Epulopiscium, a giant among bacteria

The size of a cell is limited by the ability of nutrients and gasses to diffuse in and waste products to diffuse out, as well as for the cell constituents to get to where they are needed. The diffusion problem can be solved by adjusting the surface to volume ratios, and very thin or very flat cells could in theory be infinitely large, but this only makes the internal localization problem worse. Eukaryotic cells are 1-2 orders of magnitude larger that most prokaryotes, and they have managed to achieve these sizes by developing sophisticated nutrient uptake systems, subcellular compartmentalization, and the use of a cytoskeleton to transport vesicles and proteins around the cell. Large bacterial cells maintain high surface-to-volume ratios by being long and slender, or if spherical, contain an intracellular vacuole to press the cytoplasm into a thin layer just under the outer membrane. Some big prokaryotes also have a type of cytoskeleton and extensive intracellular membranes reminiscent of eukaryotic cells.

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The genus Epulopiscium is an unusual group of cigar-shaped Gram-positive organisms which live in the guts of fish. The name Epulopiscium means “guest at a fish’s banquet”. That may not be your idea of a cozy nest, but they seem to do very well there, growing up to nearly a millimetre in length, big enough to see with the naked eye - about the size of a grain of salt and a million times bigger than Escherichia coli. Why does a bacterium want to be so big? Possibly to escape being eaten by protists, which tend to be quite fussy about the particle size of the organisms they consume. While most bacteria reproduce by binary fission (dividing into two equal-sized daughter cells), Epulopiscium species produce offspring internally, usually two, one at each end of the cell. These new cells grow within the mother cell’s cytoplasm until it eventually bursts open and releases them.

You might think that such an unusual bacterium would have an unusual genome, but the fact that no Epulopiscium species currently grows in laboratory culture has slowed down research into this area. A recently published paper gets around this problem by using quantitative PCR to count the copy number of genes in individuals and in DNA extracted from populations of Epulopiscium cells isolated from the intestinal tract of the unicornfish Naso tonganus - now that’s what I call an ecological niche (Extreme polyploidy in a large bacterium. 2008 PNSA USA 105: 6730-6734). The results show that Epulopiscium is highly polyploid throughout its life cycle, and that individual cells contain tens of thousands of copies of its genome. A single Epulopiscium cell may contain up to 250 picograms of DNA, a massive amount compared with 6 picograms of DNA in a human cell, and this represents 50,000-120,000 copies of the genome. Genome copy number is positively correlated with cell size, with the largest cells containing the most DNA. Although other bacteria are known to possess multiple genomes, polyploidy of the magnitude observed in Epulopiscium is unprecedented. The arrangement of the genomes around the cell periphery may permit regional responses to local stimuli, helping Epulopiscium to maintain its unusually large size. By copying its genome thousands of times and arranging it just under the cell membrane, Epulopiscium may be more able to respond quickly and locally to stimuli which come in contact with the cell. This arrangement may give an Epulopiscium cell the advantages of social microbes, with additional benefits such as exceptional motility and enhanced resistance to predation normally found in large eukaryotic microbes or multicellular organisms.

Related:

• Thiomargarita namibiensis, the largest known bacterium