From a short description of the work of Bonnie Bassler:
"Silverman ended his talk saying “‘Don’t you see, these bacteria are 
communicating with this molecule. They are acting multicellular.’” At 
the end of his talk, Bassler steamed to the podium and ask Silverman for
 a job. Much to her surprise, he offered her a postdoc position right 
there. She packed her stuff and returned to California and began 
studying bioluminescence in the marine bacterium V. harveyi.
During her postdoc, Bassler defined the QS circuit of V. harveyi. She
 showed that, like V. fischeri, V. harveyi communicated with other 
members of its species by using a chemical signal called an HSL 
autoinducer (3). The marine bacteria secrete this autoinducer into their
 environment. As long as bacteria are in low numbers in dilute 
suspension the autoinducer is washed away and no act is being taken, but
 when bacteria replicate and grow in number, the concentration of the 
autoinducer molecule also increases in the surrounding. When it reaches a
 certain threshold it is sensed by the bacteria and gives it an 
indication of its own population size. They then respond simultaneously 
by activating their light genes to produce bioluminescence. As Bassler 
puts it so nicely in her TED talk, “they vote with these chemical votes,
 the votes get counted and then everybody responds to the vote”.
Bassler also discovered that V. harveyi had more than one molecule 
for QS; she called this additional molecule autoinducer-2 (AI-2) (3, 4),
 and the original, found by Hastings, became autoinducer-1 (AI-1). 
........
So… you can legitimately ask what’s all the fuss about understanding 
the genetic and chemical basis of how marine bacteria talk to each other
 and produce light?
It turns out that bacteria use QS not only for bioluminescence but 
also for many other important traits, most important of which, virulence
 (5-8). Actually, in this regard bacteria act very similar to us; if you
 would want to do something that’s beyond your reach as an individual, 
you’d talk to some other guys, get the necessary quorum, and then carry 
it together. Bacteria use exactly the same strategy, or as Bonnie puts 
it, “they are just too small to have an impact on the environment if 
they simply act as individuals”. 
Thinking of it, bacteria are microscopic creatures, and probably their 
only chance to overcome a huge host is by acting together. So they count
 them self up and only when the right amount of cells is present they 
launch their virulence attack to take over their host. 
And this simple realization, has huge implications for human health. "
Her 
Ted talk, which according to Carl Zimmer is one of the good Ted talks and a description of the talk towards the end of the blog post 
Microbial Hermeneutics - 2
which also links to a 
later interview.