2.2. Salmonella spp. Strains, Growth Conditions and Inoculum Preparation

Salmonella spp. strains (total 5) isolated from dairy products (Salmonella typhimurium and Salmonella enterica subsp. enterica) and one reference strain (Salmonella enterica subsp. enterica (D) ATTC 13076, MicroBioLogics, St. Cloud, MN, USA) were used to prepare a cocktail of Salmonella spp. strains which was used for the artificial contamination of fresh cheese samples. Salmonella spp. strains were isolated from food products and species confirmation was performed at the Department of Industrial and Food Microbiology (Faculty of Food Sciences, University of Warmia and Mazury in Olsztyn, Poland).

Salmonella spp. strains were stored individually at freezing temperature (−80 °C) in BHI broth (Merck, Warsaw, Poland) with 20% (v/v) glycerol (Merck, Warsaw, Poland), further grown by transferring 0.1 mL of the culture to 10 mL of BHI broth and incubated for 24 h at 37 °C (ICP500, Memmert GmbH + Co. KG, Schwabach, Germany). The second passage was performed by transferring 0.1 mL from each 24 h culture to 10 mL BHI broth followed again by incubation at 37 °C for 24 h. The initial concentration of microorganisms in each working subculture was determined by traditional plate counts method (BHI agar, Merck, Warsaw, Poland). The cocktail of Salmonella spp. strains was prepared by combining equal volumes (3 mL) of each activated strain. Estimated final concentration of Salmonella spp. in the cocktail was ca. 9 log cfu/mL.