Heterologous production of pediocin for the control of Listeria monocytogenes in dairy foods

Development of a Tetracycline-Inducible System for Conditional Gene Expression in Lactococcus lactis and Streptococcus thermophilus

Inducible gene expression systems are invaluable tools for the functional characterization of genes and in the construction of protein overexpression hosts. Controllable expression is especially important for the study of essential and toxic genes or genes where the level of expression tightly influences their cellular effect. Here, we implemented the well-characterized tetracycline-inducible expression system in two industrially important lactic acid bacteria, Lactococcus lactis and Streptococcus thermophilus. Using a fluorescent reporter gene, we show that optimization of the repression level is necessary for efficient induction using anhydrotetracycline in both organisms. Random mutagenesis in the ribosome binding site of the tetracycline repressor TetR in Lactococcus lactis indicated that altering the expression levels of TetR was necessary for efficient inducible expression of the reporter gene. Through this approach, we achieved plasmid-based, inducer-responsive, and tight gene expression in Lactococcus lactis. We then verified the functionality of the optimized inducible expression system in Streptococcus thermophilus following its chromosomal integration using a markerless mutagenesis approach and a novel DNA fragment assembly tool presented herein. This inducible expression system holds several advantages over other described systems in lactic acid bacteria, although more efficient techniques for genetic engineering are still needed to realize these advantages in industrially relevant species, such as S. thermophilus. Our work expands the molecular toolbox of these bacteria, which can accelerate future physiological studies. IMPORTANCE Lactococcus lactis and Streptococcus thermophilus are two industrially important lactic acid bacteria globally used in dairy fermentations and, therefore, are of considerable commercial interest to the food industry. Moreover, due to their general history of safe usage, these microorganisms are increasingly being explored as hosts for the production of heterologous proteins and various chemicals. Development of molecular tools in the form of inducible expression systems and mutagenesis techniques facilitates their in-depth physiological characterization as well as their exploitation in biotechnological applications.

Food Safety through Natural Antimicrobials

Microbial pathogens are the cause of many foodborne diseases after the ingestion of contaminated food. Several preservation methods have been developed to assure microbial food safety, as well as nutritional values and sensory characteristics of food. However, the demand for natural antimicrobial agents is increasing due to consumers' concern on health issues. Moreover, the use of antibiotics is leading to multidrug resistant microorganisms reinforcing the focus of researchers and the food industry on natural antimicrobials. Natural antimicrobial compounds from plants, animals, bacteria, viruses, algae and mushrooms are covered. Finally, new perspectives from researchers in the field and the interest of the food industry in innovations are reviewed. These new approaches should be useful for controlling foodborne bacterial pathogens; furthermore, the shelf-life of food would be extended.

Application of Bacteriocins and Protective Cultures in Dairy Food Preservation

In the last years, consumers are becoming increasingly aware of the human health risk posed by the use of chemical preservatives in foods. In contrast, the increasing demand by the dairy industry to extend shelf-life and prevent spoilage of dairy products has appeal for new preservatives and new methods of conservation. Bacteriocins are antimicrobial peptides, which can be considered as safe since they can be easily degraded by proteolytic enzymes of the mammalian gastrointestinal tract. Also, most bacteriocin producers belong to lactic acid bacteria (LAB), a group that occurs naturally in foods and have a long history of safe use in dairy industry. Since they pose no health risk concerns, bacteriocins, either purified or excreted by bacteriocin producing strains, are a great alternative to the use of chemical preservatives in dairy products. Bacteriocins can be applied to dairy foods on a purified/crude form or as a bacteriocin-producing LAB as a part of fermentation process or as adjuvant culture. A number of applications of bacteriocins and bacteriocin-producing LAB have been reported to successful control pathogens in milk, yogurt, and cheeses. One of the more recent trends consists in the incorporation of bacteriocins, directly as purified or semi-purified form or in incorporation of bacteriocin-producing LAB into bioactive films and coatings, applied directly onto the food surfaces and packaging. This review is focused on recent developments and applications of bacteriocins and bacteriocin-producing LAB for reducing the microbiological spoilage and improve safety of dairy products.

Streptococcus thermophilus, an emerging and promising tool for heterologous expression: Advantages and future trends

Streptococcus thermophilus is the second most used bacterium in dairy industry. It is daily consumed by millions of people through the worldwide consumption of yogurts, cheeses and fermented milks. S. thermophilus presents many features that make it a good candidate for the production of heterologous proteins. First, its ability to be naturally transformable allows obtaining swiftly and easily recombinant strains using various genetic tools available. Second, its Generally Recognised As Safe status and its ability to produce beneficial molecules or to liberate bioactive peptides from milk proteins open up the way for the development of new functional foods to maintain health and well-being of consumers. Finally, its ability to survive the intestinal passage and to be metabolically active in gastrointestinal tract allows considering S. thermophilus as a potential tool for delivering various biological molecules to the gastrointestinal tract. The aim of this review is therefore to take stock of various genetic tools which can be employed in S. thermophilus to produce heterologous proteins and to highlight the advantages and future trends of use of this bacterium as a heterologous expression host.