MRSA, methicillin-resistant Staphylococcus aureus
Colonies of methicillin-resistant Staphylococcus aureus on ORSAB medium. Cultivation 24 hours, aerobic atmosphere, 37°C.
Oxacillin Resistance Screening Agar Base (ORSAB) is a medium for screening for methicillin resistant Staphylococcus aureus (MRSA) directly from routine swab samples. It's based on traditional mannitol salt agar with a reduction in salt concentration (from 7.5% to 5.5%). This lower level of salt is still sufficient to inhibit most bacteria other than staphylococci. This medium contains oxacillin to inhibit methicillin sensitive Staphylococcus aureus and polymyxin B for the suppression of other bacteria able to grow at such a high salt concentration (e.g., Proteus spp.).
Typical colonies of MRSA are intense blue in colour on a colourless background enabling the organism to be more easily identified in mixed culture (in the picture the medium is comletely blue due to the high concentration of MRSA colonies on the plate).