Bacteria synthesize fatty acids, membrane main constituents, via the FASII pathway. In streptococci, the transcriptional regulator FabT represses FASII gene expression. The authors examined the role of FabT during infection by Streptococcus pyogenes, also known as Group A Streptococcus (GAS). GAS causes a wide variety of diseases ranging from mild non-invasive to severe invasive infections and fatal post-infectious sequelae. GAS is responsible for over 517,000 deaths worldwide every year. The literature showed the existence of a paradox: although FabT defects are associated with attenuated virulence in animal models, spontaneous FabT mutants appear in vivo.
To resolve this paradox, we characterized the conditions and mechanisms that require FabT activity and those that favor the emergence of FabT mutants.
FabT represses fatty acid (FA) synthesis in the presence of exogenous fatty acids (eFAs). The FabT mutant strain continues to synthesize FA in the presence of eFAs, producing longer FAs than the wild-type strain and causing membrane defects. In addition, this synthesis induces higher metabolic expenditure and consequently higher energy dissipation: specific nutrients are consumed without allowing bacterial growth. These two defects lead to bacterial death. As a result, the mutant is unable to grow ex vivo on human tissue, nor on cells or in cell and tissue filtrates, where nutrients are limited. These characteristics explain the need for FabT during infection.
Conversely, fabT mutants show a marked growth advantage over the wild-type strain in the presence of saturated eFAs, FAs that are present in the host. Indeed, when its FASII pathway is repressed, the wild-type strain incorporates eFAs. Saturated eFAs are toxic to the bacterium. We have shown that, conversely, continuous FASII activity, as in the fabT mutant, prevents this incorporation. Also, spontaneous fabT mutants were obtained in vitro when the wild-type strain was cultured in the presence of saturated eFAs. Furthermore, using an ex vivo model of saturated FA-rich tissue, we showed that growth of the wild-type GAS strain was inhibited, while that of the fabT mutant was stimulated. However, the latter was abolished when the FASII pathway of the mutant strain was blocked, demonstrating the role of FabT in this growth difference.
Thus, our results elucidate the rationale for emerging fabT mutants that enhance survival in lipid-rich biotopes composed of saturated FAs, but lead to a genetic dead-end for infection.
Figure: Model for the emergence of FabT mutants with attenuated virulence. A, Saturated fatty acids (SFAs) in a lipid environment favor the emergence of FabT mutants. Indeed, saturated, toxic FAs, which are incorporated only by the wild-type strain, may be present during the first contacts of GAS with the host. Counter-selection leads to the emergence of FabT mutants that do not incorporate the FAs. B, The host cell environment during invasion limits the growth of FabT mutants. Longer FAs destabilize the membrane, and continued FASII activity in fabT mutants causes a state of futile bacterial metabolism where metabolite uptake does not allow growth. FabT mutant bacteria fail to grow and die more rapidly when exposed to human cells. Thus, FabT mutants in GAS populations may confer a survival advantage at the inoculation site, but are not resistant to host infection conditions.
Purple and red circles, wild-type and FabT mutant cocci; zoom in on phospholipids. Small yellow circles and lines represent lipids and hydrolysis products of eFAs, respectively; small pink, blue and grey circles, sugars and amino acid residues and arrows of the same color, their secretion by the cells.
Our teams had previously demonstrated that inhibiting the FASII pathway was not a therapeutic option due to the incorporation of eFAs in vivo by different Gram-positive bacterial species. Our study indicates that, conversely, inhibiting FabT, rendering FASII synthesis constitutive, is detrimental to infection in vivo. FabT is therefore a promising target for new therapies against specific Gram-positive pathogens, including GAS, Streptococcus agalactiae, Streptococcus pneumoniae et Enterococcus faecalis.
Reference
The double-edged role of FASII regulator FabT in Streptococcus pyogenes infection. Clara Lambert C, Gaillard M, Wongdontree P, Bachmann C, Hautcoeur A, Gloux K, Guilbert T, Méhats C, Prost B, Solgadi A, Abreu S, Andrieu M, Poyart C, Gruss A* and Fouet A*. Nature Communications 2024, 15(1):8593. doi: 10.1038/s41467-024-52637-3
