Using CHROMagar™ STEC medium exclusively does not recover all clinically relevant Shiga-toxin producing Escherichia coli in Aotearoa, New Zealand.

Diagnostic laboratories in Aotearoa New Zealand refer cultures from faecal samples positive for Shiga-toxin genes to the national Enteric Reference Laboratory for isolation of Shiga-toxin producing Escherichia coli (STEC) for epidemiological typing. As there was variation in the culture media being referred, a panel of 75 clinical isolates of STEC, representing 28 different serotypes, was used to assess six commercially available media and provide guidance to clinical laboratories. Recommendations were subsequently tested for a three-month period where STEC isolations and confirmations were assessed by whole genome sequencing analysis against the culture media referred. CHROMagar™ STEC (CH-STEC; CHROMagar Microbiology, Paris, France) or CH-STEC plus cefixime-tellurite sorbitol MacConkey agar (CT-SMAC) was confirmed inferior to CH-STEC plus Blood agar with vancomycin, cefsulodin and cefixime (BVCC). The former resulted in fewer STEC types (n = 18) being confirmed compared to those from a combination of CH-STEC and BVCC (n = 42). A significant (P <0.05) association with an STEC's ability to grow on CH-STEC and the presence of the ter gene cluster, and eae was observed. Culturing screen positive STEC samples onto both CH-STEC and BVCC ensures a consistently higher recovery of STEC from all clinical samples in NZ than CH-STEC alone.