Fine Read­ing: The Hy­brid Two-En­gine Sys­tem of Myxobac­te­ria

by Merry

Mi­cro­bi­ol­o­gists have been fas­ci­nated by the so­cial swarm­ing of the myxobac­te­ria since they were first iso­lated by Roland Thax­ter in 1892. Trav­el­ing in "wolf packs," these bac­te­ria glide along solid sur­faces, such as agar. En masse they se­crete enough ex­tra­cel­lu­lar en­zymes to lyse and di­gest bac­te­ria, the po­ten­tial prey they meet on the way. A great deal is now known about how the in­di­vid­ual cells in the swarm move about. In a re­cent re­view, Dale Kaiser cov­ers the topic from one end to the other – in both ge­netic and mol­e­c­u­lar de­tail. But don't let the pages of de­tails de­ter you. Even if you skim them, you will still en­joy the un­fold­ing story.

In short, Myx­o­coc­cus xan­thus "glides" us­ing two en­gines, one at each pole of the cell.

Schematic of the type IV pilus ma­chine omit­ting 7 of the pilin pro­teins "for the sake of clar­ity." Source

At the lead­ing pole is a type IV pilus en­gine that pulls the cell for­ward; ex­tru­sion of slime through noz­zles at the trail­ing pole is thought to push. Both en­gines are com­plex, each as­sem­bled into elab­o­rate struc­tures from about 20 dif­fer­ent pro­teins. The two en­gines work so well to­gether that the cells zip along at a speed that is faster than the sum of the speeds mea­sured for sin­gle-en­gine mu­tants.

SEM of M. xan­thus cells with ex­tra­cel­lu­lar fib­rils. Source (dead link in 2014)

The cells can pro­duce pili at ei­ther end, but they do so only at the lead­ing pole. When the ex­tended pili at­tach to a clus­ter of M. xan­thus cells, the pili re­tract, pulling the in­di­vid­ual cell for­ward to­wards the more-firmly-an­chored clus­ter. Thus, the cells have to be less than a pilus length apart for this to work. The tips of the pili don't at­tach to just any tar­get, but only to spe­cific ex­tra-cel­lu­lar polysac­cha­ride fib­rils pro­duced by these bac­te­ria.

Now the push. Early ob­ser­va­tions of the slime trails vis­i­ble on agar plates led to the sug­ges­tion that these bac­te­ria might be given a push by se­cret­ing slime. In­deed, there are rea­sons to think this rocket en­gine mech­a­nism is plau­si­ble. Trans­mis­sion elec­tron mi­cro­graphs of the trail­ing pole of the cell show more than a hun­dred pores in the outer mem­brane, called noz­zles, from which nar­row, amor­phous poly­sac­cha­ride fil­a­ments are ex­truded. These fine fil­a­ments fuse lat­er­ally to form the bright rib­bon of trail­ing slime vis­i­ble by phase-con­­trast mi­croscopy. The other pole of the cell has its set of noz­zles, too, but they are in­ac­tive.

stained iso­lated cell en­ve­lope of M. xan­thus show­ing the noz­zle ar­ray at one of the cell poles. Source (dead link in 2014)

Myxobac­te­ria can di­vide and glide at the same time. When the di­vi­sion sep­tum forms, one daugh­ter cell in­her­its the ac­tive pilus en­gine from its mother and as­sem­bles an ac­tive slime en­gine at its newly formed pole. Like­wise, the other daugh­ter cell in­her­its the ac­tive slime en­gine and as­sem­bles a new pilus en­gine. As the re­sult, each daugh­ter cell keeps mov­ing in the same di­rec­tion as the par­ent, with­out miss­ing a beat. There is yet an­other feat: ap­prox­i­mately every 7 min­utes, the cells re­verse their glid­ing di­rec­tion. The lead­ing pole that had been pro­duc­ing pili now ex­trudes slime, and vice versa. It takes only a minute or so for the ap­pro­pri­ate en­gines to be turned off and on.

If you ever find your­self try­ing to con­vince a skep­tic that bac­te­ria are dex­ter­ous and com­plex, you might hand them a copy of this re­view.