Metronidazole and spiramycin in abscesses caused by Bacteroides spp. and Staphylococcus aureus in mice

The activity of metronidazole and spiramycin, singly or in combination, was tested in vitro and also in vivo, in the eradication of infection caused by Bacteroides spp. and Staphylococcus aureus, alone or in combination. The MICs of metronidazole for the B. melaninogenicus and B. fragilis strains were significantly reduced by the addition of spiramycin (from 0.25 and 0.5 mg/l, to 0.062 and 0.125 mg/l, respectively). Antimicrobial synergy between metronidazole and spiramycin was noted against Bacteroides spp. in abscesses caused in mice by subcutaneous injection of Bacteroides spp. alone or in combination with S. aureus. Furthermore, an additional reduction in the number of S. aureus was noted in mixed infections with bacteroides that were treated with metronidazole alone. The antimicrobial synergy in vitro and in vivo between metronidazole and spiramycin may have clinical implications that deserve to be further investigated.