From eyelash mites to friendly viruses

It may look like an alien invasion but this is something that happens regularly inside our bodies It may look like an extraterrestrial Moon landing or something from a science-fiction film, but in fact the image below shows viruses called phages attacking bacteria – something that happens regularly inside all of our bodies, aiding our immune system.
These are just some of the 100 trillion or so non-human organisms living on or in the human body that do us no harm and even help us stay alive. So just what are these alien invaders and how do they benefit our health?
Bacteria are responsible for unpleasant diseases, from tetanus to tuberculosis. But the bacteria that reside in the human stomach and intestines help us to digest difficult-to-handle foods such as carbohydrates by breaking them down into smaller molecules.
They also keep out dangerous bacteria such as Clostridium difficile (C.diff) by covering the gut surfaces the unwanted bacteria might otherwise breed on.
The appendix, often mistakenly thought of as a useless appendage, acts as a kind of bacterial reservoir where they can rest and recuperate.
Your body even produces special antibodies, defensive chemicals that usually have the role of attacking invading microbes, to help the bacteria.
The antibody IgA (immunoglobulin A) forms a supporting structure to anchor the helpful bacteria in your stomach and prevent them being washed away.
Most cells in the body contain mitochondria. These minuscule pods, about a 50th of the width of a human hair, are sometimes called the cell’s power plants, as their job is to store the energy we get from what we eat.
They combine oxygen with the glucose and other substances your digestive system derives from food to produce a chemical called ATP.
The energy that will then be used to power your body – whether it’s to make your heart beat or your muscles flex – comes from this compound.
The molecule has bonds between atoms that are easily broken to give off energy, the chemical equivalent of releasing a coiled spring.
The most remarkable thing about mitochondria is that they appear to have once been bacteria that became part of the cell in a mutually beneficial relationship. They are structurally very similar to bacteria and have their own DNA. There is even a common bacterium called SAR11 that is so genetically similar that it is thought to share an ancestor with mitochondria.
Though mitochondria were once a separate species, they are now an essential part of your body.
Viruses cause everything from the common cold to HIV and smallpox. However, some viruses that may infect our guts or wounds are natural medics – these are bacteriophages, or phages for short. Where the viruses we dislike destroy human cells, phages, which look like miniature Moon landers, attack bacteria. They inject their own DNA into the bacterium, using it as a host to produce more of the virus, and destroying the bacterium in the process.
Phages enter the body from the air or by contact, like any other virus. We catch them all the time, and they don’t last long in your system as our own immune cells recognise and eliminate them. By that time they have reproduced and been passed on, and so the cycle continues. They can also be artificially cultured in a lab and introduced into the body as a vaccine.
Infections such as MRSA are increasingly becoming resistant to antibiotics; one hope in the war against these diseases is to use phages.
Put an eyelash or eyebrow hair under a microscope and you may get a surprise. About half the population have tiny transparent creatures about a third of a millimetre long called eyelash mites living on old skin cells and sebum (skin oil) at the base of these hairs. But unlike lice or fleas, they do little harm.
Though they cause an allergic reaction in a few people, most of us will never know they are there, scavenging dead skin and excess oil like the pilot fish that swarm around sharks to pick off their parasites.
We pick up eyelash mites by direct facial contact with other people – and they seem to prefer older hosts because they tend to have more oily skin.
The mere thought of worms makes most people shudder. Yet for the majority of our existence, all humans have had worms, and our bodies have developed defences to minimise their impact.
Science is at an early stage but there is strong evidence that conditions that increased hugely in frequency during the 20th Century – specifically autoimmune diseases such as Crohn’s and some allergies – may be an over-reaction from our bodies because our immune system expects to have to fight against worms that are no longer present.
Research at University College London suggests that appropriate use of worms can produce a more controlled immune response that is less likely to damage the host human.
One of the most recent trials of therapeutic worms (known as helminthic therapy) is being run by the University of Nottingham to monitor any reductions in multiple sclerosis symptoms after infection with a type of hookworm. Many worms are destructive, but it may be that those suffering from these conditions will benefit from the reintroduction of specially selected worms that their bodies treat as natural partners.
lThe Universe Inside You, by Brian Clegg, will be published by Icon Books later this spring.