A 28-year-old woman presents to the emergency department with acute onset dysuria, frequency, and suprapubic pain for 2 days. She denies fever, flank pain, or vaginal discharge. Urinalysis shows pyuria (>10 WBC/hpf), bacteriuria, and positive nitrites. Urine culture grows a gram-negative rod that is oxidase-negative, ferments lactose, and produces indole. The organism is resistant to ampicillin but susceptible to fluoroquinolones. What is the most likely causative organism?

Explanation

## Diagnosis: Uropathogenic E. coli (UPEC) ### Clinical Presentation This patient presents with uncomplicated acute cystitis — dysuria, frequency, suprapubic pain without systemic symptoms or upper tract involvement. The urinalysis findings (pyuria, bacteriuria, nitrites) are classic for bacterial urinary tract infection. ### Microbiological Identification | Feature | Finding | Significance | |---------|---------|---------------| | Gram stain | Gram-negative rod | Enterobacteriaceae family | | Oxidase | Negative | Rules out Pseudomonas | | Lactose fermentation | Positive | Consistent with E. coli, Klebsiella | | Indole production | Positive | **Characteristic of E. coli** | | Ampicillin resistance | Resistant | Suggests β-lactamase production | | Fluoroquinolone | Susceptible | Common susceptibility pattern | **Key Point:** Indole production is the **single most useful test to differentiate E. coli from other Enterobacteriaceae** (Klebsiella, Proteus, Enterobacter are indole-negative). ### Why UPEC? **UPEC (Uropathogenic E. coli)** is the causative agent in 80–85% of uncomplicated UTIs in non-pregnant women. UPEC strains express specialized virulence factors: 1. **P fimbriae (pili)** — bind to P blood group antigens on uroepithelial cells 2. **Type 1 fimbriae** — mediate initial adhesion 3. **Hemolysin and aerobactin** — tissue invasion and iron acquisition **Clinical Pearl:** UPEC causes acute cystitis; when upper tract involvement occurs (flank pain, fever, CVA tenderness), the diagnosis becomes pyelonephritis, but the organism remains E. coli. ### Antibiotic Resistance Pattern The ampicillin resistance with fluoroquinolone susceptibility is typical of community-acquired UPEC strains in India, where fluoroquinolones remain first-line for uncomplicated UTI (per Indian guidelines, though resistance is rising). **High-Yield:** E. coli is indole-positive; Klebsiella and Proteus are indole-negative — this single test rapidly narrows the differential in the microbiology lab.