Antagonism between chloramphenicol and penicillin in streptococcal endocarditis in rabbits

Antagonism between chloramphenicol (CHL) and penicillin (PCH) was studied in rabbits with Streptococcus viridans endocarditis. PCN alone or preceded 1 hour by CHL was injected twice a day starting 6 hours or 3 days after infection. In animals treated 6 hours after infection, vegetations contained mean log colony-forming units (CFU):3.0 per gram in PCN animals and 3.7 in PCN + CHL animals (P greater than 0.05) after 24 hours of treatment and 2.4 in PCN animals and 4.3 in PCN + CHL animals (P greater than 0.01) after 48 hours. In animlas treated 3 days after infection, vegetations contained mean log CFU: 3.8 per gram in PCN animals and 5.2 in PCN + CHL animals (P greater than 0.01) after 72 hours of treatment and 1.7 in PCN animals and 2.5 in PCH + CHL animals (P greater than 0.05) after 5 days. The antibiotic antagonism demonstrated in these studies could be reduced by injecting CHL 1 hour after PCN instead of 1 hour before PCN.