Read more: “Secrets of the body“
WELCOME to the body safari. Our destination is the adult female, a human landscape as varied and rich in biodiversity as any you will find. There is far more to explore than on the adult male. After landing on the head, we will work our way down to the feet. The tour – scheduled to take six days – stops at sites of interest, allowing us to explore the three main types of human habitat: oily, dry and moist. The local wildlife includes fungi, viruses and mites, but bacteria are the most common denizens. With hundreds of species, they dominate the skin microbiome – all the microbes and their secretions that live on the surface. There are 1 billion bacteria per square centimetre – more than 1.6 trillion over the 1.8-square-metre surface of the average person – so sightings are guaranteed.
A note of caution. For the micro-Lilliputian traveller, the dangers are considerable. We will face peril at all times, from deadly swamps of toxic bacteria to huge roaming predatory mites. Even when we are past the horrors of the anal-genital region, don’t let your guard down. The dangers of the back of the knee and the fungal kingdoms of the feet are not to be underestimated. Obey your tour guide at all times, and do not leave the path.
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Day 1
The north face of the face
Our first stop is an oily oasis for bacteria – the glabella, or space between the eyebrows. Here we will encounter one of the mainstays of the skin microbiome, Propionibacterium acnes. This rod-shaped creature makes its home in the warm pocket formed by hair follicles, feeding on sebum, a waxy, oily product secreted by the sebaceous glands that helps waterproof the skin.
P. acnes was once thought to cause the angry red spots that are the scourge of many teenagers. Recent sampling and sequencing of bacteria from acne lesions shows otherwise. In fact, healthy follicles harbour only, whereas acne-inflamed ones have a mixture of bacteria (). Here, then, is evidence for the protective role that bacteria can provide for their host: by filling the niche made by the hair follicle, P. acnes prevents more malevolent bacteria from getting in.
We camp for the night on the peak of the nose, overlooking the tear-swept valleys beneath the eyes. We will not venture into the eyebrows or eyelashes, but observe from a safe distance the mites clambering about at the base of the hairs. Mostly nocturnal, these eight-legged slug-shaped monsters are Demodex mites, a relative of spiders. There are two species that live on the human face, bivouacking in the follicles, feeding on sebum, and copulating and laying eggs in the crevices. Growing up to 0.4 millimetres long, they have sharp mouthparts and also feed on our friend P. acnes.
“Observe from a safe distance the mites clambering about in the eyebrows”
Recent research suggests that Demodex mites may contribute to rosacea, in which skin on the face becomes red and inflamed. The purported mechanism is not for the faint-hearted. When they die, the mites disintegrate and their faeces, stored for a lifetime, are sowed into the pores.
Day 2
The war zone of the nostril
Whatever you do, don’t venture too close to the edge of the nostril: you risk being inhaled or jettisoned into oblivion on a breath of hot air. The nostrils form a moist ecological niche completely different to the oily shallows of the brow. They are also strategically important sites of bacterial colonisation and the location of a relentless bacterial war, the outcome of which can have fatal consequences for the human host.
The wildlife of the nostrils is dominated by the lozenge-shaped Corynebacterium, along with various species of Staphylococcus, which bunch together in grape-like clusters. The latter include S. aureus and its notorious antibiotic-resistant strain, MRSA. Under normal circumstances, S. aureus is prevented from establishing itself by a benign resident of the nasal cavity, S. epidermidis. If there is a dip in numbers, however, can invade, but not without a fight. S. epidermidis has a chemical weapon – it secretes a special enzyme – that prevents the intruder from growing ().
Similar battles help keep other pathogens out of the nostrils, including the strain of Staphylococcus that causes pneumonia. Indeed, such conflict is rife across the human surface, with armies of bacteria coordinating their activities through a form of communication known as quorum sensing.
