Rapid detection of Shigella and enteroinvasive Escherichia coli in produce enrichments by a conventional multiplex PCR assay

• Six produce types were inoculated with Shigella post-enrichment.
• A conventional multiplex assay can accurately detect Shigella in less than 90 min.
• The assay can differentiate between the laboratory positive control and natural strains of Shigella.
• The four DNA extraction methods tested can detect at least 1.6 × 103Shigella cells/ml.
• The Bio-Rad InstaGene Matrix method surpassed others for consistency and sensitivity.

Faster detection of contaminated foods can prevent adulterated foods from being consumed and minimize the risk of an outbreak of foodborne illness. A sensitive molecular detection method is especially important for Shigella because ingestion of as few as 10 of these bacterial pathogens can cause disease. The objectives of this study were to compare the ability of four DNA extraction methods to detect Shigella in six types of produce, post-enrichment, and to evaluate a new and rapid conventional multiplex assay that targets the Shigella ipaH, virB and mxiC virulence genes. This assay can detect less than two Shigella cells in pure culture, even when the pathogen is mixed with background microflora, and it can also differentiate natural Shigella strains from a control strain and eliminate false positive results due to accidental laboratory contamination. The four DNA extraction methods (boiling, PrepMan Ultra [Applied Biosystems], InstaGene Matrix [Bio-Rad], DNeasy Tissue kit [Qiagen]) detected 1.6 × 103Shigella CFU/ml post-enrichment, requiring ∼18 doublings to one cell in 25 g of produce pre-enrichment. Lower sensitivity was obtained, depending on produce type and extraction method. The InstaGene Matrix was the most consistent and sensitive and the multiplex assay accurately detected Shigella in less than 90 min, outperforming, to the best of our knowledge, molecular assays currently in place for this pathogen.