(khoahoc.tv) – While humanity has utilized the power of yeast for fermenting bread and beer, the functions of yeast and various fungi living in and on the human body remain poorly understood.
In the first study on fungal diversity on human skin, scientists from the National Institutes of Health decoded the DNA sequence of a type of fungus that resides on healthy human skin. The goal was to identify the common fungal populations that inhabit human skin and provide insights to help study conditions caused by dermal fungi.
The human skin surface is a complex ecosystem of microorganisms, including fungi, bacteria, and viruses, collectively known as the skin microbiome (the skin microbiome is understood as the entire gene pool of microorganisms living in and on the skin). Although fungal infections affect approximately 29 million people in the United States, fungi are slow and difficult to culture in laboratories, making diagnosis and treatment complicated, even for the most common cases like toenail infections.
The research team from the National Human Genome Research Institute and the National Cancer Institute, both part of NIH, expanded their recent study of skin bacteria to use optimized DNA sequencing techniques to identify fungi. The study was published online on May 22, 2013, in the journal Nature.
Researchers discovered that a single type of fungus, belonging to the Malassezia genus, is prevalent on the scalp and torso. The human hands exhibited a high diversity of bacteria but housed relatively few types of fungi. In contrast, the feet, including toenails, heels, and the tops of the feet, showed a significant diversity of fungi.
“Using DNA sequencing to study skin fungi is a natural development in understanding the symbiotic microbial life on our bodies,” said Dr. Daniel Kastner, the scientific director of NHGRI. “Along with recent gene sequencing to identify microbial diversity, this analysis of fungal diversity provides a more comprehensive picture of the human microbiome.”
“The fungal populations reside in complex niches, even on the human body,” said Dr. Heidi Kong, a senior co-author and investigator in dermatology at the NCI. “By gaining a fuller understanding of bacterial and fungal ecosystems, we can better treat skin diseases related to fungi and bacteria, including skin conditions that may be associated with cancer treatments.”
Researchers collected samples from 14 different sites on the bodies of 10 healthy adults. The DNA sequences of the fungi in the samples were identified using DNA fragments known as phylogenetic markers, which can be calculated and used to distinguish one type of fungus from another. These sequencing efforts generated over 5 million markers from the samples, representing more than 80 species or genera of fungi. In contrast, traditional culturing methods produced only 130 fungal colonies representing just 18 genera of fungi.
A marker is a standard set of DNA fragments of a predetermined size used as a reference to estimate the size of a nucleic acid segment in electrophoresis techniques.
In 20% of study participants, researchers observed issues such as changes in heels, toes, and toenails that could be infected with fungi. From the gene sequencing analysis, the research team noted differences in heel infections among individuals with the same common fungal populations at those sites, while those with toenail infections exhibited very different fungal populations.
“DNA sequencing reveals a tremendous diversity of fungi, despite their difficulty in culturing,” said Dr. Julie Segre, a senior co-author and senior investigator in the NHGRI’s Genetics and Molecular Biology Branch. “DNA sequencing allows us to learn more about the areas where fungi dominate as part of the human skin microbiome.”
Researchers identified fungi from two groups, Ascomycetes and Basidiomycetes, as part of the investigation into the common fungal populations at 14 skin sites. The most common genus, Malassezia, was found at 11 of the 14 sampling sites on the body. Researchers discovered Malassezia fungi on every skin surface of healthy volunteers, whether on the back of the head, behind the ears, in the nostrils, or on the heels. The heels also harbor many other fungal species, including the genera Aspergillus, Cryptococcus, Rhodotorula, and Epicoccum.
“The sequencing-based identification method allows us to distinguish between different fungal species and conclude that fungal diversity depends more on body locations than on the individual from whom the samples were taken,” Dr. Kong explained. As a dermatologist, Dr. Kong elaborated on why these locations were chosen for the study: “Our research focused on areas of skin where we often see skin diseases related to fungi.”
The most complex site, the heel, is home to about 80 genera of fungi. Researchers found 60 types in samples taken from toenails and 40 types from the skin between the toes. Other areas with high fungal diversity include the inside of the elbows, the insides of the arms, and the palms, each site having 18 to 32 fungal genera. Surprisingly, locations such as the scalp and torso—including the back, nape, inside the ears, behind the ears, and between the eyebrows—had significantly fewer fungal types, with only about 10 genera.
The research team compared fungal diversity data with skin bacteria from the same healthy adults. They found that while arms contained a high diversity of bacteria, they had low diversity of fungi. Conversely, the opposite was true for the feet. The central areas of the body exhibited low diversity of both bacteria and fungi. Researchers have previously shown that the diversity of bacteria on the human body can be predicted by the nature of the skin—be it moist, dry, or oily. In contrast, fungal diversity seems to depend on the body location.
They also observed that there is greater similarity in fungal population structure on the left and right sides of the same person compared to different body parts across any two individuals. Fungal populations also appear quite stable over time, with minimal changes between two separate tests conducted three months apart.
“The data from our study provides us with a baseline for normal individuals that we have never had before,” Dr. Segre stated. “The bottom line is that your feet are full of fungi, so tiptoe in the locker room if you don’t want to mix your foot fungi with someone else’s.”
