In vitro growth of mastitis-associated streptococci in bovine mammary secretions.

Cell-free, fat-free mammary secretions were tested in vitro for ability to support growth of streptococci associated with mastitis. Secretions were obtained prior to drying off, during the dry period, at calving, and during lactation from four cow treatment groups. Treatment groups were dry cow therapy, dry cow therapy and mammary glands subjected to induced inflammation 7 d post-drying-off, no dry cow therapy and no induced inflammation, no dry cow therapy but mammary glands subjected to induced inflammation. Growth of Streptococcus uberis, Streptococcus faecalis, and Streptococcus agalactiae in secretions from nonlactating glands was unaffected by induced inflammation. Growth of Streptococcus bovis was significantly inhibited in secretion obtained 14 d after induced inflammation. Dry cow therapy had no effect on streptococcal growth in secretion obtained 7 d after therapy. Streptococcal growth was greatest in secretions from involuted glands, and there was little or no evidence for growth inhibitory factors in cell-free, fat-free secretions obtained during the dry period. Milk from lactating glands inhibited streptococcal growth, and the inhibitory factor was presumptively identified as lactoperoxidase. Apolactoferrin, immunoglobulin, or both had little effect on streptococcal growth.