The outer membrane (OM) is an essential element in Gram-negative (diderm) bacteria. Its integrity is maintained by the control of lipids asymmetry via LPS and phospholipids transport by the Lpt and Mla systems, respectively. However, outside a few model organisms, it is unknown how this is achieved in most diderm bacteria. In this project, we aim at understanding these processes and their evolution by a powerful combination of experimental and bioinformatics approaches. We will use genetics, functional and structural analyses to dissect the entire process for generation of OM lipid asymmetry in a new experimental model of the bacterial cell envelope, Veillonella parvula, an abundant component of the human microbiome and member of Negativicutes, atypical Firmicutes that strikingly possess a diderm cell envelope with LPS. We will complement these studies by a thorough evolutionary analysis of these transport systems across all Bacteria.