Description
The ability of staphylococci to grow on solid media containing 7.5% sodium chloride was utilised by Chapman1 in the development of Mannitol Salt Agar. Other bacteria were found to be so completely inhibited that it was possible to use a considerably heavier inoculum. This is advantageous when staphylococci are present in small numbers, as in faeces2.
Chapman1 noted that nearly all the organisms that grew luxuriantly were coagulase positive staphylococci, and that almost all of them were surrounded by yellow zones. This parallels the ability of Staphylococcus aureus to coagulate rabbit plasma, with the ability to ferment mannitol. Mannitol Salt Agar has been used to detect coagulase positive staphylococci in fish3, poultry4, packed bacon5, and bacon curing brines5.
Porter and Nicholson6 recommended Mannitol Salt Agar to assess environmental contamination by staphylococci using the contact slide method. An improved method using similar principles is by means of Amyl Media Press Plate. These are available as prepared plates or as empty, grided, gamma sterilised dishes. Modern practice, especially when determining multiple resistant S.aureus from clinical sources, is to incorporate methicillin at 5 to 10 ug/ml of media. This can be achieved by the addition of the appropriate volume of reconstituted Amyl Media Methicillin Supplement (SP434) to each litre of sterile molten media cooled to 50°C. Inoculate heavily then incubate for 24 to 36 hours at 37°C.
