Resumo

The bacterium Streptococcus agalactiae is recognized worldwide as a major pathogen causing clinical and subclinical intramammary infections in lactating cows. The aim of this study was to characterize the antimicrobial resistance patterns of S. agalactiae isolated from cows with subclinical mastitis. The bacteria were isolated on sheep blood agar from composite milk samples aseptically collected in 11 herds in the Zona da Mata region in Minas Gerais State - Brazil. Antibiotic resistance was determined by the agar diffusion method according to CLSI guidelines and the detection of resistance genes performed by PCR. A total of 130 isolates were identified as S. agalactiae by classical microbiological methods (colony morphology, hemolytic activity on sheep blood agar medium, Gram staining characteristics, catalase reaction, CAMP test, hippurate and esculin hydrolysis) and confirmed by PCR amplification of a 120 bp DNA fragment species-specific corresponding to part of the gene encoding for the 16S rRNA. The isolates most frequently exhibited phenotypic resistance to tetracycline and erythromycin. The ermB gene was found in 95% of isolates phenotypically resistant to erythromycin (n=40). Among the isolates phenotypically resistant to tetracycline, tetM and tetO genes were found in 23 (51%) and 29 (64%), respectively, and 06 isolates (13%) presented the genotype tetM/tetO. Moreover, resistance genes were also detected in some susceptible isolates, showing that the resistance genotype does not accurately correlate to the phenotypic resistance. On the other hand, no genetic determinant was detected in some phenotypically resistant isolates indicating that further work is needed to better document the genetic background of antimicrobial resistance in this group of bacteria.