Superbugs: Vertical Gene Transfer

"Superbugs," or antibiotic-resistant bacteria, have been in the news a lot lately. These types of bacteria can cause infections that are very difficult to treat since they are not killed by conventional antibiotics. While most of them can be eradicated, it requires very powerful (and costly) antibiotics. And most terrifying, we play a role in creating these superbugs. To see how, we first need to understand how bacteria reproduce and how they adapt (and share that adaptation to their surrounding buddies).

First, let's look at vertical gene transfer - how genes are passed from one generation to the next. For both bacteria and humans (and all other animals made of cells), genes are passed down to generations through DNA, the helix-shaped molecules that tell the protein-making machinery in cells what to build. Bacteria, made up of only one cell, have their own DNA and reproduce by dividing into two sister cells, each with the same DNA. In this way, DNA is passed on to the next generation as an almost perfect copy. Even humans started off very much like this single-celled bacteria: When we were but a tiny fertilized egg in our mother's womb, we were just a single cell that then split into two, then four, then eight cells, each with the same DNA. While humans then develop specialized cells that then become our trillion-celled bodies, bacteria are single-cell organisms, and their life cycle is limited to single cells splitting into more single cells, over and over again. Each split is a new generation.

Whereas humans reproduce through combining DNA from the parents, bacteria cells reproduce by creating clones. They divide their chromosomes in half, then each half scurries to opposite ends of the cell, which then divides into two cells, which then allows the chromosomes to reorganize into pairs again. This process does not result in perfect copies, however.

In bacteria, the rate of errors in this DNA copying is pretty high, much higher than in humans. But these errors pay off in the long run for bacteria, since some of those errors result in characteristics that help them survive better. If that error gives them a unique ability to survive their environment, including antibiotics, they pass that error, now a strength, on to future generations. For example, if a random error, called a mutation, enables the bacteria to withstand a certain antibiotic, that bacteria will pass along that mutation to its "daughter" cells upon division.

The lifespan of a single bacteria cell is short enough that scientists can observe their genetic changes over hundreds of generations. These observations have revealed that bacteria are able to adapt (become stronger warriors, or "superbugs") very quickly to their enemies, both other organisms and the environment itself. This knowledge has allowed researchers to identify the role we play in enabling these superbugs to evolve.

In next week's post about superbugs, we'll look at horizontal gene transfer, the way genes can pass on their superpowers to each other (spooky!), and then we'll wrap up superbugs with the human actions which facilitate the evolution of superbugs in two categories: Things we can't change, and things we can change.