Determination of methicillin-resistance in Staphylococcus aureus by agar dilution and disc diffusion methods

Four-hundred and seventy-six isolates of Staphylococcus aureus from patients in Hong Kong were tested for methicillin-resistance by agar dilution and disc diffusion methods, using heavy inocula. With Mueller-Hinton agar incubated at 30 degrees C for 24 h, 216 (MRSA) isolates were resistant to 8 mg/l of methicillin and grew up to the edge of a 10 micrograms methicillin disc, and 260 strains were susceptible to greater than or equal to 4 mg/l methicillin and produced inhibition zones of at least 20 mm diameter. Incubation for 48 h, the addition of sodium chloride, or the use of a 5 micrograms disc had little effect on these results, but a significant number of MRSA strains produced inhibition zones when disc diffusion tests were incubated at 35 degrees C, and many sensitive strains showed scanty growth on salt-containing agar at high methicillin concentrations in agar dilution tests. Iso-Sensitest agar did not appear to support the growth of the minority resistant populations of MRSA unless supplemented with menadione and thiamine, and even with supplemented Iso-Sensitest medium a few presumptively resistant strains appeared methicillin-sensitive in both disc diffusion and agar dilution tests.