As the titles of journal articles go, it's hard to find one more elegant, enticing and -- notably, if you've been in the business long -- succinct than "Gliding Ghosts of Mycoplasma mobile".
Jules Verne short story? Steampunk Western? No. This was the title of an article in "Cell Biology" back in 2005.
But the substance of "Gliding Ghosts" is even better than the promise. That's because the cellular propulsion system of Mycoplasma mobile does something extraordinary: with the addition of ATP, M. mobile's gutted and very much dead remains can get up and move as if they were alive.
In the following jaw-dropping video, perkily gliding, unsuspecting living cells suddenly find themselves exposed to the detergent Triton X-100, which punches holes in their cell membranes, allowing the cell's soluble contents to escape while simultaneously delivering a swift and presumably humane death*.
This is followed by immersion in DNAse, an enzyme that chews up any long sticky masses of DNA that spill out of the dying cell like entrails (watch carefully after the addition of DNAse -- you can actually see their poor little guts spilling out). At this point, pretty much all that's left are the cytoskeletal remains of the ex-Mycoplasmas covered with some tattered membrane.
Yet if you add ATP at the end of this sequence -- which you may remember from high school biology as all-purpose cellular fuel pellets -- this is what happens:
They're .... ALIIIIIVE!! Ghosts? Zombies? Whatever they are, this little microbial party trick -- and essential step in verifying that ATP is the power source for these mycoplasmal engines -- is downright amazing.
And there's more: Daniel Haeusser, the author of the post at Small Things Considered (link below) that inspired this post, told me that he found it particularly interesting that if the scientists did not treat the cells with DNAse and RNAse, the ex-cells got stuck to the slides by their spilled DNA guts -- they started moving, stretched out their "gooey" (his word) DNA/RNA tethers, and then snapped back. So apparently, the strength of the bonds in DNA is greater than the force that can be brought to bear by their locomotory engines. And what of those engines? How do they work?
The mechanism that makes this work is unique to just two species of Mycoplasma. Cells of M. mobile resemble light bulbs (or badminton birdies). The screw of the light bulb is actually the head, inside of which is a cytoskeletal matrix that gives it shape. Trailing this matrix are "jellyfish tentacles" -- long protein strands.
Each of the units on this strand is in turn attached to a grasshopper-leg like structure, which appear in scanning electron micrograph photos at "spikes" sticking out around the neck of M. mobile.
An ATPase protein attached to this apparatus cleaves the fuel, which induces a protein shape change cascade in the engine mechanism that ultimately results in the detachment, movement, and reattachment of the leg to a surface. In a word: a step.
Interestingly, artificial Mycoplasma ghosts eventually start losing their grip on the slide surface and slowing down as bits of their engines break down but fail to be replaced by the repair systems of a living cell.
The complex system of proteins that powers this feat is nearly unique to this microbe, so far as scientists know. Unlike the flagella that power sperm, protists, and several other eurkaryotic cells (a whipping, flexible tail) or bacteria (a fixed, corkscrew like rotor that turns to push bacteria forward like a ship's screw) or even the actin-myosin muscle fibers which power all animals which are all widespread systems, the engine apparatus of M. mobile is found only in it and one related species.
And the genus Mycoplasma seems to be a bit of a microbial dream factory for these sorts of motility innovations, considering how very many few such unique systems are known: in a blog post at Small Things Considered that inspired this post and that you should check out if you are interested in learning more, Daniel Haeusser tells us of at least one other unique "inchworm"-type propulsion system that has evolved in this remarkable genus in M. pneumoniae, a germ that causes bacterial pneumonia and also happens to be the smallest known free-living organism on the planet. There may be other such unique systems in this genus as well.
Why has this happened twice in Mycoplasma? No one knows for sure. Members of the genus do tend to be parasitic -- M. mobile was isolated from a fish gill. Perhaps the transition to parasitism generated pressure for new motility systems to evolve. And members of the genus also differ from most other bacteria in that they lack a cell wall. Most bacteria have a rigid wall made of peptidoglycan. Mycoplasma do not. Perhaps that gives them more ... er ... flexibility in evolving new motility systems.
Still, putting together not just one, but two motility systems like this from what appears to be scratch (none of the proteins in the system seemingly are homologous, or share ancestry, with any of the other major bacterial or eukaryotic movement protein systems) is a pretty impressive feat. But so too, you have to admit, is rising from the dead.
* Unlike most other bacteria, the genus Mycoplasma lacks a cell wall, so you don't have to disrupt that to empty the cell. The plant parasite equivalent of this genus is Phytoplasma, in which the tiny bacteria also lack cell walls and, unconstrained by a rigid wall, sometimes expand into fantastically blobby shapes (scientists call them "pleiomorphic") in the plant juices in which they live. There is also a plant and insect-parasitic genus called Spiroplasma, whose members, in spite of lacking a cell wall, still manage to form perfect corkscrews.