Molecular mechanisms underlying the structural diversity of rhamnose-rich cell wall polysaccharides in lactococci

In Gram-positive bacteria, cell wall polysaccharides (CWPS) play critical roles in bacterial cell wall homeostasis and bacterial interactions with their immediate surroundings. In lactococci, CWPS consist of two components: a conserved rhamnan embedded in the peptidoglycan layer and a surface-exposed polysaccharide pellicle (PSP), which are linked together to form a large rhamnose-rich CWPS (Rha-CWPS). PSP, whose structure varies from strain to strain, is a receptor for many bacteriophages infecting lactococci. Here, we examined the first two steps of PSP biosynthesis, using in vitro enzymatic tests with lipid acceptor substrates combined with LC-MS analysis, AlfaFold2 modeling of protein 3D-structure, complementation experiments, and phage assays. We show that the PSP repeat unit is assembled on an undecaprenyl-monophosphate (C55P) lipid intermediate. Synthesis is initiated by the WpsA/WpsB complex with GlcNAc-P-C55 synthase activity and the PSP precursor GlcNAc-P-C55 is then elongated by specific glycosyltransferases that vary among lactococcal strains, resulting in PSPs with diverse structures. Also, we engineered the PSP biosynthesis pathway in lactococci to obtain a chimeric PSP structure, confirming the predicted glycosyltransferase specificities. This enabled us to highlight the importance of a single sugar residue of the PSP repeat unit in phage recognition. In conclusion, our results support a novel pathway for PSP biosynthesis on a lipid-monophosphate intermediate as an extracellular modification of rhamnan, unveiling an assembly machinery for complex Rha-CWPS with structural diversity in lactococci.

A novel plasmid for delivering genes into mammalian cells with noninvasive food and commensal lactic acid bacteria

Using food and commensal lactic acid bacteria (LAB) as vehicles for DNA delivery into epithelial cells is a new strategy for vaccine delivery or gene therapy. However, present methods for DNA delivery with LAB have suffered low efficiency. Our goal was to develop a new system to deliver DNA into epithelial cells with high efficiency using food and commensal LAB. An Escherichia coli-LAB shuttle plasmid, pLKV1, for DNA delivery into eukaryotic cells was constructed. Two reporter plasmids with green and red fluorescent protein genes were also constructed to monitor the uptake of protein and DNA, respectively. Bacteria delivering these reporter plasmids into Caco-2 cells were monitored by fluorescence microscopy. Several methods that weaken the bacterial cell wall prior to co-culture with Caco-2 cells were evaluated for their role in the improvement of gene transfer efficiency. Treating Streptococcus gordonii with penicillin and lysozyme greatly increased its rate of gene delivery to mammalian cells compared to untreated control bacteria, while glycine pretreatment promoted the highest gene transfer rate for Lactococcus lactis. Uptake of green fluorescent bacteria by Caco-2 cells showed that the cell wall-weakening treatment promoted the internalization of the noninvasive bacteria into Caco-2 cells. In conclusion, we have developed a noninvasive system using LAB as a vehicle for vaccine delivery or gene therapy, and tested this system in vitro with Caco-2 cells.

Diversity of the surface properties of Lactococci and consequences on adhesion to food components

Bacteria possess surface properties, related to their charge, hydrophobicity and Lewis acid/base characteristics, that are involved in the attachment processes of microorganisms to surfaces. Fermentation bulks and food matrixes are complex heterogeneous media containing various components with different physicochemical characteristics. The aim of the present study was to investigate whether (i) bacteria present in a food matrix, interacted physicochemically at their surface level with the other constituents and (ii) the diversity of bacterial surface properties could result in a diversity of microbial adhesion to components and thus in a diversity of tolerance to toxic compounds. The surface properties of 20 lactic acid bacteria were characterized by the MATS method showing their relatively hydrophilic and various basic characteristics. The results obtained from a set of representative strains showed that (i) the strains with higher affinity for apolar solvents adsorbed more to lipids and hydrophobic compounds, (ii) the more the strains adsorbed to a toxic solvent, the less they were tolerant to this solvent. A diversity of bacterial surface properties was observed for the strains in the same species showing the importance of choosing bacteria according to their surface properties in function of technological objectives.

Amino acid catabolism in cheese-related bacteria: selection and study of the effects of pH, temperature and NaCl by quadratic response surface methodology

AIMS

To screen the cystathionine lyase and L-methionine aminotransferase activities of cheese-related bacteria (lactococci, non-starter lactobacilli and smear bacteria) and to determine the individual and interactive effects of temperature, pH and NaCl concentration on selected enzyme activities.

METHODS AND RESULTS

A subcellular fractionation protocol and specific enzyme assays were used, and a quadratic response surface methodology was applied. The majority of the strains, 21 of 33, had detectable cystathionine lyase activity which differed in the specificity. Aminotransferase activity on L-methionine was observed in only three strains. The cystathionine lyase activities of Lactobacillus reuteri DSM20016, Lactococcus lactis subsp. cremoris MG1363, Brevibacterium linens 10 and Corynebacterium ammoniagenes 8 and the L-methionine aminotransferase activity of Lact. reuteri DSM20016 had temperature and pH optima of 30-45 degrees C, and 7.5-8.0, respectively. As shown by the quadratic response surface methodology these enzymes retained activities in the range of temperature, pH and NaCl concentration which characterized the cheeses from which the bacteria originated.

CONCLUSION

The enzyme activities may have a role in flavour development during cheese ripening.

SIGNIFICANCE AND IMPACT OF THE STUDY

The findings of this work contribute to the knowledge about the amino acid catabolic enzymes in order to improve cheese ripening.

Simultaneous immunofluorescent detection of coentrapped cells in gel beads

An immunofluorescent method involving double color labeling and confocal microscopy was reported to specifically detect lactic acid bacteria and probiotic cells coimmobilized in gels beads. The method described is rapid (4 h) and sensitive and may be useful for studying cell dynamics during mixed-culture starter production using immobilized cells in gel beads. Microscopic observations were perfectly correlated to cell counts obtained using a sandwich enzyme-linked immunosorbent assay.

Survival, physiology, and lysis of Lactococcus lactis in the digestive tract

The survival and the physiology of lactococcal cells in the different compartments of the digestive tracts of rats were studied in order to know better the fate of ingested lactic acid bacteria after oral administration. For this purpose, we used strains marked with reporter genes, the luxA-luxB gene of Vibrio harveyi and the gfp gene of Aequora victoria, that allowed us to differentiate the inoculated bacteria from food and the other intestinal bacteria. Luciferase was chosen to measure the metabolic activity of Lactococcus lactis in the digestive tract because it requires NADH, which is available only in metabolically active cells. The green fluorescent protein was used to assess the bacterial lysis independently of death. We report not only that specific factors affect the cell viability and integrity in some digestive tract compartments but also that the way bacteria are administrated has a dramatic impact. Lactococci which transit with the diet are quite resistant to gastric acidity (90 to 98% survival). In contrast, only 10 to 30% of bacteria survive in the duodenum. Viable cells are metabolically active in each compartment of the digestive tract, whereas most dead cells appear to be subject to rapid lysis. This property suggests that lactococci could be used as a vector to deliver specifically into the duodenum the proteins produced in the cytoplasm. This type of delivery vector would be particularly appropriate for targeting digestive enzymes such as lipase to treat pancreatic deficiencies.