Yeast extract ye

The following strains of S. enterica and L. monocytogenes were used for the growth and thermal inactivation test. Of S. enterica, three food-isolated strains selected were Salmonella Derby LFMFP 872 (pork isolate), Salmonella Enteritidis LFMFP 875 (poultry isolate) and Salmonella Typhimurium LFMFP 877 (poultry isolate). Three L. monocytogenes strains (LFMFP 392, serotype 4b, liver pate isolate; LFMFP 421, serotype 4b, clinical isolate, and LFMFP 491, serotype 1/2b, tuna isolate) were used. All stock cultures were kept at –75°C in Tryptone Soy Broth (TSB, Oxoid, Basingstoke, England), supplemented with 0.6% yeast extract (YE, Oxoid) and 15% glycerol (Prolabo, Heverlee, Belgium). Working stocks were stored refrigerated at 4°C on Tryptone Soy Agar (TSA, Oxoid) slants and were renewed monthly. Working cultures were activated by transferring a loopful from the slants into BHI (Oxoid) and incubated at 37°C for 18 to 24 h. The working cultures were prepared by transferring 0.1 ml of each culture into 10 ml of BHI and incubated at 37°C for 24 h. Immediately before inoculation, a cocktail containing three strains of S. enterica or L. monocytogenes was prepared individually by mixing approximately equal population of each strain and serially diluted in Peptone Physiological Salt Solution (PPS, containing 1 g/l neutralized bacteriological peptone and 8.5 g/l NaCl).

Upon arrival, NS-agar cultures were incubated for up to two days at 37 °C and 5% CO₂. On both days cultures were screened for presence of meningococcus-like colonies (grey, round and smooth colonies with convex shape). When found, 1–3 colonies were re-plated on Columbia Blood agar (CBA, bioTRADING Benelux B.V., Mijdrecht, the Netherlands) and tested for species identification using Matrix-assisted Laser Desorption/Ionization Time-of-Flight mass spectronomy (MALDI-ToF, Bruker Daltonik GmbH, Bremen, Germany). Separately for oropharyngeal and saliva samples, a single isolate with a score ≥ 2.0 for Neisseria meningitidis (database BDAL V8.0.0.0 + SR1.0.0.0, Bruker Daltonik) was stored at − 70 °C in Brain Heart Infusion (BHI, Oxoid) supplemented with 0.5% Yeast Extract (YE, Oxoid) and 10% glycerol. NS-agar cultures displaying any microbial growth were harvested into 2 ml of Todd–Hewitt Broth (Oxoid) supplemented with 0.5% YE and 10% glycerol. These harvests were considered to be culture-enriched for meningococci, and 0.7 ml of it stored at − 70 °C.