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.
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?