L25: Peritonitis and intra-abdominal infection

Question 1

What is peritonitis?

Answer 1

• Inflammation of peritoneum
• Can be generalised/diffuse or localised/abscess infection
• Primary/spontaneous peritonitis = diffuse bacterial infection without loss of GI integrity (rare, associated with patients with liver disease)
• Secondary = acute infection from loss of GI tract integrity or from infected viscera (most common, related to visceral pathology or post-surgical infection)
• Tertiary = recurrent infection of peritoneal cavity following initial therapy (often defective immunity)

Question 2

Microbial causative agents

Answer 2

Polymicrobial infection = more than 1 species, synergistic infection (hospital acquired may be one species)

Bacteria:

1. Enterobacteriaceae (E. coli, Klebsiella, enterobacter)
2. Anaerobes
   - Gram neg bacilli (bacteriodes fragilis, prevotella)
   - Gram pos cocci (peptostreptococcus)
   - Gram pos bacilli (clostridium)
3. Enterococci

Question 3

Sources and transmission

Answer 3

Source: more bacteria further down GI (more anaerobic) - stomach, duodenum, jejunum, ileum, colon

Transmission: from GI tract to peritoneum via perforation (secondary peritonitis)
- Mainly from appendicitis, diverticulitis (less so: ulcers, infections/abscess of other organs)

Question 4

Risk factors for peritonitis

Answer 4

Primary peritonitis = liver disease, portal hypertension, ascites

Secondary peritonitis = appendicitis, diverticulitis, ulcers, CAPD (dialysis), surgery

Tertiary peritonitis = immune deficiencies, previous primary or secondary peritonitis

Question 5

Pathogenesis of peritonitis

Answer 5

1. Bacteria gain entry (normally would be phagocytosed or contained in fibrin clot)
2. Bacteria not cleared in presence of nutrients (e.g. Hb) and necrotic tissue
3. Bacteria proliferates -> inflammation -> fluid exudate in cavity -> dilutes immune factors and reduces blood volume (hypovolemia)
4. Abscess formation (e.g. bacteriodes fragilis - fibrin trap bacteria “hide”) , prevents phagocytosis and immune access (microbial growth continues)

Question 6

Presentation of peritonitis

Answer 6

• Fever
• Increase resp rate (>20/min), heart rate (>90/min)
• Nausea and vomiting
• Diffuse abdominal pain (may become more localised)
• Rebound tenderness
• Abdominal wall rigidity

Question 7

Investigations for peritonitis

Answer 7

• Increased blood leukocytes
• CT/US: fluid accumulation, inflammation
• Laparoscopy: diagnostic, may allow treatment

Question 8

Diagnostic microbiology

Answer 8

Aspirate pus (foul smelling)

Gram stains of pus from abscess:

• Gram neg rods
• Possibly gram pos cocci
• Probably more than one type

Anaerobic and aerobic cultures:
- Culture from pus
- Anaerobic transport swabs
(useful e.g. if antibiotics fail, give idea of range of bacteria present)

Question 9

Diagnostic tests - anaerobes and bacteriodes

Answer 9

• Because of their fastidiousness, bacteriodes are difficult to isolate, often overlooked
• Often mixed infections e.g. E. coli on MacConkey agar
• Isolation requires aseptic aspiration, transport in an anaerobic environment
• Selective agar (bile aesculin agar produces black colour)
• Gas-liquid chromatography can be used to detect volatile fatty acids produced by anaerobic bacteria
• PCR
• MALD-TOF mass spectrometry

Question 10

Polymicrobial infection

Answer 10

(Synergy e.g. bacteriodes fragilis and E. coli)
Bacteriodes fragilis:
- Polysaccharide capsule promotes abscess formation (fibrin deposition) and stops phagocytosis
- Produces proteases, degrades complements
- Produce enzymes to stop iron-forming enzymes

E. coli:
- Secretes factors to acquire iron (required for oxygen-forming enzymes produced by body)

Question 11

Treatment of peritonitis

Answer 11

1. Symptomatic (fluids, pain relief, removal/drain pus guided by US/CT)
2. Establish cause and control origin (drain pus, dead tissue, corrective surgery)
3. Broad spectrum antibiotics (empiric broad spectrum - 1-2 weeks)

Triple therapy or single therapy if kidney and liver problems
Danger of C. difficile infection of GI tract with longer treatment regime

Question 12

Triple therapy for peritonitis

Answer 12

Enterobacteriaceae (E. coli) = aminoglycoside, fluoroquinolone or 4th generation cephalosporin

Anaerobes (B. fragilis) = clindamycin, metranidazole

Enterococcus = amphicillin

Question 13

Metronidazole action

Answer 13

• Antibiotic = bactericidal, amoebicidal, trichomoncidal
• Not effective against aerobic and facultatively anaerobic bacteria
• Oxidised by bacteria, interfering with DNA synthesis (exact mechanism not known)

Question 14

Metronidazole effective against:

Answer 14

• Anaerobic gram-neg bacilli (B. fragilis)
• Anaerobic gram-pos cocci (Clostridium, Eubacterium, anaerobic streptococci)
• Some pathogenic protozoa (Trichomonas vaginalis, Entamoeba histolytica, Giardia lamblia)

Question 15

Prevention of peritonitis

Answer 15

Prompt diagnosis and treatment of predisposing conditions (e.g. appendicitis)