BackgroundLactic Acid Bacteria such asLactococcus lactis, Latilactobacillus sakei(basonym:Lactobacillus sakei) andLactiplantibacillus plantarum (basonym:Lactobacillus plantarum) have gained importance as recombinant cell factories. Although it was believed that proteins produced in these lipopolysaccharides (LPS)-free microorganisms do not aggregate, it has been shown thatL. lactisproduce inclusion bodies (IBs) during the recombinant production process. These protein aggregates contain biologically active protein, which is slowly released, being a biomaterial with a broad range of applications including the obtainment of soluble protein. However, the aggregation phenomenon has not been characterized so far inL. plantarum. Thus, the current study aims to determine the formation of protein aggregates inL. plantarumand evaluate their possible applications.ResultsTo evaluate the formation of IBs inL. plantarum, the catalytic domain of bovine metalloproteinase 9 (MMP-9cat) protein has been used as model protein, being a prone-to-aggregate (PTA) protein. The electron microscopy micrographs showed the presence of electron-dense structures inL. plantarumcytoplasm, which were further purified and analyzed. The ultrastructure of the isolated protein aggregates, which were smooth, round and with an average size of 250–300 nm, proved thatL. plantarumalso forms IBs under recombinant production processes of PTA proteins. Besides, the protein embedded in these aggregates was fully active and had the potential to be used as a source of soluble protein or as active nanoparticles. The activity determination of the soluble protein solubilized from these IBs using non-denaturing protocols proved that fully active protein could be obtained from these protein aggregates.ConclusionsThese results proved thatL. plantarumforms aggregates under recombinant production conditions. These aggregates showed the same properties as IBs formed in other expression systems such asEscherichia coliorL. lactis. Thus, this places this LPS-free microorganism as an interesting alternative to produce proteins of interest for the biopharmaceutical industry, which are obtained from the IBs in an important number of cases.