Egg yolk tellurite emulsion

Each sample was aseptically weighed in an analytical balance and 25 g were transferred into a sterile plastic bag. Then, 225 mL of buffered peptone water (Merck, Germany) was added and homogenized in a Stomacher Bagmixer 400 W (Interscience, Saint-Nom, France) for 2 min. Five milliliter aliquot of the enriched homogenate was transferred into 50 mL Trypticase Soy Broth (TSB, Merck, Germany) supplemented with 10% NaCl and 1% sodium pyruvate. After incubation at 35 °C for 18 h, a loopful of the culture was plated onto Baird-Parker agar supplemented with egg yolk tellurite emulsion (Merck, Germany) and incubated overnight at 37 °C. Black shiny colonies surrounded by 2 to 5-mm clear zones were further identified on the basis of Gram staining, hemolytic activity on sheep blood agar (Merck, Germany), catalase activity, coagulated test (rabbit plasma), oxidase test, glucose O/F test, resistance to bacitracin (0.04 U), mannitol fermentation on Mannitol salt agar (Merck, Germany), urease activity, nitrate reduction, phosphatase, deoxyribonuclease (DNase, Merck, Germany) test, voges-proskaver (Merck, Germany) test and carbohydrate (xylose, sucrose, trehalose and maltose, fructose, lactose, mannose) fermentation tests [11 (link)].

First, 10⁻² dilutions of samples were prepared by placing 1 mL of the samples in Stomacher bags using a sterile pipette and inoculating them into tubes containing 9 mL of buffered peptone water (Merck Millipore, 107228, Germany). Serial dilutions were prepared via similar procedures and these dilutions were analyzed.
Staphylococcus aureus was counted using a mixture of Baird–Parker agar (Merck, 1.05406) and egg yolk tellurite emulsion (Merck, 103785). The inoculated Petri dishes were incubated under aerobic conditions at 30 and 35°C for 24 and 48 h, respectively. At the end of the incubation period, samples were taken from the suspicious colonies (black colonies with a white zone around them) with the help of a sterile loop and inoculated again on a mixture of Baird–Parker agar and egg yolk tellurite emulsion and left to incubate for the same period under the same conditions. Samples were taken from the incubated colonies and a coagulase test was performed with Bactident coagulase (Merck 1.13306) and positive results were considered Staphylococcus aureus (Bennett & Lancette, 2001 ).

As many as 20 g of each collected ready-to-eat food sample was blended with 225 mL of buffered peptone water (Merck, Germany). Next, the solutions were homogenized using a stomacher (Interscience, Saint-Nom, France). Consequently, 5 mL of the produced solution was transferred to 50 mL of Trypticase Soy Broth (TSB, Merck, Germany) supplemented with 10% NaCl and 1% sodium pyruvate, which was then incubated for 18 h at 35 °C. Next, a loopful culture was transferred to the Baird-Parker agar supplemented with an egg yolk tellurite emulsion (Merck, Germany) and incubated at 37 °C for about 24 h. Black shiny colonies surrounded with 2- to 5-mm clear zones were identified based on gram staining, hemolytic activity on the sheep blood agar (Merck, Germany), catalase test, coagulase test (rabbit plasma), oxidase test, OF glucose test, bacitracin sensitivity test (0.04 U), mannitol fermentation on the Mannitol salt agar (Merck, Germany), urease activity, nitrate reduction, phosphatase, deoxyribonuclease (DNase, Merck, Germany) test, Voges-Proskauer (VP) (Merck, Germany) test, and carbohydrate (xylose, sucrose, trehalose and maltose, fructose, lactose, and mannose) fermentation test [13 (link)].