D.S.T. Agar (sulphonamide antagonist free) | DM215

D.S.T. Agar (sulphonamide antagonist free) | DM215

For all antibiotic susceptibility testing.

One of the main problems with antibiotic susceptibility testing has been the presence of sulphonamide antagonists in culture media. Traditionally, lysed blood has been added to the medium to reduce the effects of sulphonamide antagonists1 and has been successful, in part due to the conversion of thymidine, a potent sulphonamide antagonist, to thymine. In 1978, Aymes and Smith,2,3 reported that the addition of the ribonucleoside uridine to laboratory media renders it more suitable for susceptibility testing of sulphonamides and trimethoprim. They stated that the presence of uridine effectively ensures that the medium is free of sulphonamide antagonists and so the addition of lysed blood to the medium is unnecessary for non-fastidious organisms.
MAST D.S.T. Agar contains specially selected peptones which are low in sulphonamide antagonists. It also contains uridine as recommended by Aymes and Smith. In a more recent survey4 it was shown that more reliable susceptibility test results were achieved in those laboratories using a test medium supplemented by the addition of lysed blood. Errors with the
sulphonamide group were reduced by the addition of lysed blood, particularly where a D.S.T. Agar was used.
It should be noted that certain thymineless mutants will not grow on a medium containing uridine or lysed blood. If such organisms are encountered, Mueller Hinton Agar (DM170) without the addition of lysed blood may be used. MAST D.S.T. Agar is suitable for susceptibility testing of all organisms to the major groups of antibiotics by the disc diffusion and agar dilution methods. A further application is the microbiological assay of aminoglycosides in serum for the routine monitoring of therapy with this potentially toxic group of antibiotics.

2. Technical Formula*

3. Directions
1. Suspend by swirling 37.5g of powder in 1 litre or the contents of the sachet in the stated volume of distilled or deionised water. Boil to dissolve completely.
2. Autoclave at 121ºC (15 p.s.i.) for 15 minutes.
3. After cooling to 55ºC, 5-7% lysed blood may be added if required.
4. Thoroughly mix before pouring