Zbinden R, von Graevenitz A, Rossi J, Kümin E, Berger-Bächi B, Hächler H, Nadal D
Department of Medical Microbiology, University of Zürich, Switzerland.
Zentralbl Bakteriol. 1999 Jul;289(3):365-70. doi: 10.1016/s0934-8840(99)80076-0.
The Vitek AutoMicrobic system in combination with the Gram-positive susceptibility card detects beta-lactamase in staphylococci by utilizing penicillin as the substrate coupled with oxacillin as an inducer. The beta-lactamase activity of 21 clinical isolates and two reference strains of Staphylococcus lugdunensis was determined with this automated system and compared with a liquid nitrocefin assay after induction with oxacillin. Eight (38%) clinical isolates and the reference strain ATCC 49576 of S. lugdunensis showed production of beta-lactamase in both tests. Thirteen (62%) clinical isolates and the type strain ATCC 43809 were nitrocefin-negative. The Vitek AutoMicrobic system reported false-positive beta-lactamase results for 9 of those 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 known staphylococcal genes for beta-lactamase as investigated by Southern hybridization, 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 Vitek cards containing beta-lactamase tests.
Vitek自动微生物系统结合革兰氏阳性药敏卡，以青霉素为底物、苯唑西林为诱导剂，检测葡萄球菌中的β-内酰胺酶。使用该自动化系统测定了21株临床分离株和两株路邓葡萄球菌参考菌株的β-内酰胺酶活性，并在苯唑西林诱导后与液体硝基头孢菌素试验进行比较。在两项试验中，8株（38%）临床分离株和路邓葡萄球菌参考菌株ATCC 49576显示产生β-内酰胺酶。13株（62%）临床分离株和模式菌株ATCC 43809的硝基头孢菌素试验呈阴性。Vitek自动微生物系统对这13株分离株中的9株以及路邓葡萄球菌模式菌株报告了β-内酰胺酶假阳性结果。纸片扩散法（氨苄西林）结果与硝基头孢菌素试验一致。除1例外，根据美国国家临床实验室标准化委员会（NCCLS）标准，硝基头孢菌素试验阴性菌株的青霉素最低抑菌浓度（MIC）在0.06 - 0.12 mg/l的可疑范围内。然而，通过Southern杂交研究发现，硝基头孢菌素试验阴性且Vitek试验阳性的菌株均未显示任何已知的葡萄球菌β-内酰胺酶基因，这支持了Vitek自动微生物系统可能出现β-内酰胺酶假阳性结果这一事实。我们从数据中得出结论，将路邓葡萄球菌纳入含β-内酰胺酶检测的Vitek卡质量控制可能是合理的。
