Ornithine-decarboxylase activity assay during enrichment: Early identification of Escherichia coli O157:H7-free food samples

Enrichment step is a cell amplification step essential for all detection of foodborne pathogens. The present work developed an early identification of finished food samples free from Escherichia coli O157:H7 contamination. Our work exploited the activity of ornithine decarboxylase (ODC) along with its pH indicator and a selective inhibitor. In the presence of the target bacteria, the enrichment broth changed colour significantly due to the diauxic growth during sugar fermentation and ODC activity. Among all conditions, phenol red and 0.028 g/L novobiocin were found to be optimal and selective for determining the presence or absence of E. coli O157:H7. When tested on real food samples, the proposed method yielded results that agreed well with the conventional method (Cohen’s kappa = 1.00). The present work provided a new method for quickly determining if finished food products are free from E. coli O157:H7 contamination.