Decreased growth of Streptococcus uberis in milk from mammary glands of cows challenged with the same mastitis pathogen

Milk samples from mammary glands challenged with Streptococcus uberis and from unchallenged mammary glands were selected for analyses of bacterial growth, antibody response, and lactoperoxidase activity. All challenged mammary glands became infected with isolation of S. uberis and elevated somatic cell counts in milk during the first week after challenge. In vitro growth of the homologous challenge strain and a heterologous strain of S. uberis was significantly lower in milk from challenged mammary glands than in milk from control mammary glands at 3, 5, and 7 days after challenge. Removal of casein significantly reduced bacterial growth. In general, antibodies specific to S. uberis started to increase at day 3 postchallenge and were higher in milk from challenged mammary glands than in milk from control mammary glands. There was also a marked increase in total IgG in milk from challenged mammary glands. Growth of S. uberi increased following heat treatment at 56 degrees C of pooled milli or whey samples from challenged mammary glands. Growth of S. uberis correlated negatively with the specific antibody response to the bacteria (P < 0.001). Lactoperoxidase activity varied among cows and among different samples over time and did not appear to contribute to decreased growth of S. uberis. These results suggest that decreased growth of S. uberis in milk from challenged mammary glands in comparison to milk from control mammary glands could result from the interaction of antibodies with complement components.