Even harmless bacteria suck up the energy of their hosts and hasten their deaths—or so the conventional thinking went. New findings show that flies scrubbed clean of bacteria do not outlive their infested brethren. In both flies and humans, because the number of bacteria living both inside and on the body increases with age, researchers expected infestations to prove harmful by depleting their hosts' resources. University of Southern California scientists and their colleagues compared normal fruit flies with ones born from eggs washed in antibiotics and raised in bacteria-free environments. The flies even ate disinfected food. Surprisingly, normal and superclean fruit flies had the same life span of roughly 65 days. Although these experiments cannot be replicated in higher organisms, which need bacteria for proper digestion and other functions, the investigators say their results do help narrow down which factors help to limit animal longevity. The findings appear in the August 8 Cell Metabolism.