Veterinary Microbiology, 18 (1988) 41-50 41 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands Ingestion and Killing of Streptococcus agalactiae by Bovine Granulocytes in the Presence of Natural Opsonins P. RAINARD, Y. LAUTROU and B. POUTREL Institut National de la Recherche Agronomique, Station de Pathologie de la Reproduction, 37380 Nouzilly (France) (Accepted for publication 1 March 1988) ABSTRACT Rainard, P., Lautrou, Y. and Poutrel, B., 1988. Ingestion and killing of Streptococcus agalactiae by bovine granulocytes in the presence of natural opsonins. Vet. Microbiol., 18: 41-50. An enzyme-linked immunosorbent assay (ELISA) demonstrated the presence of naturally ac- quired antibodies against Streptococcus agalactiae in normal bovine serum (NBS). In milk wheys, ELISA values were much lower than in sera. Pre-colostral calf serum (PCS) was shown to lack antibodies to type II and III S. agalactiae. The opsonic requirements of 10 human and 10 bovine strains were investigated by evaluating the phagocytosis-induced reduction of the incorporation of radiolabeled thymidine by strepto- cocci. Antibodies present in NBS were required for the efficient ingestion of both human and bovine isolates type II by bovine granulocytes. Three out of five type III bovine isolates were opsonized in the absence of specific antibodies (opsonization by PCS ) and type II and III bovine isolates did not require complement for opsonization. By contrast, inactivation of complement reduced phagocytosis of human isolates and only one type III strain of human origin was opsonized by PCS. These findings suggest that human isolates had higher opsonic requirements. The phagocytic killing of 6 type III strains (5 mastitis isolates and the reference typing strain ) was investigated. Opsonization by normal serum enabled bovine blood granulocytes to ingest and kill S. agalactiae. Nevertheless, > 35% of bacteria remained viable at the end of the phagocytosis incubation in 10% NBS. Heat treatment of serum decreased the efficacy of killing for only 3 of the 6 tested strains. An IgG2 fraction of normal adult bovine serum promoted active ingestion, which was still increased in the presence of PCS. Normal wheys displayed large variations in their ability to promote ingestion of S. agalactiae by blood granulocytes. The promoting effect was systematically less than that of serum from the same cow, and this can be related to the lower ELISA values found in wheys. INTRODUCTION Streptococcus agalactiae is still a frequent cause of mastitis in dairy herds (McDonald, 1977; Wilson and Richards, 1980; Oliver and Mitchell, 1984 ). Most 0378-1135/88/$03.50 © 1988 Elsevier Science Publishers B.V.