Elio Schaechter of www.smallthingsconsidered.us has a thought provoking piece on the function of bacteria and the antibiotics they produce. Could it be that antibiotics have more to do with bacterial communication then as a defense mechanism?
"Antibiotics are now being thought about as benign compounds that, at least at low concentrations, have little to do with intraspecies warfare between organisms, and a great deal to do with the ways microbes communicate with one another. More and more examples are being reported of antibiotics that, at sub-inhibitory concentrations function, as community organizers, prodding bacteria into making protective biofilms."
"Did microbes invent these compounds and then "make a pact" not to use them for what their name implies—killing other living things? Or did these compounds function first as signaling molecules helping the "conversation" between and within microbial species? Highly appealing is the notion put forth by Julian Davies that antibiotics are "fossil molecules" that played a key role as effectors in the early evolution of life, molecules still made today and still serving to shape microbial communities."