Primary Causes of Spontaneous Bacterial Peritonitis
The primary cause of spontaneous bacterial peritonitis (SBP) is bacterial translocation from the intestinal flora to mesenteric lymph nodes and then to the bloodstream, occurring predominantly in patients with cirrhosis and ascites. 1, 2, 3
Pathophysiological Mechanisms
SBP is defined as a bacterial infection of ascitic fluid without any intra-abdominal surgically treatable source of infection. The pathogenesis involves multiple factors:
Bacterial translocation: The main route by which ascitic fluid becomes infected 3
- Increased intestinal permeability
- Gut bacterial overgrowth (particularly Gram-negative aerobic bacilli in the jejunum)
- Changes in intestinal barrier function
Cirrhosis-related factors 1:
- Liver dysfunction
- Portosystemic shunting
- Gut dysbiosis
- Cirrhosis-associated immune dysfunction (CAID)
- Genetic factors (e.g., NOD2 variants associated with impaired recognition of bacterial products)
Microbiology
The most commonly isolated organisms in SBP include 2, 3, 4:
Gram-negative bacteria (traditionally most common):
- Escherichia coli (predominant)
- Other Enterobacteriaceae family members
Gram-positive cocci (increasingly common):
- Streptococcus species
- Enterococcus species
- Staphylococcus species
Changing microbiology trends 4:
- Increasing prevalence of Gram-positive cocci
- Rising incidence of multidrug-resistant organisms, particularly in healthcare-associated and nosocomial infections
Risk Factors
Several factors increase the risk of developing SBP 1, 4:
Cirrhosis severity:
- Advanced liver disease
- Low ascitic fluid protein concentration (<1 g/dL)
- Previous episodes of SBP (69% recurrence risk within a year)
Medication-related factors 4:
- Acid-suppressive therapy (proton pump inhibitors)
- Beta-adrenergic antagonists (in patients with end-stage liver disease and resistant ascites)
Clinical scenarios:
- Gastrointestinal bleeding
- Hospitalization (nosocomial infections)
Clinical Variants
SBP can present in different forms 2:
- Classic SBP: Neutrophil count >250/mm³ with positive culture
- Neutrocytic ascites: Neutrophil count >250/mm³ with negative culture
- Bacterascites: Neutrophil count <250/mm³ with positive culture (may represent transient colonization or early SBP)
Diagnostic Considerations
It's important to distinguish SBP from secondary bacterial peritonitis, which results from perforation or inflammation of an intra-abdominal organ 1, 2:
| Parameter | SBP | Secondary Peritonitis |
|---|---|---|
| Neutrophil count | >250/mm³ | Often >1000/mm³ |
| Culture | Typically monobacterial | Often polymicrobial |
| Protein | Usually <1 g/dL | Often >1 g/dL |
| Glucose | Usually normal | Often <50 mg/dL |
Clinical Implications
Early diagnosis and treatment are critical as:
- Mortality increases by 3.3% for each hour of delay in diagnosis 2
- SBP can precipitate other complications like hepatorenal syndrome and recurrent variceal hemorrhage 5
- Without prompt treatment, mortality can be as high as 20-40% 1, 6
Understanding these causes and mechanisms is essential for implementing appropriate preventive strategies and early treatment to improve outcomes in patients with cirrhosis and ascites.