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In this work we present the design of the recombinant shuttle vector pBM02 applicable to gene expression in Lactococcus lactis and Enterococcus faecalis. This vector was constructed by using the replicon Rep264 of pCIT264 plasmid, and the pH-inducible promoter Pcit of the citMCDEFXG operon from L. lactis CRL264. As selection marker the chloramphenicol acetyltransferase gene (cat) was employed. All these elements were cloned in the vector pUC18 of Escherichia coli. In pBM02 the induction of the gene from the promoter Pcit is triggered by the lactic acid accumulated in the culture during fermentation of L. lactis. Therefore, it doesn´t require the addition of inducers. The capacity of this system to express a lactococcal enzyme CitM and the protein VP8* of rotavirus was studied by western blot assays. In this way, it was possible to determine that while in L. lactis the expression of different genes is induced at acidic condition, in E. faecalis constitutive expression levels for these genes were obtained. Thus, pBM02 could be used as an alternative tool for the economic and simple production of heterologous proteins of therapeutical or technological interest. Also in this work we developed a shuttle vector named pBVGh for quickly generating gene inactivation mutants in Lactic Acid Bacteria (LAB). The pBVGh vector was constructed by using the pG+host replicon (a thermosensitive derivative of pWV01, widely employed in the manipulation of LAB), and a fragment of the pMAD vector containing the multicloning site, the constitutive promoter and the bgaB gene. In the pBVGh vector we combined the blue-white discrimination of mutants (pMAD strategy) with the broad range replicon from pG+host vector. In addition replacing the replicon pE194ts from pMAD (non functional in LAB) allowed to establish the plasmid and then to induce recombination, increasing it´s efficiency. To assess the pBVGh vector for the generation of stable chromosomal mutations in E. faecalis, we constructed a derivative of pBVGh carrying the upstream part of a target gene and the downstream part of the target gene. This plasmid was electroporated into competent cells of E. faecalis, transformant cells were recovered at the permissive temperature (30°C). After that, one colony was grown at non-permissive temperature (42°) to induce the first event of recombination. Then, the culture was diluted into fresh medium without antibiotic and incubated a few hours at 30°C to allow the second recombination event. Subsequently the culture was incubated overnight at 42°C to ensure the loss of the plasmid. The deletion mutants were selected by serial dilutions on X-Gal plates, white colonies were isolated and verified for antibiotic sensitivity. Finally, the deletion of the target gene was confirmed by PCR analysis. In addition, we determined that the pBVGh plasmid is fully functional in the strain L. lactis IL1403, allowing the easy generation of clean deletion mutants. In outline, this plasmid overcomes the limitations of the available vectors, since it not requires a helper plasmid and allows on X-Gal plates a quick colorimetric blue-white discrimination of bacteria which have lost the plasmid, greatly facilitating clone identification during mutagenesis.