The frequency and rates of ascent and dissemination of antibiotic resistanc
e in bacterial populations are anticipated to be directly related to the vo
lume of antibiotic use and inversely related to the cost that resistance im
poses on the fitness of bacteria. The data available from recent laboratory
studies suggest that most, but not all, resistance-determining mutations a
nd accessory elements engender some fitness cost, but those costs are likel
y to be ameliorated by subsequent evolution.