Thursday, March 24, 2011

Oozing along with Beggiatoa (part II)

(See part I).
So, as my student asked, how do they do that--oozing along so hypnotically?

We don’t know. That’s the bottom line. Beggiatoa is really common, but it turns out to be kind of difficult to study in the lab—traditional laboratory work requires getting a pure culture of nothing but Bedgy, and that seems to be pretty near impossible. So it’s been difficult to answer even fairly basic questions about this guy.

We do have a bit of a clue, courtesy of another creature from the same mud sample:

It’s a cyanobacterium, a type of bacterium that does the traditional kind of photosynthesis. It kind of looks similar, and it oozes along in the same way. You can even see that it sort of spirals along like Bedgy does. Unlike Bedgy, you can grow cyanobacteria in a pure culture in the lab, and study how they do stuff—like move along oozily.

We still don’t know exactly how this cyanobacterium (and Bedgy) moves, but at least we have some clues. One set of clues comes from Egbert Hoiczyk and his coworkers, who propose what they call the “slime-extrusion” model. Each cell in that long chain of cells is girdled with a ring of pores, which they think are "slime jets". Each cell produces a lot of slime and pumps it out of the pores. Outside of the cell, the slime picks up water and expands dramatically, like a dried sponge taking up water. The expanding slime pushes the cell along.

This model has some appeal; Hoiczyk did some impressive microscopy examining these slime jets. They’re distinctive protein structures that are neatly positioned to connect the inside of the cell where slime is made with the outside world. They also note a correlation between the presence of slime jets and the ability to glide in other types of cyanobacteria (alas, they did not check on Bedgy).

There’s another line of evidence supporting the slime-extrusion model, and that is actual evidence that slime is extruded. Hoiczyk simply put India ink (a suspension of very fine, black particles) in the mix with his cyanobacteri, and watched them go:

You can see streamers of ink-stained snot trailing off of the chains of cyanobacterial cells. But this isn’t proof that jets of slime are pushing these cells along—it could be these are streamers of snot trailing after these cells as they move along by some other means. If the model is wrong, thinking that the slime is pushing the cells along could be like thinking that the plume of exhaust is pushing a car along.

An alternative to the slime-extrusion model is raised by a group led by David Adams. According to this “peristaltic” model, the slime is mainly a lubricant that helps the cell move. The work is done by fibers that lie just beneath the cells’ outer membranes; these fibers contract in waves that move along the length of the cell. These fibers have been chemically characterized, and also nicely examined by electron microscopy. Nicely, the fibers are all aligned in a lengthwise spiral, causing the cells to spiral along as they glide.

We still don't know whether either of these models really describes how cyanobacteria move, let alone whether or not that has anything to do with the way that Bedgy moves. It would be nice to do some experiments to tinker with the slime jets and see if that affects motility. It would be nice to see whether or not Bedgy even has slime jets. The best we can say is that the way Bedgy moves looks like the way these cyanobacteria move, and that Bedgy has some genes for slime synthesis that kinda, sorta look like the cyanobacterial genes.

For now, though, we can dip into almost any pond, find this beautiful pearl necklace, and just sit back and wonder.

Egbert Hoiczyk and Wolfgang Baumeister (1998). The junctional pore complex, a prokaryotic secretion organelle, is the molecular motor underlying gliding motility in cyanobacteria. Current Biology 8:1161–1168. Electron micrographs of slime jets, ink-stains of slime, and circumstantial evidence for the slime extrusion model.

Marc Mußmann, Fen Z. Hu, Michael Richter, Dirk de Beer, Andre Preisler, Bo B. Jørgensen, Marcel Huntemann, Frank Oliver Glockner, Rudolf Amann, Werner J. H. Koopman, Roger S. Lasken, Benjamin Janto, Justin Hogg, Paul Stoodley, Robert Boissy, Garth D. Ehrlich (2007). Insights into the Genome of Large Sulfur Bacteria Revealed by Analysis of Single Filaments. PLOS Biology 5 (9): e230. The suggestion that Beggiatoa has slime-producing genes similar to gliding Cyanobacteria.

Nicholas Read, Simon Connell, and David G. Adams (2007). Nanoscale Visualization of a Fibrillar Array in the Cell Wall of Filamentous Cyanobacteria and Its Implications for
Gliding Motility. Journal of Bacteriology 189 (20): 7361-7366. Striking images of the fibers that support the peristalsis model of cyanobacterial motility.

(The literature on this subject is pretty thin, and the dates are old; it's obviously not a subject that attracts much interest, cool as it is.)

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