Evaluation of enrichment cultures for detection of Escherichia coli O157 from human stool specimens by using immunomagnetic separation

Abstract

We investigated the conditions of enrichment cultures preceding the immunomagnetic separation (IMS) procedure to detect Escherichia coli O157 (E. coli O157) from human stool specimens in routine laboratory examinations. Samples were made by adding either of the three selected strains of E. coli O157 to stools from three healthy human subjects in three different doses. The enrichment cultures were done for 18 hours at 37 degrees C or 42 degrees C, using five different media such as trypticase soy broth (TSB). TSB containing cefixime, tellurite and vancomycin, modified EC broth (mEC), mEC containing novobiocin (N-mEC) and BGLB. The IMS procedure following enrichment culture increased the detection rate of E. coli O157, irrespective of the kinds of the media and the temperatures. It recovered E. coli O157 in 42 samples out of 90, while only 31 samples were positive when the IMS was not applied. The N-mEC showed the best recovery rate of the five enrichment media, and it was the only media that recovered the E. coli O157 Gunmma 298 strains at a level of 2-3 cells per ml. In 73 stool samples collected from probable patients with E. coli O157 infection and subjects who made close contact with the patients, positive results were obtained in six samples with the N-mEC enrichment followed by the IMS procedure, while only three samples were positive by the direct isolation culture. It was concluded, therefore, that, in routine laboratory examinations of E. coli O157 from human stools, the N-mEC enrichment culture for 18 hours followed by the IMS procedure is a sensitive method even when the dose of E. coli O157 in the stool is minimal.