Antibiotic concentrations in serum, serum bactericidal activity, and results of therapy of streptococcal endocarditis in rabbits.

The correlation between antibiotic concentrations in serum, serum bactericidal activity, and results of therapy was studied in rabbits with streptococcal endocarditis. Five days of procaine penicillin G reduced bacterial titers to <10 per g in 12 of 14 vegetations in rabbits receiving 75,000 U intramuscularly every 6 h and 10 of 20 in rabbits given 37,500 U. Ten days of 18,750 U every 6 h did not reduce the titers. To test for cure, rabbits were treated with 75,000 U every 6 h for 10 or 20 days and then received no therapy for 7 days. At the end of the 7-day period without therapy, vegetations were sterile in five of five and eight of eight animals, respectively. Rabbits received 37,500 U every 6 h for 5, 10, or 20 days and then no therapy for 7 days, after which vegetations were sterile in one of seven, four of nine, and seven of eight animals, respectively. The median maximal serum bactericidal dilutions at 1 h were 1/16 when 75,000 U of procaine penicillin G was administered, 1/8 to 1/16 with 37,500 U, and 1/4 to 1/8 with 18,750 U. Serum bactericidal activity could not be detected in 50% of the rabbits 6 h after administration of 37,500 U. Cure was related to a median maximal serum bactericidal dilution of at least 1/8 to 1/16 1 h after penicillin administration. A median maximal serum bactericidal dilution of 1/4 to 1/8 resulted in unsuccessful therapy.