What does it mean to have invasive and pathogenic E. coli in a stool sample?

Finding Invasive and Pathogenic E. coli in Stool Samples

Yes, it is normal to detect both invasive and pathogenic E. coli strains in stool samples when testing symptomatic patients with colitis or inflammatory diarrhea, as these represent distinct categories of disease-causing E. coli that laboratories routinely screen for when evaluating infectious diarrhea. 1

Understanding the Terminology

The terms "invasive" and "pathogenic" E. coli refer to different categories of disease-causing strains that are not mutually exclusive:

• Invasive E. coli (EIEC) penetrate and multiply within intestinal epithelial cells, causing dysentery-like illness with fever, abdominal cramps, and bloody diarrhea 2
• Pathogenic E. coli is an umbrella term encompassing multiple categories including enterotoxigenic (ETEC), enteropathogenic (EPEC), enterohemorrhagic (EHEC/STEC including O157:H7), enteroinvasive (EIEC), and enteroaggregative (EAggEC) strains 3, 4, 2

When These Organisms Are Tested

Stool cultures for invasive enteropathogens should be submitted when patients exhibit symptoms of colitis including severe fever, abdominal cramps, and/or bloody diarrhea with or without white blood cells in stool, particularly without recent antibiotic use. 1

Standard stool cultures in most laboratories are designed to detect the most frequent invasive enteropathogens:

• Campylobacter jejuni
• Salmonella species
• Shigella species
• E. coli O157:H7 (and other Shiga toxin-producing E. coli) 1

Clinical Significance of Detection

Invasive Pathogens

Diarrhea due to invasive pathogens like Salmonella, Shigella, and invasive E. coli strains is associated with:

• Fever (present in 58-100% of Salmonella cases, 53-83% of Shigella cases) 1
• Prominent inflammatory signs
• Bloody stool (25-51% of Salmonella cases, up to 37% of Shigella cases) 1
• Readily diagnosed by stool culture 1

E. coli O157:H7 and STEC

These enterohemorrhagic strains cause:

• Bloody diarrhea typically without fever
• Risk of hemolytic uremic syndrome 1
• Diagnosed by Shiga toxin assay or specialized culture methods 1

Management Implications

For patients with invasive E. coli or other invasive enteropathogens detected in stool, consider prompt transfer to acute care facilities due to the risk of bacteremia and systemic complications, particularly in elderly or immunocompromised patients. 1

Treatment Considerations

• Generally avoid antibiotics for uncomplicated E. coli infections, as they may prolong bacterial shedding, increase hemolytic uremic syndrome risk with STEC, and promote antimicrobial resistance 5
• Consider antimicrobial therapy only for patients with signs of invasive disease/sepsis, immunocompromised individuals, infants under 6 months with Salmonella, or elderly patients with significant comorbidities 5
• Empirical fluoroquinolone therapy may be appropriate for severe inflammatory diarrhea after obtaining stool specimens, though quinolone-resistant Campylobacter is increasingly common 1

Infection Control

• Implement strict hand hygiene with soap and water (alcohol preparations may not be sufficient) 5
• Notify local public health authorities, as both invasive E. coli and Salmonella are reportable diseases 5
• Food handlers and healthcare workers may require negative stool cultures before returning to work 5

Important Caveats

Do not submit stool specimens for routine culture if collected more than 3 days after hospital admission, as the yield is extremely low (accounting for 15-50% of unnecessary specimens). 1 The exception is if the patient appears involved in a nosocomial outbreak or has specific risk factors like age >65 years, immunosuppression, or HIV infection 1

For patients with recent antibiotic use (within 30 days), prioritize testing for C. difficile toxin before pursuing invasive bacterial pathogen cultures. 1