A 52-year-old man with diabetes mellitus presents to the emergency department with fever, dysuria, and flank pain. Urine culture grows a gram-negative bacillus that is oxidase-positive, motile, and produces a fruity odour in culture. The organism is resistant to cephalosporins and carbapenems. Which of the following is the most likely organism?

## Identification of Pseudomonas aeruginosa ### Clinical Presentation The patient presents with a urinary tract infection (UTI) with fever and flank pain, suggesting upper UTI/pyelonephritis. While UTIs are commonly caused by E. coli, the microbiological features point to a different organism. ### Distinguishing Microbiological Features | Feature | P. aeruginosa | E. coli | K. pneumoniae | P. mirabilis | |---------|---------------|---------|---------------|-------------| | **Oxidase test** | Positive | Negative | Negative | Negative | | **Motility** | Motile (polar flagella) | Motile | Non-motile | Motile (peritrichous) | | **Odour** | Fruity/grape-like | None | None | Ammonia-like | | **Carbapenem resistance** | Often resistant | Sensitive | Sensitive | Sensitive | | **Cephalosporin resistance** | Often resistant | Sensitive | Sensitive | Sensitive | **Key Point:** The **oxidase-positive** result is the single most discriminating feature. Pseudomonas aeruginosa is one of the few gram-negative bacilli that produces cytochrome c oxidase, making it oxidase-positive. All other common gram-negative bacilli (E. coli, Klebsiella, Proteus, Enterobacter) are oxidase-negative. ### Resistance Pattern **High-Yield:** Pseudomonas aeruginosa is intrinsically resistant to many beta-lactams, including: - Cephalosporins (all generations) - Carbapenems (except meropenem and imipenem, which it may still resist) - Amoxicillin This is due to chromosomal beta-lactamase production and reduced outer membrane permeability [cite:Jawetz, Melnick & Adelberg's Medical Microbiology Ch 21]. ### Clinical Context **Clinical Pearl:** Pseudomonas aeruginosa UTIs are more common in: - Diabetic patients (as in this case) - Patients with indwelling catheters - Immunocompromised hosts - Hospital-acquired settings ### Diagnostic Confirmation The fruity (grape-like) odour is a classic feature of Pseudomonas aeruginosa cultures, produced by volatile compounds including 2-aminoacetophenone. This, combined with oxidase positivity and motility, makes the identification highly specific. **Mnemonic:** **OPFR** = Oxidase Positive, Fruity odour, Resistant to beta-lactams → **Pseudomonas**