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Background   Lactococcus lactis , a lactic acid bacterium traditionally used to ferment milk and manufacture cheeses, is also, in the biotechnology field, an interesting host to produce proteins of medical interest, as it is “Generally Recognized As Safe”. Furthermore, as  L. lactis  naturally secretes only one major endogenous protein (Usp45), the secretion of heterologous proteins in this species facilitates their purification from a protein-poor culture medium. Here, we developed and optimized protein production and secretion in  L. lactis  to obtain proteins of high quality, both correctly folded and pure to a high extent. As proteins to be produced, we chose the two transmembrane members of the HtrA protease family in  Staphylococcus aureus , an important extra-cellular pathogen, as these putative surface-exposed antigens could constitute good targets for vaccine development.    Results  A recombinant ORF encoding a C-terminal, soluble, proteolytically inactive and tagged form of each staphylococcal HtrA protein was cloned into a lactococcal expression-secretion vector. After growth and induction of recombinant gene expression,  L. lactis  was able to produce and secrete each recombinant rHtrA protein as a stable form that accumulated in the culture medium in similar amounts as the naturally secreted endogenous protein, Usp45.  L. lactis  growth in fermenters, in particular in a rich optimized medium, led to higher yields for each rHtrA protein. Protein purification from the lactococcal culture medium was easily achieved in one step and allowed recovery of highly pure and stable proteins whose identity was confirmed by mass spectrometry. Although rHtrA proteins were monomeric, they displayed the same secondary structure content, thermal stability and chaperone activity as many other HtrA family members, indicating that they were correctly folded. rHtrA protein immunogenicity was established in mice. The raised polyclonal antibodies allowed studying the expression and subcellular localization of wild type proteins in  S. aureus : although both proteins were expressed, only HtrA 1  was found to be, as predicted, exposed at the staphylococcal cell surface suggesting that it could be a better candidate for vaccine development.    Conclusions  In this study, an efficient process was developed to produce and secrete putative staphylococcal surface antigens in  L. lactis  and to purify them to homogeneity in one step from the culture supernatant. This allowed recovering fully folded, stable and pure proteins which constitute promising vaccine candidates to be tested for protection against staphylococcal infection.  L. lactis  thus proved to be an efficient and competitive cell factory to produce proteins of high quality for medical applications.    Electronic supplementary material  The online version of this article (doi:10.1186/s12934-015-0271-z) contains supplementary material, which is available to authorized users.

Production, secretion and purification of a correctly folded staphylococcal antigen in Lactococcus lactis