A 68-year-old man with chronic obstructive pulmonary disease is admitted with acute exacerbation and fever. Sputum culture grows a gram-negative rod that is oxidase-positive and produces a blue-green pigment. Which is the most common site of infection with this organism in hospitalized patients?

Explanation

## Most Common Site of Pseudomonas aeruginosa Infection in Hospitalized Patients **Key Point:** Pseudomonas aeruginosa most commonly infects the respiratory tract in hospitalized patients, particularly in those on mechanical ventilation (ventilator-associated pneumonia, VAP). ## Epidemiology of P. aeruginosa Infections | Site | Frequency | Clinical Context | Risk Factors | |------|-----------|------------------|---------------| | **Respiratory tract** | Most common | VAP, chronic lung disease, CF | Mechanical ventilation, ICU stay | | Urinary tract | Second most common | Catheter-associated UTI | Indwelling catheter, prolonged catheterization | | Bloodstream | Less common | Sepsis, immunocompromised | Central lines, severe infection | | Surgical wound | Least common | Post-operative infection | Contaminated environment, poor wound care | **High-Yield:** In the ICU setting, P. aeruginosa is the leading cause of ventilator-associated pneumonia (VAP), accounting for 20–30% of all VAP cases. The organism thrives in moist environments (ventilator circuits, condensate) and biofilm formation on endotracheal tubes. **Clinical Pearl:** The blue-green pigment (pyoverdine and pyocyanin) is pathognomonic for P. aeruginosa and aids rapid identification on culture plates. The oxidase-positive test confirms the diagnosis. ## Why Respiratory Tract is Most Common 1. **Environmental preference:** P. aeruginosa is an aerobic, gram-negative rod that colonizes moist surfaces—the respiratory tract provides an ideal niche. 2. **Nosocomial prevalence:** In hospital settings, especially ICUs, respiratory tract colonization precedes infection in ventilated patients. 3. **Biofilm formation:** The organism readily forms biofilms on endotracheal tubes, making it difficult to clear and leading to persistent infection. **Mnemonic:** **PABCDE** — P. aeruginosa sites (in order of frequency): **P**ulmonary (respiratory) > **A**bdominal (rare) > **B**loodstream > **C**atheter-related (urinary) > **D**ebridement (wound) > **E**ar (otitis externa in swimmers). [cite:Mandell, Douglas, Bennett's Principles and Practice of Infectious Diseases Ch 221]