A patient admitted with 50% burns develops an infection at the burn site. The swab was cultured, the isolate is a strict aerobe and the test shown in the image is positive. What is the likely aetiology of the burn infection?

Explanation

## Correct Answer: C. Pseudomonas aeruginosa Pseudomonas aeruginosa is the most common gram-negative aerobic rod causing burn wound infections in Indian hospitals. The discriminating feature here is the **strict aerobe** characteristic combined with a **positive oxidase test** (the image reference). P. aeruginosa is an obligate aerobe that produces the enzyme cytochrome c oxidase, which turns the oxidase test positive (blue-black color within 10 seconds). This combination—strict aerobic metabolism + oxidase positivity—is pathognomonic for P. aeruginosa among the gram-negative rods listed. Burn wounds create an ideal environment for P. aeruginosa colonization: the necrotic tissue is poorly vascularized (low oxygen tension paradoxically favors biofilm formation), the wound is moist, and the patient is immunocompromised. P. aeruginosa produces multiple virulence factors (exotoxins A and S, elastase, alkaline protease) that cause rapid tissue destruction and systemic sepsis. In Indian burn units, P. aeruginosa accounts for 40–60% of burn infections and is the leading cause of burn mortality. The organism is intrinsically resistant to many beta-lactams and aminoglycosides, making empiric coverage with anti-pseudomonal agents (piperacillin-tazobactam, carbapenems, fluoroquinolones) essential in burn management protocols per Indian guidelines. ## Why the other options are wrong **A. Klebsiella** — Klebsiella is a facultative anaerobe (not a strict aerobe) and is **oxidase-negative**. While Klebsiella can colonize burn wounds, it does not fit the strict aerobe + oxidase-positive profile. Klebsiella is more commonly associated with respiratory and urinary tract infections in hospitalized patients, not as the primary burn pathogen. **B. Salmonella** — Salmonella is a facultative anaerobe and is **oxidase-negative**. Although Salmonella can cause systemic infections in immunocompromised hosts, it is not a typical burn wound pathogen and does not match the strict aerobe + oxidase-positive criteria. Salmonella is primarily a gastrointestinal pathogen. **D. Escherichia coli** — E. coli is a facultative anaerobe and is **oxidase-negative**. While E. coli is a common nosocomial pathogen and can colonize burn wounds, it does not fit the strict aerobe phenotype and fails the oxidase test. E. coli is more associated with secondary infections (UTI, bacteremia) in burn patients rather than primary wound infection. ## High-Yield Facts - **Pseudomonas aeruginosa** is the most common gram-negative aerobic rod in burn wound infections (40–60% of cases in Indian burn units). - **Oxidase test positive** (blue-black color within 10 seconds) is the key biochemical test that distinguishes P. aeruginosa from other gram-negative rods. - **Strict aerobe** metabolism makes P. aeruginosa thrive in poorly vascularized, necrotic burn tissue despite low oxygen tension due to biofilm formation. - **Virulence factors** (exotoxin A, elastase, alkaline protease) cause rapid tissue destruction and systemic sepsis in burn patients. - **Anti-pseudomonal coverage** (piperacillin-tazobactam, carbapenems, fluoroquinolones) is mandatory in empiric burn infection therapy per Indian burn management guidelines. ## Mnemonics **OXIDASE-POSITIVE Gram-Negative Rods** **P**seudo (Pseudomonas aeruginosa) + **V**ibrio + **A**lkaligenes = OXI-POSITIVE. All others (E. coli, Klebsiella, Salmonella, Shigella, Proteus) are oxidase-NEGATIVE. **Burn Wound Pathogen Hierarchy (Indian Context)** **P**seudo (1st—40–60%) → **S**taph aureus (2nd—MRSA common) → **A**cinetobacter (3rd—MDR) → **E**. coli (4th). Remember: PSA-E for burn priority. ## NBE Trap NBE pairs "strict aerobe" with "gram-negative rod" to lure students into picking E. coli or Klebsiella (which are facultative anaerobes). The oxidase test is the discriminator that separates P. aeruginosa from all other gram-negative rods in this list. ## Clinical Pearl In Indian burn units, P. aeruginosa sepsis is the leading cause of death after the first 48 hours. Early recognition via oxidase test and rapid initiation of anti-pseudomonal therapy (often empiric in >20% TBSA burns) is critical for survival. Biofilm formation on eschar makes topical antimicrobials (silver sulfadiazine, mafenide) less effective than systemic therapy. _Reference: Jawetz, Melnick & Adelberg's Medical Microbiology (Ch. Pseudomonas); Robbins Pathology (Ch. Infectious Diseases); Indian Burn Management Guidelines (ISBI)_