Life is tough all over, but particularly bad for microbes. Without a glimmer of consciousness, and with hardly any tools at hand, (or hands), they still struggle, suffer, and die in astronomical numbers. One of the more fascinating and classic areas of discovery in the field is the beauty and complexity of the T4 phage, which is a virus that infects bacteria like E. coli. It has a lunar-lander like structure that docks to its victim and injects the DNA (from a highly pressurized head chamber) which then kills it while producing hundreds of new viruses.
But it is only in recent years that a connection has been drawn between this phage injection mechanism and what bacteria do to each other. What has been sedately called the type VI bacterial secretion system, used by Vibrio cholerae and Pseudomonas aeruginosa, among many other species of bacteria, turns out to actually be a violent spearing mechanism they use to kill competing bacteria and also mammalian host cells during an infection.
This was recently described in an NIH talk by John Mekalanos, and also in various publications from his group. One representative film clip is linked below.
|Still from movie of the Vibrio spear in action, link here.|
One of the more remarkable aspects of this mechanism is its scale. Sausage-shaped bacteria typically are closest to each other at the side, so the apparatus begins assembling at the side of the Vibrio cell. This injector assembles clear across the interior of the cell, forming a thin thread almost 500nm long which has, in addition to the base plate that nucleates the process, an inside spear and an outside sheath.
The outside sheath is spring-loaded, and when triggered (how that happens is unknown) winds down in a matter of milliseconds to half its original length, thrusting out the spear, while also turning it like a drill. In his talk, Mekalanos showed that the spear is tipped by a pointed protein his lab had recently discovered, that contains a zinc-coordinated domain that gives it particular stability. Behind the tip, the spear also is festooned with a variety of toxins, because simply spearing a nearby cell is not enough to kill it. Vibrio injects both eukaryotic-directed toxins such as one that cross-links actin and thus paralyzes the cell and another that modifies the cAMP signalling system causing massive ion and water efflux, as well as several bacterial-directed toxins to clean out the competition, such as inhibitors of cell wall (peptidoglycan) synthesis.
|Sample killing, where Vibrio (red) were mixed with Pseudomomas (green) cells. The spearing system sheath is labeled in red and green respectively. In each horizontal set of time lapse images, a spear from a Pseudomonas cell (green) impales a neighboring Vibrio cell and either causes it to swell locally or to lyse entirely, losing its optical contrast (arrows).|
An interesting wrinkle in the story is that each bacterium that has this kind of system also has a complement of immunity proteins that neutralize the various toxins that it creates. The bacteria are not terribly bright, and live in close proximity, so they frequently spear each other. One wouldn't want that kind of thing to be fatal. But Vibrio doesn't need immunity from the eukaryotic-specific toxins, which do not affect bacteria, including itself.
Once the spear is thrown, another protein comes along to quickly disassemble the spent apparatus, and another one re-assembles from a new base plate somewhere else inside the bacterium. Quite a bit more is waiting to be learned here, like the triggering mechanisms, and the details of assembly, but not only is this knowledge helpful in addressing a significant pathogen, (though one we hope to not meet in the developed world), but it is an example of the breathtaking complexity, and even beauty, in biology, even in the midst of the most desperate dramas.
- Description of the system.
- General review of the system.
- Immunity and warfare issues.
- Investigation of the system in a plant pathogen.
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