Resistance to antimicrobials and biofilm formation in Staphylococcus spp. isolated from bovine mastitis in the Northeast of Brazil

Mastitis is the principal disease affecting dairy herds worldwide. The aim of the present study was to characterize phenotypic and genotypic features associated with resistance to antimicrobials in Staphylococcus spp. isolated from 2064 milk samples of 525 lactating cows in the Northeast of Brazil. Of the 218 isolates analyzed, 57.8 % were characterized as Staphylococcus aureus, 28 % as coagulase-positive staphylococci other than S. aureus (oCPS), and 14.2 % as coagulase-negative staphylococci (CNS). The test for susceptibility to antimicrobials showed amoxicillin (32.6 %) to be the less effective drug in vitro, and the multi-drug resistance (MDR) rate for beta-lactams varied from 0 to 0.75. The genotypic characterization showed that 93.1 % of the samples were tested positive for the blaZ gene, while none amplified mecA. The antibiotic efflux mechanism was observed in 0.9 % of isolates. The biofilm formation was found in 3.7 and 96.3 % of samples, respectively, on Congo red agar and on the microplate adhesion test, while the icaD gene was present in 92.2 % of Staphylococcus spp. The high frequency of blaZ gene observed in this study was associated with the resistance of most Staphylococcus spp. to one or more of the beta-lactams tested, which are routinely used in Brazilian herds for mastitis treatment. The biofilm formation was also detected in the isolates analyzed being an important characteristic for pathogenicity and antimicrobial resistance of bacteria.