Experimental endocarditis in rabbits. 7. Results of long-term combined therapy of Streptococcus faecalis endocarditis with penicillin and gentamicin.

The purpose of this study was to investigate the efficacy of long-term penicillin-gentamicin treatment on experimental S. faecalis endocarditis due to selected strains; one strain was homogeneously resistant to 8000 micrograms/ml streptomycin (IC50 and MIC greater than 8000 micrograms ml), the other strain heterogeneously resistant to 8000 micrograms/ml streptomycin (IC50: 3300 micrograms/ml, MIC: greater than 8000 micrograms/ml. Both strains showed low-grade resistance to gentamicin (MIC 10.5 and 25 micrograms/ml, respectively). The results showed that there was significant effect of the treatment in rabbits with endocarditis due to both strains, as measured by lowered mortality and by high bacteriologic cure rate. Despite effective antibiotic treatment, a high incidence (57%) of congestive heart failure was noted in rabbits late in the treatment period or after termination of antibiotic treatment, probably due to healing processes on the aortic valves. In human cases of S. faecalis endocarditis the antibiotic treatment should be either penicillin plus streptomycin or penicillin plus gentamicin. However, in cases where the infecting strain is homogeneously resistant to 8000 micrograms/ml streptomycin, the treatment with penicillin plus gentamicin is clearly the drug combination of choice.