The word “Epulopiscium” looks like a mouthful, but the roots of the word have an interesting meaning. The first half, “epulo” means “guest of.” If you’re interested in astrology, you might notice that the latter half of the word, “piscium,” sounds a lot like pisces, as in the fish. So quite literally, Epulopiscium are the “guest of a fish.”
Epulopiscium sp. are guests of different types of surgeonfish. The bacterium has different morphologies that live in different surgeonfish based on the diet of the omnivorous fish. The bacteria being discussed in this article are Epulopiscium sp. type B, which is an intestinal symbiont of Naso tonganus (Figure 1), also known as the “Humpnose Unicornfish.”
Originally, scientists thought Epulopiscium was a eukaryote because of its large size and the appearance of organelles contained within it (spoiler alert: the “organelles” are actually the daughter cells). To understand the size of this bacterium, see Figure 3, where it is compared to a Paramecium, which is a eukaryote, and Escherichia coli, which is a prokaryote and also bacteria. Epulopiscium can actually be seen with the naked eye (Figure 4) as the cells range from approximately 100–300 µm in length. Epulopiscium truly are Godzilla in the bacterial world. Elizabeth Hutchison, a professor at SUNY Geneseo and the lead author of the paper which inspired this blog post, stated during a 2021 video, if regular bacteria were the size of a baby, Epulopiscium are the Empire State Building. On top of their size and internal appearance, Epulopiscium exhibits a mode of motility similar to Paramecium, so it's understandable why Epulopiscium was thought to be a eukaryote!
This bacterium cannot even be grown in a lab and exclusively lives in the intestines of N. tonganus. That means scientists either have to go work with them out in the field, or they have to be fixed with chemicals in the field and then bring them back to the lab.
But if this Epulopiscium is so sensitive to different environments, why do we care about it? Besides helping that cutie N. tonganus digest it’s dinner, Epulopiscium is a marvel among the microbial world for its mode of genetic reproduction. They are highly polyploidy, which means they have a ton of chromosome copies.
The study, “Developmental stage influences chromosome segregation patterns and arrangement in the extremely polyploid, giant bacterium Epulopiscium sp. type B,” which is the subject of discussion in this post, was done in joint by the Department of Microbiology at Cornell University in the Angert Lab, and the Department of Biology at SUNY Geneseo. The researchers hypothesized that in such polyploid cells, the locations (loci) of chromosomes in relation to the poles is not as important for the chromosome to function as for smaller cells with less volume and limiting genetic resources that have to completely isolate DNA in order to reproduce. I’d just like to take a moment to recognize the irony in the scientists using fluorescence in situ hybridization (aka FISH) to study a bacterium that can only live in the guts of surgeonfish. This technique is used in a wide variety of studies, but it makes me happy that it is so poetically fitting. FISH was used to locate and track the position of individual chromosomes throughout several developmental stages of this bacterium.
Most bacteria use binary fission to reproduce, so Epulopiscium is an odd occurrence within the bacterial world because it does not. It is fairly common knowledge that a female is born with all the egg cells they will ever have in their body, meaning that a grandmother carries her grandchildren as egg cells if her offspring is female. The way this bacteria divides has a similar reproductive aspect to it. In Epulopiscium, the reproduction is heavily dependent on the poles of the cells. This is depicted in Figure 6, which shows the process of the polar division of the granddaughter cells. The black lines indicate cell outlines and the blue portions show DNA (more on that later). At stage A, the process of two fully formed mother cells within a grandmother cell, dividing at the poles where DNA has collected. Those divisions are engulfed by the mother cell. The grandmother cell begins to deteriorate and a slit forms in the cell wall. Then the two fully formed mother cells emerge, already containing daughter cells of their own. Once they emerge from their mother cell, stage B, the two new daughter cells begin to elongate and grow. When they have grown large enough, as shown at stage C, the process begins again.
Figure 6. The life cycle of Epulopiscium sp. type B. In this illustration, cell outlines are shown in black and DNA is shown in blue.
This process evolved from endospore formation, which my fellow microbiology students will know is a process that many bacteria can use for survival during times of harsh environmental conditions. In Epulopiscium, it has evolved into their primary mode of reproduction.
Now back to that DNA mentioned in Figure 6. The blue portion on that figure corresponds with the lighter portions of Figure 7. Using FISH, the chromosomal DNA was located and highlighted for microscopy imagining. Figure 7 shows that DNA gathering at the poles of the mother cell, and then growing into the new daughter cells after the portions divide and are engulfed. What is most interesting about this study in my opinion is the changes in chromosome replication origins (oriC) spacing in Epulopiscium type B cells throughout development. Every bacteria will have hundreds of thousands of oriC. The average distance between oriC foci in Epulopiscium type B cells is < 1 µm, suggesting that the cytoplasm around the outer edges in Epulopiscium is similar to smaller bacterium in its chromosome packing density. This specifically can be seen in Figure 5 below.
Figure 8. Average distance between oriCs at different life cycle stages. A, B, and C represent the stages previously mentioned respectively.
Figure 8 shows that the spacing of the oriCs is consistent within stages of development, but that each stage has different spacing from each other, indicating that the microbe changes chromosome density as it cycles through its lifespan. Stage A and C were more similar to each other than either was to stage B, but A and C were still statistically different from each other. This essentially means that oriC‐per‐µm3 packing density is a little higher in the earlier stages of offspring development and then decreases once the daughter cells start to elongate. This is important for other large bacterium in the microbial sphere such as Thiomargarita namibiensis, Spirochaeta plicatilis, and Achromatium oxaliferum. We now understand the need for larger bacteria that have a low surface‐to‐volume ratio to keep their chromosomes at the edges of the cytoplasm.
Some questions still remain about our friend, the “Guest of Fish,” Epulopiscium, but there is a great model for future study of chromosomal dynamics in large, polyploidy bacterium found in this article. Understanding DNA replication of bacteria can help us discover new ways they evolve and adapt, their impact on fish health and nutrition, and that in turn can help humans stay protected if need be.
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