Most people host the common yeast Candida albicans at some point in their lives, with about 40% to 60% carrying it in their mouth or guts, according to researchers at the University of Georgia in Athens and Quadram Institute Bioscience in the United Kingdom. Yet this normally harmless cohabitant can be lethal in immunocompromised people.
Generally thought to only grow in warm-blooded animals, C albicans has occasionally been isolated from plants, prompting researchers to ask if these instances are misplaced yeast or if the fungus can thrive outside a warm body. Examining the genomes of three C albicans strains isolated from the barks of the oldest oak trees in an ancient wood pasture, these researchers have found genetic evidence that this yeast can live on plants for extended periods of time.
After ensuring these strains were new tree-based isolates and not laboratory contaminants, the researchers conducted a phenotypic investigation showing that all three had most of the standard traits of C albicans, including the ability to grow at the elevated temperatures expected in a mammal. However, they weren’t identical. One wasn’t as salt-tolerant as the others, wouldn’t grow on soluble starch, and switched to a different growth form under particular nutritional conditions.
Next, the researchers sequenced the genomes of the new strains. This was the first time that C albicans from a non-animal source has been sequenced. Analysis showed that they were relatively distantly diverged from each other. The new sequences also were compared with more than 200 yeast sequences previously isolated from humans and other animals to create a phylogenetic tree, where all three of the tree strains showed more similarity with clinical strains than with each other.
The researchers further analyzed the levels of heterozygosity, a measure of genetic variation, within the tree strains and found that they were more heterozygous than typical clinically isolated strains, suggesting that life on trees subjects the years to different selection or mutation pressures than life in humans.
The researchers said the higher heterozygosity could be a result of yeast evolving in conditions where it has to reproduce asexually, which would make mutations more likely to accumulate, increasing allelic variation. This difference also supports the idea that these yeast grow in the wild, rather than being recent emigrants from a warm-blooded host.
These findings may have implications beyond forestry, the researchers noted, as understanding the wild life of C albicans could shed light on the evolution and lifestyle of the yeast found in humans and help us better understand how virulent strains emerge and damage human health.
The study, “Diverse Lineages of Candida Albicans Live on Old Oaks,” was published by Genetics.
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