Penicillin Therapy for Treatment of Experimental Endocarditis Caused by Viridans Streptococci in Animals

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Abstract
We studied the efficacy of penicillin and penicillin combined with streptomycin in the treatment of experimental endocarditis caused by viridans streptococci that are susceptible, tolerant, or relatively resistant to penicillin. Rabbits with experimental endocarditis were treated with procaine penicillin (1.5 × 105 U/kg) administered twice daily or with procaine penicillin (1.5 × 105 U/kg) plus streptomycin (20 mg/kg) administered twice daily for five days. Compared with control animals, animals treated with penicillin alone experienced a significant reduction (P < .001) of colony forming units per gram of cardiac valve vegetations when infected with streptococci that are susceptible, tolerant, or resistant to penicillin. This antibiotic alone was less effective against streptococci that were tolerant or resistant to penicillin than against streptococci susceptible to the drug (P < .01). The combination of penicillin and streptomycin was more effective therapy than was penicillin alone in animals with penicillin-tolerant or penicillin-resistant streptococci causing endocarditis (P < .01). Penicillin-streptomycin therapy was less active against penicillinresistant strains than against either penicillin-tolerant (P < .04) or penicillin-susceptible (P < .01) strains. The results of our study suggest that tolerance or relative resistance to penicillin in strains of viridans streptococci influences the response to therapy with penicillin alone or penicillin combined with streptomycin in the treatment of experimental endocarditis caused by viridans streptococci.