In Vitro and in Vivo Studies of Streptomycin-Resistant, Penicillin-Susceptible Streptococci from Patients with Infective Endocarditis

The combination of penicillin and streptomycin did not act synergistically in vitro against three streptomycin-resistant strains (MIC, ⩾1,OOO μg of streptomycin/ml) of penicillin-susceptible streptococci. Using a model of experimental infective endocarditis, weinfected rabbits with a control streptomycin-susceptible strain, with an intermediately streptomycinresistant strain (MIC, 1,000 μg/ml), and with a highly streptomycin-resistant strain (MIC, >32,000 μg/ml), Treating animals with a combination of procaine penicillin and streptomycin was more effective (P < .01) than treating them with procaine penicillin alone only for those animals infected with the control streptomycin-susceptible strain. Treatment with procaine penicillin plus gentamicin was more effective (P < .01)than treatment with procaine penicillin alone for all three treatment groups and was more effective (P < .01)than treatment with procaine penicillin and streptomycin for those animals infected with an intermediately or highly streptomycin-resistant strain of streptococci.