Submitted by BethBradshaw on 31 October 2017
We’re used to thinking of fungi as falling into two groups: the ‘good’ industrial fungi that produce beer, yoghurt and Quorn, and the ‘bad’ species that cause athlete’s foot, spoiled food and serious infections such as aspergillosis. But what about species that do both?
C. krusei lives naturally in the environment and has been used in the chocolate-making process since time immemorial. Piles of de-podded cocoa beans are wrapped up in banana leaves to ferment for up to a week to neutralise bitterness and bring out the delicious flavours that we enjoy in the finished product.
But while normally harmless, this yeast carries a dark secret: it causes opportunistic infections in immunocompromised patients. Worryingly, it is also naturally resistant to the first-line antifungal fluconazole, and isolates have even been found that were resistant to amphotericin B. There have been scattered clues as to how this resistance works, but it is difficult to put these pieces of the puzzle together, in part because much of the existing whole-genome data we have has come from environmental strains such as those living on fruit.
Last month, scientists in the USA announced the first genome sequence from a clinical isolate of C. krusei. While it had only few virulence genes, as expected, the genome revealed a variety of genes that encode transporter proteins. It is possible that the yeast could be using these to pump antifungal drugs out of its cells to avoid being affected.
Click here to read more about the microbiology of chocolate production. Note that the Pichia kudriavzevii in this article is the same species as C. krusei – fungi change their name during their sexual phase.
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