Image Credit: AJE Terzi, CC BY-SA 4.0, Image Cropped

While many biologists have to spend arduous days in the field, often sweating, getting stuck in mud or bitten by a million mosquitos, Dr. Benjamin Clanner-Engelshofen carries his study species wherever he goes: on his own forehead.

He finished his first PhD in October 2020 and is both a pharmacist and medical doctor, currently practicing at the clinic and polyclinic for dermatology and allergy of the Ludwig Maximilian University Hospital in Munich. His work focuses on the little mites that live on all of us.

Eva Paulus (EP): You study an animal that most people probably think is gross or creepy, how did you get into this topic?

Dr. Benjamin Clanner-Engelshofen (BCE): I have always been interested in model organisms such as Drosophila (flies) or highly resilient animals such as tardigrades (so-called water bears). While I was working at a medical clinic at the beginning of my PhD, I asked if I could look at skin samples to try and find some mites. The results were quite meager – each sample contained either no mites or barely alive ones. It was quite a frustrating start to this project. But then I had the brilliant idea to check my own skin and it turns out I am one of the few young people who have a lot of mites! Since I realized I have so many mites on my face a few years ago, I haven’t changed anything about my skin care routine out of fear they might disappear.

EP: Can you give us a quick rundown on what these mites are?

BCE: They were first discovered by a German scientist, Jakob Henle, in 1841 in human ear wax and assumed to be a tardigrade. Another German scientist, Gustav Simon, correctly identified them as mites and therefore as arachnids, which means they’re related to spiders and ticks. More than 100 years later, in 1963, two species of human mites, Demodex folliculorum and Demodex brevis, were correctly identified and described. Both spend most of their life time in hair follicles: D. folliculorum is found above the sebaceous gland close to the skin surface, and the shorter D. brevis sits deep inside the pores. They are not even half a millimeter in size and live for about 14 days according to the literature. They are described to be human commensals, which is a kind of symbiosis in which the mite has a benefit (in this case shelter and food), while the host neither benefits from the interaction nor is harmed. While I think that a healthy population of mites on your face may actually be beneficial, they can also become “parasitic” if they experience extremely favorable conditions and reproduce rapidly. This can eventually cause skin diseases such as demodicosis or rosacea.

EP: What do they eat?

BCE: There are three hypotheses: First, they might just simply eat the bacteria on your face, such as in pores with acne bacteria. However, they’re also found in sterile pores, which discounts this hypothesis a bit. They might also eat sebum (the oily substance produced by sebaceous glands), but sebum doesn’t contain a lot of the basic vitamins animals need to survive. My theory is that they eat keratinocytes, the most common cells in the epidermis, which would explain why they face downwards and towards the walls of the follicle instead of the hair in the middle.

They eat through extracellular digestion – they excrete digestive enzymes and slurp up the remaining mush afterwards. Something we’re currently looking into is which chemicals they use to eat and why those chemicals don’t seem to affect humans at all. It’s quite hilarious to try and get enough of their saliva to characterize it chemically, we need 30-50 mites to get any results at all.

Further Reading: Efficient isolation and observation of the most complex human commensal, Demodex spp.

EP: So these mites are super tiny – how difficult is it to study their biology?

BCE: We have to improvise quite a bit and we’re often using methods from tardigrade or C. elegans (roundworm) papers, since there is no established methodology for these mites other than the methods we described. For example, one of my papers shows how we can tell how well they’re doing: They sort of inch along like a caterpillar (see a video here), and if you place them on an agar plate you can tell from the prints left in the agar whether they are still moving along normally or slowing down and losing their “inner pressure” (turgor). We can also see that bacteria grow right in the tracks that the mites walked in, so we were able to show which bacteria they carry. We’ve also tested how well they respond to obstacles in the agar, and they are able to go around them very effectively. Something else we’re trying to do is to keep them alive ex vivo, so we’re trying to manufacture tiny pores with nutrients inside.

EP: What can the mites teach us about their human hosts?

BCE: So they’re passed on most commonly from mother to baby, as mothers often have the most direct skin-to-skin contact. Of course, within families and between partners there is also an exchange of mites, but always through smear transferral (don’t worry, the mites don’t crawl up on you from other human beings!). Studying the genome of these mites can teach us a lot about their human hosts – for example, the history of human movement. The mites can prove the out-of-Africa hypothesis, that the earliest Homo sapiens originated in Africa, is likely correct. Studying genetics of the mites is an efficient way to tell us where their hosts are from, as the lineages are clearly differentiated between the continents.

Further Reading: Global divergence of the human follicle mite Demodex folliculorum: Persistent associations between host ancestry and mite lineages

EP: Do other animals also have these mites?

BCE: Yes! Since the mites need hair follicles to survive, only mammals carry mites, and curiously they often also have two different species just like we do. In total, over 140 species of these mites have been discovered so far: on dogs, cats, livestock such as cows, and even on aquatic mammals.

When amniotes first evolved – animals that can lay their eggs on land rather than water – the outermost layer of the skin (stratum corneum) evolved to become more rigid to avoid water loss. Hair then likely developed as a way to protect from the cold as an insulation, and this created a perfect habitat for the mites. The evolution of hair on the predecessors of mammals was around 220 million years ago, and we hypothesize that mites have been with our mammal ancestors for somewhere between 100 and 200 million years.

EP: What is a common myth you encounter about our little friends?

BCE: The most annoying thing to me is that when you google pictures of Demodex, a horrible picture is among the first results that pops up and it isn’t even a mite, it’s actually a moth larvae! But it looks mean and it creeps people out. An easy way to tell that it can’t possibly be a mite is that it has eyes, while mites don’t need eyes inside the hair follicle. Another thing that irks me is that many people are scared of their own mites and want them dead – while I believe that they’re really helping you keep your pores clean from bacteria and fungi, so you should be happy to have them living with you!

Eva Paulus is a marine biologist who heard of Benjamin’s research over a year ago and has been fascinated with this topic ever since. Three hours on Zoom later, all the questions were answered, and hopefully a lot of other people are hooked now as well! For more on Eva’s work, check out her Ecology for the Masses profile, or follow her on Twitter @Deep_Sea_Dirndl.