Antibiotic susceptibility testing of Staphylococcus aureus using the Biolog OmniLog® system, a metabolic phenotyping assay

Use of the Biolog OmniLog® phenotyping system for antibiotic susceptibility testing (AST) was evaluated using 51 clinical isolates of Staphylococcus aureus. MIC testing by broth microdilution was compared to results generated using the OmniLog® system for oxacillin, daptomycin, vancomycin, gentamicin, linezolid, and tetracycline. There was >90% essential and categorical agreement between methods for all antibiotics, except gentamicin, which had 83.6% essential agreement, although very major errors occurred with linezolid (n = 3) and daptomycin (n = 1). Precision was satisfactory, with 5 triplicate measurements in agreement. A quantitative threshold allowed automated interpretation of MICs yielding results comparable to manual interpretation; oxacillin, gentamicin, and tetracycline resistance could be identified at a median of 7.13, 5.25, and 7.25 hours, respectively. Limitations include the small number of isolates, and especially resistant isolates tested, and the focus on a single species. Overall, the OmniLog® system was a precise method for AST of S. aureus, although accuracy was imperfect.