Imagine a club scene—a bouncer at a velvet rope selects which individuals get into the club. This, explains Eva Kanso, a professor of mechanical engineering at USC Viterbi School of Engineering, is what cilia do in an organism. Kanso applied the analogy to explain her new paper, “Motile cilia create fluid-mechanical microhabitats for the active recruitment of the host microbiome,” co-authored with researchers from the Pacific Biosciences Research Center at the University of Hawaii at Manoa and from Stanford, to explain the active role that cilia have in ensuring certain bacteria are kept out of an organism while other symbiotic bacteria are selectively permitted to enter.