Activation of the type 3 secretion system of enteropathogenic E. coli leads to remodeling of its membrane composition

The type III secretion system (T3SS) is critical for the virulence of enteropathogenic E. coli (EPEC), an important human pathogen. T3SS is activated upon host attachment, leading to effector injection, which is associated with repression of CsrA. CsrA repression results in gene expression remodeling, which contributes to EPEC adaptation to a cell-adherent lifestyle. How these changes in gene expression influence EPEC physiology and virulence remains poorly understood. We reasoned that this influence may be mediated by bacterial membrane, and that aside from its well documented influence on central carbon metbaolism, CsrA regulation also confers changes in membrane lipid metabolism. By combining lipidomics with genetic analyses of mutant strains that simulate T3SS activation, we show that activation of the EPEC T3SS is associated with remodeling of lipid metabolism, and in particular, a shift from phospholipids towards lysophospholipids, and from menaquinones and ubiquinones to undecaprenyl lipids. Concomitant to the remodeling of membrane lipid composition, we found higher O-antigen levels, as well as increased cell size and negative cell surface charge in the mutant strains. The shifts we found in membrane lipid compositoion and in O-antigen content are expected to cause changes in memebrane permeability and function. In line with this notion, we found increased sensitivity to vancomycin in the mutant strains. Importance EPEC is an important human pathogen. The characterization of its membrane lipid composition upon attachment to host is an important step towards understanding the biology of EPEC and other pathogenic bacteria. We predict that the lipid remodeling upon attachment to host cells and T3SS activation contributes to bacterial fitness and promotes host colonization. ### Competing Interest Statement The authors have declared no competing interest.