A 38-year-old woman develops a surgical site infection following an open appendectomy performed 4 days ago. Culture of wound fluid grows multiple organisms including gram-negative rods and anaerobes. Which is the most common organism isolated from SSIs of the lower gastrointestinal tract?

Question

Accepted Answer

B. Bacteroides fragilis. ## Most Common Organism in Lower GI Tract SSI

**Key Point:** Bacteroides fragilis is the most common anaerobic organism isolated from surgical site infections of the lower gastrointestinal tract, including appendectomy, colectomy, and rectal surgery.

### Microbiology of Lower GI SSI

| Organism | Category | Frequency | Clinical Significance |
|----------|----------|-----------|----------------------|
| **B. fragilis** | Anaerobic gram-negative rod | 40–60% of anaerobic SSI | Most virulent anaerobe; β-lactamase producer |
| C. difficile | Anaerobic gram-positive rod | <5% of SSI | Toxin-mediated; rare in acute SSI |
| P. mirabilis | Aerobic gram-negative rod | 5–10% | Urease producer; indole-negative |
| K. pneumoniae | Aerobic gram-negative rod | 8–12% | Encapsulated; nosocomial |

### Why B. fragilis Dominates Lower GI SSI

1. **Endogenous source:** Normal colonic flora; concentration ~10^11 CFU/mL in colon
2. **Virulence factors:**
   - Polysaccharide capsule (antiphagocytic)
   - β-lactamase production (resistance to penicillins)
   - Lipopolysaccharide endotoxin
3. **Synergy:** Often isolated with aerobic gram-negatives (E. coli, Klebsiella) in polymicrobial infections
4. **Timing:** Manifests 3–7 days postoperatively (as in this case)

**High-Yield:** Lower GI surgery SSI is characteristically **polymicrobial** — anaerobes + aerobes. B. fragilis is the anaerobic component in >50% of cases. This is why prophylaxis for colorectal surgery includes agents covering anaerobes (cefoxitin, cefotetan, ampicillin-sulbactam).

### Clinical Pearl

The presence of **multiple organisms (gram-negative rods + anaerobes)** on culture is a red flag for lower GI source. B. fragilis is often isolated alongside E. coli or Klebsiella in a synergistic polymicrobial infection, which is more difficult to treat and has higher morbidity than monomicrobial infection.

**Mnemonic: BAGEL** — **B. fragilis = Bacteroides (anaerobic), Abdominal (lower GI), Gram-negative, Endogenous flora, Lower gut**

### Antibiotic Implications

- B. fragilis is resistant to clindamycin (~20% resistance) and cephalosporins (intrinsic β-lactamase)
- Agents of choice: Carbapenems (meropenem), piperacillin-tazobactam, ampicillin-sulbactam, metronidazole (if combined with aerobic coverage)
- Prophylaxis in colorectal surgery: Cefoxitin or cefotetan (covers both aerobes and anaerobes)

Answer

A. Klebsiella pneumoniae

Answer

C. Clostridium difficile

Answer

D. Proteus mirabilis

A 38-year-old woman develops a surgical site infection following an open appendectomy performed 4 days ago. Culture of wound fluid grows multiple organisms including gram-negative rods and anaerobes. Which is the most common organism isolated from SSIs of the lower gastrointestinal tract?

Explanation

Most Common Organism in Lower GI Tract SSI

Microbiology of Lower GI SSI

Why B. fragilis Dominates Lower GI SSI

Clinical Pearl

Antibiotic Implications

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Organism	Category	Frequency	Clinical Significance
B. fragilis	Anaerobic gram-negative rod	40–60% of anaerobic SSI	Most virulent anaerobe; β-lactamase producer
C. difficile	Anaerobic gram-positive rod	<5% of SSI	Toxin-mediated; rare in acute SSI
P. mirabilis	Aerobic gram-negative rod	5–10%	Urease producer; indole-negative
K. pneumoniae	Aerobic gram-negative rod	8–12%	Encapsulated; nosocomial