What is the best antibiotic for treating infectious colitis?

Best Antibiotic for Infectious Colitis

For non-severe Clostridium difficile colitis, use oral metronidazole 500 mg three times daily for 10 days; for severe C. difficile colitis, use oral vancomycin 125 mg four times daily for 10 days. 1, 2 For other bacterial causes of infectious colitis (Shigella, Salmonella, Campylobacter), azithromycin 1000 mg as a single dose is the empiric treatment of choice. 3

Disease Severity Assessment for C. difficile Colitis

Determining severity is the critical first step that dictates antibiotic choice. 1, 2

Non-severe disease is characterized by:

• Stool frequency <4 times daily 1
• White blood cell count <15 × 10⁹/L 1
• Absence of systemic inflammatory signs 1

Severe disease includes any of the following:

• Core body temperature >38.5°C with rigors 4, 1
• Hemodynamic instability or septic shock 4, 1
• Signs of peritonitis (decreased bowel sounds, abdominal tenderness, rebound, guarding) 4, 1
• Ileus with vomiting or absent stool passage 4, 1
• Marked leukocytosis (>15 × 10⁹/L) or left shift (>20% bands) 4, 1
• Serum creatinine rise >50% above baseline 4, 1
• Elevated serum lactate 4, 1
• Pseudomembranous colitis on endoscopy 4, 1
• Colonic distension or wall thickening on imaging 4, 1

Treatment Algorithm for C. difficile Colitis

Initial Episode - Oral Therapy Possible

For non-severe disease:

• Metronidazole 500 mg orally three times daily for 10 days 4, 1, 2
• This is the most cost-effective option and prevents vancomycin resistance 5, 6

For severe disease:

• Vancomycin 125 mg orally four times daily for 10 days 4, 1, 2, 7
• Vancomycin is superior to metronidazole in severe cases 4
• Higher doses (up to 500 mg four times daily) may be used in fulminant disease, though evidence is limited 4

Initial Episode - Oral Therapy Impossible

For non-severe disease:

• Metronidazole 500 mg intravenously three times daily for 10 days 4

For severe disease:

• Metronidazole 500 mg intravenously three times daily PLUS 4, 2
• Intracolonic vancomycin 500 mg in 100 mL normal saline every 4-12 hours 4, 2 AND/OR
• Vancomycin 500 mg four times daily by nasogastric tube 4, 2

Recurrent C. difficile Infection

For first recurrence:

• Treat the same as initial episode based on severity (metronidazole for non-severe, vancomycin for severe) 1, 2

For second and subsequent recurrences:

• Vancomycin 125 mg orally four times daily for at least 10 days 4, 1, 2
• Consider a taper strategy (decreasing daily dose by 125 mg every 3 days) 4, 1, 2
• Consider a pulse strategy (125 mg every 3 days for 3 weeks) 4, 1, 2
• Fidaxomicin 200 mg twice daily for 10 days is an alternative, particularly for high-risk patients (elderly with multiple comorbidities receiving concomitant antibiotics) 4, 1, 2

For multiple recurrences unresponsive to antibiotics:

• Fecal microbiota transplantation (FMT) with 70-90% success rates 4, 1

Treatment for Other Bacterial Infectious Colitis

For empiric treatment of febrile dysenteric diarrhea (suspected Shigella, Salmonella, or Campylobacter):

• Azithromycin 1000 mg as a single oral dose 3
• This covers the most common invasive bacterial enteropathogens 3

Important exception - Do NOT treat Shiga toxin-producing E. coli (STEC) with antibiotics:

• STEC typically presents with acute dysentery but only low-grade or no fever 3
• Antibiotics may increase risk of hemolytic uremic syndrome 3

Critical Management Principles

Immediately discontinue the inciting antibiotic if the colitis was clearly induced by antibiotic use, particularly in mild cases. 4, 1, 2 In mild C. difficile infection with stool frequency <4 times daily and no severe signs, stopping the offending antibiotic alone may be sufficient with close observation. 4

Avoid antiperistaltic agents (loperamide, diphenoxylate) and opiates entirely as they can precipitate toxic megacolon. 4, 1, 2

Narrow antibiotic spectrum when possible based on culture results to minimize further disruption of gut flora. 4

If continued antibiotic therapy is required for a primary infection other than C. difficile, use agents less frequently implicated in antibiotic-associated colitis: parenteral aminoglycosides, sulfonamides, macrolides, vancomycin, or tetracycline/tigecycline. 4

Surgical Intervention Criteria

Colectomy should be performed urgently for any of the following: 4, 1, 2

• Perforation of the colon 4, 1, 2
• Toxic megacolon 4, 1
• Severe ileus 4, 1
• Systemic inflammation with deteriorating clinical condition not responding to antibiotic therapy 4, 1, 2
• Operate before serum lactate exceeds 5.0 mmol/L 4, 2

Diverting loop ileostomy with colonic lavage is a useful alternative to total colectomy. 4

Common Pitfalls to Avoid

Do not use parenteral vancomycin for C. difficile colitis - it is not excreted into the colon and is completely ineffective. 1, 7 Vancomycin must be given orally or intraluminally. 7

Do not repeat stool testing after treatment to assess response - clinical improvement (decreased stool frequency, improved consistency) is the primary measure of success. 1

Do not delay surgery in severe cases waiting for antibiotic response - early colectomy before severe deterioration improves outcomes. 4, 1

Do not assume all antibiotic-associated diarrhea is C. difficile - confirm diagnosis with stool toxin testing before treating. 1

Monitor for systemic absorption in high-risk patients - clinically significant serum vancomycin concentrations can occur with oral therapy in patients with inflammatory intestinal mucosa, renal insufficiency, or those receiving concomitant aminoglycosides. 7 Consider monitoring serum vancomycin levels in these patients. 7

Monitor renal function in elderly patients - nephrotoxicity can occur during or after oral vancomycin therapy, particularly in patients >65 years of age. 7 Serial renal function monitoring is recommended in this population. 7