Bacterial ecology, antibiotics and selection for virulence

The evolution of antibiotic resistance in bacteria is well known. Here I de scribe possible mechanisms by which an increased rate of re-colonization of vertebrate guts by microbes caused by antibiotic use could lead to selecti on for increased virulence in currently mutualistic or benign microbes. The importance of understanding both the source and the frequency of colonizat ion in such mutualisms is stressed and the possible importance of pseudo-ve rtical transmission in the evolution of these systems is discussed. A numbe r of areas requiring experimental investigation are identified.