Synergy of mecillinam (FL1060) with penicillins and cephalosporins against Proteus and Klebsiella, with observations on combinations with other antibiotics and against other bacterial species.

Thirty-five strains each of Klebsiella and Proteus were tested for susceptibility to mecillinam alone and in combination with ampicillin, carbenicillin, cephalothin, and cefazolin. Antibiotics were considered to be synergistic when there was a >/=fourfold reduction in minimum inhibiting concentration of both antibiotics in the combination as compared with that of each antibiotic alone. Synergy of mecillinam with ampicillin, carbenicillin, cephalothin, and cefazolin was demonstrated against 2, 3, 7, and 8 of the 35 strains of Klebsiella and against 14, 14, 19, and 24 of the 35 strains of Proteus, respectively. Synergy against the isolates of Proteus was related to species, whereas against Klebsiella it was related to susceptibility of the isolates to mecillinam. Tests of combinations of mecillinam with other antibiotics on the same and different species indicated that synergy was related to the antibiotic, the species, and the strains of organisms tested.