Response of Streptococcus pyogenes to therapy with amoxicillin or amoxicillin-clavulanic acid in a mouse model of mixed infection caused by Staphylococcus aureus and Streptococcus pyogenes.

The response of Streptococcus pyogenes to amoxicillin or amoxicillin-clavulanic acid (Augmentin; Beecham Group) therapy of a mixed streptococcal-staphylococcal infection was studied in a surgical wound in mice. A superficial wound was produced on the backs of anesthetized mice, and a suture infected with S. pyogenes, Staphylococcus aureus, or a mixed inoculum of both organisms was inserted. Oral therapy was started 4 h after infection and continued for 3 days. Both amoxicillin and amoxicillin-clavulanic acid were effective in eliminating the streptococci from the pure wound infection. In contrast, amoxicillin failed to eliminate the streptococci from a mixed infection in which a beta-lactamase-producing strain of S. aureus was also present, wound counts reaching 10(7) streptococci per wound by 80 h, whereas amoxicillin-clavulanic acid reduced the count to less than 33 streptococci per wound by 24 h. Numbers of S. aureus were also reduced by amoxicillin-clavulanic acid therapy, controlling the infection, whereas amoxicillin was ineffective. Also of significance was the fact that successful therapy was achieved with blood and tissue concentrations of amoxicillin and clavulanic acid of the same order as those measured in humans. These results show that amoxicillin therapy failed to eliminate S. pyogenes from a wound infection in the presence of a beta-lactamase-producing strain of S. aureus and suggest the potential of amoxicillin-clavulanic acid in the treatment of mixed bacterial skin infections involving beta-lactamase-producing organisms.