CDC urges culturing suspected E coli samples

Oct 3, 2006 (CIDRAP News) – The Centers for Disease Control and Prevention (CDC) is urging clinical laboratories to use bacterial culture of stool specimens in suspected cases of Escherichia coli O157:H7 infection to facilitate diagnosis and proper control measures.

Failure to culture specimens can cause labs to mistake some other pathogen for E coli O157:H7, leading to inappropriate public health measures, the CDC warns in the Sep 29 issue of Morbidity and Mortality Weekly Report.

That happened last year in North Carolina, where a clinical lab used enzyme immunoassay (EIA) to look for Shiga toxin—which is produced by E coli O157:H7 and other pathogenic E coli strains—in a specimen from a baby. Test results were positive, leading authorities to take public health measures appropriate for Shiga toxin–producing E coli (STEC) strains. Those included barring the baby and several other sick children from their day care center until two stool cultures showed them to be free of STEC.

"Some days" after the EIA result, the lab sent the patient sample to the state public health lab, which tested it and found no STEC, the report says. The sample was then sent to the CDC, where an EIA again showed Shiga toxin, but further tests for Shiga toxin genes were negative. Subsequently the state lab tested more samples from the index patient and four other patients and found they had norovirus.

After this finding, authorities changed the control measures, and the sick children were allowed to return to the day care center as soon as they were well, the report says.

"As the North Carolina outbreak demonstrates, occasional false-positive results from the Stx [Shiga toxin] EIA test can result in inappropriate and unnecessary public health action," the CDC states. If the lab had cultured for E coli O157 at the same time as it ran the EIA or immediately afterward, a negative result might have prompted an earlier search for norovirus, the report says.

The article also reports on an outbreak in a New York state prison last year that turned out to have been caused by E coli O45:nonmotile, a far less common strain than O157:H7. In that episode, an EIA pointed to Shiga toxin, but cultures were negative for E coli O157:H7. Eventually the CDC tested isolates from three patients and identified the O45 serotype.

The CDC recommends that clinical labs use EIA to screen stool samples for Shiga toxins and, ideally, culture samples for STEC O157 at the same time, which facilitates rapid diagnosis and treatment of patients and rapid subtyping by public health labs.

Labs that don't simultaneously culture samples for STEC O157 should culture any Shiga toxin–positive broths for STEC O157 as soon as possible and send the isolates to a public health lab for confirmation and subtyping, the CDC advises. If a Shiga toxin–positive broth does not yield STEC O157, it should be sent to a public health lab for identification of non–O157 STEC. Public health labs should send all non–O157 STEC isolates to the CDC for confirmation, the agency says.

CDC. Importance of culture confirmation of Shiga toxin–producing Escherichia coli infection as illustrated by outbreaks of gastroenteritis—New York and North Carolina, 2005. MMWR 2006 Sep 29;55(38):1042-5 [Full text]

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