Why Be In­tra­cel­lu­lar and Be Motile?

by Elio

Why would a strict in­tra­cel­lu­lar or­gan­ism be motile? In the case of rick­ettsiae (and some not strict in­tra­cel­lu­lar par­a­sites such as the lis­te­riae and shigel­lae), motil­ity is car­ried out through the ac­tiv­ity of the host's actin, appa­rently for the pur­pose of in­fect­ing ad­ja­cent cells. I am not talk­ing about that, but rather I will con­sider here the in­trinsic motil­ity of those ex­em­plars of in­tra­cel­lu­lar­ity, the chlamy­diae and the rick­ettsiae. Why would they sprout fla­gella and move around, pos­si­bly ex­tra­cel­lu­larly? But be­fore I will deal with this puz­zle, I want to in­tro­duce as­pects of the chlamy­diae, a group of or­gan­isms that I find par­tic­u­larly at­ten­tion-grab­bing.

Fig­ure 1. Source. Front­page: Source

About the Chlamy­diae

Every­body, es­pe­cially col­lege stu­dents, know about the chlamy­diae for their abil­ity to cause the most com­mon sex­u­ally-trans­mit­ted dis­ease in the US and else­where. But there is much more to this group of or­gan­isms, some­thing that is not al­ways known to many mi­cro­bi­ol­o­gists ei­ther. It turns out that chlamy­diae are nearly ubiq­ui­tous, be­ing found within a large va­ri­ety of an­i­mals and plants. In­cluded are ver­te­brates, such as peo­ple or the leafy sea-dragon and the bar­ra­mundi, in­vertebrates like the sweet potato white­fly, and many pro­tists. Free-liv­ing amoe­bas are fre­quent hosts for chlamy­diae and their ilk, which are the best stud­ied of the en­vi­ron­men­tal chlamy­diae. In ad­di­tion, chlamy­dial rRNA se­quences have been found in a re­mark­able va­ri­ety of habi­tats, such as soils, sed­i­ments, hy­drother­mal vents, sludge, plus, plus. In other words, they seem to be every­where. They are surely un­der­ap­pre­ci­ated, de­spite some of en­vi­ron­men­tal strains ap­par­ently be­ing able to in­fect hu­mans. (Prov­ing Koch's pos­tu­late here as else­where is not al­ways easy). One spe­ci­es, Chlamy­dophila pneu­mo­niae, is said not only to cause pneu­mo­nia but also to be as­so­ci­ated – in ways as yet un­cer­tain – with ath­er­o­scle­ro­sis, asthma, and Alzheimer's dis­ease no less.

Chlamy­diae un­dergo a rather com­plex life cy­cle that com­prises a tough, en­vi­ron­men­tally re­sis­tant stage called 'El­e­men­tary Body' and a re­pro­duc­tive one, known as 'Retic­u­late Body.' The mor­pho­lo­gy of these de­vel­op­men­tal stages varies con­sid­er­ably with the species, from rod- to cres­cent- to star-shaped. In­side host cells, these forms tend to live in clumps known as 'In­clu­sion Bod­ies,' which are read­ily vis­i­ble in patho­log­i­cal spec­i­mens. What else? Chlamy­diae tend to have small ge­nomes, about 1 Mb in size, and to lack the genes for amino acid biosyn­the­sis and for mak­ing some nu­cleotides. They have ADP/ATP translo­cases that al­low them to im­port these en­ergy rich com­pounds. Phy­lo­ge­net­ics sug­gest that they have been strict in­tra­cel­lu­lar par­a­sites since their early days.

Did Chlamy­diae Help Plants Evolve?

Fig­ure 2. Ma­rine Sin­gle Am­pli­fied Genomes (SAGs) rep­re­sent­ing deeply branch­ing chla­my­di­ae. A Bayesian 16S rRNA-based phy­lo­ge­netic tree is shown, in­di­cat­ing the re­la­tion­ship of the three ma­rine SAGs with known chlamy­diae. Source

Re­cent work on this group of or­gan­isms has re­vealed a whole lot of in­ter­est­ing things about them but per­haps the main one is that they have played a key role in the ori­gin of plants. This cu­ri­ous in­fer­ence arises from chlamy­dial pro­teins be­ing re­mark­ably like those of cyanobac­te­ria and plants and, even more sur­pris­ing, be­ing able to funct­ion in plas­tids. In one species, over 150 pro­teins have high se­quence sim­i­lar­ity to those of cyanobac­te­ria and plants. In ad­di­tion, some of the chlamy­diae have a 23S rRNA in­tron, which are char­ac­te­ris­tic of al­gal plas­tids, ar­chaea, and lower eu­kary­otes, but ab­sent from most bac­te­ria. So, what hap­pened here? Hy­pothe­ses abound, but per­haps the most cred­i­ble one at present is that chlamy­dia par­tic­i­pated in a tri­par­tite af­fair, along with the ur-eu­kary­ote and the bac­te­ria that be­came mi­to­chon­dria and plas­tids. The work of Hueng and Gog­a­rten sup­ports this pos­si­bil­ity. They found that 21 genes were trans­ferred be­tween chlamy­diae, a red alga and plants. They and oth­ers (see for ex­am­ple here and here) pro­posed that the chlamy­diae worked to help the fu­ture plas­tid (a cyanobac­terium) be­come es­tab­lish­ed in the plant. This has been called the MAT (for Mé­nage á Trois) hy­poth­e­sis, al­though the way it worked has not been fully worked out. Not sur­pris­ingly, this dar­ing no­tion has been force­fully con­tested.

