Type II fatty acid synthesis pathway and cyclopropane ring formation are dispensable during Enterococcus faecalis systemic infection

Enterococcus faecalis, a multiple antibiotic-resistant Gram-positive bacterium, has emerged as a serious nosocomial pathogen. Here, we used a genetic approach to characterize the strategies used by E. faecalis to fulfill its requirements for endogenous fatty acid (FA) synthesis in vitro and in vivo. The type II fatty acid synthesis (FASII) pathway is encoded by two operons and two monocistronic genes. Expression of all of these genes is repressed by exogenous FAs, which are incorporated into the E. faecalis membrane and modify its composition. Deletion of nine genes of the 12-gene operon abolished growth in an FA-free medium. Addition of serum, which is lipid rich, restored growth. Interestingly, the E. faecalis membrane contains cyclic fatty acids that modify membrane properties but that are unavailable in host serum. The cfa gene that encodes the cyclopropanation process is located in a locus independent of the FASII genes. Its deletion did not alter growth under the conditions tested, but yielded bacteria devoid of cyclic FAs. No differences were observed between mice infected with wild-type (WT) or with FASII or cyclopropanation mutant strains, in terms of bacterial loads in blood, liver, spleen, or kidneys. We conclude that in E. faecalis, neither FASII nor cyclopropanation enzymes are suitable antibiotic targets. IMPORTANCE Membrane lipid homeostasis is crucial for bacterial physiology, adaptation, and virulence. Fatty acids are constituents of the phospholipids that are essential membrane components. Most bacteria incorporate exogenous fatty acids into their membranes. Enterococcus faecalis has emerged as a serious nosocomial pathogen that is responsible for urinary tract infections, bacteremia, and endocarditis and is intrinsically resistant to numerous antibiotics. E. faecalis synthesizes saturated and unsaturated fatty acids, as well as cyclic fatty acids that are not found in the human host. Here, we characterized mutant strains deficient in fatty acid synthesis and modification using genetic, biochemical, and in vivo approaches. We conclude that neither the fatty acid synthesis pathway nor the cyclopropanation enzyme are suitable targets for E. faecalis antibiotic development.