The Godzilla Bug
A couple weeks ago, I was rooting around in different journals searching for the latest papers on virulence factors when I stumbled upon something that was so different and strange that I just had to sit down and say, “Huh?” This something in question was a paper by Angert and Clements on the reproductive weirdness of Epulopiscium, a bacterium that I had never heard of before.
When most people think of bacterial reproduction, they think of the asexual kind called binary fission. In a way, it’s like when your somatic cells undergo mitosis, the DNA replicates and then the cell divides forming two daughter cells. However, there are ways in which bacteria can exchange genetic material instead of making clones of themselves. In a transformation, you can shock the bacteria via a heat bath or electroporator into taking up foreign DNA. Or it could involve conjugation (“bacterial sex” if you want to get giggles from the undergrads) where one bacteria forms a pilus to another bacteria to exchange DNA. And yet a third way would be transduction where a virus is used as an intermediary (during an infection, the virus may take up some bacterial DNA so when it goes to infect another cell, that DNA will be transferred). Epulopiscium, on the other hand, gives birth to live offspring.
Okay, we’ll eventually get to that, but first let’s just consider, what is Epulopiscium?
The first species, Epulopiscium fishelsoni, was discovered by Lev Fishelson and colleagues while they were working on identifying the gut microflora of a surgeonfish species (Acanthurus nigrofuscus) in the Red Sea in the early 1980s. But among the gamut of symbiotic unicellular prokaryotes and eukaryotes, they found a gigantic cigar-shaped microorganism that could easily be seen just with a light microscope. This microbe, which was initially tagged as a protozoan, was covered in cilia and flagella which helped it to move around. Other Epulopiscium species were soon found as symbionts in other surgeonfish around the world.
To get a sense of how large Epulopiscium is, we should compare it to the “average” bacteria which can range from 0.2 to 1.5 microns. You would definitely need a microscope to see that. Epulopiscium is 250 microns long–large enough to be seen by the naked eye. But how can they get so large? A size of a cell is limited by how much material can diffuse or be transported through the surface to the interior. So a cell reaches a threshold when the interior or the volume of the cell increases faster than the surface area (consider the equations for the surface area and the volume of a sphere–the volume increases by r3 while the surface area only increases by a factor of r2). Epulopiscium gets past this rule by wrinkling or folding in its membrane to increase its surface area–not unlike the folds of mitochondria or the wrinkling of the brain.
Scientists, though, began to reconsider Epulopiscium classification. Was it really a protozoan? Pace et al. checked this by comparing genetic sequences rather than cell morphology. As a result, not only did they prove that Epulopiscium was a bacterium and not a protozoan but that it is closely related to another symbiotic bacterium, Metabacterium polyspora, which is found in the guinea pig gut as well as the more common soil bacteria Clostridium. This is going to be important later on.
So back to Epulopiscium and its unusual life cycle. Currently, it’s unknown how the bacterium is transmitted from fish to fish–it hasn’t been found in sea water or fish eggs or fish feces. The bacterium also follows a circadian cycle. During the day, it moves along the surgeonfish’s digestive tract, following the food and growing. At night, the bacterium reproduces giving rise to smaller cells which can go through the cycle again the next day when the fish feeds. In various species of Epulopiscium, the mother cell gives birth to one to seven daughter cells. The offspring grow inside the parent until they are so large that they burst through, killing the mother.
How did Epulopiscium come up with this bizarre birthing process? Its closest relative, Metabacterium polyspora, doesn’t do such a thing. But wait, Metabacterium as well as the soil bacteria Clostridium (famous members of this family cause botulism and tetanus), form endospores. Endospores form roughly the same way as Epulopiscium offspring, but they aren’t exactly live. They are dormant forms of the bacteria that are released into the environment when conditions are not optimal but start up their cellular processes again when conditions are favorable. Angert and Clements performed protein and DNA localization studies comparing Epulopiscium reproduction and endospore formation. And interestingly enough, both lined up. So what does this mean? Perhaps bacterial live births derived from endospore formation. Or maybe it was the other way around.
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More (Yet Unrelated) Links:
Takagism. A very cool locked room mystery. I hope the author finishes the next one soon.
4096 Color Wheel. Yep, another color scheme website. You can never have too many of those.
Dialect Survey Results. Geographical breakdowns of what people say and where they say it.