Two-tiered biological containment strategy for Lactococcus lactis-based vaccine or immunotherapy vectors

Safety Aspects of Genetically Modified Lactic Acid Bacteria

Lactic acid bacteria (LAB) have a long history of use in the food industry. Some species are part of the normal human microbiota and have beneficial properties for human health. Their long-standing use and considerable biotechnological potential have led to the development of various systems for their engineering. Together with novel approaches such as CRISPR-Cas, the established systems for engineering now allow significant improvements to LAB strains. Nevertheless, genetically modified LAB (GM-LAB) still encounter disapproval and are under extensive regulatory requirements. This review presents data on the prospects for LAB to obtain 'generally recognized as safe' (GRAS) status. Genetic modification of LAB is discussed, together with problems that can arise from their engineering, including their dissemination into the environment and the spread of antibiotic resistance markers. Possible solutions that would allow the use of GM-LAB are described, such as biocontainment, alternative selection markers, and use of homologous DNA. The use of GM-LAB as cell factories in closed systems that prevent their environmental release is the least problematic aspect, and this is also discussed.

Chromosomal integration of heterologous oxalate decarboxylase in Lactobacillus plantarum WCFS1 using mobile genetic element Ll.LtrB

Lactobacillus plantarum WCFS1 (L. plantarum WCFS1) is commonly used as a potential cell factory because of its 'generally recognized as safe' status. The plasmid instability and the presence of antibiotic selection marker complicate the application of genetically modified L. plantarum in human clinical trials. In the present study, we aimed to integrate oxalate decarboxylase (oxdC) gene of Bacillus subtilis origin by targeted chromosomal mutation in L. plantarum using mobile genetic element Ll.LtrB as a therapeutic tool against calcium oxalate stone disease. oxdC expression cassette was constructed and integrated into a targeted gene, thymidylate synthase (thyA) in the L. plantarum genome. The dependence on external thymidine for growth and survival was established by live dead population assay and SEM (scanning electron microscopy) analysis. The western blotting assay showed the secretion of 44 kDa OxdC protein in the culture supernatant of L. plantarum ∆thyA:OxdC. The biologically contained recombinant strain significantly reduced the oxalate concentration by 53% and exhibited a loss of viability when introduced to environmental samples. Biologically contained L. plantarum secreting OxdC constructed using group II intron has the ability to degrade oxalate present in the extracellular environment and could be used as a therapeutic tool for the calcium oxalate stone disease.

Development of a gene delivery system in Streptococcus gordonii using thymidylate synthase as a selection marker

Streptococcus gordonii, a commensal bacterium of the human oral cavity, is a potential live vaccine vector. In this study, we have developed a system that delivers a vaccine antigen gene onto the chromosome of S. gordonii. The system consisted of a recipient strain, that is a thymidine auxotroph constructed by deletion of a portion of thyA gene, and a linear gene delivery construct, composed of the functional thyA gene, the vaccine antigen gene, and a DNA fragment immediately downstream of thyA. The construct is assembled by a ligation and polymerase chain reaction strategy. Upon introduction into the thyA mutant, the vaccine antigen gene integrated into the chromosome via a double crossing-over event. Using the above strategy, a test vaccine antigen gene coding for a fusion protein composed of the Bordetella pertussis filamentous hemagglutinin type I domain and the single chain antibody against complement receptor 1 was successfully delivered to S. gordonii. The resulting S. gordonii expressed the fusion protein and the delivered gene was stable in the bacterium in vitro and in a mouse colonization experiment. Mice colonized by the fusion protein-expressing S. gordonii developed antibodies that recognized the native filamentous hemagglutinin protein suggesting that an immune response was elicited.