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    Antibiotics targeting enterococcal fatty acid biosynthesis are ineffective

    Team Claire Poyart - Agnès Fouet

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

     

    The team of Claire Poyart and Agnès Fouet, in collaboration with Alexandra Gruss (Micalis, INRAE), tested the possibility of using the biosynthetic pathway of fatty acids, major constituents of the bacterial membrane, or the formation of the cyclopropane ring within fatty acids, as antibiotic targets in a model of Enterococcus faecalis sepsis. 

    Our team had previously demonstrated that the biosynthesis of fatty acids in Streptococci could not be an antibiotic target because these bacteria incorporate fatty acids present in their environment and in particular in the blood. On the other hand, E. faecalis has, in its membrane, cyclic fatty acids which are absent in the blood. Therefore, it was possible that their biosynthesis pathway could be a therapeutic target. By genetic, biochemical and in vivo approaches we show that these pathways are not targets for the design of new antibiotics. This work was published in J. Bacteriol.

     

    Membrane lipid homeostasis is crucial for bacterial physiology, adaptation and virulence. Fatty acids (FAs) are constituents of phospholipids that are essential membrane components. Bacterial FAs are generally synthesized by the type II fatty acid synthesis pathway (FASII). Defects in the FASII pathway prevent bacterial growth in vitro in media not supplemented with FAs; this suggests that FASII enzymes may be essential for bacterial viability and thus possible targets for antibiotic design. However, most bacteria incorporate exogenous FAs into their membranes. Thus, as we have shown, FA incorporation can bypass FASII-targeted antibiotics in the neonatal pathogen Streptococcus agalactiae, as well as in other Streptococcaceae. Nevertheless, antibiotic bypass may vary depending on whether the bacteria can survive with exclusively exogenous FAs.

    Enterococcus faecalis, a multi-antibiotic-resistant Gram-positive bacterium, has emerged as a serious nosocomial pathogen. It is responsible for urinary tract infections, bacteriemia and endocarditis. It synthesizes FAs harboring a cyclopropane ring, FAs that are absent in the human host. These FAs are more rigid than unmodified FAs and contribute to the homeostasis of the E. faecalis membrane. We therefore wondered whether the FA biosynthetic pathway or the cyclopropanation process could be targets for antimicrobial agents in this pathogen.

    We used a genetic approach to characterize the strategies used by E. faecalis to meet its requirements for endogenous fatty acid synthesis in vitro and in vivo. The FASII pathway is encoded by two operons, of 12 and two genes, and two monocistronic genes. The expression of all these genes is repressed by exogenous FAs, which are incorporated into the membrane of E. faecalis and modify its composition. Deletion of nine genes of the 12-gene operon abolishes growth in a FA-free medium. Addition of serum, which is lipid-rich, restores growth. The cfa gene, which encodes the enzyme responsible for the cyclopropanation process, is located in a locus independent of the FASII genes. Its deletion did not alter growth under the conditions tested, but resulted in bacteria with membranes lacking cyclic fatty acids.

     

    We then tested the virulence of E. faecalis in a mouse model of systemic infection. No differences were observed between mice infected with wild-type, FASII- or cyclopropanation-mutant strains in terms of bacterial loads in blood, liver, spleen or kidney. We conclude that in E. faecalis, neither the FASII nor the cyclopropane fatty acid synthase are suitable antibiotic targets.

     

     

    Legend: Mice are infected with Enterococcus faecalis wild type strain or with mutants of the FASII or cyclopropanation pathway. The infection develops similarly, regardless of the strain used.

     

     

     

     

    This article was selected for a Spotlight by a J. Bacteriol editor.

     

    Reference

    Constantin Hays, Clara Lambert, Sophie Brinster, Gilles Lamberet, Laurence du Merle, Karine Gloux, Alexandra Gruss, Claire Poyart, Agnes Fouet. Type II fatty acid synthesis pathway and cyclopropane ring formation are dispensable during Enterococcus faecalis systemic infection. J. Bacteriol 2021.  doi: 10.1128/JB.00221-21

     

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