A resistance pattern across treatments
An experiment with the yeast Candida tropicalis found a broad cross-resistance pattern after agricultural azole exposure. All 12 evolved strains were less susceptible than the starting strain, C23, to the other agricultural azoles and to one or more medical azoles. The study’s question was whether agricultural-fungicide exposure was associated with this wider pattern in C. tropicalis. The result describes an association in laboratory cultures; it does not by itself establish that exposure caused resistance elsewhere.
Researchers began with C23, an azole-susceptible progenitor, and repeatedly passaged the cultures under agricultural azole exposure. Passaging continued until the optical density measured at 600 nanometres reached at least 5.0. The azole concentration was escalated to 64 times the MIC and cells were plated at 128 times MIC. MIC, or minimum inhibitory concentration, was defined as the lowest drug concentration that reduced growth by at least 50% compared with a drug-free control.
For each evolution condition, 10 colonies from each of three biological repeats were screened for susceptibility. The researchers selected the three strains with the highest MICs from each azole group for follow-up, producing a comparison panel of 12 evolved strains alongside C23. That choice means the downstream analysis concentrated on the strongest resistance phenotypes selected in each group.
The strains did not all behave alike
Resistance was not accompanied by one uniform growth pattern. Strains evolved under difenoconazole and flutriafol grew more slowly than C23. By contrast, two diniconazole-evolved strains and one hexaconazole-evolved strain grew faster, while the remaining strains were slower. On phloxine-B medium, most evolved colonies changed from white to pink or red. Hexaconazole-evolved colonies were wrinkled and more aggregated, but cell size and yeast-form structure remained similar.
The response also extended to selected non-azole drugs. Every evolved strain had reduced susceptibility to micafungin. The flutriafol- and hexaconazole-evolved strains also had elevated MICs for anidulafungin. The study reported no significant changes for amphotericin B, caspofungin or 5-flucytosine.
Multiple layers of change
The evolved strains also differed from C23 in chromosome content. C23 was diploid, meaning it had two chromosome sets, while all 12 derivatives were aneuploid, meaning they carried an irregular chromosome complement. Their measured DNA content ranged from 2.1 N to 2.9 N. Comparative genomic analysis found between 135 and 1,063 genes present in multiple copies across the evolved strains. Nine of the 12 had aneuploidy involving chromosome 3, which contained several resistance-associated genes.
Changes in gene activity were similarly uneven. RNA sequencing found more than 1,700 differentially expressed genes, meaning genes whose activity changed, in FLT-Evo1, compared with approximately 250 in HXC-Evo1. Across the 12 strains, 33 such genes were shared by at least 10 strains, 27 were shared by 11, and 16 were shared by all 12. Several resistance-associated genes were more active even though their copy number had not changed, indicating that gene copy number alone did not fully determine the expression pattern.
Why the result needs caution
These findings do not identify a single mechanism. The 12 follow-up strains were the highest-MIC representatives from each group, and the study did not functionally separate the effects of aneuploidy, copy-number changes and sequence variants. The data therefore show changes that accompanied reduced susceptibility, but they do not show that any one change caused it. Because the experiment used one C. tropicalis progenitor and selected laboratory derivatives, it also does not establish that the same pattern occurs in environmental or clinical isolates.
Paper data and sources
Original title: Agricultural azole fungicides induce cross-resistance to medical antifungals in Candida tropicalis: a One Health perspective on antifungal resistance
Authors: Li S, Hu T, Li C et al.
Journal/Repository: Mycology
Status: Peer-reviewed
First online: 2026-10-09
DOI: Not available
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