Phagocytosis of Streptococcus uberis by bovine mammary gland macrophages

The ability of macrophages isolated from the bovine mammary gland to phagocytose and kill Streptococcus uberis was investigated. The strains of S uberis used were selected on the basis of their known resistance (C197C) or sensitivity (EF20) to phagocytosis by bovine peripheral blood neutrophils in the same assay system. Macrophages isolated from mammary secretions collected during the mid-dry period were capable of phagocytosing both strains of S uberis in the presence of serum and skimmed milk. The removal of complement from serum by heat-inactivation did not influence the opsonization of S uberis for phagocytosis by mammary macrophages and both IgG1 and IgG2 isotypes were found to opsonise both strains. The uptake of S uberis into the cells was confirmed by electron microscopy. Potential mechanisms by which S uberis could resist phagocytosis by neutrophils but not by macrophages are discussed.