Use of the Autobac system for detection of methicillin-resistant Staphylococcus aureus.

Using 490 strains of Staphylococcus aureus divided into methicillin-susceptible, -resistant, and -heteroresistant varieties, we compared the results obtained by the agar disk method with those obtained with the automated Autobac system. Susceptible strains exhibited a perfect correlation, whereas there were numerous discrepancies with resistant and still more with heteroresistant varieties. When incubation was increased to 18 h at 37 degrees C (Autobac incubation temperature), 35 degrees C, or 30 degrees C, these differences disappeared, but other problems may arise when incubation is prolonged, especially with erythromycin. We thus recommend carrying out two readings, a normal one after 3 h of incubation and a special reading after 18 h, solely for the detection of heteroresistance to methicillin.