Is E. coli Normal in an Infectious Stool?
E. coli is a normal inhabitant of the human gastrointestinal tract, but specific pathogenic strains cause infectious diarrhea and are the targets of diagnostic testing—not the commensal E. coli that comprises normal intestinal flora. 1
Understanding E. coli in Stool
Normal vs. Pathogenic E. coli
Commensal E. coli is universally present in the intestinal flora of all warm-blooded animals at concentrations of 10^5-10^7 organisms per gram of stool and represents a normal, non-pathogenic cohabitant. 2, 3
Pathogenic E. coli strains are distinct from normal flora and cause specific disease syndromes through virulence factors including toxins, adhesion factors, and invasive properties. 2, 3, 4
What Laboratories Actually Test For
Routine stool cultures for infectious diarrhea are designed to detect only specific pathogenic E. coli strains, not the normal commensal E. coli present in everyone's stool. 1
The targeted pathogenic strains include:
Shiga toxin-producing E. coli (STEC), including O157:H7, which laboratories should routinely test for in all stool specimens submitted for bacterial culture. 1
E. coli O157 is specifically sought using specialized media (sorbitol-MacConkey agar) or Shiga toxin detection assays, as this pathotype causes bloody diarrhea and hemolytic uremic syndrome. 1
Other pathogenic categories (ETEC, EPEC, EIEC, EAggEC) require specialized testing beyond routine stool culture and are not part of standard workup. 2, 5
Clinical Interpretation
When E. coli Matters in Infectious Stool
The presence of pathogenic E. coli strains (particularly STEC O157:H7) should be specifically tested in patients with acute bloody diarrhea or hemolytic uremic syndrome. 1
Key clinical indicators for STEC testing include:
Gross blood in stool (present in 63% of STEC O157 cases) or history of bloody diarrhea (91% of cases). 1
Less severe fever but more abdominal pain compared to Salmonella, Shigella, or Campylobacter infections. 1
Hemolytic uremic syndrome or severe bloody diarrhea warrants testing stool samples after broth enrichment with an EIA kit for Shiga toxin. 1
Common Pitfall to Avoid
Do not interpret the isolation of generic "E. coli" from stool as pathogenic—laboratories must specifically identify pathogenic strains through serotyping (O157:H7) or toxin detection (Shiga toxin). 1
The finding of non-pathogenic E. coli in stool culture represents normal colonization, not infection, and does not explain diarrheal illness. 6
Diagnostic Algorithm
When evaluating infectious diarrhea:
Order routine stool culture which targets Salmonella, Shigella, Campylobacter, and STEC (including E. coli O157:H7). 1
Specifically request STEC/Shiga toxin testing if your laboratory does not routinely perform this on all specimens, particularly for bloody diarrhea. 1
Confirm with your laboratory what their "routine stool culture" includes, as practices vary regarding automatic STEC testing. 1
If STEC is detected, public health confirmation of the serotype is essential for outbreak detection and epidemiologic purposes. 1