Bacteria in and on your body

(With thanks to Alfonso Jiménez-Sánchez for the article "ADN libre" in his blog "La Ciencia también es Cultura" and to Hans Bergmans, whose comments and corrections I gratefully incorporated)

Many different bacteria (microorganisms) inhabit all those parts of the human body that are in frequent contact with the outside world. Under certain health conditions some bacteria (so-called pathogens) can cause illnesses, but most of the time they live with us in harmony, providing functions that are vital for our survival (like food digestion and breaking down poisons).

Studies have estimated that there are about 10 bacteria for every human cell in our body. But how many cells do we have? This is difficult to determine. If our body would contain 10 trillion cells (10 x 10¹²) the weight of 100 trillion bacteria (average weight = 5 x 10^-12 gram) would be 500 gram or half a kilogram. There are also estimates that the total weight of bacteria in and on the body of an adult person amounts to 1 to 3 kg.

 

A bacterium from our gut, like Escherichia coli, can have many different sizes and shapes. These bacteria can weigh between 0.5 to 10 x 10^-12 gram depending on their growth condition.

Previously a few hundred bacterial species were isolated from the human body. In 2012, researchers from the "Human Microbiome Project" published reports describing 10,000 species in our body, based on analysis of  their DNA. The vast majority of these species have not yet been cultured. Taken together their DNA codes for some 8 million unique proteins, that is 360 times the number of proteins coded by our own DNA. Many of those proteins help us to digest and absorb the food we are eating. But, in addition, the bacteria produce compounds like vitamins that our genes cannot produce (genes are "instruction manuals" present in every single cell of our body; they direct the building of proteins, the "machines" that make our body function).

When we eat our food consisting of plant and animal tissue, we eat cells that contain DNA. This DNA is broken down into small fragments. According to a recent study (Spisák and others, July 2013) some of these fragments can contain genes and can end up in our blood stream (see also Alfonso Jiménez Sánchez, who pointed out this article to me in his article "ADN libre").

The article of Spisák in PLOS ONE (July, 2013)

How should we read such a difficult, technical article? Either, as a human being with deep-seated fears for the uncertainties life brings us, or, as a skeptic who accepts the uncertainties of life, but does not immediately accept the remarkable results in this article?

The first category is well supplied by, for instance, anti-GMO web sites (links 1 and 2) like ISIS (Institute of Science In Society), in which "the hazards of GM and other unknown nucleic acids introduced into the human food chain by GMOs" are emphasized. The second category is supplied, for instance, by a pro-GMO blog "The skeptical Raptor", in which it is pointed out that this mystery has not yet been confirmed by an independent study and that, if it is confirmed, it must always have been so. (See also "The Conversation".)

There is a strange tendency amongst critics of genetic engineering, to assume that the DNA that has been added or changed in a transgenic organism (GMO) has a special status amongst the vastly larger amount of DNA that is naturally present in the organism. The transgenes do not differ in physical and chemical properties from the DNA of other organisms. It is likely that fragments of DNA of any organism that we eat will find its way into our body. In fact, during millions of years of evolution we have developed as organisms that, for our maintenance and multiplication, have to rely on eating other organisms, and thus on eating their DNA. Vast amounts of DNA have entered our system, but there has never been any evidence that this DNA is doing something wrong. No form of cancer has been identified that results from the plant or animal DNA we are eating. Why would the transgenic DNA in a trangenic organism then behave differently?

This last question will certainly not take away the anxiety in people who do not like to live with uncertainties.