Epulopiscium spp. are large, bacterial symbionts found in the intestinal tract of certain species of tropical marine surgeonfish (Family Acanthuridae). They may play a role in digestion of food consumed by their fish host and tend to be associated with surgeonfish species that primarily eat algae or detritus. The largest Epulopiscium cells can reach lengths of 600 µm (0.6 mm) or more. These unusual microbes were discovered by a group of biologists studying Brown Surgeonfish (Acanthurus nigrofuscus) of the Red Sea . Their work characterized this symbiotic association and examined some of the peculiar biology of these exceptional microbes. In a follow-up report, the Brown Surgeonfish intestinal symbionts were named Epulopiscium fishelsoni. "Epulopiscium" means "guest at a banquet of a fish" . The authors also described in detail an unusual mode of cellular reproduction used by E. fishelsoni in which multiple offspring cells form inside a mother cell.
An extensive survey of intestinal biota of tropical reef fish caught in the South Pacific revealed a variety of microbes similar to E. fishelsoni, all of which were associated with surgeonfish . These intestinal symbionts were classified based on their cell shape, size and their method of reproduction. Some cells appeared to reproduce by forming multiple internal offspring, some cells used binary fission and some used a combination of both of these methods. Large cells, similar to E. fishelsoni, were found in several species of Pacific surgeonfish (including A. nigrofuscus and A. lineatus), and were classified as the "A" morphotype.
Small subunit ribosomal RNA gene sequence comparisons were used to place these usual intestinal microbes in an evolutionary framework . These analyses demonstrated that the largest Epulopiscium spp. belong to the low G+C Gram-positive bacteria (also known as the Firmicutes) and that E. fishelsoni is comprised of at least two genetically distinct populations of closely related bacteria. Additional analyses have shown that the large and morphologically diverse bacterial symbionts of surgeonfish form a genetically diverse group within the Firmicutes.
As we continue to study the lives of Epulopiscium spp. and related surgeonfish symbionts we are characterizing a variety of life cycle strategies used by different lineages. Our observations indicate that some features and mechanisms are conserved between live intracellular offspring formation in Epulopiscium spp. and endospore formation in Bacillus subtilis.
Finally, Epulopiscium spp. are among the largest known bacteria. In terms of cell volume, the largest Epulopiscium is a million times bigger than a bacterium the size of Escherichia coli or Bacillus subtilis. Simple cellular modifications appear to help these cells attain their enormous size.
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