Unicellular giants
(May 8th 2008) How does one of the world's largest bacteria, Epulopiscium, attain its relatively enormous size? Melanie Estrella reports that scientists may have unravelled a possible explanation.
Records of any kind never fail to impress us. In biology we tend to have a soft spot for extremes such as the biggest or the oldest. From the world of microbes, the number one ranking for the "Biggest" bacterium is held by
Thiomargarita namibiensis, a unicellular prokaryote visible to the naked eye with dimensions up to 750 micrometers. And second on this list of microbial giants is
Epulopiscium fishelsoni measuring in at upto 600 micrometers. However, this runner-up currently finishes first when it comes to DNA content since the cigar-shaped bacterium possesses an incredible 50,000-120,000 copies of its genes, that is, about 25 times the total amount of DNA in a human cell!
Epulopiscium fishelsoni was first discovered in 1985. It lives in the gut of most surgeonfish species. In 1993, Esther Angert, currently at the Department of Microbiology, Cornell University (USA), proved that the organism was a gram-positive bacterium. Since then she has been brooding over the next question - how could this unicellular bacteria grow to such enormous sizes? In fact,
Epulopiscium can be as much as a million times larger than a bacterium like
Escherichia coli!
In theory, the size of bacteria is limited due to their dependence upon passive diffusion for the passage of nutrients and biomolecules across their membranes. Consequently the total surface area of bacteria needs to be large compared to their volume.
In the case of Epulopiscium, however, its oversized way of life seems possible thanks to its tremendous DNA content: each cell carries tens thousands of copies of its genome. Angert and her team previously showed that large amounts of DNA are located near the membrane and suggested that this peripheral DNA location might help the giants to efficiently maintain a highly localised metabolism in proximity to the membrane despite their low-surface-to-volume ratio.
To find out more about the supersizing mechanism of
Epulopiscium, which still does not exist in lab culture, Jenifer Mendell in Angert's lab has now analysed the DNA from a few thousand
Epulopiscium sp. Type B which populates the bowels of the unicornfish,
Naso tonganus (
PNAS, April 29th, 2008).
Using quantitative single-cell PCR to measure the frequency of three genes,
dnaA,
recA, and
ftsZ, that are present in single copies on bacterial genomes, Mendell discovered that the big bug contains tens of thousands of these genes per cell, indicative that there are thousands of genome copies. Similarly, the tremendous copy numbers positively correlated with the individual size of the bacteria. Hence
Epulopiscium seems to be highly polyploid. But why does
Epulopiscium need that many genomic duplicates?
Mendell and Angert have suggested various explanations. Firstly, polyploidy in
Epulopiscium could be favoured by selective pressures, since it could be advantageous to the fishy host if these supersized bacteria offered a proportionately increased contribution to its metabolism. Alternatively, the extra helping of genes and the increased bacterial size might enhance each bacterium's survival since their huge size serves to prevent their being engulfed and broken down by predatory cells that are quite prevalent in the sugeonfish's guts. Additionally, it could allow the bacteria to move more readily along the fish's intestines towards parts that are richest in food. Furthermore, polyploidy confers higher tolerance to genetic mutations.
Hence, by copying its DNA many thousandfolds, this unicellular, prokaryotic companion of the surgeonfish may in fact be taking advantage of features that are normally reserved for large eukaryotic microbes and multicellular organisms.
Homepage of Esther Angert's Lab
