Some methicillin-resistant cultures of give equivocal results when tested in disk diffusion tests or tube dilution MIC tests incubated at 37°C.

Practically all resistant cultures have the following characters: (1) they give growth up to or to within 1 mm of a 10 μg methicillin disk in a disk diffusion test on nutrient agar or blood agar incubated at 30°C, and (2) they show a MIC of 25 μg methicillin per ml or greater after incubation for 18 hr at 30°C, or for 48 hr at 37°C, in a tube dilution test in nutrient broth.

Tests for growth at 37°C on methicillin salt agar, or on salt agar with a methicillin disk, are also generally reliable, but some batches of methicillin salt agar are relatively inhibitory for resistant staphylococci.

Routine sensitivity tests with cloxacillin give unsatisfactory results.


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