According to research published in Proceedings of the National Academy of Sciences by Johanna Bjorkman, Diarmaid Hughes and Dan Andersson, biologists at Uppsala University and the Swedish Institute for Infectious Disease Control strains resistant to antibiotics are initially less virulent than their susceptible counterparts, as you would expect, but the researchers found that virulence can be rapidly acquired without any loss of resistance. In other words, the evolutionary burden imposed by resistance may not be significant after all. It means that judicious use of antibiotics will help prevent the appearance of new antibiotic-resistant mutants, but it may not have any effect on the populations of antibiotic resistant bacteria that already exist.