Antimicrobial susceptibility of Streptococcus species isolated from clinical mastitis in dairy cows.

The antimicrobial susceptibility was determined for 50 Streptococcus uberis, 42 S. dysgalactiae subsp. dysgalactiae and eight S. agalactiae strains isolated from cow mastitis. Only 27% of the strains were susceptible to all antimicrobial compounds tested. Resistance to tetracycline was most frequent (particularly for S. dysgalactiae strains), then macrolide and/or lincomycin resistance. High level resistance to streptomycin and kanamycin was detected. All S. dysgalactiae and S. agalactiae strains were susceptible to beta-lactams but 44% of the S. uberis strains showed an elevated penicillin G MIC. All strains were susceptible to chloramphenicol and rifampicin.