Why should cell biologists study microbial pathogens?

Welch, Matthew D.

The study of pathogens that mobilize actin for movement has also led to the discovery of fundamental mechanisms of cytoskeletal dynamics and regulation. Such pathogens include the bacteria Listeria monocytogenes, Shigella flexneri, and Escherichia coli, which cause food-borne illnesses, as well as vaccinia virus, which is the smallpox vaccine strain. These microbes undergo actin-based motility either within or on the surface of cells, which enables cellto-cell spread during infection (Welch and Way, 2013) . Through biochemical reconstitution of L. monocytogenes and/or S. flexneri motility, the host Arp2/3 complex and its activators (the WASP family proteins) were revealed as key actin-nucleating factors for bacterial pathogens and host cells (Welch et al., 1997 (Welch et al., , 1998 , and a minimal set of proteins that is sufficient to drive actinbased motility was defined (Loisel et al., 1999) . Moreover, the study of enteropathogenic/enterohemorrhagic E. coli and vaccinia virus, which induce actin assembly from outside the cell through the plasma membrane, revealed important roles for tyrosine kinase signaling and protein clustering in regulating actin assembly (Frischknecht et al., 1999; Campellone et al., 2008) .