## 3 Results and Discussion
### Cell Envelope Modifications in Response to HXT Stress  
_L. plantarum_ adjusted the expression of a set of genes coding for proteins involved in the biogenesis of the cell wall, including several glucosyltransferases that were differently regulated by HXT. Genes _lp_2844_ or _tagE6_ (poly (glycerol-phosphate) \(\alpha\)-glucosyltransferase) and _lp_1818_ or _tagB1_ were upregulated whereas _lp_1248_ or _tagD2_, _lp_1372_ or _gtCA1_ (a protein involved in teichoic acid glycosylation), and _lp_2716_ or _ica3_ as well as _lp_2783_ (both glycosyltransferases), were downregulated. This profile could indicate modification of the wall teichoic acid (WTA) by glycosyl substitution in response to HXT. Genes coding for other proteins involved in cell wall biogenesis were also differently expressed upon HXT. Gene _lp_1713_ or _lysA_ (diaminopimelate (DAP) decarboxylase, which catalyzes the final step in the meso-DAP/lysine biosynthetic pathway) was downregulated, which can avoid DAP dissipation towards lysine production and provide more meso-DAP to be incorporated as constituent of the peptidoglycan (PG) layer of _L. plantarum_ cell walls. Interestingly _L. plantarum_ uses a similar strategy to respond to the damage incurred by tannic acid in the cell wall, i.e., overproducing DapF protein (DAP epimerase) which epimerizes L, L-DAP into meso-DAP to be incorporated into the PG [5]. The strategy of increasing meso-DAP for PG biosynthesis in response to HXT may similarly reflect damage in the cell wall caused by this phenolic compound. Another enzyme that participates in PG biosynthesis of _L. plantarum_, _lp_2199_ or _nmaY_ (phospho-N-acetylmuramoyl-pentapeptide-transferase) was downregulated. MraY is an essential enzyme that catalyzes the first membrane step of bacterial cell wall PG synthesis and is a promising target for the search of new antibacterial compounds [37]. Hence the observed profile identifies MraY as a target for the antibacterial action of HXT. A set of different genes coding for enzymes that can hydrolyze specific bonds in PG was downregulated, probably as strategy to maintain cell wall integrity. Among these, three code for enzymes of the peptidoglycan hydrolase complement (PGH) including two extracellular lytic transglycos (_lp_3015_; _lp_0302_) and the N-acetylmuramoyl -L-alanine amidase (_lp_1982_ or _lytH_). Also, a diaminopellate mureopeptidase (_lp_2162_) as well as a gene couple (_lp_3254_ or _lgrA_, _lp_3255_ or _lgrB_) which code for a murein hydrolase export protein and a murein hydrolase regulator, were also downregulated. Amassing of compatible solutes in the extracellular space seems to be a further strategy to avoid damage in the cell envelope caused by HXT, judging by the downregulation of several transporters for these substances. These included the _lp_3324_ (glycine betaine/carnitine/choline transporter), _lp_1722_ (GABA transporter) and _lp_0265_ (trehalose PTS transport system) genes, which is coincident with the response of _L. plantarum_ to p-coumaric acid [7], a phenolic compound that turns leaky the membrane of several bacteria. It was also observed an increased expression of three subunits of the H\({}^{+}\)-ATPase (_lp_2365_ or _atpG_, _lp_2367_ or _atpH_ and _lp_2365_ or _atpF_). The increased expression of F\({}_{0}\)F\({}_{1}\)-ATPase subunits may be required to compensate for a potential proton motive force dissipation caused by HXT damage on membrane integrity, as it has been speculated to occur upon exposure to bile acids [38], albeit this hypothesis needs to be experimentally supported. Interestingly _atpF_ subunit was downregulated in the presence of EVOO [10]. In this case, F\({}_{0}\)F\({}_{1}\)-ATPase subunit downregulation is modulated by fatty acids and is probably linked to diminished respiration metabolism triggered by fatty acid pathway disfunction.