Resistance to Methicillin in Coagulase-negative Staphylococci and Its Detection
Acta Veterinaria Brno
Resistance of staphylococci to methicillin is important especially in the case of Staphylococcus aureus isolates. Its impact in veterinary medicine is not exactly specified in coagulase-negative staphylococci; however, these staphylococci may represent an important reservoir of resistance genes. The study aimed at detecting resistance to methicillin in coagulase-negative staphylococci from raw materials and foodstuffs of animal origin and assessing the tests frequently used to determine this
... o determine this resistance. Coagulase-negative staphylococci (198 isolates of 12 species) were tested. Resistance to methicillin was determined by the disk diffusion method using oxacillin and cefoxitin disks, microdilution method, detection of PBP2a and the mecA gene. Of the tested isolates, 109 (55.1%) were classified as resistant by the diffusion test with oxacillin, 32 isolates (16.2%) by the test with cefoxitin and 50 isolates (25.3%) on the basis of oxacillin minimum inhibitory concentration (MIC). No resistant isolates were incorrectly identified as susceptible when using the disk diffusion method with oxacillin (sensitivity of 100%). However, apart from 22 correctly classified resistant isolates, another 87 isolates were incorrectly identified as resistant as well (specificity of 50.6%). The test with cefoxitin showed the lowest (45.5%) sensitivity in determination of resistant isolates. By contrast, this test was the most precise in classification of resistant isolates (specificity of 87.5%). When using the microdilution method, resistant strains were identified with the sensitivity and specificity of 68.2% and 80.1%, respectively. The results revealed substantial variability of methicillin-resistant isolates ranging from 16.2% to 55.1%, depending on the phenotyping methods and recommended interpretation criteria used. Therefore, it is advisable to reconsider the current interpretation criteria in the case of coagulasenegative staphylococci of animal origin (with the exception of S. epidermidis).