Bacteria are minimalist (http://www.sciencedirect.com/science/article/pii/S0092867402...) which tends to amplify the use it or loose it evolutionary principle. There's a certain cost to keeping a "feature" in an organism that small with a high rate of mutation and adaptability.

"Strains of bacterial pathogens that have acquired mutations conferring antibiotic resistance often have a lower growth rate and are less invasive or transmissible initially than their susceptible counterparts." http://aac.asm.org/content/54/5/2085.full

If evolutionary logic holds (and there's little doubt about that, though research on the rate of resistance lose with time varies), switching to new classes of antibiotics and not using old ones until resistance to them de-evolves is a viable strategy.