Morphological heterogeneity of Aspergillus terreus cultures was observed during continued cultivation of AMB-resistant isolates on drug-free medium. Outgrowth leads to the emergence of multiple sectors that might result from increased growth rates at drug-free conditions. We evaluated the differences in AmB susceptibility and virulence between sector subcultures (ATSec), AmB-resistant strains (ATR), and AmB-susceptible strains (ATS). By comparing AmB resistant A. terreus strains (ATR) and A. terreus sector cultures (ATSec) a highly significant reduction of AmB MICs in ATSec was observed (ATR: MIC: 2-32μg/ml, ATSec 0.12-5 μg/ml). Furthermore Galleria mellonella survival studies revealed an enhanced virulence of ATSec, which was comparable with AmB-sensitive Aspergillus terreus strains (ATS) (median survival rate for ATS: 72 h, ATSec G1: 84 h, and ATR: 144 h).
Our findings clearly demonstrate that spontaneous culture degeneration occurs in A. terreus and most importantly crucially impacts drug efficacy and virulence.
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