concepts
Bacteriology questions reward a structured approach: organism → virulence factor → syndrome → identification method. Know the classic Gram-positive cocci (staphylococci: catalase positive, coagulase distinguishes S. aureus; streptococci: bacitracin/optochin for S. pneumoniae, bile esculin for group D), Gram-negative bacilli (lactose fermentation on MacConkey separates E. coli/Klebsiella from Salmonella/Shigella), and the fastidious organisms (HACEK, Neisseria, Haemophilus requiring X and V factors).
High-yield mechanisms
Understand toxin-mediated disease: C. difficile toxins A/B, C. botulinum and C. tetani neurotoxins, S. pyogenes streptolysin and superantigens, B. anthracis tripartite toxin. Know intracellular pathogens (Listeria, Salmonella, Legionella, Mycobacterium) and the immunity each requires (cell-mediated for mycobacteria and listeria).
Traps
- Confusing colonization with infection - a positive culture alone is not disease.
- Mixing up catalase vs. coagulase and oxidase results.
- Forgetting that Mycoplasma lacks a cell wall (beta-lactams useless) and Legionella needs charcoal-buffered media.
- Assuming all diarrhea is inflammatory; non-inflammatory vs. inflammatory patterns guide workup.
Worked rule of thumb
A CSF with low glucose, high protein, and neutrophils in an adult most often means bacterial meningitis; in a neonate, add Listeria and group B Streptococcus to empiric coverage. In a patient with gram-positive rods in blood and a history of eating unpasteurized cheese, think Listeria and treat with ampicillin (plus gentamicin in severe disease).