J. Vet. Med. B zyxwvutsrq 40,707-714 (1993) zyxwvut 0 1993 Paul Parey Scientific Publishers, Berlin and Hamburg ISSN 093 1 zyxwvutsrqponm - 1793 zyxwvutsrq From the Department of Animal Science, Institute of Agriculture, The University of Tennessee, Knoxville, TN Antiphagocytic Effect of the Capsule of zyx Streptococcus uberis R. A. ALMEIDA and S. P. OLIVER Address of authors: Department of Animal Science, Institute of Agriculture, The University of Tennessee, P.O. Box 1071, Knoxville, TN; USA 37901-1071 With one figure and zyxw fi tables (Received forpublication July 5, 1993) Summary Non-opsonized encapsulated and non-encapsulated strains of Streptococcus uberis were incubated with bovine mammary macrophages and the percentage of phagocytosis and intracellular killing were determined. Seventy-seven percent of macrophages ingested non-encapsulated organisms with a killing rate of 75 %. In contrast, 48 % of macrophages ingested encapsulated bacteria with a killing rate of 35 %. When strains were opsonized with homologous antiserum, differences were detected in the percentage of phagocytosis (84 % vs. 48 %) and intracellular killing (52 % vs. 35 %) of the encapsulated strain only. Effects were partially abolished when antiserum was absorbed with purified capsule, or when macrophages were pre-treated with purified capsular material. Electron microscopy of mammary macrophages incubated with the encapsulated strain of S. uberis showed the microorganism in contact with the macrophage cell membrane without signs of membrane activation. In contrast, the non-encapsulated strain induced formation of pseudopods and membrane ruffling. These results suggest that capsule may protect S. uberis from phagocytosis which may be due to a direct interaction of capsular material with macrophages. Introduction Macrophages and polymorphonuclear leukocytes are the predominant cells in milk and secretions from non-lactating mammary glands and are important in defense against new intramammary infections (JENSEN and EBERHART, 1975). These cells exert their phagocytic activity through receptors that recognize the Fc fraction of immunoglobulins, non-immunological receptors and receptors for opsonic complement fractions (WAT- SON, 1976; WEISSMANN, 1983). Presence of specific antibodies enhance the phagocytic activity of these cells (POUTREL and CAFFIN, 1983; TARGOWSKI and KLUCINSKI, 1985). However, most encapsulated bacteria can be phagocytized only after opsonization with anticapsular antibodies (DENSEN and MANDELL, 1980; HORWITZ and SILVERSTEIN, 1980; HORWITZ, 1982; GUIDRY et al., 1991). Bacterial capsule is considered a virulence factor primarily because of its anti- phagocytic role. The capsule protects bacteria from phagocytosis by masking surface antigens, by causing consumption of complement fractions or altering complement binding to the bacterial surface, or by blocking receptors on the surface of the phagocyte (VERBRUGH et al., 1979; DENSEN and MANDELL, 1980; WILKINSON, 1983; SPITZNAGEL, 1983). zyxwvut US. Copyright Clearance Center Code Statement: 093 1 - 1793/93/4009 - 0707$02.50/0