In a patient with acute bacterial diarrhea, should I always test for Shiga toxin‑producing Escherichia coli (STEC) before starting antibiotics, or are there circumstances where empiric antibiotic therapy is appropriate?

Testing for Shiga Toxin Before Starting Antibiotics in Acute Bacterial Diarrhea

You do NOT need to test for Shiga toxin before starting empiric antibiotics in most cases of bloody diarrhea, but you should avoid empiric antibiotics altogether in immunocompetent patients with bloody diarrhea unless specific high-risk criteria are met. 1

When Empiric Antibiotics Are Appropriate (Without Waiting for STEC Testing)

The IDSA guidelines provide clear circumstances where empiric antibiotics should be started immediately, even before STEC results are available 1:

High-Risk Scenarios Requiring Empiric Treatment:

• Infants <3 months of age with suspected bacterial etiology and bloody diarrhea 1
• Bacillary dysentery presentation: Ill patients with fever documented in a medical setting, abdominal pain, bloody diarrhea, and classic dysentery symptoms (frequent scant bloody stools, fever, abdominal cramps, tenesmus) presumptively due to Shigella 1
• Recent international travelers with body temperature ≥38.5°C and/or signs of sepsis 1
• Immunocompromised patients with severe illness and bloody diarrhea 1
• Suspected enteric fever with clinical features of sepsis (after obtaining blood, stool, and urine cultures) 1

Empiric Antibiotic Choices:

• Adults: Fluoroquinolone (ciprofloxacin) or azithromycin based on local susceptibility patterns and travel history 1
• Children: Third-generation cephalosporin for infants <3 months or those with neurologic involvement; azithromycin for others based on local susceptibility patterns 1

When to Avoid Empiric Antibiotics

In immunocompetent children and adults with bloody diarrhea, empiric antimicrobial therapy while waiting for test results is NOT recommended unless the above high-risk criteria are met 1. This is a strong recommendation with low-quality evidence from the IDSA.

Critical Rationale:

The concern about STEC is paramount because antibiotics administered to patients with STEC O157 or other STEC producing Shiga toxin 2 significantly increase the risk of hemolytic uremic syndrome (HUS) 1, 2. A 2016 meta-analysis of low-risk-of-bias studies demonstrated that antibiotic use in STEC infections is associated with a 2.24-fold increased odds of developing HUS (95% CI 1.45-3.46) 3. This occurs because antibiotics trigger prophage induction, leading to increased Shiga toxin production and release 4, 5.

Testing Strategy for STEC

When clinical or epidemic history suggests possible Shiga toxin-producing organisms, you should apply diagnostic approaches that detect Shiga toxin (or genes encoding them) and distinguish E. coli O157:H7 from other STEC 1. Specifically:

• Use sorbitol-MacConkey agar or chromogenic agar to screen for O157:H7 STEC 1
• Employ Shiga toxin detection or genomic assays for non-O157 STEC 1
• If available, distinguish between Shiga toxin 1 and Shiga toxin 2 (the more potent form) 1

Watery Diarrhea: A Different Algorithm

For acute watery diarrhea without recent international travel, empiric antimicrobial therapy is NOT recommended in most patients 1. Exceptions include:

• Immunocompromised patients 1
• Young infants who appear ill 1

Key Clinical Pitfall

The most dangerous mistake is giving antibiotics to a patient with undiagnosed STEC infection. Once STEC O157 or other STEC producing Shiga toxin 2 is identified (or if toxin genotype is unknown), antimicrobial therapy should be avoided entirely 1, 2. This is a strong recommendation with moderate-quality evidence from the IDSA.

If you start empiric antibiotics in one of the appropriate high-risk scenarios listed above, you must modify or discontinue treatment immediately when STEC is identified 1.

Monitoring After STEC Diagnosis

If STEC is confirmed, implement frequent laboratory monitoring including 2:

• Daily hemoglobin and platelet counts
• Close monitoring of electrolytes, blood urea nitrogen, and creatinine
• Peripheral blood smear examination for red blood cell fragmentation (schistocytes)