Capsular polysaccharide antigenemia in rats with experimental endocarditis due to Staphylococcus aureus

In vivo expression of the type 8 capsular polysaccharide (CP) of Staphylococcus aureus was studied in the rat model of aortic valve endocarditis. An enzyme-linked immunosorbent assay (ELISA) with monoclonal antibodies to type 8 CP was sensitive to 0.16 ng/ml in serum. All (23 of 23) animals infected with the prototype type 8 strain Becker had CP detected in one or more serum samples. Peak CP levels ranged from 0.16 to 240 ng/ml (median, 25 ng/ml). Among the 14 rats alive on day 2, antigenemia and bacteremia were significantly correlated (r = .71, P less than .005). CP was also present in serum of rats infected with three additional type 8 bacteremic isolates. All serum samples from animals infected with the type 5 prototype strain were negative in the type 8 ELISA. These studies document that the type 8 CP of S. aureus, previously identified only in vitro, is produced and released during in vivo infection and can be detected in serum of infected animals.