Staphylococcal food poisoning ranks as one of the most prevalent causes of gastroenteritis worldwide. Staphylococci ferment glucose to produce acid from glucose. Tryptone Soya Broth with 10% sodium chloride and 1% sodium pyruvate is used for enumeration of Staphylococcus aureus in dairy products (1, 2) and is recommended for enumeration by MPN technique.
Casein enzymic hydrolysate and papaic digest of soyabean meal provide essential nutrients. Dextrose serves as an energy source. Sodium pyruvate protects injured cells, helps recovery and also enhances growth of S.aureus . Many other bacteria except staphylococci are inhibited by 10% sodium chloride (3).
For MPN Technique: Inoculate 3 tubes of Tryptone Soya Broth w/ 10% NaCl and 1% sodium pyruvate (M1229) of each test dilution with 1 ml aliquots of decimal dilutions of sample. Incubate at 35-37°C for 48 hours. Following incubation, transfer a loopful from each positive growth tube to Baird-Parker Agar (M043) plates. Colonies of S.aureus on Baird Parker Agar are typically circular, smooth, convex, moist, 2-3 mm in diameter, grey-black to jet black and surrounded by an opaque zone.
Directions: Suspend 135 grams in 1000 ml distilled water. Heat if necessary to dissolve the medium completely. Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes. Mix well and dispense as desired.