Comparison of four enrichment broths for the detection of non-O157 Shiga-toxin-producing Escherichia coli O91, O103, O111, O119, O121, O145 and O165 from pure culture and food samples

Aims: We compared the efficiency of universal pre-enrichment broth (UPB), modified Escherichia coli broth containing novobiocin (mEC + n), modified Tryptic Soy Broth (mTSB) and mTSB with novobiocin (mTSB + n) for the enrichment of non-O157 Shiga-toxin-producing E. coli (STEC). Methods and Results: Freeze-injured and control non-O157 STEC (O91, O103, O111, O119, O121, O145 and O165) strains were used to artificially contaminate beef and radish sprout samples, which were then cultivated in each of the four enrichment media. After incubation, STEC strains were detected by loop-mediated isothermal amplification (LAMP) and plating assays. Enrichment in mEC + n was least effective for facilitating the detection of uninjured STEC strains in radish sprouts, while mTSB + n was least effective for enriching freeze-injured non-O157 STEC strains from beef samples for detection by LAMP assay. UPB and mTSB were superior to mEC + n and mTSB + n for the enrichment of non-O157 STEC from food samples. Conclusions: The enrichment of non-O157 STEC was negatively affected by the addition of novobiocin to enrichment broths. Significance and Impact of the study: Novobiocin should not be added to media used for the enrichment of non-O157 STEC in food when cell injury is anticipated.