Comparative efficacies of mezlocillin and ampicillin alone or in combination with gentamicin in the treatment of Streptococcus faecalis endocarditis in rabbits.

The in vitro and in vivo activities of mezlocillin and ampicillin, alone and in combination with gentamicin, against Streptococcus faecalis were compared. In vitro, relative bactericidal and activities for 10 strains of S. faecalis were as follows: mezlocillin plus gentamicin greater than ampicillin plus gentamicin (P is not significant) greater than mezlocillin (P less than 0.01) greater than ampicillin (P less than 0.01) greater than gentamicin (P less than 0.01) greater than control (P less than 0.01). One of the strains was used to induce endocarditis in rabbits. Infected rabbits were treated by random selection with ampicillin, mezlocillin, ampicillin plus gentamicin, mezlocillin plus gentamicin, or saline (control) for 7 days. Among antibiotic-treated rabbits, survival rates were the same, but ampicillin was less effective than the other regimens in sterilizing both blood cultures (P less than 0.001 on day 4) and cardiac vegetations (P less than 0.05 on day 7). Differences in quantitative vegetation cultures among rabbits treated with mezlocillin, mezlocillin plus gentamicin, or ampicillin plus gentamicin and sacrificed on day 7 were not significantly different. Peak inhibitory and bactericidal antibacterial activity titers in serum determined during treatment were higher with mezlocillin (with or without gentamicin) than with ampicillin (with or without gentamicin) (P less than 0.01). Mezlocillin may be more effective than ampicillin in the treatment of S. faecalis infections in humans.