Environmental mastitis pathogens isolated from bulk tank milk collected on California dairies

The objective of this prospective study was to determine the prevalence of potential environmental bacterial mastitis pathogens in bulk tank milk, and to determine their antibiotic susceptibility patterns. Bulk tank milk samples from over 400 California dairies routinely submitting samples to the Milk Quality Laboratory were initially screened. Once potential environmental bacterial mastitis pathogens were found in bulk milk from a dairy, the dairy was repeatedly sampled on a monthly basis. Over a nine-month period, 93 dairies were identified with these bacteria, and 381 isolates were collected. Most common isolates were Streptococcus uberis (42.3%), Streptococcus dysgalactiae (16.3%) and Enterococcus faecium (10.5%). Bacterial isolates (335) from 73 dairies which had at least three isolates were subjected to Minimum Inhibitory Concentration (MIC) testing using 10 antibiotics. A wide range of susceptibility to antibiotics was found. Streptococcus dysgalactiae tended to have lower MIC values to the test antibiotics than the other isolates. The non-streptococcal bacteria tended to have the highest MIC values.