Ciliate 007

SEM of the ciliate Euplotidium
itoi with bacterial ectosymbionts
on its surface. Courtesy of
Giovanna Rosati.

Certain marine ciliates (i.e., Euplotidium) use devices worthy of James Bond. Or so it looks like. These creatures carry bacterial ectosymbionts belonging to the Verrucomicrobia that have extraordinary attributes. Before revealing what this is about, I need to pause for an admiring gaze at an Euplotidium. What is so startling is that these are single celled organisms with a cellular complexity rivaling that of many a multicellular animal. Legs on a unicellular organism? Actually what look like legs really act like legs. They are bundles of cilia called cirri that permit the organism to walk or crawl on surfaces. Ciliates are like that─remarkable in looks and variety. See where Euplotidium fits in taxonomy.

Back to the bacteria on Euplotidium...

Giovanna Rosati and colleagues at the University of Pisa. discovered that these ciliates carry bacteria on a band in an outer portion of the back. These are not your run-of-the-mill epibionts (organisms that live on the surface of a host): when provoked by as yet poorly identified signals, they become a tethered projectile. Each bacterium eviscerates itself and shoots off a 40-μm long filament. Aggregates of these filaments cover one end of the ciliate with what look like long thin flexible barbs. In the figure, you can see that the filaments of extruded bacteria cover part of the ciliate. Interestingly, in the laboratory euplotidia that carry these symbionts (termed epixenosomes) resist predation by other ciliates; those that don’t are readily ingested. The way protection works is not yet known.

TEM of ectosymbiotic bacteria known as
epixenosomes, on the surface of E. itoi.
Courtesy of Giovanna Rosati.

How does the bacterium extrude its contents? The bacteria on the ciliates differentiate into unusual cells that contain a coiled protein ribbon (reminiscent of unrolled fly paper), surrounded by a basket-like structure made up of tubulin proteins. During extrusion, the bacteria open up and the tube unrolls, carrying on its tip densely packed DNA. This is a unique structural modification, the first of its kind to be described for bacteria (Note that Caedibacter, the endosymbionts of another ciliate, Paramecium, as well as some free-living bacteria, also makes rolls of protein called R-bodies.)

Extrusion of body contents, however, is not news in biology. Various single and multicellular organisms display such mechanisms. Best known are the nematocysts ejected by stinging jellyfish, the occasional bane of swimmers in the ocean. Less appreciated but perhaps more startling is the “gun cell” of the oomycete Haptoglossa mirabilis, which shoots off a whole cell into passing rotifers (more about H. mirabilis some other time, I promise). In these cases the extrusion machinery is made by the organism itself. Instead, the Euplotidium uses bacteria on its surface to provide the extrusion apparatus. This is news.

Many questions are left to answer. How do the barbs protect the host from predation? How does the extrusion of the filament take place? What is the role of tubulin in extrusion? Why is there DNA at the end of the filament? Do other organisms use bacteria for this kind of defense? But, mechanisms aside, isn’t this a handsome story?

Comments

[Hael]

how small is the smallest ciliate? how large is the larget ciliate? {important details}

Elio responds:
Go to this site for an answer
http://www.sizes.com/natural/protists.htm.
Here you;ll see that the largest is many cm long, the smallest about 1 micrometer.

[anonymous]

The idea of Samantha is the most credible on the basis of our present knowledge. It must be also considered that differentiated epixenosomes (form II) are probably doomed to dye when the ciliate host Euplotidium divides. The cell cycle of epixenosomes is well coordinated with that of the host: the number of dividing epixenosomes (form I) increases before the ciliate daughter cell separation, soon after division most bacteria show a morphology half-way between the form I and form II, while during the non dividing stage form II epixenosomes are more abundant then form I, but the total number of symbiont per cell is roughly constant during generations. It is possible that a spontaneous loss (detachment or ejection?) of the old form II epixenosomes takes place. In this case the heroism of epixenosomes would not be so extreme!

[Rosie Redfield]

Bacterial tubulin?

My response:
Indeed! A number of bacterial proteins homologous to tubulins have been found. See Gitai, Z. Curr Opin Cell Biol. 2007:5-12. Diversification and specialization of the bacterial cytoskeleton. This is a hot area of investigation. MS

[Samantha]

Any ideas on what's in it for the bacteria? Do only some of the bacteria "sacrifice" themselves so that the rest of the bacterial population and their ciliate host survive? Samantha.

Samantha: Yours is a fine thought, at least as good as any I could come up with. Anybody else has a better one? Moselio