Landscape of fungal diversity in healthy nails and onychomycosis
Onychomycosis (OM) is a common fungal nail infection. Based on current diagnostic tools, such as microscopy and culture instated that OM is commonly caused by a single pathogen. The advent of molecular diagnostics, however, indicated that in many cases, more than one fungal pathogen is present in OM. Yet, an in-depth characterization of fungal diversity in OM is lacking and, fungal diversity of healthy nails has not been extensively investigated. To analyse the landscape of fungal diversity in healthy and onychomycotic nails we performed unbiased next-generation sequencing (NGS) of DNA samples from clinically healthy toenails and from those with suspected OM. Additionally to NGS, samples underwent routine and molecular laboratory OM testing. Samples from healthy nails were only considered for NGS if OM testing was negative, while detection of a fungal infection in at least one diagnostic test was a prerequisite for allocation into the OM case group. After quality controls, the complete data set comprises 324 operational-taxonomic units with no difference in alpha-diversity among the two groups, indicating that fungi are part of the healthy human nail microbiome. Regarding beta-diversity, cases and controls differ. Within cases, two distinct sub-clusters are present: Those with a high number of Trichtophyton rubrum reads, and those with an undistinguishable fungal beta-diversity from healthy controls. Thus, we hypothesize that there are two types of OM, one pathogen driven, and one in which OM develops due to host- and/or environmental factors. Collectively, we here provide detailed insights into the landscape of fungal diversity in health and OM.