Penicillin-Aminoglycoside Synergy in Experimental Streptococcus viridans Endocarditis

In vitro synergism has been described for many strains of Streptococcus viridans with penicillin and streptomycin; however, the in vivo correlation of this phenomenon is lacking. Rabbits with catheter-induced aortic valve endocarditis caused by S. viridans (MIC, 0.06 μg/ml) were treated with penicillin, penicillin plus streptomycin, and penicillin plus gentamicin. The bacteria were eradicated from the cardiac vegetations in approximately one-half of the time with the penicillin-aminoglycoside combinations (three to four days) than was required with penicillin alone (seven to 10 days), as determined by titrations of viable bacteria from vegetations and relapse after discontinuation of therapy.