Bacteria abundance - (Dec/11/2008 )
Quick question: what limits bacterial numbers to the point that they're not taking over the world? 
is this a philosophical question or you just saw (or read) the war of the worlds?? 
and i don't really see why you say they haven't taken over the world yet....they're everywhere!! maybe you mean if they were some gigantic organisms that we could see without a microscope they'd be taking over the world (so as it happens in "evolution")
perhaps slightly philosophical 
and no, i've not seen or read war of the worlds.
it's tricky i suppose. we had a little debate at a recent lab meeting. the question of whether one should save rhino's, elephants, etc (the big five in south africa) or whether one should rather allocate the funds to save bacteria came up. the post-docs decided to save bacteria. of course i failed to see the logic. so i asked, what's special about this bacteria they want to save, what does it do? they replied: nitrogen fixing bacteria. i still didn't understand their logic, hence the start of the debate.
finally i said, i don't know of bacteria on the endangered list. besides bacteria can multiply to be millions in a day, while something like rhino's can not (the turn-over of the latter is slow compared to the former). i don't think i convinced them to let go of saving bacteria (very odd), but then someone asked the question of bacteria abundance. in retrospect, the question is general and the answer to it is likely view dependent. i suppose the counter question is then: who says we aren't 
Limited nutrients. A good thing, too -- given sustained perfect growing conditions and doubling every twenty minutes, a single bacterium would give rise to a colony the size of the Earth in about a week.
Although bacteria are very hardy and can adapt to many situation, nature still comes up with ways to control them. One of them are predators, such as viruses (phages). One theory is called "kill the winner", which basically means that phages attack bacteria that are most abundant (of course, it's a little bit more complex than that, but this is a way to control bacteria).
Cheers!