Relevance of Secondary Enrichment in the Detection of Salmonella spp. in Food Samples by qPCR According to DIN 10135.

Background: When detecting Salmonella spp. in food samples, unlike with culture-based procedures where there are solid standards, PCR techniques are generally dominated by commercial solutions, often backed by the conformity of reference organizations, and based on rigorous validation studies. The few independent standards that exist are not subject to revision and improvement to the same extent as the manufacturer's methods. Moreover, since commercial networks do not promote them, they are implemented less in everyday practice. The German standard DIN 10135 is an example of this. In this method, before PCR detection, a primary enrichment (16-20 h) followed by a secondary selective enrichment of at least 6 h is needed. Nevertheless, it allows the possibility of only applying the first step if evidence of their correct operation is provided.

Objective: To evaluate how necessary is the secondary enrichment for DIN 10135 standard.

Methods: Short and complete enrichment steps were compared in the context of the evaluation of the limit of detection for 11 types of food. Additionally, a blind assay was performed with 75 food samples.

Results: The data show that a simple primary enrichment may be sufficient and that the second selective enrichment with the tested matrixes would not be strictly essential. The blind study obtained a 98.6% of trueness and precision of 100%.

Conclusions: At least for the end consumer products, a secondary enrichment of 6 h is not necessary for all the products tested.

Highlights: In the context of the DIN 10135 standard, the primary enrichment (16-20 h, 37 ± 1°C) can be enough for detecting Salmonella spp.