CHAPMAN STONE AGAR

Description

Staphylococcus aureus is one of the pathogens most frequently isolated from clinical specimens. In fact, S.aureus is currently the most common cause of nosocomial infections. Treatment of infection caused by S.aureus has become more problematic since the development of multiple drug resistant strains. To identify S. aureus from contaminated samples more easily and reliably, selective media have been developed. Chapman Stone Agar 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. It is similar to Staphylococcus Medium 110, previously described by Chapman, except that the sodium chloride concentration is reduced to 5.5% and additionally ammonium sulfate is included in the formulation. The main modification consists 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. 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 surrounded by clear zones are presumptively identified as S.aureus. White or non-pigmented colonies, with or without a clear zone, are presumptively identified as S. epidermidis. Coagulase activity should be performed to confirm the findings. Enterococci and/or Group D streptococci may exhibit growth on the medium and show slight mannitol fermentation. The colonies, however, are tiny and can easily be differentiated from staphylococci by gram stain and the catalase test.