Fine Read­ing: In the Com­pany of Cil­i­ates

by Merry

Or­gan­isms such as cil­i­ates that dine daily on bac­te­ria run the risk of get­ting an in­fec­tion. In­deed, cil­i­ates – large, com­plex cells – are host in na­ture to gazil­lion dif­fer­ent bac­te­ria that they ac­quire pri­mar­ily through phago­cy­to­sis at their "mouth." Res­i­dent bac­te­ria are gen­er­ally termed sym­bionts even though only a few of them have been stud­ied enough to re­veal the na­ture of their in­ti­mate re­la­tion­ship. The sto­ries of those cho­sen few are re­counted in an en­gross­ing re­view by Hans-Di­eter Görtz.

Some cil­i­ated pro­to­zoa found in lakes and rivers. Those placed to­wards the top left are ty­pically found in the open wa­ter of lakes; those close to the cen­tre at the base are all anaer­o­bic. The re­main­der are gen­er­ally found in sed­i­ments and de­tri­tus, and at­tached to sub­merged sur­faces (e.g. aquatic an­i­mals and plants). All are drawn to scale (bar at the right-hand side = 1 mm). Source

These ap­pear to gen­er­ally be an­cient re­la­tion­ships, on-go­ing long enough that the part­ners have adapted to one an­other in com­plex ways. For the bac­te­ria, the relation­ship tends to be oblig­a­tory. The cil­i­ates, on the other hand, can mostly get along quite well when cured of their in­fec­tions, at least un­der lab con­di­tions. But, as usual, there are ex­cep­tions. One Para­me­cium has its needs for folic acid met by its Lyticum fla­gel­la­tum sym­biont; sev­eral fresh­wa­ter Eu­plotes species die with­out their Polynuc­leo­bacter nec­es­sar­ius sym­bionts (aka "omikron par­ti­cles"). And some­times, as sug­gested in the case of the endonuc­lear sym­bionts to be fea­tured in our next post, the sym­bionts may be ben­e­fi­cial un­der par­tic­u­lar en­vi­ron­men­tal con­di­tions.

Some sym­bionts run of­fense for their host. Wit­ness the Caed­ibac­ter, first known as "kappa par­ti­cles," that con­vert their Para­me­cium hosts into "killers" that make tox­ins that kill "sen­si­tives" of the same or other strains. Those that kill only dur­ing con­ju­ga­tion are termed "mate killers." Enough said. Oth­ers run de­fense, no­tably the Verruco­microbium that re­sides as an ec­tosym­biont on Eu­plo­tid­ium and was fea­tured in one of our first posts. Those as­so­ci­ated with cil­i­ates that live in hy­dro­gen sul­fide-rich anaer­o­bic en­vi­ron­ments (such as in an­i­mal ru­mens or sewage sludge) are at­tract­ing more at­ten­tion in this era of cli­mate change be­cause many of them pro­duce methane.

Many more fas­ci­nat­ing sto­ries are un­doubt­edly yet to come. Op­ti­misti­cally, the au­thor notes that the lag in their in­ves­ti­ga­tion "may be partly due to the fact that in­tra­cel­lu­lar bac­te­ria could not be in­ves­ti­gated with clas­si­cal mi­cro­bi­o­log­i­cal meth­ods, since they can­not be grown out­side their host cells. With the new pow­er­ful tech­niques for de­tec­tion and phy­lo­ge­netic clas­si­fi­ca­tion such as po­lymerase chain re­ac­tion (PCR) and flu­o­res­cence in situ-hy­bridiza­tion (FISH) now avail­able, this is chang­ing. We can hardly wait."

Our thanks to Mark O. Mar­tin for call­ing this fine pa­per to our at­ten­tion.