Comparison of nine agar media for the recovery of shigatoxin-producing Escherichia coli (STEC) from the food

Pathogenic Shiga toxin-producing Escherichia coli (STEC) are a major public health concern and a key issue for the agri-food sector. However, their isolation remains challenging because STEC lack distinctive biochemical characteristics that clearly differentiate them from other Escherichia coli. This study evaluated the performance of nine commonly used agar media for STEC isolation using three types of samples: pure cultures, experimentally spiked food matrices, and naturally contaminated foods. Two isolation approaches were compared: direct streaking and immuno-separation followed by plating. In pure cultures, five media (TBX, SMAC, Rainbow®AgarO157, CHROMID®EHEC Agar, and IDEA™ STEC with or without cefixime-tellurite) achieved complete recovery, whereas CT-CHROMID®EHEC, CT-SMAC and CHROMagarTMSTEC agar inhibited the growth of several strains, particularly those sensitive to tellurite. In food matrices, immunomagnetic separation resulted in high and comparable detection rates across media (80–95%; n = 16/20–19/20). In contrast, direct streaking showed highly variable recovery depending on the medium (29–100%; n = 7/20–20/20). These results demonstrate that combining selective and non-selective media improves STEC detection. Immuno separation enhances recovery of key serogroups, while IDEA STEC enables additional differentiation through halo formation. The observed variability in toxin production suggests that environmental and physiological factors influence STEC detection.