The secret life of mites in our facial skin

Microscopic mites that live in human pores and mate on our faces at night are becoming such simplified organisms due to their unusual lifestyle that they could soon become one with humans, new research has found.

The mites are passed on during childbirth and are carried by almost everyone, with numbers being highest in adults as the pores enlarge. About 0.3mm long, they reside in the hair follicles on the face and nipples, including the eyelashes, and eat the sebum naturally released by the cells in the pores. They become active at night and move between the follicles to mate.

The first-ever genome sequencing study of the D. folliculorum mite found that its isolated existence and resulting inbreeding causes it to shed unnecessary genes and cells and move towards a transition from external parasites to internal symbionts.

dr Alejandra Perotti, Associate Professor of Invertebrate Biology at the University of Reading, who co-led the research, said: “We found that these mites have a different arrangement of body part genes than other similar species as they adapt to sheltered living in pores. These changes to their DNA have resulted in some unusual body traits and behaviors.”

Detailed analysis of Demodex folliculorum DNA revealed:

• Due to their isolated existence, without contact with external threats, without competition for hosts and without encounters with other mites with different genes, genetic reduction has meant that they have become extremely simple organisms with tiny legs supported by only 3 unicellular muscles are driven. They survive on the minimal repertoire of proteins – the lowest numbers ever observed in this and related species.
• This gene reduction is also the reason for their nocturnal behavior. The mites lack UV protection and have lost the gene that makes animals wake up to daylight. They are also unable to produce melatonin – a compound that keeps small invertebrates active at night – but they can fuel their nocturnal mating sessions with the melatonin secreted by human skin at dusk.
• Their unique gene arrangement also leads to the mites’ unusual mating habits. Their reproductive organs have moved forward, and males have a penis that projects up from the front of their bodies, meaning that when mating, they must position themselves beneath the female and copulate while both of them cling to human hair .
• One of their genes is inverted, giving them a peculiar arrangement of mouth appendages that protrude particularly for gathering food. This helps them survive at a young age.
• The mites have many more cells when they are young than when they are adults. This contradicts the previous assumption that parasitic animals reduce their cell number early in development. The researchers argue that this is the first step in making the mites symbionts.
• The lack of potential mates who could add new genes to their offspring may have put the mites on course for an evolutionary dead end and eventual extinction. This has previously been observed in bacteria living in cells, but never in an animal.
• Some researchers had hypothesized that the mites don’t have an anus and therefore must accumulate all of their feces throughout their lives before releasing them when they die, leading to skin inflammation. However, the new study confirmed that they do have anus and have therefore been unfairly blamed for many skin conditions.

The research was led by Bangor University and the University of Reading in collaboration with the University of Valencia, the University of Vienna and the National University of San Juan. It will be published in the magazine molecular biology and evolution.

dr Henk Braig, co-lead author from Bangor University and the National University of San Juan, said: “Mites have been blamed for a lot of things. The long association with humans may suggest that they may also play simple but important useful roles, such as keeping the pores on our face unclogged.”