## 4 Discussion
The amount of acetate kinase (_ackA_) increased during growth on cheese broth without pH control [42]. There is some evidence that Lactobacillus modify the pyruvate metabolism and increase the synthesis of basic compounds (e.g., lysine, diacetyl/acetoin), energy-rich intermediates (such as ATP and NADH), exopolysaccharides, and/or glycogen at the expense of lactic acid [43]. According to the literature, _L. rhamnus_ strains can utilize pyruvate as a growth substrate during cheese ripening, in this way they are able to increase the amount of pyruvate oxidase (POX) or phosphotransacetylase for synthesizing acetate [44]. Additional ATP generated by acetate kinase is an alternative carbon source for _Lactobacillus_ that permits an increment of biomass production [45; 46]. Throughout fermentation, the phenolic acid profile of corn steep liquor used as nitrogen source in the culture medium was modified with an increase of caffeic and protocatechuric acid and a reduction of ferulic acid concentration in the medium. At the same time, the bioconversion of phenolic acids showed the over-expression of _pcaC_ genes involved in the conversion of protocatechuate to 3-oxoadipate. Over-expression of gene _rhoA_ was observed for _L. rhamnus_ strains in co-culture with baker's yeast. Network analysis of _L. rhamnus_ strains revealed co-expression of (_rhoA_) with carboxymuconolactone (_pcaC_) and/or xylanase (_R0011_11803_). Recently, studies have shown that lactic acid bacteria can use phenolic acids as external electron acceptors to gain additional metabolic energy to counteract the stressful conditions generated by phenolics [47; 48]. On the other hand, the utilization of corn steep liquor in the culture medium leads to bioactive substances with potential health benefits for humans through bacterial metabolism of phenolic acids [48]. The PCA indicates that EPS production was principally associated with growth and lactic acid production for _L. rhamn_ strains, as previously shown [4; 10]. The stimulation of kefiran production and the growth of _L. kefiranofaciens_ JCM 6985 by yeast began in the early exponential growth phase [20]. Previous reports indicated that the largest EPS production of _L. rhamn_s RW-9595M (about 85% of the maximum EPS concentration) occurred during the growth phase with a concentration of 2.3 g/L in immobilized conditions and under pH control [13]. In this study, the EPS production increased in co-culture relative to monoculture for the three strains. It was demonstrated that co-culture of _L. paracasi_ with _S. cerevisiae_ resulted in over-expression of a gene coding for polyphenyl glycosylphosphotransferase linked to EPS production [49]. The same authors suggested that EPS production could be induced by direct contact of bacteria and yeast through recognition of mannan in the yeast cell wall and that adhesion of LAB cells to yeast cells triggers EPS production in the LAB cells because the adhesion leads to more efficient consumption of lactic acid by the yeast cells. Despite the positive effect of co-culture, the quantity of EPS produced was different for each strain. The most productive strain was _L. rhamn_s RW-9595M with 1.35 g/l in co-culture without pH control in this study, corresponding to a production five times greater than the other strains. Transcriptome analysis of the three strains indicates that after 6 h (exponential phase) of co-culture, each strain over-expressed the gene responsible for EPS polymerization: phosphatase (_wzb_). However, for _L. rhamn_s RW-9595M, this gene is over-expressed and the network analysis of co-expression indicated it as an important node in target gene interactions. Network analysis also revealed a common behavior among all three strains with the co-expression of _wzb_ (G2) and _lacT_ (G1). The _wzb_ protein is involved in chain length determination and _LacT_ acts as an anti-terminator protein of the _galKETRM_ operon. The co-expression of genes coding for _rmlA_ (G2) with _galR_ and/or _bgaC_ (G1) represent a complementary aspect of network analysis.