Cil­i­ate 007

by Elio

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.

Fig­ure 1. SEM of the cil­i­ate Eu­plo­tid­ium itoi with bac­te­r­ial ec­tosym­bionts on its sur­face. Cour­tesy of G. Rosati.

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.

Fig­ure 2. TEM of ec­tosym­bi­otic bac­te­ria known as epixeno­somes, on the sur­face of E. itoi. Cour­tesy of G. 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; we fea­ture R‑bodies here in STC.

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?