Determination of the synergy of antibiotic combinations by an overlay inoculum susceptibility disc method.

A method involving an overlay inoculum agar and antibiotic susceptibility discs in the determination of synergy of gentamicin and disc antibiotics against clinical isolates of Pseudomonas aeruginosa has been described. Gentamicin combinations with carbenicillin showed synergism against all test isolates. Combinations containing fosfomycin had indifferent effects. Netilmicin, ceftriaxone and ampicillin, respectively, produced interisolate variations in the synergy of their combinations with gentamicin. In all cases synergy was determined on the basis of correlations between inhibition zone diameter (IZD) increments and fractional inhibitory concentration (FIC) indices. An IZD increment of 19% or more predominantly corresponded with synergy. Cases showing less than 19% increase in IZD produced additive effects, while cases showing no variations in IZD had indifferent effects.