To spice things up a bit, I decided to end this chapter with a brief section on sex in bacteria. In this case, I'm talking about sex in the sense of genetic reassortment — that is, the combining of genetic material from different individuals.
Bacteria don't need sex to reproduce. For the most part, they reproduce by dividing in half. One bacterium makes two bacteria; then these two make four, and so on. But occasionally, a bacterium acquires DNA from other bacteria and combines that DNA with its genome; henceforth, all its descendents have this new combination of genes.
Here are a few interesting tidbits about the product of bacterial sex:
i Bacteria don't seem to be very picky about which bacteria they get the new genes from. Some bacteria even absorb free DNA from the environment and incorporate these genes into their own genomes. This phenomenon is especially puzzling from a fitness perspective, because the free DNA has most likely come from bacteria that died and ruptured, releasing their genetic material into the environment. Because these particular bacteria died, it's not clear why other bacteria would want their genes.
i Bacteria occasionally incorporate large numbers of genes from distantly related bacteria. A trick like this wouldn't even be possible for most organisms, because even if they had a way to transfer some of the genes — from a pine tree to a duck, for example — the resulting combination wouldn't actually be functional. Most organisms have tightly interconnected sets of genes, and it's not possible to just splice into a genome a bunch of genes from some other organism and expect the resulting combination to work. You probably wouldn't end up with a functional organism.
Although scientists don't completely understand the selective forces that favor bacteria's ability to incorporate foreign genes, they have the tools to see its results. With the increase in the use of DNA sequencing — the technique that determines the specific sequence of an organism's DNA — scientists have discovered that the new genes the bacteria acquire often confer new function.
Take the common intestinal bacteria E. coli. You have some of it living in you right now. It keeps to itself and doesn't do you any harm. It's even possible that by taking up space, this particular strain of E. coli keeps more harmful bacteria from taking over your intestines.
But some strains of E. coli don't just sit quietly in your gut; they disrupt the intestinal walls and cause illness. E. coli O157:H7, commonly known as Jack in the Box E. coli, is one such strain, and it's an especially nasty one. The reason this particular E. coli strain causes illness is that it has picked up a variety of toxin-producing genes from other bacteria, including a large cluster from the bacterial species Shigella, which causes dysentery in humans and animals.
E. coli O157:H7 was first noticed in 1982, and the Centers for Disease Control and Prevention estimates that it causes approximately 73,000 cases of illness and 61 deaths in the United States each year.
Scientists don't have a clear understanding of what regulates bacterial sex. But at least in the case of E. coli O157:H6, the new combination of genes seems pretty good from the bacteria's perspective. O157:H7 has been extremely successful. Today, it can be found in most cattle farms and most petting zoos. So remember to wash your hands regularly!
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