Quantification of Listeria monocytogenes in salads by real time quantitative PCR

A real time quantitative PCR (RTQ-PCR) was carried out purifying DNA extracts of Listeria monocytogenes using a High Pure Listeria Sample Preparation Kit and quantifying in a LightCycler system with hybridisation probes. A standard curve was constructed with serial dilutions. A range linear relationship, from 10 to 105L. monocytogenes colony forming units (CFU), was observed between threshold cycle (Ct) and logarithmic concentration of the serial dilutions. The assay was linear in a range from 10 to 105L. monocytogenes CFU and the coefficient of determination (r2) was > 0.98. RTQ-PCR presented an efficiency of > 85%. The accuracy of the PCR-based assay, expressed as % bias, ranged from 9% to 26% and the precision, expressed as % CV, ranged 9–22%. Intraday and interday variabilities were studied at 102 CFU/g and resulted in 12% and 14%, respectively. The proposed RTQ-PCR method and classical cultural methods were applied to analyse 77 salads from restaurants in Valencia (Spain). All culture positive samples were also RTQ-PCR positive.