Day 3
Mount Nipple and Armpit valley
Today we journey up the slopes of the breast, then down to the dark depths of the underarm. From the summit of the breast you will be able to view one of the famous spectacles of the upper body: the biological “shooting stars” known technically as dandruff. Formed by slabs of dead skin falling from the scalp, they are the work of a fungus called Malassezia, which feeds on the oils produced by the skin. All humans harbour Malassezia but in some it grows out of control and a by-product of its oily feast, oleic acid, irritates the scalp causing it to dry and flake away.
The areola and the nipple form another specialised habitat. Rich in apocrine sweat glands that secrete a viscous fluid containing proteins and lipids, this is home to a variety of bacteria including species of Staphylococcus and Corynebacterium. Other mammals have apocrine glands all over their body, but humans have them only in a few locations, including the nipples, armpits and groin. The sweat glands everywhere else produce only salty water.
As we descend towards the armpit you will notice the smell. Don’t be alarmed! It is generated by the rich local bacterial fauna as they feast on the apocrine fluid. Rumoured to be attractive to some, nevertheless, humans go to great lengths to expunge these odours, and the regular application of deodorants, antiperspirants and anti-bacterial soaps makes this one of the most dynamic ecosystems on the body.
Day 4
Abseiling in the navel
Next on our tour is the treacherous pit formed by the umbilicus, a warm, moist oasis that usually escapes the ravages of soap. We will abseil a short way into the umbilical cave on spider-silk ropes. Observe the myriad bacteria forming beautiful growths on the cavern walls. , run out of North Carolina State University in Raleigh, took swabs from hundreds of people and after analysing only the first 60 samples have found more than 2300 types of bacteria, many of which were unique to their host. Beware! Some people even harbour Clostridium, the family that includes bacteria responsible for gangrene and botulism.
We camp for the night on the smooth scar tissue left by the caesarean section. Tomorrow we will find out how babies acquire their skin flora – how their surface ecosystem is seeded.
Day 5
The anal-genital badlands
As tourists we don’t have the specialist equipment necessary for a foray into the vagina but we will venture as close as possible. An encounter with the fearsome pubic louse is, mercifully, rare these days but the vagina itself remains densely populated and home to some of the most important microbes of the entire human body.
The keystone species of this ecosystem are rod-shaped bacteria called Lactobacillus– it is no coincidence that they can digest milk. By passing through the vagina, the newborn gets its first inoculation of skin fauna. Babies born vaginally have skin bacteria similar to that living in the mother’s vagina, whereas babies born by caesarean are inoculated by bacteria from the environment (). This may help explain why C-section babies have higher levels of asthma, eczema and obesity than babies born vaginally.
We will not venture to the backside as the latest reports suggest a war has broken out between rival populations of bacteria on the left and right buttock.
Day 6
The final leg
As we shimmy down towards journey’s end, observe the inflammation at the base of shaved leg hairs – fungal and bacterial infections are to blame – and note the odd fungal pustule.
Take care not to fall into the red crevasses on the back of the knee: these are lesions formed by eczema. It can occur anywhere on the skin but loves the flexures at the elbows and knees. Although eczema’s cause is unknown, the presence of S. aureus in the cracked skin is a smoking gun. Around half of people with eczema have a mutation in the gene that produces a protein called filaggrin, which predisposes their skin to dryness. The rest don’t, so there must be something else that encourages S. aureus to “bloom” – perhaps a sudden decrease in other, protective bacteria. There are plenty to choose from. The back of the knee harbours some of the most diverse collections of bacteria on the entire skin surface, as does the heel. Both sites are unstable, with constant invasions and colonisations taking place.
And so to the toes and our drop-off point. Here flaky skin with an angry red flush indicates the presence of the fungus Trichophyton rubrum– the fabled athlete’s foot. The same fungus can also be found in the groin where its handiwork goes by the unappealing name jock itch. It thrives in the moist spaces between the smaller toes, so for safe passage we will avoid these areas.
As our safari ends, we would like to thank you for joining us. We hope that the trip has left you with a better appreciation for the diversity of life on the human savannah.
This article appeared in print under the headline “Skin safari”
