## Spontaneous Bacterial Peritonitis — Microbiology **Key Point:** SBP is a bacterial infection of ascitic fluid without a secondary source (no perforation, no localized peritoneal inflammation). The causative organisms are predominantly gram-negative enteric bacteria. ### Most Common Organisms in SBP | Organism | Frequency | Notes | | --- | --- | --- | | **Escherichia coli** | **40–50%** | **Most common; gram-negative enteric** | | Klebsiella pneumoniae | 15–20% | Second most common | | Streptococcus pneumoniae | 10–15% | Gram-positive; less common than E. coli | | Staphylococcus aureus | <5% | Rare in SBP; more common in secondary peritonitis | | Anaerobes | <5% | Uncommon in SBP | **High-Yield:** E. coli accounts for nearly half of all SBP cases. The pathogenesis involves bacterial translocation from the gut, facilitated by impaired opsonic activity in ascitic fluid and reduced complement levels in cirrhosis. ### Diagnostic Criteria for SBP - Ascitic fluid polymorphonuclear (PMN) cell count ≥250 cells/μL - Positive bacterial culture (though culture may be negative in ~40% of cases) - No secondary cause (no perforation, no localized peritoneal inflammation) **Clinical Pearl:** SBP can occur with a negative ascitic fluid culture in up to 40% of cases, so clinical suspicion and elevated PMN count are critical for diagnosis. Empiric antibiotic therapy should not be delayed pending culture results.
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