Global patterns in culturable soil yeast diversity

Seasonal Dynamics of Culturable Yeasts in Ornithogenically Influenced Soils in a Temperate Forest and Evaluation of Extracellular Enzyme Secretion in Tausonia pullulans at Different Temperatures

The culturable yeast communities in temperate forest soils under the ornithogenic influence were studied in a seasonal dynamic. To investigate the intense ornithogenic influence, conventional and "live" feeders were used, which were attached to trees in the forest and constantly replenished throughout the year. It was found that the yeast abundance in the soil under strong ornithogenic influence reached the highest values in winter compared to the other seasons and amounted to 4.8 lg (cfu/g). This was almost an order of magnitude higher than the minimum value of yeast abundance in ornithogenic soils determined for summer. A total of 44 yeast species, 21 ascomycetes and 23 basidiomycetes, were detected in ornithogenic soil samples during the year. These included soil-related species (Barnettozyma californica, Cyberlindnera misumaiensis, Cutaneotrichosporon moniliiforme, Goffeauzyma gastrica, Holtermanniella festucosa, Leucosporidium creatinivorum, L. yakuticum, Naganishia adeliensis, N. albidosimilis, N. globosa, Tausonia pullulans, and Vanrija albida), eurybionts (yeast-like fungus Aureobasidium pullulans, Debaryomyces hansenii, and Rhodotorula mucilaginosa), inhabitants of plant substrates and litter (Cystofilobasidium capitatum, Cys. infirmominiatum, Cys. macerans, Filobasidium magnum, Hanseniaspora uvarum, Metschnikowia pulcherrima, and Rh. babjevae) as well as a group of pathogenic and opportunistic yeast species (Arxiozyma bovina, Candida albicans, C. parapsilosis, C. tropicalis, Clavispora lusitaniae, and Nakaseomyces glabratus). Under an ornithogenic influence, the diversity of soil yeasts was higher compared to the control, confirming the uneven distribution of yeasts in temperate forest soils and their dependence on natural hosts and vectors. Interestingly, the absolute dominant species in ornithogenic soils in winter (when the topsoil temperature was below zero) was the basidiomycetous psychrotolerant yeast T. pullulans. It is regularly observed in various soils in different geographical regions. Screening of the hydrolytic activity of 50 strains of this species at different temperatures (2, 4, 10, 15 and 20 °C) showed that the activity of esterases, lipases and proteases was significantly higher at the cultivation temperature. Ornithogenic soils could be a source for the relatively easy isolation of a large number of strains of the psychrotolerant yeast T. pullulans to test, study and optimize their potential for the production of cold-adapted enzymes for industry.

Culturable yeast diversity in urban topsoil influenced by various anthropogenic impacts

In urban ecosystems, processes associated with anthropogenic influences almost always lead to changes in soil micromycete complexes. The taxonomic structure of soil micromycete complexes is an important informative parameter of soil bioindication in the ecological control of urban environments. Unicellular fungi, such as culturable yeasts, are a very suitable and promising object of microbiological research for monitoring urban topsoil. This review aims to give an overview of the yeast communities in urban topsoil in different areas of Moscow (heating main area, household waste storage and disposal area, highway area) and to discuss the changes in the taxonomic structure of culturable yeast complexes depending on the type and intensity of anthropogenic impact.

Seasonal Variation in Fungi in Beach Sand in Summertime: Stintino (Italy)

BACKGROUND

The goal of this study was to monitor the microbial biodiversity in beach sand that is heavily visited by tourists during the summer, and to determinate whether the high presence of bathers (around 5000 per day) can modify sand microbial composition.

METHODS

Between 2016 and 2020, 150 sand samples were collected from nine different points at La Pelosa beach in Sardinia, Italy. Non-culturing methods were used; DNA extraction and meta-barcode sequencing were performed. All samples were analyzed with sequencing methods for 16S and ITS sequences.

RESULTS

Fungal genera differ on the three beaches and in the winter/summer zones. The ITS sequence showed the most common presence of Candida during summer and Paradendryphiella in the winter. The greatest diversity was found in the dune during winter, while in other parts of the beach, there are differences between bacteria and fungi, particularly in the wash zone during the winter, with high diversity for 16S sequences but low diversity for ITS sequences.

CONCLUSIONS

It appears reasonable that the sands, even on non-urban beaches, should be included in health monitoring programs in addition to the waters, and that access to them should be regulated by limiting the number of bathers with the aim of reducing the presence of pathogenic fungal species.

Assessing global fungal threats to humans

Fungi are an integral part of the earth's biosphere. They are broadly distributed in all continents and ecosystems and play a diversity of roles. Here, I review our current understanding of fungal threats to humans and describe the major factors that contribute to various threats. Among the 140,000 or so known species out of the estimated six million fungal species on Earth, about 10% directly or indirectly threaten human health and welfare. Major threats include mushroom poisoning, fungal allergies, infections of crop plants, food contamination by mycotoxins, and mycoses in humans. A growing number of factors have been identified to impact various fungal threats, including human demographics, crop distributions, anthropogenic activities, pathogen dispersals, global climate change, and/or the applications of antifungal drugs and agricultural fungicides. However, while models have been developed for analyzing various processes of individual threats and threat managements, current data are primarily descriptive and incomplete, and there are significant obstacles to integration of the diverse factors into accurate quantitative assessments of fungal threats. With increasing technological advances and concerted efforts to track the spatial and temporal data on climate and environmental variables; mycotoxins in the feed and food supply chains; fungal population dynamics in crop fields, human and animal populations, and the environment; human population demographics; and the prevalence and severities of fungal allergies and diseases, our ability to accurately assess fungal threats will improve. Such improvements should help us develop holistic strategies to manage fungal threats in the future.

