Gram-Negative Bacteria

By completing this question set, you will be able to identify any gram-negative organism in this curriculum from a combination of gram stain morphology, oxidase test, lactose fermentation, motility, H₂S production, and selective media growth, explaining the reasoning at each decision step. For Neisseria, you will explain why N. meningitidis requires a polysaccharide capsule but N. gonorrhoeae uses Opa protein antigenic variation — and why complement terminal pathway deficiency specifically predisposes to Neisseria infection. For E. coli, you will distinguish all six pathotypes by mechanism and predict why EHEC requires withholding antibiotics while Shigella requires administering them. For Salmonella and Shigella, you will explain the molecular basis of each organism's invasive strategy and predict why Salmonella causes osteomyelitis in sickle cell disease. You will explain why Bordetella pertussis's lymphocytosis is a direct consequence of pertussis toxin Gi-inactivation, not immune hyperactivation. For Pseudomonas, you will map each virulence mechanism to a specific clinical niche and explain why intrinsic multi-drug resistance is structural. For H. pylori, you will apply T4SS biology to explain how CagA transforms a mucosal pathogen into an oncogenic one. For Campylobacter, you will explain molecular mimicry with peripheral nerve gangliosides as the basis for post-infectious GBS. Throughout, you will apply the gram-negative identification cascade with mechanistic understanding of why each biochemical test reflects organism biology.