A microbroth kinetic model based on turbidity measurements was developed in order to analyze the growth characteristics of three species of filamentous fungi (Rhizopus microsporus, Aspergillus fumigatus, and Scedosporium prolificans) characterized by different growth rates in five nutrient media (antibiotic medium 3, yeast nitrogen base medium, Sabouraud broth, RPMI 1640 alone, and RPMI 1640 with 2% glucose). In general, five distinct phases in the growth of filamentous fungi could be distinguished, namely, the lag phase, the first transition period, the log phase, the second transition period, and the stationary phase. The growth curves were smooth and were characterized by the presence of long transition periods. Among the different growth phases distinguished, the smallest variability in growth rates among the strains of each species was found during the log phase in all nutrient media. The different growth phases of filamentous fungi were barely distinguishable in RPMI 1640, in which the poorest growth was observed for all fungi even when the medium was supplemented with 2% glucose. R. microsporus and A. fumigatus grew better in Sabouraud and yeast nitrogen base medium than in RPMI 1640, with growth rates three to four times higher. None of the media provided optimal growth of S. prolificans. The germination of Rhizopus spores and Aspergillus and Scedosporium conidia commenced after 2 and 5 h of incubation, respectively. The elongation rates ranged from 39.6 to 26.7, 25.4 to 20.2, and 16.9 to 9.9 μm/h for Rhizopus, Aspergillus, and Scedoporium hyphae, respectively. The germination of conidia and spores and the elongation rates of hyphae were enhanced in antibiotic medium 3 and delayed in yeast nitrogen base medium. In conclusion, the growth curves provide a useful tool to gain insight into the growth characteristics of filamentous fungi in different nutrient media and may help to optimize the methodology for antifungal susceptibility testing.
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