Microbial Cultivation and Enumeration

The following microorganisms were tested: Staphylococcus aureus ATCC 6538P, Escherichia coli ATCC 25922, Salmonella enteritidis ATCC 13076 and Listeria monocytogenes ATCC 19114. Each strain was grown in a test tube containing 10 mL sterile nutrient broth (Oxoid Ltd., Basingstoke, Hampshire, UK) at 37 °C for 24 h. The purity of the inoculum was confirmed by microscopic examination of the Gram-stained smear. A loopful of inoculum was transferred to selective medium: Baird-Parker agar base supplemented with egg yolk tellurite emulsion for S. aureus, TBX agar for E. coli, XLD agar for Salmonella enteritidis (Oxoid Ltd., Basingstoke, Hampshire, UK) and Palcam agar (Oxoid Ltd., Basingstoke, Hampshire, UK) for Listeria monocytogenes. Plates were incubated for 24 h at 37 °C. Bacterial morphology was confirmed by optical microscopy. Several colonies were transferred to sterile saline solution (8.5 g/L), and adjusted to match the turbidity of McFarland 0.5 standard (1.5 × 10⁸ CFU/mL) [36 (link),37 (link)].