(khoahoc.tv) – While humans have harnessed the power of yeast for baking bread and brewing beer, the role of yeast and other fungi living on and in the human body remains 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 the skin of healthy individuals, aiming to identify the common fungal populations that inhabit human skin and provide insights to study conditions related to fungal skin diseases.
The surface of human skin is a complex ecosystem of microorganisms, including fungi, bacteria, and viruses, collectively known as the skin microbiome (the skin microbiome refers to the entire gene set of microorganisms living on and in the skin). Although fungal infections affect approximately 29 million people in the United States, fungi are slow-growing and difficult to culture in the laboratory, making diagnosis and treatment complicated even for the most common fungal skin infections, such as nail infections.
The research team from the National Human Genome Research Institute and the National Cancer Institute, both part of NIH, expanded their recent bacterial skin genome sequencing study using 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 exhibit a large diversity of bacteria but host only a few fungal species. In contrast, the feet, including the toenails, heels, and the tops of the feet, show a significant diversity of fungi.
“Using DNA sequencing to study skin fungi is a natural development to understand the life of symbiotic microorganisms on our bodies,” said Dr. Daniel Kastner, 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, senior co-author and a dermatologist investigator at the NCI’s Cancer Research Center. “By gaining a better 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 treatment therapies.”
Researchers sampled 14 different sites on the bodies of 10 healthy adults. The DNA sequences of the fungi in the samples were identified using DNA segment markers, known as phylogenetic markers, which can be calculated and used to differentiate one type of fungus from another. These sequencing efforts produced over 5 million markers from the samples, representing more than 80 fungal species or genera. In contrast, traditional culturing methods yielded 130 fungal colonies representing only 18 fungal genera.
Markers are standard sequences of DNA with defined and known sizes used as a reference to estimate the size of a nucleic acid segment in electrophoresis techniques.
In 20% of the study participants, researchers observed issues such as changes in the heels, toes, and toenails that could be fungal infections. From the gene sequencing analysis, the research team noted differences in the fungal populations on heels among individuals sharing the same common fungal populations at those sites, while those with nail infections displayed significantly different fungal populations.
“DNA sequencing revealed a vast diversity of fungi, even though they are difficult to culture,” said Dr. Julie Segre, 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 divisions, Ascomycetes and Basidiomycetes, as part of the investigation of the fungal populations at 14 locations on the skin. The most common genus, Malassezia, was found at 11 of the 14 sampled sites on the body. Researchers discovered Malassezia fungi on all skin surfaces of healthy volunteers, whether on the back of the head, behind the ears, inside the nostrils, or on the heels. The heels also hosted many other fungal species, including the genera Aspergillus, Cryptococcus, Rhodotorula, and Epicoccum.
“The sequence-based identification method allows us to distinguish between different fungal species and conclude that fungal diversity is largely dependent on body sites rather than on the individual from whom the sample is taken,” said Dr. Kong. As a dermatologist, Dr. Kong explained why these sites were chosen for the study: “Our research focused on areas of skin where we often see skin diseases associated with fungi.”
The most complex location, the heel, is home to approximately 80 fungal genera. Researchers found 60 types in samples taken from toenails and 40 types from the spaces between the toes. Other high-diversity fungal locations include the inside of the elbows, the inner arms, and the palms, with each site hosting 18 to 32 fungal genera. Surprisingly, areas such as the scalp and torso – including the back, nape, inside the ears, behind the ears, and between the eyebrows – had far fewer fungal types, with only about 10 genera.
The research team compared fungal diversity data with skin bacteria on the same healthy adults. They found that while the arms had high bacterial diversity, they had low fungal diversity. Conversely, the opposite was true for the foot locations. The central body locations exhibited low diversity for both bacteria and fungi. Researchers previously indicated that bacterial diversity on the human body could be predicted by the nature of the skin being moist, dry, or oily. In contrast, fungal diversity seems to depend on the location on the human body.
They additionally observed that there was greater similarity in the fungal community structure on the left and right sides of the same individual compared to different body parts of any two different individuals. The fungal populations also appeared to be quite stable over time, showing very few changes between two separate tests conducted three months apart.
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“The data from our study provide a baseline for normal individuals that we have never had before,” Dr. Segre said. “The bottom line is that your feet are filled with fungi, so tiptoe in the locker room if you don’t want to mix your foot fungi with someone else’s.”
