Factors affecting the results of a broth microdilution antifungal susceptibility testing in vitro

In experiments involving 10 antifungal drugs and 46 strains of potentially pathogenic fungi, the factors affecting the results of in vitro susceptibility testing were studied. The composition of the test medium, inoculum size, temperature and length of incubation were the most pronounced effects influencing the results of testing in vitro. Minimal inhibitory concentrations (MIC) of the antimycotics tested were lowest in complex media (Brain Heart Infusion, Antibiotic Medium 3, Sabouraud broth) except for 5-fluorocytosine which was most effective in Yeast Nitrogen Base medium. Inoculum sizes of 10(3) to 10(4) cfu* mL(-1) had no marked effect on MIC but starting from a final concentration of 10(5) cfu*mL(-1), an abrupt increase in MIC in azole derivatives and 5-fluorocytosine was observed. There was a direct relationship between the duration of incubation and MIC of fungistatic antimycotics. The influence of the incubation temperature became generally manifest primarily in fungi with retarded growth at elevated temperature (>35 degrees C). In these fungal species, a tendency towards a decrease in MIC with increasing temperature was apparent. The other factors studied (medium pH, buffer, solvent) had no substantial influence on the antifungal activity of the drugs tested.