Which condition is usually caused by a single organism: pelvic inflammatory disease, perforated diverticulitis, acute cholecystitis, primary peritonitis, or diabetic foot infections?

Primary Peritonitis is Usually Caused by a Single Organism

The correct answer is (d) primary peritonitis, which is typically monomicrobial (caused by a single organism), in contrast to the other conditions listed which are characteristically polymicrobial infections. 1, 2

Understanding the Microbiology of Each Condition

Primary Peritonitis (Spontaneous Bacterial Peritonitis)

• Primary peritonitis is characteristically monomicrobial, occurring as a diffuse bacterial infection without loss of gastrointestinal tract integrity, typically in cirrhotic patients with ascites or those with peritoneal dialysis catheters 1, 3
• E. coli is the predominant pathogen (32-67% of culture-positive cases), followed by Klebsiella species and Streptococcus species, which together account for nearly three-fourths of all cases 2, 4
• The single-organism nature of this infection is a defining characteristic that distinguishes it from secondary peritonitis 4

Why the Other Options Are Polymicrobial

Perforated Diverticulitis:

• This represents secondary peritonitis resulting from loss of gastrointestinal tract integrity 5
• Colon-derived infections are typically polymicrobial, involving both facultative/aerobic organisms and obligate anaerobes 5
• The infection extends beyond the organ into the peritoneal space with mixed flora 5

Acute Cholecystitis:

• While uncomplicated cholecystitis may involve a single organ, when infection develops it typically involves multiple organisms from the biliary tract 5
• Gram-positive and gram-negative aerobic and facultative pathogens are commonly isolated together 5

Pelvic Inflammatory Disease:

• This is characteristically a polymicrobial infection involving multiple sexually transmitted and endogenous organisms
• Multiple pathogens from the lower genital tract ascend to cause upper tract infection

Diabetic Foot Infections:

• These are typically polymicrobial infections involving aerobic and anaerobic bacteria
• Mixed flora from skin and environmental sources contribute to the infection

Clinical Implications for Diagnosis

• When multiple organisms are isolated from ascitic fluid, consider secondary bacterial peritonitis (due to intestinal perforation) rather than primary peritonitis 2
• Diagnostic paracentesis showing polymorphonuclear leukocyte count >250/mm³ with a single organism on culture confirms spontaneous bacterial peritonitis 1, 3
• Low ascitic fluid glucose (<50 mg/dL) and multiple organisms suggest secondary rather than primary peritonitis 3

Treatment Considerations

• Third-generation cephalosporins (cefotaxime 2g IV every 8 hours) remain first-line for primary peritonitis due to excellent coverage of the typical single causative organisms 1, 2, 4
• Primary peritonitis is managed with antibiotics alone without surgical intervention, unlike secondary peritonitis which requires source control 1, 3
• The monomicrobial nature allows for more targeted antibiotic therapy compared to the broad-spectrum coverage needed for polymicrobial infections 4