In an adult with acute gastroenteritis, when are antibiotics indicated and what are the first‑line antimicrobial choices for common bacterial pathogens?

Antimicrobial Therapy for Acute Gastroenteritis in Adults

Most adults with acute gastroenteritis do not require antibiotics; reserve empiric antimicrobial therapy for patients with fever ≥38.5°C plus bloody or mucoid stools, recent international travel with sepsis signs, immunocompromised status with severe illness, or suspected enteric fever. 1

When to Initiate Antibiotics

Specific indications for empiric antimicrobial therapy include:

• Bacillary dysentery syndrome (frequent bloody stools, high fever, severe abdominal cramps, tenesmus) presumptively due to Shigella 1, 2
• Recent international travelers presenting with temperature ≥38.5°C and/or signs of sepsis 1, 2
• Immunocompromised patients with severe illness and bloody diarrhea 1, 2
• Suspected enteric fever with clinical features of sepsis (sustained high fever, altered mental status, hypotension) 1, 2
• Infants <3 months of age with suspected bacterial etiology 1, 2

Absolute Contraindications to Antibiotics

Never administer antibiotics for suspected or confirmed Shiga toxin-producing E. coli (STEC O157:H7 or other Shiga toxin 2-producing organisms), as this markedly increases the risk of hemolytic uremic syndrome. 1, 2 Obtain stool culture and Shiga toxin testing before initiating antibiotics in any patient with bloody diarrhea. 1

Asymptomatic household contacts of patients with diarrheal illness should not receive antibiotics. 1, 2

First-Line Antimicrobial Choices

Adults

Azithromycin is the preferred first-line empiric agent for adults requiring antimicrobial therapy. 1, 2 This recommendation is based on widespread fluoroquinolone resistance in Campylobacter (exceeding 90% in Southeast Asia, India, and Thailand). 1

Dosing regimens:

• Azithromycin: 500 mg daily for 3 days, or single 1-gram dose for moderate-to-severe cases 1, 2
• Fluoroquinolones (second-line): Ciprofloxacin 500 mg twice daily for 3 days, or 750 mg single dose 1, 2

Fluoroquinolones should only be used when local susceptibility patterns are favorable, azithromycin is contraindicated, or in regions with documented low resistance. 1

Infants and Children

• Infants <3 months: Third-generation cephalosporin (ceftriaxone 50 mg/kg/day) 1, 2
• Older children: Azithromycin based on local susceptibility patterns and travel history 1, 2

Pathogen-Specific Antimicrobial Therapy

Shigella Species

• First-line: Azithromycin 500 mg daily for 3 days 1, 2
• Alternative: Ciprofloxacin 500 mg twice daily for 3 days (if susceptible) 1, 2
• Severe cases: Ceftriaxone 2 g daily IV 2

Campylobacter Species

• First-line: Azithromycin 500 mg daily for 3 days 1, 2
• Alternative: Fluoroquinolones only if documented susceptibility (resistance rates ~19% in U.S., >90% in Southeast Asia) 1, 2

Non-typhoidal Salmonella

Antibiotics are NOT routinely recommended for uncomplicated non-typhoidal Salmonella gastroenteritis. 1, 2 Treatment is indicated only for:

• Severe infection with systemic toxicity 2
• Age <6 months or >50 years 1
• Immunocompromised status (HIV, transplant, chemotherapy) 1
• Prosthetic vascular materials, valvular heart disease, or severe atherosclerosis 2

When treatment is indicated:

• Ciprofloxacin 500 mg twice daily for 5-7 days 2
• Alternatives: Ceftriaxone 2 g daily IV, or TMP-SMX 160/800 mg twice daily 2

Vibrio cholerae (Cholera)

• First-line: Azithromycin single 1-gram dose (superior to ciprofloxacin, reducing diarrhea duration by >1 day) 1
• Alternatives: Doxycycline or tetracycline 1

Yersinia enterocolitica

• Ciprofloxacin 500 mg twice daily 2
• Alternatives: TMP-SMX or doxycycline 2

Rehydration: The Cornerstone of Management

Rehydration prevents morbidity and mortality more effectively than antibiotics and must never be deprioritized. 1

• Mild-to-moderate dehydration: Reduced-osmolarity oral rehydration solution (ORS) containing 50-90 mEq/L sodium 3, 1
• Severe dehydration, shock, altered mental status, or ileus: Isotonic intravenous fluids (lactated Ringer's or normal saline) 3, 1
• Continue rehydration until pulse, perfusion, and mental status normalize 3

Adjunctive Symptomatic Therapy

Loperamide may be used in immunocompetent adults with watery diarrhea after adequate rehydration, but is absolutely contraindicated when fever or bloody stools are present (risk of toxic megacolon). 3, 1 Never give loperamide to children or adolescents <18 years of age. 3

Antiemetics (ondansetron) may facilitate oral rehydration in children >4 years with vomiting. 3

Antibiotic Stewardship and Follow-Up

Modify or discontinue antimicrobial therapy once a specific pathogen is identified and susceptibilities are available. 1, 2 If no clinical improvement occurs within 48-72 hours, reassess for:

• Antibiotic resistance 1
• Inadequate rehydration or electrolyte imbalances 1
• Non-infectious causes (inflammatory bowel disease, irritable bowel syndrome) 2
• Need for hospitalization 1

Critical Pitfalls to Avoid

• Never initiate antibiotics for bloody diarrhea before ruling out STEC with Shiga toxin testing 1
• Never prioritize antibiotics over rehydration—dehydration drives mortality, not diarrhea 1
• Never use fluoroquinolones empirically for travelers returning from Southeast Asia due to >90% Campylobacter resistance 1
• Never prescribe empiric antibiotics for uncomplicated watery diarrhea without fever or blood, as this promotes resistance without benefit 1
• Never use loperamide when fever or bloody stools are present 1
• Never treat non-typhoidal Salmonella routinely—reserve for high-risk patients only 1, 2

Dietary Management

After rehydration, resume a normal, age-appropriate diet immediately with small, light meals; avoid heavy, fatty, spicy foods and caffeine. 1 Continue breastfeeding in infants throughout the diarrheal episode. 3