In 2018 Kourosh Lanater-Zadeh, an engineer with the University of Melbourne, Australia and his colleagues developed an electronic pill which you can swallow.

As it meanders its way through your stomach and intestines, it measures the gases found along the way until it is expelled. The results show gases differ by individuals and different diets modify the environment in our stomach.

The conclusions are not new, but the fact that we can monitor them in real time opens up a unique opportunity in the science of gut bacteria.

Carolyn Bohach, a microbiologist at the University of Idaho, estimates that there are 10 times more bacterial cells in your body than human cells. Smaller than human cells, they weigh only one to three per cent of our body weight, but these 500-1,000 different types of bacteria have evolved with us for millions of years. We have co-evolved.

Although bacteria are all over our body – inside and out – it seems more crucial in the human gut. As we age, there are fewer physical changes in our gut than any other system in the body. Although the stomach loses its elasticity and might be more prone to damage – primarily as a result of taking some medications – the small and large intestine, pancreas, liver and gall bladder change minimally with age. The main changes in our stomach come from bacteria that inhabit this internal world.

In the gut there are 100 trillion microorganisms that engage in fermenting, killing off other harmful bacteria and viruses, enhancing our immune system and producing vitamins and hormones. This bacterial activity is so necessary to the body that they function as an independent organ – a virtual, ‘forgotten’ organ.

Here, bacteria help extract energy and nutrients from our food and maintain order. This sharing of benefits shows in experiments where bacteria-free mice have to consume nearly a third more calories than normal mice to maintain their body weight. Less well understood is the role of fungi and protozoa that are also part of this hidden world in our gut.

There are 10 times more bacterial cells in your body than human cells

In 2012 Marcus Claesson and Ian Jeffery from University College Cork in Ireland and their colleagues reported this world in our gut changes among older adults. The authors suggest that changes in bacteria cause frailty and mortality. They found that older adults in nursing homes or hospitals have different gut bacteria than those living at home.

There are good and bad bacteria. And they related this change in bacteria to more bad bacteria – caused by having a less healthy diet – to becoming physically weaker. But even age by itself seems to change the bacteria.

In a unique study, Elena Biagi and her colleagues with the University of Bologna in 2016 looked at the stomach bacteria of semi-supercentenarians – those 105 years and older – and found numerous beneficial good bacteria.

The bacteria they found was different between these semi-supercentenarians and younger (99-104 years old) individuals. Biagi’s work reported this ‘invasion’ of bacteria from other parts of the body. Extremely long-living people seem to experience an increase in several health-promoting bacteria that help the immune system, protect against inflammation and promote a healthy metabolic balance. But there is a question puzzling scientists. Have these bacteria stayed on from birth or are they new bacteria? The answer is likely to be both.

When (and perhaps where) you are born determines the bacteria in your body. But this is influenced by our behaviour and diet. With increasing age bacteria seems to change as well, influenced by bacteria from other parts of the body, especially the mouth. Gum disease allows bad bacteria to migrate to our stomachs. Whether these bacteria changes promote or demote health is determined by still-unknown factors.

Bacteria reside everywhere on the body and in large numbers in the lungs, mouth, skin and sex organs. Recent evidence suggests that the age-related changes in bacteria in the gut and lung may also influence health in other parts of the body. Of particular interest is the finding that poor oral hygiene results in oral bacteria invading the heart as well as the brain.

We are just now looking at the body not just as human but also as alien. By opening our eyes we see that we live with other organisms, that we depend upon them as much as they do us, then we see our biology from a different angle. We also start to see how we age, as orchestrated by larger events rather than just our genes. Which are also partly bacteria.

First discovered by Barbara McClintock in the 1940s, bacteria that ‘exchanged’ their genes with humans were known as ‘jumping genes’. Even when these bacterial genes have become human, so to speak, they move around the chromosome to find the best fit. Sometimes they even jump across to other chromosomes within the same genome.

In the 1940s and 1950s this idea – which became known as plasmids or transposons – was met with ‘puzzlement, even hostility’ from the scientific community. But by 1983, McClintock’s work was recognised for its brilliance, and she was awarded the Nobel Prize for Medicine.

It took 40 years to recognise the influence of bacteria on genes. We are just now starting the journey to understand how bacteria affects ageing. But one thing is for certain, we are not on this journey alone.

Mario Garrett was born in Malta and is currently a professor of gerontology at San Diego State University in California, US.

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