Background: Inoculum preparation for hydrophobic fungi, as AF, is facilitated by adding Tween 20 (TW). The NCCLS M38-P document recommends to prepare inoculum suspensions by a spectrophotometric method and adding a 5% of TW, thus final inoculum contains a TW concentration of 0.025%. However, TW can have an effect on AST of AF. The objective of this study was to evaluate the combined influence of TW and inoculum size on AST of AF. Method: A microdilution procedure was performed. Antifungal agents: Amphotericin B (AmB), Itraconazole (ITZ) and Voriconazole (VZ). Assay medium: RPMI 2% of glucose. Organisms: 7 clinical isolates of AF; 3 in vivo resistant to Itraconazole (RS), and 4 susceptible (SS). AF ATCC 9197 was used as quality control. Inocula preparation: a hemacytometer count method was employed. Three different concentrations of TW were utilized (0.25%, 0.025% and 0.005%). Two inoculum sizes were analyzed (104 and 105 CFU/mL). Each experiment was repeated three times on three different days. Conclusions: (1) Inoculum size (104 or 105) has no significant influence on AST of AF. (2) MICs obtained with higher TW concentrations were proportionally more elevated than those obtained with lower concentrations, mainly for ITZ. (3) The utilization of final inoculum with a TW concentration higher than 0.025% can falsely elevate MICs. (4) The use of higher inocula than that recommended by NCCLS must take into account the influence of TW concentration on AST of AF.
Full conference title:
11th European Congress of Clinical Microbiology and Infectious Diseases
- ECCMID 11th (2001)