Reply to Kidd et al., “New Names for Fungi of Medical Importance: Can We Have Our Cake and Eat It Too?”

REPLY

We are grateful to Dr. Kidd and her colleagues for the sensible and supportive stance adopted in their letter (1) in reply to our recent manuscript summarizing proposed name changes for fungi of medical importance (2), and for offering voices of reason in the heated Twitter exchanges that ensued following the publication of said taxonomy update. Since neither author of the original manuscript is particularly active on social media, we only became aware of the resultant Twitter maelstrom after alerts from colleagues overseas. Like Kidd et al., we were surprised by some predictions warning of impending patient harm if such names were adopted, and the implicit suggestion in several Tweets that clinicians might ignore reports of isolation of an organism if it was one that they had not previously encountered.

As Kidd et al. pointed out in their reply to our original nomenclatural update, nomenclatural changes are not new to mycology, and thousands have been introduced, and accepted by the medical community over recent decades. To the example of Cryptococcus neoformans (formerly Torula histolytica) cited by Kidd et al., one could also question who now would refer to Candida albicans as “Monilia albicans,” which was still in use by some of the old school when we started our careers, as was the disease name “moniliasis” in place of candidiasis (or more correctly candidosis)? Who would refer to Candida glabrata (now Nakaseomyces glabrata) as “Torulopsis glabrata” which many were fighting to retain late into the 1990s? As we explained in the original update, and as echoed by Kidd et al., the potential for taxonomic and clinical confusion engendered by such changes has always been successfully managed in the past by repetitive iteration of the new name together with the most recent previous incarnation, and this is the approach that we have suggested that laboratories continue to adopt with future taxonomic revisions.

While it is reassuring that a number of highly respected mycology reference laboratories worldwide have also chosen to implement these long-overdue changes and drip-feed them to their users, we share the disappointment of Kidd et al. that the recent reference document updates from both the Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) continue to refer to all “Candida” species by their previous (now often obsolete) names (3, 4). As pointed out in the update, and restated by Kidd et al., medical mycologists have accepted for decades that the genus “Candida” is an artificial and highly heterogeneous construct, containing species from dozens of extremely unrelated teleomorph genera. What was largely ignored in the social media outcry that followed the recent taxonomic update (presumably because a significant proportion of those offended by the suggestions had not read the full update and references) is that many of the proposed changes are neither recent nor novel. For example, the genus Clavispora was erected in 1979 with Clavispora lusitaniae (ex-Candida lusitaniae) as the type strain on the basis of sexual compatibility studies (5). Similarly, Candida krusei was reassigned to the genus Pichia in 2008 based on comprehensive phylogenetic analyses (6) and formally shown to be indistinguishable from P. kudriavzevii by whole-genome sequence analyses in 2018 (7). Although some commercial identification platforms and databases are slow in update implementation, genetic databases are much more up-to-date. As an example of such, a BLASTN nucleotide alignment of GenBank sequence KC601852 (which corresponds to the rDNA genes of the type strain of Candida krusei ATCC 6258) against the public synchronized databases retrieves >1,000 highly scoring hits, all matching isolates named as Pichia kudriavzevii. Thus, the recent taxonomic update attempted to summarize all such proposals/taxonomic reaffiliations in a single place, as sooner or later the wider medical mycology community and the clinicians that this community serves will be confronted by them.

The fact that Candida krusei has now been properly reclassified as a Pichia species makes perfect sense when you realize that many Pichia species are fluconazole resistant, unlike most true Candida species. Equal clarity should also come from the reclassification of Candida glabrata as Nakaseomyces glabrata and many of the other suggested changes as the underlying taxonomic relationships become more apparent. These changes may seem unnecessarily complicated and unfamiliar now but are well supported by genomic analyses and will help to predict expected antifungal susceptibility profiles and many other phenotypic characteristics. We should of course for many years continue to refer to the old name in parentheses, until the next generation of physicians, infectious disease specialists, microbiologists, and mycologists become familiar with the new taxonomy. Change is often challenging but when it leads ultimately to greater clarity and understanding of the genotypic and phenotypic interrelatedness of the mycological world and its patient interface, it should be embraced as a helpful development and not dismissed as an irrelevant nuisance dreamt up by taxonomists to enable mycologists to maintain their professional mystique.