Reliability of Selective Media for Recovery of Staphylococci from Cheese

Abstract

The reliability of 12 selective media for recovery of four different strains of Staphylococcus aureus inoculated at manufacture into three cheese types was determined. Selective medium and time of ripening had a highly significant effect (p < 0.001) on reliability of the staphylococcal count. In addition, highly significant interaction effects were observed. The most reliable medium in the overall analysis was mannitol salt agar. However, this medium was not equally reliable at all times during ripening, and use of both mannitol salt agar and Staphylococcus medium no. 110 is recommended. The tellurite- and azide-based selective media were generally unsatisfactory, however tellurite glycine agar, Vogel Johnson (VJ) agar, and azide blood agar base were totally unreliable. In general, the salt-based selective media were most reliable. This applied also to the egg yolk media that use salt as the selective agent. Salt egg yolk agar and Colbeck's egg yolk medium generally gave higher recoveries of S. aureus than did Baird-Parker medium, Crisley et al. tellurite polymyxin egg yolk agar, and Hopton egg yolk azide agar, except in the unripened cheeses. The debilitating effect of cheese ripening on the staphylococcal cells was not eliminated by the egg yolk tellurite and azide media.