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°C for 24 h, 216 (MRSA) isolates were resistant to 8 mg/1 of methicillin and grew up to the edge of a 10 pg methicillin disc, and 260 strains were susceptible to> 4 mg/1 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 fig disc had little effect on these results, but a significant number of MRS A strains produced inhibition zones when disc diffusion tests were incubated at 35°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-scnsitive in both disc diffusion and agar dilution tests