**Core Concept**
The most common organism causing peritonitis is typically a gram-negative bacterium that is commonly found in the gastrointestinal tract. This organism can translocate from the gut lumen to the peritoneal cavity, leading to peritonitis.

**Why the Correct Answer is Right**
Escherichia coli (E. coli) is the most common cause of spontaneous bacterial peritonitis (SBP) in patients with cirrhosis. This is due to the translocation of bacteria from the gut lumen to the peritoneal cavity through a process known as the gut-liver axis. The bacteria adhere to the peritoneal surface and release endotoxins, leading to an inflammatory response. E. coli produces capsular polysaccharides that protect it from phagocytosis, allowing it to persist in the peritoneal cavity.

**Why Each Wrong Option is Incorrect**
**Option A:** Klebsiella pneumoniae is also a gram-negative bacterium, but it is less common than E. coli in causing peritonitis.
**Option B:** Streptococcus pneumoniae is a gram-positive bacterium that can cause peritonitis, but it is more commonly associated with aspiration pneumonia.
**Option C:** Staphylococcus aureus is a gram-positive bacterium that can cause peritonitis, but it is more commonly associated with skin and soft tissue infections.

**Clinical Pearl / High-Yield Fact**
In patients with cirrhosis, the development of spontaneous bacterial peritonitis is a significant complication that can lead to increased mortality. Early detection and treatment of SBP are crucial to improve outcomes.

**Correct Answer: C. E. coli**