Sunday, February 28, 2010
Symbiotic bacteria found to defend its host with cell structures previously thought to be exclusive to more complex organisms
The ciliate Euplotidium has what appears to be symbiotic bacteria on its surface that defend it against predators with extrusive spears
These bacteria were found to contain structures that react to tests for eukaryotic microtubules.
However, microtubules had never been found in bacteria before, and are in fact one of the defining features of the more complex kingdom of organisms called eukaryotes (animals, plants and protozoa), serving to organize their more complex cell structures and processes such as the dance of mitosis, another defining feature exclusive to eukaryotes.
  
If these turn out to be microtuble containing bacteria, the discovery would be a major one for biology. The Eukaryotes likely evolved from the bacteria and their relatives. While we understand some of the steps, so far we have no clues as to the origins of the complex strtuctures and processes involving microtubules.    
Euplotidium arenarium and E. itoi are marine hypotrich ciliates (eukaryote) found in tide pools along rocky shores. The defensive bacteria, called epixenosomes, are arranged in a distinct band around the host cells.
There are many cases of bacterial ectosymbionts on eukaryotes, and these at first seemed to be another case.
But probes designed to detect bacterial ribosomal RNA genes (rRNA) wouldn't bind these epixenosomes. This, together with the fact that they contained structures that reacted to tests for eukaryotic microtubules, led biologists to conclude that they were of eukaryotic origin and not bacteria.
The life cycle of these ectosymbiotic bacteria are keyed to the life cycle of the host ciliate. They start off looking like typical small spherical bacteria, then they develop the complex coiled ribbon structures.
When the predatory ciliate Litonotus attempts to capture and ingest Euplotidium cells, these coiled ribbons spring open to form filaments up to 8 times longer than the epixenosomes, repelling the predator. When the epixenosomes are experimentally removed, Litonotus has no trouble ingesting Euplotidium.
These defensive structures are different than the trichocysts typically found on ciliates.
  
In a recent study Giovanna Rosati and coauthors make a strong case that these bodies are bacteria.  She and her team used a novel technique to develop a new bacterial rRNA probe that bind to these bacteria.
For comparison, DNA was extracted from cells of Euplotidium with and without their symbionts. Then both DNA samples were tested for bacterial 16S rRNA genes.
Just as previous attempts to detect these genes in the epixenosomes had failed, so did their initial trials using standard bacterial probes on both DNA samples.
So they modified the standard probe to include gene sequences from the other prokaryotic kingdom called Archaea. With these they obtained positive results from the samples with the symbionts, and not from those without.
They could then create probes attached to fluorescent green molecules to use on complete Euplotidium cells. Their photomicrographs show the bacterial probes binding only to the cortical band of epixenosomes, and their extruded barbs.
When they compared the genes extracted from the epixenosomes to other bacterial genes, they found the closest matches to be in the little known newly discovered bacterial division Verrumicrobia. So far, only two species of bacteria have been identified in this group.
They conclude that these epixenosomes are in fact bacteria.
The relationship to the Verrucomicrobia is interesting because most other bacterial symbionts belong to the large well known division Proteobacteria.
It was also found that these 16S rRNA genes have two mutations not found in the genes routinely used for bacterial probes, and is likely the cause of the previous failure of this probe to bind to these bacteria.
This result points to the possibility that there are other undetected Verrucomicrobia bacteria out there awaiting discovery.
So there is a mystery afoot! One possiblity is that these are still eukaryotic organelles that for some reason contain bacterial rRNA genes.
A more interesting possibility is that these bacteria at some point acquired genes for microtubules from a eukaryote, possibly their host.
But the most exciting possibliity is that these bacteria are ancient relicts of a lineage of bacteria which had something to do with the evolutionary appearance of the eukaryotes, which is an event still shrouded in mystery.
Images by Giavanna Rosati from
 Giulio Petroni, Stefan Spring, Karl-Heinz Schleifer, Franco Verni, and Giovanna Rosati (2000) "ectosymbionts of Euplotidium (Ciliophora) that contain microtubule-like structures are bacteria related to Verrucomicrobia", Proceedings of the National Academy of Sciences, 97 no. 4, 1813-1817
 Ahmadjian, Vernon and Paracer, Surindar (1986) "Symbiosis an introduction to Biological Associations" pg 117
 Harumoto, T. & Miyake, A. (1991) J. Exp. Zool. 260, 84-92[ISI].
 Knoll, G., Haacke-Bell, B. & Plattner, H. (1991) Eur. J. Protistol. 27, 381-385[ISI].
 Miyake, A. & Harumoto, T. (1996) Eur. J. Protistol. 32, 128-133[ISI].
 Bermudes, D., Hinkle, G. & Margulis, L. (1994) Microbiol. Rev. 58, 387-400
 Alberts, Bruce et. al. (1994) "Molecular Biology of the Cell", 3rd ed. pg 23-24, pg 787
 Margulis, Lynn and Karlene V. Schwartz (1998) "Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth" 3rd ed. pg 10-14
 Alberts (1994) pg 17-21
 Ahmadjian (1986) pg 57-62
 Rosati, Giovanna (2004) "Ectosymbiosis in Ciliated Protozoa"