Rapid real-time PCR detection of Listeria monocytogenes in enriched food samples based on the ssrA gene, a novel diagnostic target.

A real-time PCR assay was designed to detect a 162-bp fragment of the ssrA gene in Listeria monocytogenes. The specificity of the assay for L. monocytogenes was confirmed against a panel of 6 Listeria species and 26 other bacterial species. A detection limit of 1-10 genome equivalents was determined for the assay. Application of the assay in natural and artificially contaminated culture enriched foods, including soft cheese, meat, milk, vegetables and fish, enabled detection of 1-5 CFU L. monocytogenes per 25g/ml of food sample in 30h. The performance of the assay was compared with the Roche Diagnostics 'LightCycler foodproof Listeria monocytogenes Detection Kit'. Both methods detected L. monocytogenes in all artificially contaminated retail samples (n=27) and L. monocytogenes was not detected by either system in 27 natural retail food samples. The method developed in this study has the potential to enable the specific detection of L. monocytogenes in a variety of food types in a time-frame considerably faster than current standard methods. The potential of the ssrA gene as a nucleic acid diagnostic (NAD) target has been demonstrated in L. monocytogenes. We are currently developing NAD tests based on the ssrA gene for a range of common foodborne and clinically relevant bacterial pathogens.