## 4 Discussion
In the EPS operon, _rmlA_ and _wzb_ are co-expressed for R0011 and ATCC 9595 and _rmlD_ and _wzr_ are co-expressed for ATCC 9595 and RW-9595M. The expression of _rmlA-D_ genes was activated independently of EPS biosynthesis to form the cell wall polysaccharides, but co-transcription of these genes with the EPS biosynthesis gene cluster might increase the dTDP-L-rhamnose pool available for biosynthesis of EPS [10]. The levels of EPS production and even the repeating unit sugar composition of EPS could be influenced by using a conditional _rfbBD_ (_rmlBD_) mutant of _L. lactis_[50]. Other potential targets for acid stress exist, such as the cell wall and cytoplasmic membrane. In co-culture, an over-expression of the gene coding for enzyme cyclopropane-fatty-acyl-phospholipid synthase (_cfa_) or the gene encoding the enzyme regulating non-saturated fat acids elongation (_fabG_) for _L. rhamn_s RW-9595M and _L. rhamn_s ATCC 9595 was observed. Acetyltransferase (_R0011_RS11250_) coding for membrane fatty acid biosynthesis was over-expressed for the three strains. Cyclopropanation increases membrane stability when facing stress conditions affecting the membrane, such as acid stress [51]. The same strategy to resist stress conditions was described for _Lactobacillus plantarum_ at pH 3 [52]. From Group 3, the overexpression of the gene coding for cysteine synthase (_cysK_) which participates in cysteine and methionine biosynthesis was observed for _L. rhamn_s R0011 and _L. rhamn_s RW-9595M. This gene is coded by the _metaA-cysK_ operon and permits cysteine production from serine. These genes form the cysteine-cystathionine cycle, allowing NH\({}_{3}\) production and stimulating lactic and acetic acid synthesis by the pyruvate pathway. Genes linked to pH homeostasis regulation such as the proton pump F0F1-ATPase, expression of general proteins associated with stress and the chaperone proteins that repair damaged DNA were monitored during fermentation (G4). Genes coding for F0F1 ATP synthase (_atpD_ or _atpA_), proteins responsible for maintaining the proton gradient throughout the cell membrane, were over-expressed in all three strains. In the same manner, _L. plantarum_ under acid stress showed increased expression of the _atpA_ gene, which was proposed to increase acid tolerance [53]. The gene coding for alkyl hydroperoxyde reductase (_alhpC_), a catalyzer for the reduction of organic hydroxide to protect cells, was over-expressed by the three strains. Genes _dnaK_, _groES_ and _clpP_ were over-expressed in _L. rhamnus_ RW-9595M and ATCC 9595. Transcriptomic and proteomic studies highlighted that _dnaK_ and _groESL_ operons were positively regulated by the catabolite control protein [54]. Proteases linked to _clp_ (ATP dependent) such as sub-unit _clpP_ are highly active under general cell stress (heat, acid, ethanol, or p-coumaric acid). Similarly, a set of genes involved in tolerance to one or more environmental stresses was induced during the acid stress responses (class I and III stress response pathways) for _Lb. plantarum_[52]. The involvement of the heat-shock proteins with the _Clp_ proteins, pertinent to acid-mediated stress, is a feature across _Lactobacillus_ species [43].