Effects of exopolysaccharide production by viridans streptococci on penicillin therapy of experimental endocarditis.

The influence of exopolysaccharide production by viridans streptococci on penicillin therapy of endocarditis was studied in rabbits with experimental aortic-valve endocarditis. Animals were first infected with penicillin-susceptible streptococci known to produce varying amounts of exopolysaccharide in cardiac vegetations. Treatment with procaine penicillin, 300,000 U every 12 hr, was begun one to two days after infection. After five days of therapy, animals infected with Streptococcus sanguis II and Streptococcus morbillorum, both vigorous exopolysaccharide producers, continued to have infected vegetations, while animals infected with Streptococcus salivarius and a different Streptococcus sanguis II, both deficient in exopolysaccharide production, had sterile vegetations. These findings indicate that failure to eradicate streptococci from vegetations correlates with exopolysaccharide production and that this property may be important in determining outcome of therapy.