Use of the E test for methicillin sensitivity testing.

Performance of the E test for methicillin sensitivity testing was investigated using 21 methicillin-resistant and 28 methicillin-sensitive staphylococci. Thirty of the strains had previously given equivocal results with a strip diffusion method. The E test performed well with Mueller-Hinton agar plates incubated at 30 degrees C for 24 h but only after supplementation with 5% NaCl. Testing on agar without salt was necessary for the recognition of methicillin resistance in one strain. We recommend a breakpoint of 4 mg/L and heavy inocula. The E test can be controlled by testing a control strain on the same plate.