A 68-year-old man with diabetes mellitus type 2 presents to the emergency department with fever (39.2°C), dysuria, and flank pain for 3 days. He has a history of recurrent urinary tract infections. Urinalysis shows pyuria, bacteriuria, and nitrites. Urine culture on MacConkey agar grows large, mucoid, pink colonies. Gram stain of the isolate shows gram-negative rods. The organism is oxidase-negative, indole-negative, and ferments lactose. What is the most likely causative organism?

Explanation

## Identification of Klebsiella pneumoniae ### Clinical Presentation The patient presents with acute pyelonephritis (fever, flank pain, dysuria) with a history of recurrent UTIs — a classic risk factor for Klebsiella infection in diabetic patients. ### Microbiological Features | Feature | Klebsiella pneumoniae | E. coli | Proteus mirabilis | Pseudomonas aeruginosa | |---------|----------------------|---------|-------------------|------------------------| | **Gram stain** | Gram-negative rod | Gram-negative rod | Gram-negative rod | Gram-negative rod | | **Oxidase** | Negative | Negative | Negative | Positive | | **Indole** | Negative | Positive | Negative | Negative | | **Lactose fermentation** | Positive (pink on MacConkey) | Positive (pink on MacConkey) | Negative (colorless) | Negative | | **Colony morphology** | Large, mucoid, viscous | Small, non-mucoid | Swarming (motile) | Green/blue pigment | | **Urease** | Negative | Negative | Positive | Negative | **Key Point:** The combination of **oxidase-negative, indole-negative, lactose-fermenting gram-negative rod with mucoid colonies** is pathognomonic for *Klebsiella pneumoniae*. ### Clinical Context **High-Yield:** *Klebsiella pneumoniae* is the second most common cause of hospital-acquired UTIs and is particularly prevalent in: - Diabetic patients - Patients with indwelling catheters - Immunocompromised hosts - Patients with prior antibiotic exposure (often resistant strains) **Clinical Pearl:** The mucoid appearance of *Klebsiella* colonies is due to its prominent polysaccharide capsule, which is a major virulence factor and also confers resistance to phagocytosis and complement-mediated killing. ### Pathogenesis in UTI The organism produces fimbriae (type 1 and type 3) that enable adherence to uroepithelial cells, and its capsule helps evade immune clearance — explaining the propensity for recurrent infections in this patient. ### Antibiotic Susceptibility Considerations **Warning:** Extended-spectrum β-lactamase (ESBL)-producing *Klebsiella* strains are increasingly common in nosocomial settings. Empiric therapy should account for local resistance patterns; carbapenems are often required for confirmed ESBL producers.