When Bacteria Get It On

I've alluded to this phenomenon in the previous blog post about e. coli (check it out b/c it's awesome, like everything I write). It is known as Horizontal Gene Transfer, or HGT for short. There are four processes that are included in HGT which are conjugation, transduction, transposons and plasmids, and transformation. In the world of pathogens this is the way that new traits are obtained quickly and efficiently between organisms and it is probably the most important principle in the quick evolution of pathogens. This exponentially increases the genetic variation within the species of bacteria and produces bacteria with brand new phenotypes we've never seen before.

Let's start with conjugation. Enter Marvin Gaye because they are about to get it on! Ok so bacterial sex may be a bit of an exaggeration but it's easy to see why the process of conjugation has become perceived in this way. In this process a bacteria that contains the "sex pilus," a series of proteins on the bacteria membrane, extends and makes contact with a neighboring bacteria. The pilus can then contract which draws the bacteria closer together and finally, their membranes make contact. Now, essentially their is a direct link between the bacteria called a conjugation bridge. This allows easy passage of genetic material from the donor cell to the recipient and the recipient cell now contains whatever genes were sent through the conjugation bridge.


Plasmids and transposons are transmittable genetic elements that can be transferred through conjugation and really any other process in HGT. Plasmids are uber important...seriously, plasmids run the show in terms of bacterial evolution. These are small, circular bits of DNA that is now contained within the genome of a bacteria. These plasmids usually have genes that allow bacteria to survive in a different niche. Antibiotic resistance is usually conferred to bacteria via plasmids. More importantly, these plasmids are highly stable and easily transferred through the conjugation bridge. The end result is that both bacteria end up with the plasmid! BOOM!



Transposons are like Rick James...super freaky. These things are small bits of DNA that can actually "jump" out of a genome! Yep that's right, these little guys jump out of one bacteria's genome and can recombine into another's, bringing whatever genes it carries along with it. Again, conjugation allows easy transfer through the conjugation bridge. Transposons are pretty nuts and I don't even know the entire details of how these things work, but they are very important in bacterial evolution. In fact, a ton of the human genome has been found to be derived from transposons!



The next method of Horizontal Gene Transfer is transduction. This is the injection of genetic material into a bacteria via a bacteriophage. Those are those weird alien looking things that you've probably seen somewhere before labeled "virus." Yes, those are viruses but they only infect bacteria which is why when you see them in movies infecting humans, it's wrong! After injecting the genetic material into the cell, it can recombine within the genome of the bacteria and it now has whatever genes it gets from the bacteriophage. The shiga toxin from e.coli is one of many genes controlling toxin production that is believed to have entered the bacteria via this method.

Transformation is the last form of HGT. Here, a bacteria becomes "competent," or able to take up extracellular genetic material. What happens here is that bacteria that have holes in their membranes are able to take up genetic material from outside of the cell and then recombine it into their genome to obtain the traits. The DNA is obtained after cells die and rupture, releasing it's genetic material to surrounding cells. It's important to understand however that not all bacteria are able to do this naturally. Labs take advantage of an electric shock to make cells competent so they can insert whatever plasmids or other DNA they want to insert into bacteria. Some bacteria are naturally competent, like neisseria gonorrhoeae (last time I'm spelling that), and can take up DNA if it's available.


Through these methods, bacteria can obtain a wide variety of genes very quickly. Instead of waiting millions of years for evolution to produce better mechanisms of survival, bacteria can obtain new genes and immediately express new phenotypes without having to wait. This has been a scourge to the medical field due to the increase in antibiotic resistance in bacteria seen in hospitals. What's awesome here is that genes can cross genus and species lines from bacteria to bacteria, an impossible feat for humans. Imagine how awesome it would be to be able to exchange DNA with tigers, wolves, or birds and be able to recombine and express those genes! People could have bird wings, tiger strips, and wolf fur!