Determination of in vitro synergy when amphotericin B is combined with various antimicrobial agents against yeasts by using a colorimetric microdilution checkerboard

: We determined in vitro interactions when amphoteric B (AMPH) is combined with various antimicrobial agents against yeast by using a colorimetric microdilution transfer plate technique, principally based on the current National Committee for Clinical Laboratory Standards (NCCLS) M27-T. An oxidation-reduction dye, sodium resazurin, was used as a color indicator to detect the growth of yeasts, and a standard two-dimensional, two-agent microdilution checkerboard in RPMI 1640 was employed to determine in vitro interactions; synergistic, indifferent or antagonistic. The study included 125 clinical isolates of Candida species and nine reference strains of American Type Culture Collection described in M27-T. Among the 34 antimicrobial agents first tested, polymyxin B (PL), rifampicin (RFP), tetracycline (TC) and erythromycin (EM) showed significant synergism. The fractional inhibitory concentration (FIC) indices of the respective agents were; PL 0.16 to 0.51 (mean, 0.315), RFP 0.13 to 0.56 (0.255), TC 0.06 to 0.75 (0.353) and EM 0.27 to 1.0 (0.550). RFP was the most potent agent, 118 of 125 clinical isolates (94.4%) showing synergism (FIC, < or = 0.5). With these results, we can conclude that several antibacterial agents are potentially effective when combined with AMPH against yeasts, probably due to alteration in the permeability barrier of the surface membrane by AMPH. Antifungal synergism may be promising for more effective, and less toxic therapy, and thus in vivo study will be necessary to determine their clinical significance.