Activity of penicillin combined with an aminoglycoside against group B streptococci in vitro and in experimental endocarditis.

Group B streptococci were less susceptible in vitro to penicillin and to aminoglycosides with an inoculum size of 10(7)-10(8) cfu/ml than with an inoculum size of 5.5 X 10(5)-10(6) cfu/ml. With a rabbit model of experimental group B streptococcal endocarditis, after one or three days of therapy with procaine penicillin alone, the mean log10 cfu/g of valve vegetation was significantly lower (P less than 0.01) than that of the control groups. After one day of therapy, procaine penicillin combined with streptomycin was significantly more effective (P less than 0.01) than was treatment with procaine penicillin alone. There was no significant difference (P greater than 0.05) in the results of treatment of animals for one day with procaine penicillin combined with streptomycin compared with those of animals treated for three days with procaine penicillin alone.