Microbiologic tests for detecting Shiga toxin-producing Escherichia coli (STEC)

Microbiologic tests for detecting Shiga toxin-producing Escherichia coli (STEC)

Approach	Methodology	STEC detected		Advantages	Limitations
Approach	Methodology	O157:H7	Non-O157:H7	Advantages	Limitations
Isolate-directed tests	Sorbitol MacConkey agar inoculation*^[1]	Yes	No	Permits pathogen recovery, so isolate can be provided to public health laboratories for fingerprinting More sensitive than Shiga toxin antigen detection for E. coli O157:H7 If E. coli O157:H7 is detected, the patient can be assumed to be infected with a high-risk STEC without further testing	Non-O157:H7 STEC, including sorbitol fermenting E. coli O157:HNM (a pathogen in Germany and central Europe) is not detected by sorbitol MacConkey agar
Isolate-directed tests	Chromogenic STEC agar inoculation^[2]	Yes	Most	Permits pathogen recovery, so isolate can be provided to public health laboratories for fingerprinting	Depending on serotype, less sensitive than broth culture antigen testing
Shiga toxin antigen-detection	Overnight broth culture and antigen detection by enzyme immunoassay or lateral flow device^[3]	Most	Yes	Can theoretically detect O157:H7 and non-O157 STEC Some antigen detection systems will specify which Shiga toxin(s) is (are) present	Some Shiga toxin antigen tests are less sensitive than agar culture for detecting E. coli O157:H7 Some antigen detection systems do not specify the Shiga toxin(s) present^¶ Isolate must be recovered from broth before pathogen can be fingerprinted for disease control efforts
Nucleic acid amplification tests	Genes encoding Shiga toxins 1 and 2 and DNA encoding the O157 lipopolysaccharide antigen	Yes	Yes	Rapid Probably more sensitive than sorbitol MacConkey agar and antigen testing Can provide serotype if pathogen expresses the O157 LPS antigen, thereby demonstrating that the STEC signal is coming from a high-risk pathogen	Expensive Many platforms do not report the genotype(s) of the toxin(s) detected^¶ Isolate must be recovered from stool before pathogen can be fingerprinted for disease control efforts
Serology^Δ	Humoral immune response to O antigen^[4]	Yes	Yes (limited at present to O121 and O145)	Rapid Can be performed at point of care as a lateral flow test Requires minimal laboratory infrastructure	Limited to specific serogroups Day of illness on which seroconversion occurs not yet known Isolate must be recovered from stool before pathogen can be fingerprinted for disease control efforts

HUS: hemolytic uremic syndrome.

* The sensitivity of sorbitol MacConkey agar screening can be improved by inoculating specimen on agar containing tellurite and cefixime.

¶ When the test platform fails to specify the presence or absence of Shiga toxin 2 (or its corresponding gene), ambiguous reports ("Shiga toxin is detected," "Shiga toxin 1 and/or 2 is detected," "Shiga toxin-producing E. coli are detected") are generated. In these situations, the physician cannot assess the likelihood that the patient will develop HUS, because it is not known if a high-risk STEC (ie, one that produces Shiga toxin 2) underlies the positive antigen or toxin gene signal. If the submitted stool was from a patient with nonbloody diarrhea, the risk of a high-risk STEC being present is lower. If E. coli O157:H7 was sought and was negative, the risk is also lower (at least in North America where E. coli O157:H7 remains the predominant high-risk STEC). However, these are not absolute criteria, because high-risk non-O157:H7 STEC can cause HUS, and high-risk STEC O157 and non-O157 can cause nonbloody diarrhea.

Δ These tests are not commercially available outside Argentina.

References:

March SB, Ratnam S. Latex agglutination test for detection of Escherichia coli serotype O157. J Clin Microbiol 1989; 27:1675.
Zelyas N, Poon A, Patterson-Fortin L, et al. Assessment of commercial chromogenic solid media for the detection of non-O157 Shiga toxin-producing Escherichia coli (STEC). Diagn Microbiol Infect Dis 2016; 85:302.
Schindler EI, Sellenriek P, Storch GA, et al. Shiga toxin-producing Escherichia coli: a single-center, 11-year pediatric experience. J Clin Microbiol 2014; 52;3647.
Fiorentino GA, Miliwebsky E, Ramos MV, et al. Etiological diagnosis of post-diarrheal hemolytic uremic syndrome (HUS): humoral response contribution. Pediatr Nephrol 2022.

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Microbiologic tests for detecting Shiga toxin-producing Escherichia coli (STEC)