In a surveillance study of gram-negative bacteremia in a tertiary care hospital in India, which Enterobacteriaceae species is most commonly isolated from blood cultures?

Explanation

## Epidemiology of Enterobacteriaceae Bacteremia **Key Point:** E. coli is the most common gram-negative bacterium isolated from blood cultures globally and in India, accounting for 40–50% of Enterobacteriaceae bacteremia cases. ### Sources of E. coli Bacteremia 1. **Urinary tract**: Most frequent source (40–50% of E. coli bacteremia); ascending infection from cystitis or pyelonephritis. 2. **Biliary tract**: Cholecystitis, cholangitis, especially in elderly patients. 3. **Gastrointestinal tract**: Perforation, peritonitis, appendicitis. 4. **Intravascular devices**: Catheter-related bloodstream infection (CRBSI). 5. **Neonatal sepsis**: E. coli is a leading cause in newborns, particularly strains with K1 capsule. ### Why E. coli Dominates Bacteremia - **Ubiquitous colonization**: E. coli is a normal commensal of the human gut and genitourinary tract, providing multiple portals of entry. - **Virulence factors**: Lipopolysaccharide (LPS), hemolysin, and aerobactin facilitate systemic invasion and survival in blood. - **Frequency of UTI and intra-abdominal infections**: These are the most common sources of bacteremia, and E. coli is the leading pathogen in both. ### Comparative Frequency in Bacteremia | Organism | Frequency in Bacteremia | Primary Source | |----------|------------------------|----------------| | **E. coli** | 40–50% | Urinary tract, biliary, GI | | **Klebsiella pneumoniae** | 15–20% | Respiratory, urinary, biliary | | **Enterobacter species** | 10–15% | Nosocomial, intravascular devices | | **Salmonella typhi** | 5–10% (India) | Enteric fever (GI tract) | | **Proteus mirabilis** | 3–5% | Urinary tract | | **Citrobacter freundii** | <2% | Nosocomial, immunocompromised | **Clinical Pearl:** In a patient presenting with fever and positive blood cultures, if a gram-negative rod is isolated, E. coli is the most likely organism. Source control (drainage of abscess, removal of infected catheter, antibiotic therapy for UTI) is critical in management. **High-Yield:** While Salmonella typhi is endemic in India and causes enteric fever, it is NOT the most common gram-negative bacteremia isolate overall. Enteric fever bacteremia occurs in a specific clinical context (fever, rose spots, relative bradycardia, splenomegaly), whereas E. coli bacteremia is more frequent across diverse clinical presentations. **Mnemonic:** **ECKP** — E. coli, Citrobacter, Klebsiella, Proteus (in descending order of frequency in bacteremia).