Are Chlamy­diae Motile?

Fig­ure 3. Fla­gel­lar genes de­tected in chlamy­dial SAGs. a Schematic overview of com­po­nents of the fla­gel­lar ap­pa­ra­tus en­coded in three dif­fe­rent chlamy­dial SAGs of ma­rine ori­gin. No­tably, they con­tain or­thologs for both the fla­gel­lar sys­tem and the NF-T3SS (which orig­i­nally evol­ved from the fla­gel­lar sys­tem). All other known chlamy­diae lack the ma­jor­ity of fla­gel­lar genes. b Il­lus­tra­tion of the syn­tenic re­gion of a fla­gel­lar gene clus­ter in two SAGs. Con­tig ends are indi­ca­ted. Source

Now for the gist of this post. A re­cent pa­per presents da­ta for some ma­rine chlamy­diae pos­sess­ing genes for a com­plete fla­gel­lar ap­pa­ra­tus, just like that of free-liv­ing bac­te­ria. How was this re­vealed? The au­thors se­quenced three genomes from sin­gle cells in sam­ples from three ma­rine sites from the Pa­cific and At­lantic oceans. They are dis­tinct from other chlamy­diae and prob­a­bly re­pre­sent deep branches within the phy­lum. In these genom­es, the re­searchers were sur­prised to find 5 ex­am­ples in one sam­ple and 20 in the oth­ers of genes that match those of bac­te­r­ial fla­gella. In­cluded are those for all the cy­to­plas­mic, trans­mem­brane, and ex­tra­cel­lu­lar com­pon­ents. These genes ap­pear to be of an­cient ori­gin and not to have been ac­quired by re­cent hor­i­zon­tal trans­fer. How­ever, they are phy­lo­ge­net­i­cally dis­tinct from those of other bac­te­ria. So, what are these genes for? Do chla­my­di­ae ac­tively move about, per­haps chemo­tac­ti­cally?

The re­searchers looked for genes in­volved in chemo­taxis and in­deed found them in all three sam­ples. Given that chlamy­diae have re­duced genomes, find­ing such sets of genes sug­gest that they are re­quired for sur­vival in these or­gan­isms. Is it pos­si­ble that these or­gan­isms, even if in­ca­pable of re­pro­duc­ing out­side of host cells, are able to move about in the en­vi­ron­ment? The rea­son could be that if you are motile you may have an eas­ier time find­ing new hosts. The en­ergy re­quire­ment in the tran­sit forms (ele­men­ta­ry bod­ies) could be stored in the glyco­gen they carry. This con­tra­dicts the gen­er­ally held view that ex­tra­cel­lu­lar chlamy­diae are meta­bol­i­cally in­ert. Most of them may be. But a few, per­haps just ma­rine ones, may use motil­ity gene prod­ucts to ad­here to par­ti­cles, to act as sen­sors or to funct­ion as part se­cre­tion sys­tems. Keep in mind that bac­te­r­ial fla­gella are in­volved in many func­tions. This as­pect of chlamy­dial bi­ol­ogy clearly awaits re­triev­ing the or­gan­isms, not just their genomes.

Should We Re­de­fine Strict In­tra­cel­lu­lar Par­a­sitism?

Figu­re 4. Trans­­mission elec­tron mi­croscopy im­ages of the ri­ck­ettsia "Can­di­da­tus Tri­chorick­ettsia mo­bilis". A In­side the macronu­cleus of a para­me­cium, show­ing sever­al novel cylin­dri­cal elec­tron dense par­ticles, arranged in a reg­u­lar way. These un­usual struc­tur­ers may be phage con­stituents. Ar­rows in­di­cate the fla­gella of the sym­bionts. B In­side the cy­to­plasm of the para­me­cium. Icosa­he­dral elec­tron dense par­ti­cles are vis­i­ble. C Neg­a­tive stain­ing of the rick­ettsiae show­ing nu­mer­ous and long fla­gella. Bars: 0.5 µm. Source

Sur­pris­ing as the find­ing of pos­si­ble chlamy­dial motil­ity may be, it is not unique. The other ma­jor group of strict in­tra­cel­lu­lar bac­te­ria, the rick­ettsiae, not only have fla­gellar genes but carry ac­tual hon­est-to-good­ness fla­gella. These or­gan­isms are found in cil­i­ates (one in a Eu­plotes, an­other in a Para­me­cium ), where they dis­play a peri­tri­chous mor­phol­ogy. They carry out an ac­tive cir­cu­lar mo­tion within their host's nu­cle­o­plasm in one case, and more or less lin­ear mo­tion in the cy­to­plasm in the other. Move­ment ceases when the host cells are crushed, so it may be con­fined to the in­te­rior of the cells. Why they move re­mains a mys­tery.

Chlamy­diae and rick­ettsiae share fla­gel­lar genes but what else they have in com­mon is not yet known. How­ever, they share hav­ing a more so­phis­ti­cated ge­netic com­ple­ment than has been ex­pected from 'sim­ple' in­tracellular par­a­sites. Motil­ity may well be a com­mon fea­ture be­tween them, al­beit with dis­tinc­tive cha­rac­ter­is­tics in each kind. Are we sur­prised? Hardly. The mi­cro­bial world tends to re­veal in­creas­ing com­plex­ity and the more we study, the more sur­pris­ing and of­ten won­drous the things we un­cover.

 

This pa­per was dis­cussed in the Oc­to­ber 12 episode (#162) of 'This Week in Mi­cro­bi­ol­ogy' (TWiM). Press play to lis­ten now.