Or maybe not. A recent paper in PNAS presents the provocative thesis that low doses of antibiotics can stimulate nasty traits in pathogenic bacteria. Using a microarray and low doses of three structurally unrelated antibiotics, they detected switching on of a number of unpleasant genetic programs.
All three antibiotics induce biofilm formation; tobramycin increases bacterial motility, and tetracycline triggers expression of P. aeruginosa type III secretion system and consequently bacterial cytotoxicity. Besides their relevance in the infection process, those determinants are relevant for the ecological behavior of this bacterial species in natural, nonclinical environments, either by favoring colonization of surfaces (biofilm, motility) or for fighting against eukaryotic predators (cytotoxicity)
The authors go on to suggest that antibiotics may be important signalling molecules in natural communities. This is in contrast to the older model of antibiotics as weapons in microbial battles for dominance. It is a provocative thesis worth watching for stronger evidence. In my mind, their data still fits the weapons model -- what they see is the same sort of signalling as my blood in the ocean signals a shark. Or, a bacterial Captain Kirk detecting an unseen ship raising its shields, prompting a defense posture.