Isogenic variants of Staphylococcus aureus strain Reynolds expressing either no capsule or capsular polysaccharide (CP) type 5 (CP5) or type 8 (CP8) were used to assess the effect of CP on bacterial killing and the respiratory burst of bovine neutrophils. The effects of antisera specific for CP5 and CP8 were also evaluated. The killing of live bacteria by isolated neutrophils was quantified in a bactericidal assay, while the respiratory burst after stimulation with live bacteria in whole blood was measured by flow cytometry. The expression of a CP5 or CP8 capsule protected the bacteria from being killed by bovine neutrophils in vitro (P < 0.001), and the capsule-expressing variants did not stimulate respiratory burst activity in calf whole blood. The addition of serotype-specific antisera increased the killing of the capsule-expressing bacteria and enhanced their stimulating effect in the respiratory burst assay (P < 0.01). When the S. aureus variants were grown under conditions known not to promote capsule expression, there were no significant differences between them. The present study demonstrates that the expression of S. aureus CP5 or CP8 confers resistance to opsonophagocytic killing and prevents the bacteria from inducing respiratory burst of bovine neutrophils in vitro and that these effects can be reversed by the addition of serotype-specific antisera.Many bacterial species have well-characterized capsules which are important virulence factors that potentiate infections (20). In addition to being a major human pathogen, Staphylococcus aureus is the cause of a variety of infections in animals. Most important is mastitis of dairy cows, which compromises animal welfare and causes large economic losses for the dairy industry. S. aureus produces various surface polysaccharides (18,19,22,40), and most strains express capsular polysaccharides (CPs) in vivo or under defined culture conditions (24). Although at least 11 serotypes based on CPs have been proposed, only four CPs (CP1, CP2, CP5, and CP8) have been chemically characterized (15).Phagocytosis and killing by neutrophil granulocytes play a key role in defense against S. aureus infections. CP1 and CP2 have been shown to have antiphagocytic properties (9,17,25,41), but such heavily capsulated strains are rarely isolated clinically (2,31,43). Most clinical isolates from both human and bovine infections produce either CP5 or CP8, although considerable geographic variations exist in the prevalence of these types in bovine isolates (26,32,37).The relatively small amount of CP produced by serotype 5 and 8 strains has made it difficult to define the role of these capsules in virulence. However, the antiphagocytic properties of the S. aureus CP5 capsule, as measured in in vitro opsonophagocytic killing assays, have been shown to play an important role in the pathogenesis of S. aureus infections, most likely by allowing the organism to resist uptake and killing by phagocytes (21, 35).Opsonization is of major importance for effective phagocytosis of most bacterial ...