Phenogenotypical characterization of antimicrobial resistance in Staphylococcus spp. isolated from bovine mastitis

The use of antibiotic in the control of intramammary infections and in the elimination of its possible sources in dairy farms is an important control measure. However, the inappropriate use of antibiotics can result in the appearance of resistant strains and compromise the efficiency of the treatment. Besides Staphylococcus spp. are among the main pathogens of bovine mastitis, they are often resistant to antibiotics, especially beta-lactamics, mainly by two distinct mechanisms: the production of extracellular enzyme beta-lactamase, encoded by the blaZ gene, and production of PBP2a or PBP2' a penicillin-binding protein with low affinity, encoded by the mecA gene. The expression of mecA gene is constitutive or induced by beta-lactamic antibiotics, such as oxacillin and cefoxitin. The mecA gene is inserted into the chromosome through a staphylococcal mobile genetic element, called staphylococcal cassette chromosome mec (SCCmec). The present study evaluated the phenogenotypical resistance profile to beta-lactam antibiotics of 250 Staphylococcus spp isolates, using oxacillin and cefoxitin as markers in order to produce data to the knowledge of resistance in dairy farms located in the South-Fluminense and the Metropolitan regions of the State of Rio de Janeiro to support the implementation of measures to control this disease. The assessment of resistance was made through 8 different phenotypic tests and yielded 54 profiles. Disk diffusion and agar screen with oxacillin were used as "gold standard" for the calculation of sensitivity, specificity and prediction once they are recommended by the CLSI veterinarian as standardized tests. Disk diffusion with cefoxitin achieved the best performance in the prediction of oxacillin resistance. Genotypic detection of mecA do not provided any positive isolate, otherwise mecI and mecRI genes were also detected in 11.6% (29/250) of the studied Staphylococcus spp. Four cassette mec types were detected (I, II, III and IV), being type I the most disseminated one. Gene blaZ was detected in 5.2% (13/250) isolates. From these 13 blaZ positive isolates, the whole system comprising blaR1-blaI-blaZ was detected in 23.1% (3/13) isolates.

Bovine mastitis; antibiotic resistance; Staphylococcus spp.

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