[About microbiological methods for detection of oxacillin resistance in coagulase-negative staphylococci]. / Sobre los métodos microbiológicos para la detección de la resistencia a oxacilina en Staphylococcus coagulasa negativos.

OBJECTIVE: Coagulase-negative staphylococci (CoNS) take part of the human skin and mucous membranes, but they are also involving in infections with the increasing use of prosthetic, indwelling devices or intravascular catheter-related bacteraemia. They are more resistance than Staphylococcus aureus against a wide range of antimicrobial agents, and it have been observed an increase in morbidity and mortality of patients with incorrect treatment. METHODS: To analyze the results obtained by different commercial techniques: two automatic microdilution systems (MicroScan and Vitek2 Compact), PBP2a agglutiation test, with and without 1 µg oxacillin disk induction, and detection of mecA gene by nucleic acids amplification techniques, for the diagnosis of methicillin resistance staphylococci in 170 strains of CoNS isolated from blood cultures. RESULTS: One hundred and seventy methicillin resistance staphylococci were detected by MicroScan, 167 strains by Vitek 2 Compact, 115 strains were PBP2a positive without oxacillin induction and 168 after oxacillin induction. Finally, 167 strains were mecA gene positive detected by nucleic acids amplification techniques. CONCLUSIONS: It is necessary to do oxacillin induction before PBP2a test to avoid false negatives. There are a great variability in the phenotypic expression of methicillin resistance in CoNS.