What’s the Significance of Enteroinvasive E. coli (EIEC), Enteropathogenic E. coli (EPEC), and Enteroaggregative E. coli (EAEC) in Stool?

March 11, 2019

Infectious gastroenteritis is a leading cause of morbidity and mortality worldwide. Although viruses are the major cause of infectious gastroenteritis, 15-20% is caused by bacteria. Identification of the specific etiology of gastroenteritis is helpful for individual patient targeted care. It is also important for public health measures such as outbreak investigations. Until a decade ago, there were a limited number of bacteria detectable in diarrheal stool by clinical microbiology laboratories, as most laboratories performed culture to identify gastrointestinal (GI) pathogens. Pathogens detected by stool culture included Salmonella, Shigella, Campylobacter, and Shiga toxin-producing E. coli O157:H7 (photo). Some other bacterial causes of diarrhea, such as Vibrio, could be detected in stool with the use of additional media, enrichment techniques, or careful examination by the technologist. However, diarrheagenic strains of E. coli other than O157:H7 cannot be differentiated phenotypically from normal E. coli microbiota in stool culture.
 
E. coli O157:H7 colonies on sorbitol MacConkey agar. The organism does not ferment sorbitol and so, unlike most E. coli, does not form dark pink colonies on this agar. Courtesy of Clinical Microbiology Laboratory, Mayo Clinic, Rochester.

Many clinical microbiology laboratories now use multiplex molecular GI pathogen panel assays in place of routine stool culture. These assays can detect anywhere between 9 and 22 bacterial, viral, and parasitic agents. The advantages of these multiplex molecular assays include faster turnaround time, higher sensitivity than culture, and a broad testing menu. A couple of recent ASM blogs list the promises and perils of these multiplex GI panels. However, there are some issues with use of these panels, such as whether all results are actionable. In other words, is there guidance on the actions clinicians should take when certain targets are reported for their patients? In 2017, ASM released a White Paper on the clinical utility of multiplex tests, and some of these issues were covered. Some of the pathogens for which there is the least guidance include the diarrheagenic E. coli pathotypes or strains.
 
There has been much discussion recently about the clinical significance of some of these pathotypes in stool. Shiga toxin-producing E. coli strains (STEC) can be detected through targeting Shiga toxins via molecular or immunoassays. Other diarrheagenic strains (enterotoxigenic E. coli (ETEC), enteropathogenic E. coli (EPEC), enteroinvasive E. coli (EIEC), and enteroaggregative E. coli (EAEC)) require different detection methods, since they do not produce Shiga toxin. The role of STEC and ETEC in disease has been firmly established. The roles of the other E. coli pathotypes, EIEC, EPEC, and EAEC, however, are less clear.
 
Currently, there are two companies - BioFire and BD - which offer FDA-cleared commercial multiplex molecular GI panels targeting several of the diarrheagenic pathotypes of E. coli. BioFire’s FilmArray GI Panel is FDA-cleared and includes EAEC, EIEC, EPEC, ETEC, and STEC targets, alongside other targets. The BD MAX Enteric Bacterial Panel includes EIEC and STEC, while its Extended Enteric Bacterial Panel includes ETEC, alongside other targets. Both of the BD MAX panels are FDA-cleared for in vitro diagnostic (IVD) use.   In a study of the BioFire GIP, 2 or more pathogens (bacterial, viral, or parasitic) were detected in 116/709 (16%) of stool samples. Of the samples positive for 2 or more pathogens, 98/116 (84%) were positive for either EPEC or EAEC. Coinfection rates in other studies have been high as well, with up to 45.7% of EPEC and 61.5% of EAEC associated with coinfection. Since clinical laboratories have not historically tested for several of these pathotypes, laboratories and clinicians are sometimes at a loss as to whether to assign clinical relevance to these pathogens, and how to approach treatment of the patient.
 
EIEC is closely related to Shigella and is thought to cause watery diarrhea through invasion of the epithelial cells of the colon. It does not produce enterotoxins. Symptoms include abdominal cramps, malaise, tenesmus, and occasionally fever. Bloody diarrhea or dysentery is an uncommon outcome. In fact, Shigella spp. and EIEC cause similar gastrointestinal illnesses. Infections due to EIEC are rare in the U.S., although outbreaks have occurred. Rarely, it is a cause of traveler’s diarrhea. There is limited information on the efficacy of antimicrobial therapy for EIEC strains, and antimicrobial therapy is not routinely suggested.
 
First described in the 1980’s, EAEC causes epidemic and sporadic diarrhea among children and adults in developing and developed countries. It has been associated with traveler’s diarrhea, as well as persistent diarrhea in infants. EAEC produces enterotoxins and cytotoxins. Its name was derived from its ability to adhere to certain cells in cell culture in a pattern resembling stacked bricks. Distinct from several other diarrheagenic pathotypes of E. coli, EAEC strains are not as clearly associated with disease. In a U.S. study of 1267 children in Tennessee, the prevalence of EAEC strains was no different in cases versus controls (17/857 [2%] vs. 15/410 [4%], respectively; OR 0.52, range 0.26-1.07).  In a matched case-control study in Africa and Asia, there was no clear association between EAEC and diarrhea tested with an in-house multiplex PCR run on stool.
 
There is more supportive clinical evidence for EPEC as cause of GI infections. However, some studies have found similar rates of EPEC in healthy controls as compared to patients with GI disease. Infections due to EPEC occur more commonly in resource-limited settings and in children younger than two years of age. They are an important cause of infant diarrhea in South America, sub-Saharan Africa, and Asia but are not believed to be important causes of gastroenteritis in North America and Europe. EPEC does not produce Shiga toxins or enterotoxins. Thus, the disease burden due to EPEC has been relatively poorly understood, but its role in disease has been established based on studies of immune responses, disease association, and studies of virulence factors. Antimicrobial therapy may be helpful for the treatment of infection due to EPEC strains. Furthermore, different EPEC strains exist; some are “typical” while others are “atypical.” The strains differ in the rates of antimicrobial susceptibility and virulence factors. Some studies have noted a higher prevalence of atypical EPEC in patients with diarrhea, while others have demonstrated a higher prevalence of atypical strains as GI colonizers. Further studies will be helpful in delineating these differences.
 
In summary, there is some evidence to support a pathogenic role for EPEC and EIEC as causes of infectious gastroenteritis. However, clinical guidelines are unclear as to the need for antimicrobial treatment for gastroenteritis due to these bacteria. The evidence supporting clinical significance of EAEC is less clear. These issues are important, as EPEC and EAEC represent some of the most common targets co-detected with other possible GI targets in multiplex stool panels. Additionally, there are no testing guidelines addressing the possible need for repeat or confirmatory testing of such targets. At least one company on the market has included these targets on their panels, and studies will hopefully be able to further characterize the clinical importance of these targets.

The above represent the views of the author and does not necessarily reflect the opinion of the American Society for Microbiology.
 

Author: Audrey Schuetz 

Audrey Schuetz 
Audrey Schuetz is Associate Professor of Laboratory Medicine and Pathology at Mayo Clinic College of Medicine and Science in Rochester, Minnesota.