False-positive beta-lactamase results with Staphylococcus lugdunensis in the Vitek AutoMicrobic system

The Vitek AutoMicrobic system in combination with the Gram-positive suscept ibility card detects beta-lactamase in staphylococci by utilizing penicilli n as the substrate coupled with oxacillin as an inducer. The beta-lactamase activity of 21 clinical isolates and two reference strains of Staphylococc us lugdunensis was determined with this automated system and compared with a liquid nitrocefin assay after induction with oxacillin. Eight (38 %) clin ical isolates and the reference strain ATCC 49576 of S. lugdunensis showed production of beta-lactamase in both tests. Thirteen (62 %) clinical isolat es and the type strain ATCC 43 809 were nitrocefin-negative. The Vitek Auto Microbic system reported false-positive beta-lactamase results for 9 of tho se 13 isolates and for the type strain of S. lugdunensis. Results for disk diffusion (ampicillin) were concordant with the nitrocefin assay. With one exception, the MICs for penicillin of the nitrocefin-negative strains were in the equivocal range of 0.06-0.12 mg/l according to NCCLS. However, none of the nitrocefin-negative and Vitek-positive strains revealed any of the k nown staphylococcal genes for beta-lactamase as investigated by Southern hy bridization, supporting the fact that false-positive beta-lactamase results may occur in the Vitek AutoMicrobic system. We conclude from our data that it may be justified to include S. lugdunensis in the quality control of Vi tek cards containing beta-lactamase tests.