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Methicillin Resistance Strip

Certain staphylococci exhibit heterogeneous sensitivity to some Penicillinase resistant β-lactam antibiotics; some colonies are inhibited while others from the same isolate are not. This is most noted with Methicillin. Other representatives of these drugs (Cloxacillin, Flucloxacillin, Oxacillin, Cephalosporins etc) do not show the phenomenon so readily. It is not evident in a normal 24 hour disc sensitivity test; the zone of inhibition around the disc appears typical. But if the plate is incubated for 48 hours some colonies with a high MIC appear within the zone. If the isolate is cultured on 5% salt agar, or at 30°C, a small or no zone of inhibition is produced and the organism is clearly resistant. Such conditions are not suitable as a universal procedure and the use of one plate per isolate is not cost-effective. The Methicillin strip overcomes these difficulties.

Strips of filter paper, 6.8cm * 0.6cm, impregnated with Cefoxitin, which is now used to determine Methicillin resisitance, are used to test a number of strains of staphylococci on a single plate. The length of the strip is such that it almost covers the plate diameter, and maximises the available surface area.

A strip is placed on either normal or salt agar. The test organisms, and known sensitive and resistant strains, are streaked across the strip at right angles to it. After overnight incubation at 37°C (using salt agar) or 30°C (using normal agar), the inhibition of the growth streaks are compared.