What is Horizontal Gene Transfer?

Long ago during the early stages of life on Earth, bacteria were trying to solve a difficult problem. Over the course of their lives, certain lucky bacterial genes would evolve mutations which would help them survive in their habitats. However, the only way for these genes to be spread through the population was via reproduction. While this strategy is generally viable, it is not particularly fast, especially if competition for resources is steep. As a result, these genes needed some way to spread to other bacteria.

Horizontal gene transfer is essentially the solution that bacteria came up with to this problem. Instead of spreading genes solely via division, bacteria gained the capacity to transmit and uptake genes from their neighbors. After all, the only way for organisms to improve is via random mutations, which are detrimental way more often than they are beneficial. Development of horizontal gene transfer was beneficial to the bacterial community as a whole, since all of them had the chance to obtain genes that may help in survival.

Equipped with this technology, bacteria were able to adapt to rapidly changing environments much faster. If one member of the community solved a life-threatening issue, it could share the solution with its neighbors, allowing the whole population to thrive. Antibiotic resistance is a good example of this mechanism in action.

Mechanisms of Transfer

There are three ways that bacteria are able to transfer genes amongst each other:
1. Transformation: Transformation involves taking up “naked” DNA from the environment. Naked DNA is basically just genetic information that is floating around in the environment. These DNA molecules begin their journey when bacteria die and degrade or when DNA is excreted by a living bacteria. When these molecules encounter another (living) bacteria, it is absorbed and integrated into the new organism’s genome or into a plasmid.
2. Transduction: This process of DNA uptake is more a game of chance than intentional transfer. When bacteria get infected with viruses (known as a bacteriophage) they are forced to make thousands of copies of the intruder, much like human viruses. Sometimes, instead of wrapping a viral protein around the viral DNA, bacteria accidentally package their own DNA into the shell. This modified virus leaves the cell and infects another bacteria, delivering this random genetic information.
3. Conjugation: Conjugation involves the physical interaction between two bacteria. In this mode of transfer, two bacteria connect with each other using a tube known as a sex pilus. This tube is then used to transfer DNA from one bacteria to another. This transfer is a one way process; some bacteria are recipients while some are donors.

The Microbiome and Horizontal Gene Transfer

Since much of the human microbiome is bacterial, horizontal gene transfer is an essential part of its function. The gut environment is a lot like that of ancient Earth, it is constantly changing and requires constant adaptation. Some days we eat food with lots of fats, carbs, and/or proteins, and some days we eat little to nothing. Sometimes we eat foods that are toxic to bacteria and other days we don’t. All of these changes in the environment place a lot of stress on our microbiome. As a result, horizontal gene transfer plays a big role in maintaining the population of our gut bacteria.

Transfer Among Neighbors vs. Food Bacteria

While all of the bacteria in our gut occupy specific niches, the general context of their habitats is very similar. This means that the most basic needs for most of the bacteria in our gut are similar or identical. As a result, horizontal gene transfer with neighbors has had lots of time to evolve throughout our evolution. The science supports this claim; there is a lot more evidence of DNA transfer between microbes with the gut than between food microbes and gut microbes. This makes sense, since the genes that help food microbes survive in the outside world have little to contribute to the bacteria surviving in our gut.

Gene Transfer Across the Body

While there is little horizontal gene transfer between food borne bacteria and gut bacteria, there is a lot of gene transfer between bacteria on different body regions. For example, when members of the skin microbiome make their way into the gut, gene transfer does seem to be observed. The same is true for bacteria in the airway and the blood. As of right now, there is not enough evidence to hypothesize the purpose of gene transfer across body systems, although it is a very interesting phenomenon to consider.

Conclusion

All in all, the genes that are most often transferred across the microbiome pertain to metabolic processes. This makes sense, since the source of food for these organisms changes many times a day, spending on what you eat. Without horizontal gene transfer it would be much more difficult for these bacteria to maintain a population in the gut. The environment is simply too dynamic for any single organism to stick it out on their own. It is thanks to this amazing evolutionary technology that we are able to live in harmony with our bacterial neighbors.