Evaluation of the automicrobic system for detection of resistance of Staphylococcus aureus to methicillin.

The AutoMicrobic system (AMS) (Vitek System, Inc., Hazelwood, Mo.) was tested for its ability to determine oxacillin and gentamicin susceptibility of 98 known oxacillin-susceptible and 103 known oxacillin-resistant Staphylococcus aureus isolates. AMS and reference oxacillin susceptibility results were in agreement for all 95 (100%) oxacillin-susceptible isolates. In contrast, only 23 (22.3%) of the 103 known oxacillin-resistant isolates were correctly reported. For the known oxacillin-resistant isolates, 65 received AMS reports at 3 to 4 h, with only 9% being correct, whereas 38 were reported at 5 to 6 h, with 47% being correct. The reliability of AMS gentamicin susceptibility results was evaluated by testing the 198 S. aureus isolates in parallel with MIC-2000 broth dilution tests. AMS gentamicin susceptibility results were found to be reliable and essentially identical to MIC-2000 results. The possibility of improving AMS oxacillin resistance detection by using gentamicin resistance as a linked screening marker for oxacillin resistance was evaluated with data from the parallel AMS and MIC-2000 gentamicin susceptibility tests and from data accrued on recent clinical laboratory isolates. By these two approaches, respective sensitivities of 97 and 99.8%, and specificity of 72%, were found for detection of oxacillin-resistant isolates by using gentamicin resistance as a marker.