Comparative activity of amoxycillin and ampicillin in an experimental bacterial infection in mice.

Against experimental infections in mice with strains of Escherichia coli and with Proteus mirabilis, amoxycillin was found to be more active than ampicillin both by the oral and the subcutaneous routes of administration. With the bacteria used in these experiments, ampicillin and amoxycillin showed the same minimal inhibitory concentrations and, after subcutaneous administration, the levels of ampicillin and amoxycillin in the plasma and tissue homogenate were also similar. However, counts of the number of viable bacteria present in the infected tissue showed that amoxycillin exerted a more rapid and a more marked bactericidal effect than did ampicillin, and this could be correlated with the difference in therapeutic effect. When given by mouth, amoxycillin was more completely absorbed than ampicillin and gave rise to higher levels of antibiotic in the plasma. This may have accounted in part for the difference in therapeutic activity seen when these penicillins were given by the oral route. However, when appropriate oral dosages of ampicillin and amoxycillin were used so as to achieve similar levels of antibiotic in the plasma and tissue homogenate, amoxycillin was again found to exert a more marked bactericidal effect than did ampicillin, and this was accompanied by greater therapeutic activity. In experiments in vitro, amoxycillin also showed a more rapid bactericidal effect than did ampicillin against the bacteria which were used to produce the experimental infections.