I recently wrote about a paper that surveyed the diversity of bacteria in public restrooms using metagenomic techniques. While that paper focused on bacteria on bathroom surfaces, another recent paper – “Widespread Occurrence of Diverse Human Pathogenic Types of the Fungus Fusarium Detected in Plumbing Drains”, authored by Dylan Short and colleagues – focused specifically on probing the diversity of the large Ascomycete genus Fusarium found in sink drains, with specific focus on isolates that are human pathogens.
The authors sampled 471 drains – more than 95% of which were from public bathroom sinks – from 131 buildings throughout the mid-eastern to southern United States (and California too). They selectively isolated Fusarium species from sink drains using cotton swabs and then streaked petri plates of Nash-Snyder Agar, which is a semi-selective medium containing the fungicide pentachloronitrobenzene. The plates were inspected after the fungi had some time to grow, were propagated, and then verified as Fusarium species using microscopic morphology and DNA sequencing.
Six different loci – translation elongation factor (TEF), the internal transcribed spacer region (ITS) into the large ribosomal subunit (LSU), the nuclear rDNA intergenic spacer region (IGS), the RNA polymerase II large subunit (RPB2), portion of the alpha-tubulin (TUB) gene, and calmodulin (CAM) – were identified using Sanger sequencing to assess the diversity of Fusarium in the sink drains. The sequence data was compared to an extensive database of the genus Fusarium maintained by the Geiser Lab and others.
Fusarium species were extremely common in sink drains; 66% of the sink samples – and 82% of all the buildings sampled – yielded at least one isolate. These isolates could largely be placed within three Fusarium species complexes: the Fusarium solani species complex (62% of samples), the F. oxysporum species complex (28%), and the F. dimerum species complex (8.5%). Sink drains from 91% of private residences and 80% of public buildings yielded Fusarium isolates. Of all the buildings that yielded Fusarium within sink drains, approximately 80% contained one of the six major isolates recognized from human infections.
It is interesting to note that human infections from Fusarium species are rare, but the six most common Fusarium isolates found in sink drains are also the six most common involved in human infection. The authors note that it’s apparent that people are in constant contact with these fungi within indoor environments. It’s also notable that novel species complexes were identified using these techniques and that there was a wide phylogenetic breadth to the Fusarium isolates that were sampled from sink drains.
This paper is a substantial contribution to the growing literature documenting the indoor environment for fungi. The next step would be to use metagenomic techniques – and marker loci for fungi to encompass a meta-taxonomic assessment – to identify all the fungi found in sink drains.