A 58-year-old man with chronic obstructive pulmonary disease is admitted with community-acquired pneumonia. Sputum gram stain shows gram-negative rods. Culture grows *Pseudomonas aeruginosa*. What is the drug of choice for this infection?

Explanation

## *Pseudomonas aeruginosa* Pneumonia ### Organism Identification **Key Point:** Gram-negative rods in sputum culture, especially in a patient with COPD or risk factors, raise concern for *Pseudomonas aeruginosa*. This is an opportunistic pathogen associated with healthcare exposure, structural lung disease, and immunosuppression. ### Treatment of *Pseudomonas* Infection **High-Yield:** Piperacillin-tazobactam (or anti-pseudomonal cephalosporins like cefepime, or carbapenems) is the first-line agent for *Pseudomonas aeruginosa* pneumonia. Piperacillin-tazobactam provides broad spectrum coverage with activity against *Pseudomonas* and anaerobes. ### Comparative Drug Efficacy | Agent | Pseudomonas Coverage | Mechanism | Clinical Use | | --- | --- | --- | --- | | Piperacillin-tazobactam | Yes, excellent | Extended-spectrum β-lactam + β-lactamase inhibitor | First-line for *Pseudomonas* pneumonia | | Amoxicillin-clavulanate | No | Aminopenicillin + β-lactamase inhibitor | Community-acquired pneumonia (non-*Pseudomonas*) | | Cephalexin | No | First-generation cephalosporin | Skin/soft tissue; not for *Pseudomonas* | | Trimethoprim-sulfamethoxazole | Limited | Folate pathway inhibitor | Urinary tract infection; not for pneumonia | **Clinical Pearl:** *Pseudomonas aeruginosa* is intrinsically resistant to many β-lactams because of its low cell wall permeability and constitutive β-lactamase production. Only anti-pseudomonal agents (piperacillin-tazobactam, cefepime, ceftazidime, carbapenems, fluoroquinolones) are effective. **Mnemonic:** **SPACE** agents for *Pseudomonas* — **S**pectinomycin, **P**iperacillin, **A**minopenicillins (no), **C**arbapenems, **E**xtended-spectrum cephalosporins (cefepime, ceftazidime). [Note: Spectinomycin is rarely used; the key agents are piperacillin, carbapenems, and anti-pseudomonal cephalosporins.] **Warning:** Do not use amoxicillin-clavulanate, cephalexin, or trimethoprim-sulfamethoxazole for *Pseudomonas* — all are ineffective and will lead to treatment failure. [cite:KD Tripathi 8e Ch 51]