Illustration of the gut microbiota

The genomes in us

Like it or not, our bodies are teeming with billions of microorganisms

We carry many more genomes than just our own. Thousands of different kinds of bacteria live on and in humans, and there are perhaps ten times as many bacterial cells present as there are cells in our bodies.

The Human Microbiome Project is an international effort uniting the labs sequencing the genomes of these passengers, both welcome and unwelcome. The total number of genes involved is probably more than a hundred times as big as in the human genome.

Meanwhile, the number of studies of what these bacteria do for us – or to us – is growing. Some bacteria are essential, such as those needed for digestion. Some are harmful. Some can be benign but turn nasty.

An example of the last kind is Neisseria meningitidis, which lives in the throats of between one and two people in every ten, mostly harmlessly. But the bacterium (the genome of which was sequenced in 2000) also causes meningitis epidemics. Between January and May 2013, meningococcal bacteria caused 9,249 cases of meningitis across 18 African countries and killed 857 people – more than 9 per cent of those infected.

Much current research focuses on the bacteria in our intestines, most of which cannot live anywhere else. There’s a whole ecosystem in there, and it varies from person to person. A 2012 study showed that the DNA belonging to all the bacteria in one person’s gut microbiome was substantially different to another’s. Variations in this vast population – the gut microbiota – seem to affect our risk of obesity, as well as subtler responses to our diet.

The microbial populations in other areas of our body vary too. One study that sequenced the DNA of bacteria in men’s urine found that men with sexually transmitted infections (STIs) had distinct urine microbiomes when compared to those without STIs.

A growing number of serious medical conditions that were formerly thought to be unlinked to microbes may actually have bacterial culprits. For instance, a gut bacterium called Helicobacter pylori can cause stomach ulcers, and the bacteria in hardened arteries – when disturbed by stress – may play a part in triggering heart attacks.

Lead image:

Illustration © Glen McBeth


About this resource

This resource was first published in ‘Genes, Genomes and Health’ in January 2010 and reviewed and updated in December 2014.

Microbiology, Ecology and environment, Genetics and genomics, Medicine
Genes, Genomes and Health
Education levels:
16–19, Continuing professional development