## Microbiology of Acute Pyelonephritis **Key Point:** *Escherichia coli* (E. coli) accounts for approximately 80–90% of community-acquired acute pyelonephritis cases in non-pregnant, non-catheterized women. ### Pathogenic Mechanism E. coli possesses virulence factors that enable ascending infection: - **P fimbriae (pili)** — bind to P blood group antigens on uroepithelial cells - **Type 1 fimbriae** — mediate adherence to mannose-containing receptors - **Hemolysin and aerobactin** — enhance tissue invasion and iron acquisition ### Organism Distribution in UTI | Organism | Prevalence in Acute Pyelonephritis | Key Feature | |----------|-------------------------------------|-------------| | *E. coli* | 80–90% | P fimbriae; uropathogenic strains | | *Proteus mirabilis* | 5–10% | Urease production; staghorn calculi | | *Klebsiella pneumoniae* | 3–5% | Nosocomial; catheterized patients | | *S. saprophyticus* | 5–15% (young women) | Coagulase-negative; acute uncomplicated cystitis | **Clinical Pearl:** *Staphylococcus saprophyticus* is the second most common cause in sexually active young women (15–24 years) but remains rare in pyelonephritis; it predominantly causes acute uncomplicated cystitis. **High-Yield:** In pregnant women with asymptomatic bacteriuria, *E. coli* and *Klebsiella* are most common; untreated asymptomatic bacteriuria progresses to pyelonephritis in 20–40% of pregnant patients. ### Risk Factors for Ascending Infection 1. Vesicoureteral reflux (VUR) 2. Urinary obstruction (stones, tumors, strictures) 3. Neurogenic bladder 4. Pregnancy 5. Diabetes mellitus 6. Immunosuppression **Tip:** When a question asks about "community-acquired" or "uncomplicated" pyelonephritis in a non-pregnant woman without instrumentation, default to *E. coli* as the answer.
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