Mice Are Not Furry Petri Dishes

The evolution of drug resistance in bacteria depends on a rapid mutation rate. However, drug resistance mutations frequently incur a cost to the microbe, usually a reduced growth rate. Unfortunately, this cost cannot be exploited because those crafty microbes usually develop compensatory mutations, which offset the growth disadvantage while maintaining drug resistance. In a Perspective, Bull and Levin discuss new findings ( BjA¶rkman et al. ) showing that different compensatory mutations evolve depending on whether pathogenic bacteria are grown in culture or in an animal host. They caution that care should be taken when trying to extrapolate in vitro findings about bacterial drug resistance to in vivo situations.