Evaluation of immunomagnetic separation and polymerase chain reaction for culture-independent detection of Listeria monocytogenes low numbers in cheese

Listeria monocytogenes continues to be a major food-borne bacterial pathogen. Therefore, an improvement of rapid and reliable methods for its detection in food is still required. In our study, a potential of immunomagnetic separation (IMS) for culture-independent determination of low numbers of L. monocytogenes in cheese was evaluated. IMS using Dynabeads anti-Listeria (Thermo Fischer Scientific, Waltham, Massachusetts, USA) followed by simple thermal cell lysis and specific real-time polymerase chain reaction (PCR) was applied to various steamed cheeses artificially contaminated with L. monocytogenes strains. The described protocol was applied to six smoked and non-smoked steamed cheese products artificially contaminated at L. monocytogenes calculated levels ranging from 8.8 x 104 CFU.g-1 to 2.2 x 101 CFU.g-1. Results were obtained in five hours with PCR detection limit of L. monocytogenes numbers (3.8–7.4) × 102 CFU.g-1 as estimated in individual artificially contaminated test portions. An additional step of 5-fold concentration of the filtered sample homogenate by centrifugation prior to IMS improved the detection to the level of (1.6–2.0) × 102 CFU.g-1 with no negative effect due to relatively higher content of natural microflora in cheese made from raw milk. The evaluated procedure proved to be a fast method for L. monocytogenes low numbers detection in cheese.