A highly selective two-stage isolation method for the enumeration of Staphylococcus aureus in foods

A plate method for enumerating Staphylococcus aureus is described which combines a 1-h recovery period for stressed cells on a relatively non-selective Baird-Parker agar base followed by a 24-h growth phase in a highly selective, supplemented Baird-Parker medium added as an overlay. Tests with pure cultures showed satisfactory recovery of stressed Staph. aureus and other bacteria. Similar results were obtained with the conventional Baird-Parker procedure and with the two-stage isolation method for shrimps and poultry neck skins, but for raw minced meat, recoveries were higher with the combined method than with the conventional medium. All colonies visible after 24 h on the two-stage medium can be counted as Staph. aureus, whereas longer incubation times and confirmatory tests are necessary to differentiate it from other organisms on conventional Baird-Parker medium.