However, if done, they can be handled by using the same screening agars as are used for patient specimens. 5.0 Testing S. aureus isolates for oxacillin resistance 5.1 Identification and speciation of S. aureus isolates must follow the established standard protocol. The protocol must address the recent observation that MRSA epidemics have been caused by a variant strain which is difficult to distinguish from coagulase negative staphylococci. For this reason, coagulase slide tests alone are not to be advocated because of false negative and false positive results. It should be used immediately to spot or streak inoculate the screen plate.
Any growth is considered resistant.
The importance of the full 24 hours incubation cannot be overemphasized. The media should be labelled with the time to avoid early or late readings. After 24 hours of incubation, the plate should be read using transmitted light. Regardless of zone size any discernable growth within the zone of inhibition is indicative of resistance. All S. aureus isolates should be tested on oxacillin plates.