Pathogenic bacteria often kill by producing toxins, some of which are the most lethal poisons known. But what is the goal of killing one's own host? Why do the bacteria produce these toxins? The textbooks, after going into the excrutiating details of the action of cytotoxins, typically end on a philosophical note:...why certain bacteria produce such potent toxins is mysterious. The production of a toxin may play a role in adapting a bacterium to a particular niche, but it is not essential to the viability of the organism. Most toxigenic bacteria are free-living in nature and in associations with humans in a form which is phenotypically identical to the toxigenic strain but lacking the ability to produce the toxin. http://www.textbookofbacteriology.net/proteintoxins.html
a quick review of the action of toxins is here http://www.cdc.gov/ncidod/eid/vol5no2/schmitt.htm
I've just read an article that suggests, without providing much detail, an interesting idea, which is new to me:http://www.scientificamerican.com/article.cfm?id=the-art-of-bacterial-warfare
The toxins are not aiming at the host but rather at the competition: the inflammation creates the conditions when benign bacteria cannot survive providing the lethal bacteria with their only opportunity to spread. So the cholerae vibrio induce diarrhea not to spread (as I've been told when I was a student), as they will get into water anyway, but to flush the intestines of the benign gut bacteria that get in the way and even help the body to get rid of the invasive guests. The need to kill the fellow bugs is so great, as these are super sturdy, and it requires such extraodinary means that sometimes the host simply gets in a way; it is just too weak. Our immune system is seldom the problem for bacterial proliferation: our own cells are greatly outnumbered by the bacterial ones. The other microorganisms may be the problem.
Come to think of it, there could be something to this idea, but it does not have to be limited to other bacteria. Take botulism, for example: C. botulinum are anaerobic soil bacteria that have no intention of living in a human. Why is it producing neurotoxins? The toxin targets SNARE proteins that regulate vesicle fusion (taking transport vesicles across the membranes) with some of these vesicles (in motor neurons) having neurotransmitters. This type of protein is also involved in the absoprtive feeding in soil fungi http://www.biochemsoctrans.org/bst/037/0787/0370787.pdf
Perhaps it is another case of getting in a way. The bacteria seek to kill yeasts in the soil that are feeding on them. But we (metazoa) have the misfortune of sharing with our fungi sisters the opishtokont ancestry that includes the particular design of SNARE proteins. The two SNAREs are pretty close despite 900 Myr of divergent evolutionhttp://www.pnas.org/content/103/18/6958.full.pdf+html
The bacteria try to stunt the growth of its mortal enemy and accidentally kill the harmless weakling that crosses their path?
This is so unfair. We are not even important enough to be wanted
Why do bacteria kill?