A 48-year-old woman with Child–Pugh class C cirrhosis secondary to hepatitis C develops spontaneous bacterial peritonitis. Which organism is the most common causative agent?

Explanation

## Most Common Organism in Spontaneous Bacterial Peritonitis **Key Point:** Escherichia coli is the single most common causative organism in spontaneous bacterial peritonitis (SBP), accounting for approximately 40–50% of culture-positive cases in cirrhotic patients. ### Microbiology of SBP SBP is predominantly a monomicrobial infection caused by gram-negative aerobic bacteria. The typical organisms and their relative frequencies are: | Organism | Frequency | Clinical Notes | | --- | --- | --- | | E. coli | 40–50% | Most common; translocation from gut | | Klebsiella pneumoniae | 15–20% | Second most common; often nosocomial | | Streptococcus pneumoniae | 10–15% | Gram-positive; associated with poor prognosis | | Other gram-negatives | 10–15% | Enterobacter, Proteus, Pseudomonas | | Anaerobes | <5% | Rare in SBP; more common in secondary peritonitis | | Staphylococcus aureus | <5% | Uncommon; suggests secondary peritonitis or catheter infection | **High-Yield:** The predominance of gram-negative organisms (E. coli, Klebsiella) reflects bacterial translocation from the gut across the intestinal barrier in the setting of portal hypertension, ascites, and impaired host immunity. ### Pathophysiology of Bacterial Translocation 1. Portal hypertension → intestinal edema and increased permeability 2. Impaired reticuloendothelial system function (reduced opsonization) 3. Low ascitic fluid opsonic activity and complement levels 4. Gram-negative organisms from the colon translocate to mesenteric lymph nodes and peritoneal fluid **Clinical Pearl:** While E. coli is most common overall, Klebsiella pneumoniae has become increasingly prevalent in nosocomial and healthcare-associated SBP, particularly in patients with recent antibiotic exposure or hospitalization. ### Diagnostic Approach - **Ascitic fluid analysis:** PMN count >250 cells/μL (or >500 cells/μL in some guidelines) suggests SBP - **Culture:** Inoculate ascitic fluid into blood culture bottles at bedside for optimal yield - **Empiric therapy:** Third-generation cephalosporin (cefotaxime 2 g IV q6h) covers E. coli and Klebsiella **Warning:** ~~Anaerobes~~ are NOT typical in SBP; their presence suggests secondary peritonitis (perforation of a viscus). Similarly, Staphylococcus aureus is uncommon and should raise suspicion for secondary peritonitis or catheter-related infection. [cite:Harrison 21e Ch 297]