What are the broad-spectrum antibiotics for an adult patient with persistent diarrhea and no known allergies or underlying conditions?

Broad-Spectrum Antibiotics for Persistent Diarrhea

For an adult with persistent diarrhea and no known underlying conditions, broad-spectrum antibiotics are generally NOT recommended unless specific high-risk features are present, such as fever ≥38.5°C, bloody stools, signs of sepsis, or recent international travel. 1, 2 When antibiotics are indicated, azithromycin is the preferred first-line broad-spectrum agent due to its effectiveness against the most common bacterial pathogens and favorable resistance profile. 2, 3

When Broad-Spectrum Antibiotics Are Indicated

Empiric broad-spectrum antibiotic therapy should be initiated in the following specific scenarios:

• Bacillary dysentery syndrome: Fever, bloody diarrhea with mucus, abdominal cramping, and tenesmus suggesting Shigella infection 1, 2
• Recent international travelers with fever ≥38.5°C and/or signs of sepsis 1, 2
• Suspected enteric fever with clinical features of sepsis (requires blood, stool, and urine cultures before treatment) 1, 2
• Severe diarrhea with moderate to severe cramping, nausea, vomiting, diminished performance status, fever, bleeding, or dehydration requiring hospitalization 1

Preferred Broad-Spectrum Antibiotic Regimens

First-Line: Azithromycin

Azithromycin is the preferred empiric broad-spectrum antibiotic for adults with persistent diarrhea requiring treatment. 2, 3

• Dosing options: Single 1-gram dose OR 500 mg once daily for 3 days 2, 3
• Coverage: Effective against Shigella, Campylobacter, Salmonella, enterotoxigenic E. coli (ETEC), and Vibrio cholerae 2, 4
• Rationale: Fluoroquinolone-resistant Campylobacter now exceeds 90% in many regions including Thailand and India, making azithromycin superior to fluoroquinolones 2, 3

Second-Line: Fluoroquinolones (Use Only When Azithromycin Unavailable)

Fluoroquinolones should be reserved as second-line options due to widespread resistance 2, 3:

• Ciprofloxacin: 750 mg single dose OR 500 mg twice daily for 3 days 2, 5
• Levofloxacin: 500 mg once daily for 3 days 3
• FDA-approved indications: Infectious diarrhea caused by enterotoxigenic E. coli, Campylobacter jejuni, Shigella species, and Salmonella typhi 5

Critical Contraindications: When to AVOID Antibiotics

Never give antibiotics for suspected Shiga toxin-producing E. coli (STEC O157 or other Shiga toxin 2-producing strains), as this significantly increases the risk of hemolytic uremic syndrome. 1, 2, 4 This includes fluoroquinolones, β-lactams, trimethoprim-sulfamethoxazole, and metronidazole 1.

Key warning signs suggesting STEC:

• Bloody diarrhea with absent or low-grade fever 4, 6
• Recent consumption of undercooked ground beef or unpasteurized dairy 1
• Always obtain stool culture and Shiga toxin testing BEFORE starting antibiotics in bloody diarrhea 4

Special Populations Requiring Different Approaches

Neutropenic or Immunocompromised Patients

For severely immunocompromised patients with persistent diarrhea, fever, and neutropenia, consider neutropenic enterocolitis requiring broader coverage 1:

• Monotherapy options: Piperacillin-tazobactam OR imipenem-cilastatin 1
• Combination therapy: Cefepime or ceftazidime PLUS metronidazole 1
• Coverage required: Enteric gram-negative organisms, gram-positive organisms, and anaerobes 1
• Add antifungal: Consider amphotericin if no response to antibacterials, as fungemia is common 1

Infants Less Than 3 Months

Use a third-generation cephalosporin rather than azithromycin for suspected bacterial diarrhea in this age group 2, 6

Essential Complementary Management

Rehydration is the cornerstone of treatment for ALL diarrhea cases, regardless of antibiotic use. 2, 6 Antibiotics are only adjunctive therapy 4.

• Mild to moderate dehydration: Reduced osmolarity oral rehydration solution (ORS) containing 50-90 mEq/L sodium 1, 2, 6
• Severe dehydration, shock, or altered mental status: Intravenous fluids 2, 6
• Fluid resuscitation rate: Must exceed ongoing losses (urine output + 30-50 mL/h insensible losses + GI losses) 1
• Target urine output: >0.5 mL/kg/h 1

Common Pitfalls to Avoid

• Do NOT assume fever + blood = automatic antibiotics: Always rule out STEC first by obtaining stool cultures before starting treatment 4
• Do NOT use fluoroquinolones empirically for travelers returning from Southeast Asia due to >90% Campylobacter resistance 2, 3
• Do NOT treat non-typhoidal Salmonella routinely, as antibiotics prolong the carrier state; only treat high-risk patients (age <6 months or >50 years, immunocompromised, severe infection) 2, 6
• Do NOT use antidiarrheal agents (loperamide) in bloody diarrhea with fever, as they may worsen outcomes 1
• Do NOT continue empiric therapy without modification once a specific pathogen is identified 2, 6

When to Reassess or Hospitalize

If no clinical improvement within 48-72 hours of starting antibiotics, reassess for 2, 6:

• Antibiotic resistance or wrong pathogen coverage
• Inadequate rehydration or electrolyte imbalances
• Non-infectious causes (inflammatory bowel disease, ischemic colitis)
• Complications requiring hospitalization

Hospitalize immediately if patient develops 1, 4:

• Signs of sepsis or hemodynamic instability
• Severe dehydration despite oral rehydration attempts
• Significant gastrointestinal bleeding
• Suspected neutropenic enterocolitis in immunocompromised patients