How does environmental fungicide exposure influence fungal pathogen evolution and drug resistance?

Environmental fungicide exposure significantly influences fungal pathogen evolution and drug resistance, particularly as observed with Candida tropicalis. A study published in PLoS Biology revealed that tebuconazole, an azole-related fungicide widely used by farmers and gardeners, which can accumulate and persist in the environment, has driven the increase in azole-resistant C. tropicalis infections seen in clinics.

Here's how environmental fungicide exposure influences fungal pathogen evolution and drug resistance:

•Emergence of Drug Resistance: Exposure to tebuconazole in the environment has led to the emergence of C. tropicalis strains that exhibit high resistance not only to tebuconazole but also to clinically-used azole anti-fungal drugs like fluconazole and voriconazole. This is known as cross-resistance.

•Ploidy Changes (Aneuploidy):

◦The researchers found that tebuconazole-resistant strains exhibited aneuploidy, meaning their chromosome number differed from the normal count for the organism. This deviation from the normal chromosome complement is referred to as ploidy plasticity.

◦While most human cells are diploid (two sets of the genome), and aneuploidy in humans often leads to serious consequences like Down syndrome or prenatal death, C. tropicalis was also long thought to be diploid. The discovery that most tebuconazole-resistant strains had altered ploidy was surprising.

◦The ploidy of these resistant strains ranged from haploid to triploid (three copies of the genome). Those initially identified as diploid were found, upon closer analysis, to be segmental aneuploids, possessing duplications or deletions of specific chromosome segments.

•Genetic Mechanisms of Resistance through Aneuploidy:

◦Duplication of Chromosomal Segments: Resistant strains often had duplications of segments carrying genes whose overexpression is known to increase azole resistance. For instance, several tebuconazole-resistant strains showed duplications of a chromosomal segment carrying the TAC1 gene. This gene encodes a protein that helps the cell produce more of an ABC-transporter protein, which then pumps toxic compounds like azoles out of the cell.

◦Haploidisation (Deletion of Chromosome Segments): Conversely, other segmental aneuploids showed haploidisation, which involves the deletion of one copy of a segment of another chromosome carrying the HMG1 gene. Previous studies had shown that reduced expression of HMG1 stimulates ergosterol synthesis, leading to elevated resistance to fluconazole.

◦These aneuploidies, while creating imbalances in the C. tropicalis genome that reduced their growth rate in the absence of antifungals, enabled the strains to better resist antifungals. The resistant strains grew much better in the presence of antifungals, effectively trading cell growth for antifungal resistance.

•Increased Virulence: Strains with altered ploidy were also verified to be more virulent than their progenitor strains in mice treated with fluconazole.

•Discovery of Stable Haploid Strains: An unexpected finding was the discovery of stable haploid strains of C. tropicalis among the tebuconazole-resistant strains. These haploid cells were capable of mating, providing researchers with a useful tool for future genetic analyses. The researchers even found naturally occurring haploid C. tropicalis strains from clinical isolates.

•Implications for Resistance Spread: The fact that some resistant strains were haploid and could mate means they are capable of introducing their resistance mechanisms into new genetic backgrounds, potentially accelerating the spread of drug resistance.

In conclusion, the research highlights that the reckless use of triazole antifungals in agriculture can unwittingly promote the emergence of pathogenic strains showing cross-resistance to azoles of clinical importance, fulfilling a "sow the wind, reap the whirlwind" prophecy.