A 48-year-old Indian woman with a 10-year history of cholelithiasis presents with acute right upper quadrant pain, fever, and leukocytosis. Blood cultures grow a gram-negative rod. Which is the most common causative organism in acute bacterial cholecystitis?

Explanation

## Most Common Organism in Acute Bacterial Cholecystitis ### Microbiology of Acute Cholecystitis **Key Point:** *Escherichia coli* is the most common causative organism in acute bacterial cholecystitis, isolated in 40–60% of culture-positive cases. ### Frequency of Organisms | Organism | Frequency | Gram Stain | Notes | | --- | --- | --- | --- | | **E. coli** | 40–60% | Gram-negative rod | Most common; ascending infection from biliary tract | | Klebsiella pneumoniae | 15–20% | Gram-negative rod | Second most common | | Enterococcus faecalis | 10–15% | Gram-positive coccus | Often polymicrobial | | Clostridium perfringens | <5% | Gram-positive rod | Emphysematous cholecystitis (rare, high mortality) | | Salmonella typhi | <2% | Gram-negative rod | Rare in modern era; associated with chronic carrier state | | Vibrio cholerae | <1% | Gram-negative rod | Rare; endemic areas only | ### Pathophysiology of Bacterial Invasion 1. **Stone-induced obstruction** → bile stasis 2. **Increased intraluminal pressure** → mucosal ischemia 3. **Bacterial translocation** from biliary tract (ascending cholangitis) 4. **Secondary infection** in ~50% of acute cholecystitis cases **High-Yield:** E. coli is part of normal gut flora and ascends via the biliary tree, especially when bile flow is obstructed. It is an **enteric gram-negative rod** — the classic organism in biliary tract infections. ### Clinical Pearl **Mnemonic for common biliary organisms: "KECK"** - **K**lebsiella - **E**scherichia coli (most common) - **C**lostridium (emphysematous cholecystitis) - **K**lebsiella (second most common) In acute cholecystitis, cultures are often **polymicrobial** (mixed aerobic and anaerobic flora), reflecting the ascending infection from the colon via the biliary tree. ### Why NOT the Other Organisms? **Clostridium perfringens** → causes **emphysematous cholecystitis** (gas in gallbladder wall), a rare but severe form with high mortality. It is NOT the most common organism in routine acute cholecystitis. **Salmonella typhi** → associated with chronic biliary carriage (especially in endemic areas like India) and can cause cholecystitis, but is rare in modern series and not the most common. **Vibrio cholerae** → causes watery diarrhea and dehydration, not cholecystitis. Extremely rare in biliary tract infections.