Every person has an individual microbial fingerprint. The composition of the microorganisms living in our bodies differs from person to person – and cannot just be transferred or changed. On top of this, the first three years of a person’s life determine their body’s microbial populations over the long term. This short period of time can have far-reaching consequences for our development.

All of the microorganisms living in and on us fall under the term “microbiota”. These organisms can live in our bodies without harming it – often they are beneficial. In other cases they are pathogenic agents. There is currently no complete list of all the bacteria in the human body and their effects. This is because many of these bacteria are difficult to cultivate in the lab, making research on them challenging. However, thanks to modern DNA analysis methods we are able to identify their genetic makeup. Meanwhile “microbiome” has become the term used to describe all of the genes of the microorganisms related to the human body. Since these state-of-the-art methods have only been available for a few years, we are just now understanding, for example, how bacteria influence our bodies.

It is well known that a balanced gut flora is good for digestion. Bacteria also play a decisive role in the way our body develops. In lab trials researchers have bred mice that have grown up completely free of germs. These mice had the identical genetic makeup to mice that had contact with bacteria. However, they were much smaller, their brains were less developed and their heart function was impaired. Bacteria do not directly change our genome; however, they do impact how it is read and used in our cells.

This is how it works: bacteria have a primary metabolism – they process substances from our food into other substances that work in our body. These potentially important signaling substances can be identified by combining predictions from the analysis of the microbiome with lab trials on metabolic pathways. One example is hydrogen sulphide, which is toxic in high concentrations but can be beneficial to human development in low concentrations. This is a signalling substance that is partly produced by bacteria, such as Salmonella, and which promotes blood circulation. This means the substance can have a positive effect on our circulation system.

In periods of inactivity, or when there is an insufficient supply of food, many bacteria carry out a secondary metabolism – producing very different, chemically more complex metabolic products. In many cases, little is known about these processes. We don’t know which bacteria produce which substances, nor do we know the consequences they have for our bodies.

More recent studies have found that there is a correlation between certain bacteria and brain development.  The signalling substances from the gut enter the brain via the bloodstream and nervous system and can potentially lead to depression or neurodegenerative diseases like Parkinson’s. Currently this topic is receiving a lot of attention in the media; however, a lot of research is still required in order to better understand exact correlations. Nevertheless, the subject can be summarized well by the phrase “you are what you eat”. Transcript: Tom Leonhardt

This article appeared in print as part of the series "Context", in which researchers of Martin Luther University explain recent topics in their respective fields, providing background and contextual information.