 This study used temporal functional metagenomic sequencing, TFUMSEQ, to identify genes from the gut-associated bacterium bacteroids theteotium ikran, BT, that confer fitness advantages when introduced into E. coli. The authors found that these genes enabled E. coli to better utilize sugar such as galactose, which was essential for successful colonization of the mouse intestine. This work demonstrates how TFUMSEQ can be used to identify genes from commensal bacteria that confer fitness advantages in other species, providing insight into the mechanisms underlying their success in the host environment. This article was authored by Stephanie J. Yong, Lu Xiu Edeng, Ning Li, and others.