Palcam agar

The presence of L. monocytogenes was examined according to the procedures described in ISO 11290-2:1998. Firstly, 100 µL of selective culture enrichment (25 g sample plus 225 mL Half Fraser medium) was transferred to tubes with 10 mL of Fraser broth with Fraser supplement (1:10 v/v) (Biokar Diagnostics) and incubated for 18 ± 2 h at 30 ± 1 ° C. In parallel, 100 μL of the homogenate was spread on Palcam Agar (Biokar Diagnostics) and Compass Listeria Agar (Biokar Diagnostics). Both the tubes with Fraser broth and the plates were incubated for 48 h at 37 °C. Once this time had elapsed, 100 μL culture taken from the Fraser tubes was spread on Palcam and Compass, repeating the same procedure as previously mentioned. Plates were counted, and the possible suspected colonies of L. monocytogenes were isolated on TSAYE plates and incubated at 37 °C for 24 h with the same procedure criteria explained above for detection of Salmonella. The experiment was monitored with a positive control of L. monocytogenes strain CECT 4031.
For the confirmation of L. monocytogenes, one typical colony per plate and positive (CECT4031) and negative control were subjected to biochemical tests: Gram staining and tested for the production of catalase, beta-hemolysis, and fermentation of carbohydrates (xylose, mannitol, and rhamnose). Finally, API LISTERIA® (BioMérieux SA, France) was performed.