Going wild: ecology and genomics are crucial to understand yeast evolution

Improved and more accessible genome-sequencing approaches have allowed the analysis of large sets of natural yeast isolates. As a consequence, this unprecedented level of description of yeast-genome characteristics and variations in natural environments has provided crucial insights on yeast ecology and evolution. Here, we review some of the most relevant and intriguing aspects of yeast evolution pointed out, thanks to the combination of yeast ecology and genomics, and critically examine the resulting improvement of our knowledge on this field. Only integrated approaches, taking into consideration not only the characteristics of the microbe but also those of the hosting environment, will significantly move forward the exploration of yeast diversity, ecology, and evolution.

Candida auris on Apples: Diversity and Clinical Significance

Candida auris is a multidrug-resistant nosocomial fungal pathogen. While the marine environment was recently identified as a natural niche for C. auris, the environment(s) that might have contributed to the development and spread of antifungal resistance in C. auris remains a mystery. Because stored fruits are often treated with fungicides to prevent postharvest spoilage, we hypothesized that stored fruits could serve as a possible selective force for and a transmission reservoir of antifungal-resistant isolates of pathogenic yeasts, including C. auris. To test this hypothesis, we screened fruits to study the diversity of pathogenic yeasts and their antifungal susceptibility profiles. Among the 62 screened apples, the surfaces of 8 were positive for C. auris, and all were stored apples. Whole-genome sequencing (WGS) showed that C. auris strains from apples were genetically diverse and exhibited broad phylogenetic distribution among the subclades within clade I. Interestingly, strains from apples had closely related strains from other sources in India, including from patients, hospitals, and marine environments, and from clinical strains from other parts of the world. A broad range of fungicides, including dimethyl inhibitors (DMIs), were detected in stored apples, and all C. auris isolates exhibited reduced sensitivity to DMIs. Interestingly, C. auris was not isolated from freshly picked apples. Together, the results suggest a potentially complex ecology for C. auris with agriculture fungicide application on stored fruits as a significant selective force for drug resistance in clinics.

Extensive Diversity and Prevalent Fluconazole Resistance among Environmental Yeasts from Tropical China

Yeasts play important roles in both the environment and in human welfare. While some environmental yeasts positively contribute to nutrient cycling and food production, a significant number of yeast species are opportunistic human pathogens, including several that are tolerant/resistant to commonly used antifungal drugs. At present, most of our understanding of environmental yeasts has come from a few terrestrial environments in selected geographic regions. Relatively little is known about yeast diversity in tropical environments and their potential impacts on human health. Here, we characterize culturable yeasts in 968 environmental samples from eight regions in tropical China. Among the 516 soil, 273 freshwater, and 179 seawater samples, 71.5%, 85.7%, and 43.6% contained yeasts, respectively. A total of 984 yeast isolates were analyzed for their DNA barcode sequences and their susceptibilities to fluconazole. DNA sequence comparisons revealed that the 984 yeast isolates likely belonged to 144 species, including 106 known species and 38 putative novel species. About 38% of the 984 isolates belonged to known human pathogens and the most common species was Candida tropicalis, accounting for 21% (207/984) of all isolates. Further analyses based on multi-locus sequence typing revealed that some of these environmental C. tropicalis shared identical genotypes with clinical isolates previously reported from tropical China and elsewhere. Importantly, 374 of the 984 (38%) yeast isolates showed intermediate susceptibility or resistance to fluconazole. Our results suggest that these environmental yeasts could have significant negative impacts on human health.

Is Natural Population of Candida tropicalis Sexual, Parasexual, and/or Asexual?

Candida tropicalis is one of the most common opportunistic yeast pathogens of humans, especially prevalent in tropical and subtropical regions. This yeast has broad ecological distributions, can be found in both terrestrial and aquatic ecosystems, including being associated with a diversity of trees, animals, and humans. Evolutionary theory predicts that organisms thriving in diverse ecological niches likely have efficient mechanisms to generate genetic diversity in nature. Indeed, abundant genetic variations have been reported in natural populations (both environmental and clinical) of C. tropicalis. However, at present, our understanding on how genetic diversity is generated in natural C. tropicalis population remains controversial. In this paper, I review the current understanding on the potential modes of reproduction in C. tropicalis. I describe expectations of the three modes of reproduction (sexual, parasexual, and asexual) and compare them with the observed genotypic variations in natural populations. Though sexual and parasexual reproduction cannot be excluded, the analyses suggest asexual reproduction alone could explain all the observations reported so far. The results here have implications for understanding the evolution and epidemiology of C. tropicalis and other related human fungal pathogens.