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Beyond Binary Fission: Some Bacteria Reproduce by Alternative Means
Some gram-positive bacterial symbionts form multiple offspring intracellularly in a process resembling endospore formation
Esther Angert

any bacteria reproduce by binary fission, with each cell doubling in size, replicating and segregating its genetic material, and dividing to form two equivalent daughter cells. However, somebacteria follow alternative reproductive strategies, and researchers are steadily gaining insights about some of these other processes. My collaborators and I study two groups of related low-G C, gram-positive bacteria that have the extraordinary capacity to produce multiple offspring inside mother cells. Our findings suggest to us that symbiotic associations drove the development of reproductivestrategies that appear to be based on, and certainly resemble, how endospores form.

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Epulopiscium and Related Intestinal Symbionts from Surgeonfish Epulopiscium spp. are large, bacterial symbionts found in the intestinal tracts of several species of tropical marine surgeonfish belonging

Summary • Some microbes, including two groups of related
low-G C, gram-positive, typically symbioticbacteria, reproduce not by binary fission but by producing multiple offspring inside mother cells.

• Viviparity in Epulopiscium resembles sporeforming steps in Bacillus subtilis.

• In general, growth, development, and reproduction in symbiotic Epulopiscium spp. appear to be coordinated with the feeding behavior of the host.

to the Acanthuridae family. These bacteria tend to be associatedwith herbivorous and detritivorous surgeonfish, and they probably help digest the food that their host fish consume. Epulopiscium cells are cigar-shaped and can reach lengths in excess of 600 m, which is slightly bigger than the period at the end of this sentence. The first examples of these unusual microbes were discovered in the brown surgeonfish, Acanthurus nigrofuscus, of the Red Sea, by researchersLev Fishelson of Tel Aviv University in Tel Aviv, Israel, Linn Montgomery of Northern Arizona University (NAU) in Flagstaff, and Arthur Myrberg, Jr. at the University of Miami in Miami, Fla. These early studies helped characterize this symbiotic association, as well as the peculiar biology and ultrastructure of these microbes. Subsequently, Linn Montgomery and Peggy Pollack of NAU named theintestinal symbionts Epulopiscium fishelsoni and described in detail their unusual mode of reproduction. A later survey of intestinal biota of tropical reef fish from the South Pacific revealed a variety of microbes similar to Epulopiscium, all of which are specifically associated with surgeonfish, according to a team of biologists from James Cook University in Townsville, Australia, including KendallClements (now at the University of Auckland in New Zealand), David Sutton, and Howard Choat. They classified these intestinal symbionts based on cell shape, size, the number of internal offspring they produce, or whether they reproduce through binary fission. They assigned the “A” morphotype to large cells similar to E. fishelsoni that associate with several species of Pacific surgeonfish.

Esther Angertis an Assistant Professor in the Department of Microbiology at Cornell University, Ithaca, N.Y.

Volume 1, Number 3, 2006 / Microbe Y 127

FIGURE 1

Reproduction in the gram-positive symbionts Metabacterium polyspora and Epulopiscium sp. type B.

Putting Epulopiscium into a Phylogenetic Context To place these unusual intestinal microbes in a phylogenetic context Clements, Norm Pace, who isnow at the University of Colorado, Boulder, and I compared the sequences of the smallsubunit ribosomal RNA genes of several large Epulopiscium morphotypes to sequences available in GenBank. According to our analysis, Epulopiscium spp. belong to the low-G C, gram-positive bacteria, a surprising result in itself. Moreover, E. fishelsoni consists of at least two genetically distinct populations of...