GI Infections

GI infections cause disease through distinct pathogenic mechanisms that determine clinical presentation and guide management. Non-inflammatory (secretory) diarrhea is caused by enterotoxin-producing organisms (enterotoxigenic E. coli, Vibrio cholerae, Staphylococcus aureus, Bacillus cereus emetic toxin) that stimulate intestinal chloride secretion via cAMP or cGMP second messenger pathways in enterocytes without mucosal invasion — producing profuse watery diarrhea without blood, leukocytes, or fever. Inflammatory (invasive) diarrhea results from organisms that invade and destroy the intestinal mucosa (Shigella, Salmonella, Campylobacter, enteroinvasive E. coli, Entamoeba histolytica), causing bloody diarrhea with fecal leukocytes, fever, and tenesmus — Shigella produces Shiga toxin that inhibits 60S ribosomal protein synthesis in endothelial cells, causing hemolytic uremic syndrome (HUS). Clostridioides difficile represents a unique pathogenesis: antibiotic-induced disruption of normal colonic microbiota allows C. difficile spore germination and vegetative cell proliferation; toxin A (enterotoxin) attracts neutrophils and increases intestinal permeability, while toxin B (cytotoxin) disrupts the actin cytoskeleton causing epithelial cell death and pseudomembrane formation — the characteristic yellowish-white plaques on erythematous mucosa. Helicobacter pylori colonizes the gastric antrum by producing urease (which generates a protective ammonia cloud neutralizing local...