Environmental mastitis has increased particularly in well-managed or low somatic cell count herds that have successfully controlled contagious pathogens. Major pathogens of environmental mastitis are Escherichia coli (E. coli) and Streptococcus uberis. The present study was conducted to investigate the isolation and antibiotic susceptibility of 406 (21.8%) gram-negative bacteria isolated mastitis milk from 1,865 quaters of 241 Korean dairy farms from 2001 to 2004. Prevalence of major gram-negative bacteria isolated from mastitis milk were E. coli (22.7%) and Enterobacter spp. (16.3%) in coliforms and Pseudomoas spp. (10.3%) and Serratia spp. (7.9%) in non-coliforms.

Streptococcus spp. comprising Streptococcus (S.) uberis and S. dysgalactiae strains is major cause of bovine mastitis from particularly well-managed or low somatic cell count herds that have successfully controlled contagious pathogens. In this study, antimicrobial susceptibility and genetic characteristics of S. uberis isolated from clinical or subclinical mastitis milk at 2003 were investigated. Eighty seven isolates of Streptococcus spp. were identified by the conventional biochemical methods. The antimicrobial susceptibility by disk diffusion method was determined for 46 S. uberis, 11 S. bovis, 10 S. oralis, 6 S. uberis and 14 other Streptococcus spp., Overall, the tested strains were susceptible to tetracycline (11.5%), amikacin (14.9%), streptomycin (16.1%), neomycin (26.4%), kanamycin (35.6%), gentamicin (65.2%), oxacillin (70.1%), ampicillin (75.9%), chloramphenicol (78.2%), and cephalothin (97.7%).