Effects of incubation temperature, inoculum size, and time of reading on broth microdilution susceptibility test results for amphotericin B against Fusarium

Abstract

In vitro antifungal susceptibility testing for filamentous fungi remains unstandardized and is unreliable for determining adequate therapy. A study was performed to evaluate the effect of inoculum size (102, 103, 104, and 105 conidia/ml), incubation time (48 and 72 h), and temperature (25, 30, and 35°C) on MICs of amphotericin B for Fusarium spp. (20 strains). The inoculum size showed the clearest effect: when the inoculum was varied from 102 to 105 conidia/ml, the geometric mean MICs showed increases of between 10- and 19-fold in all the combined conditions of temperature and incubation time assayed. Time of incubation had less effect (increases of between two- and threefold in approximately half of the geometric mean MICs), and temperature especially had little effect (the increases were no higher than twofold). The effects of interaction between inoculum size and temperature on MICs were not statistically significant, while the combined effects of inoculum size and time of reading and of time of reading and temperature produced systematic variation in MICs.