Chapman Stone Medium (without Membrane Filter)

Principle And Interpretation

Field of Application: Food, pharmaceuticals, cosmetics and other samples. DriFilter Membrane Nutrient Pad Medium are ready to use sterile culture media in the form of a 50 mm biological inert absorbent pads impregnated with standard culture medium, then dried and sterilized in 55 mm Petri plate. They eliminate the need of laborious media preparation and autoclaving procedures. The nutrient pads are to be just rewetted with sterile distilled water and are ready to use. Use of nutrient pads allows larger sample volumes to be tested at a time. Interpretation of results is directly by counting the CFUs and also quantifies the microbial load present in the sample. Chapman Stone medium is a selective media used for the isolation of food poisoning staphylococci. Foods commonly contaminated with S. aureus included synthetic creams, custards and high-salted food. Chapman Stone Agar is prepared according to the modification of Staphylococcus Medium 110 described by Chapman(1). It is similar to Staphylococcus Medium 110, previously described by Chapman (2), except that the sodium chloride concentration is reduced to 5.5% and additionally ammonium sulfate is included in the formulation. The main modification consist the inclusion of ammonium sulfate in the medium that allows the direct observation of gelatin hydrolysis, instead of adding reagents to the plate medium. Chapman Stone Medium is especially recommended for suspected food poisoning studies involving Staphylococcus (3). It is selective, due to the relatively high salt content, and is differential due to pigmentation, mannitol fermentation and the presence or absence of gelatin liquefaction. Material under test is inoculated on the surface and incubated at 30°C for 48 hours to produce separated colonies. After incubation, cream to golden yellow colonies are presumptively identified as S. aureus. While, white or non-pigmented colonies, are presumptively identified as S. epidermidis.