How human are you?

A core subject in Biology, and a favourite discussion topic for Oxbridge interviewers, is exploring the differences between eukaryotic and prokaryotic cells. Just by reading those words, many of your minds are probably already triggered into listing the key differences you memorised for your exams; difference in cell wall types, different sized ribosomes and so on. What is more interesting, and still very much a hot topic of debate, is what exactly is the interplay between the two kingdoms and cells and what is their impact, particularly in the human body?

The BBC recently reported on new research updating our assessment of the ratio of human to non-human cells in our body. According to this most recent research, human cells make up only 43% of the body’s total cell count. The rest are microscopic colonists. No matter how well you wash, what you eat, or what you do for a living, nearly every nook and cranny of your body is covered in microscopic creatures. This includes bacteria, viruses, fungi and archaea (organisms originally misclassified as bacteria) and the greatest concentration of this microscopic life is in the dark murky depths of our oxygen-deprived bowels. However, simply counting all the individual cells and determining the ratio of human to non-human cells is only one way of looking at the relative contributions of the different cells, and one might argue that from a scientific perspective, it’s a relatively limited one. Indeed, if we were to look at the number of genes contained by human versus non-human cells, one would realise that genetically we’re even more outgunned! The human genome – the full set of genetic instructions for a human being – is made up of around 20,000 genes. However, if we add all the genes in our microbiome together and the figure comes out between two and 20 million microbial genes.

As such, it would be a fair assessment to consider that what makes us human could be the sum of contributions from the combination of our own DNA, plus the DNA of our gut microbes. It would be naive to think we carry around so much microbial material without it interacting or having any effect on our bodies at all. A significant amount of recent scientific effort has focused on uncovering the role the microbiome plays in digestion, regulating the immune system, protecting against disease and manufacturing vital vitamins. Within the world of pharmaceuticals, there is a new interest in understanding how a patient’s gut microbiome may impact or influence their ability to absorb and process ingested medications. Further intriguing results have shown that differences in the gut microbiome of identical twins can result in different levels of weight gain even when fed identical diets, suggesting that we may be able to lay (some!) of the blame for our expanding waistlines on our resident gut bacteria!