A 28-year-old woman from Delhi presents to the emergency department with acute onset dysuria, urinary frequency, and suprapubic pain for 2 days. She is afebrile with a blood pressure of 110/70 mmHg. Urinalysis shows pyuria (>10 WBC/hpf), bacteriuria, and nitrites. Urine culture grows a gram-negative rod that is oxidase-negative, ferments lactose, and produces indole. The organism is susceptible to fluoroquinolones but resistant to ampicillin. Which is the most likely organism?

Explanation

## Organism Identification **Key Point:** The clinical presentation of acute uncomplicated cystitis with a lactose-fermenting, indole-positive, oxidase-negative gram-negative rod is pathognomonic for E. coli, specifically uropathogenic E. coli (UPEC). ### Diagnostic Features | Feature | Finding | Significance | |---------|---------|---------------| | Gram stain | Gram-negative rod | Enterobacteriaceae family | | Oxidase test | Negative | Rules out Pseudomonas | | Lactose fermentation | Positive | E. coli, Klebsiella | | Indole production | Positive | **E. coli (90% of strains)** | | Nitrites in urine | Present | Indicates nitrate-reducing bacteria | | Antibiotic pattern | Ampicillin-resistant, FQ-susceptible | Common UPEC resistance pattern | ### UPEC vs Other E. coli Pathotypes **Key Point:** UPEC is the causative agent of >80% of uncomplicated urinary tract infections in non-pregnant women. - **UPEC (Uropathogenic E. coli):** Possesses P fimbriae (pili) and type 1 fimbriae for uroepithelial adherence; causes acute cystitis and pyelonephritis; isolated from urine cultures in UTI. - **EPEC (Enteropathogenic E. coli):** Causes infantile diarrhea via adherence-and-effacement lesions; does NOT cause UTI; isolated from stool, not urine. - **Klebsiella pneumoniae:** Indole-negative (key differentiator); causes nosocomial UTI, pneumonia, and sepsis; more common in catheterized or immunocompromised patients. - **Proteus mirabilis:** Urease-positive (produces ammonia, alkaline urine); causes staghorn calculi; indole-negative; less common than E. coli in community UTI. ### Clinical Pearl **High-Yield:** The combination of **indole-positive + lactose-fermenting + oxidase-negative** in a UTI isolate is virtually diagnostic of E. coli. UPEC strains express virulence factors (P fimbriae, hemolysin, CNF) that enable uroepithelial invasion and persistence, explaining recurrent UTI in women. ### Antibiotic Susceptibility Context Ampicillin resistance in E. coli is increasingly common due to TEM-type β-lactamases. Fluoroquinolone susceptibility is typical in community-acquired UPEC (though resistance is rising). This pattern guides empirical therapy: fluoroquinolones, trimethoprim-sulfamethoxazole (if susceptible), or cephalosporins are first-line for uncomplicated cystitis.