Nuclear Magnetic Resonance Structure and Mutational Analysis of the Lactococcin A Immunity Protein

Structural Basis of the Mechanisms of Action and Immunity of Lactococcin A, a Class IId Bacteriocin

Lactococcin A (LcnA), a class IId bacteriocin, induces membrane leakage and cell death by specifically binding to the membrane receptor-mannose phosphotransferase system (man-PTS), as is the case for pediocin-like (class IIa) bacteriocins. The cognate immunity protein of bacteriocins, which protects the producer cell from its own bacteriocin, recognizes and binds to the bacteriocin-man-PTS complex, consequently blocking membrane leakage. We previously deciphered the mode of action and immunity of class IIa bacteriocins. Here, we determined the structure of the ternary complex of LcnA, LciA (i.e., the immunity protein), and its receptor, i.e., the man-PTS of Lactococcus lactis (ll-man-PTS). An external loop on the membrane-located component IIC of ll-man-PTS was found to prevent specific binding of the N-terminal region of LcnA to the site recognized by pediocin-like bacteriocins. Thus, the N-terminal β-sheet region of LcnA recognized an adjacent site on the extracellular side of ll-man-PTS, with the LcnA C-terminal hydrophobic helix penetrating into the membrane. The cytoplasmic cleft formed within the man-PTS Core and Vmotif domains induced by embedded LcnA from the periplasmic side is adopted by the appropriate angle between helices H3 and H4 of the N terminus of LciA. The flexible C terminus of LciA then blocks membrane leakage. To summarize, our findings reveal the molecular mechanisms of action and immunity of LcnA and LciA, laying a foundation for further design of class IId bacteriocins. IMPORTANCE Class IId (lactococcin-like) bacteriocins and class IIa (pediocin-like) bacteriocins share a few similarities: (i) both induce membrane leakage and cell death by specifically binding the mannose phosphotransferase system (man-PTS) on their target cells, and (ii) cognate immunity proteins recognize and bind to the bacteriocin-man-PTS complex to block membrane leakage. However, class IId bacteriocins lack the "pediocin box" motif, which is typical of class IIa bacteriocins, and basically target only lactococcal cells; in contrast, class IIa bacteriocins target diverse bacterial cells, but not lactococcal cells. We previously solved the structure of class IIa bacteriocin-receptor-immunity ternary complex from Lactobacillus sakei. Here, we determined the structure of the ternary complex of class IId bacteriocin LcnA, its cognate immunity protein LciA, and its receptor, the man-PTS of Lactococcus lactis. By comparing the interactions between man-PTS and class IIa and class IId bacteriocins, this study affords some clues to better understand the specificity of bacteriocins targeting the mannose phosphotransferase system.

Biosynthesis and Production of Class II Bacteriocins of Food-Associated Lactic Acid Bacteria

Bacteriocins are ribosomally synthesized peptides made by bacteria that inhibit the growth of similar or closely related bacterial strains. Class II bacteriocins are a class of bacteriocins that are heat-resistant and do not undergo extensive posttranslational modification. In lactic acid bacteria (LAB), class II bacteriocins are widely distributed, and some of them have been successfully applied as food preservatives or antibiotic alternatives. Class II bacteriocins can be further divided into four subcategories. In the same subcategory, variations were observed in terms of amino acid identity, peptide length, pI, etc. The production of class II bacteriocin is controlled by a dedicated gene cluster located in the plasmid or chromosome. Besides the pre-bacteriocin encoding gene, the gene cluster generally includes various combinations of immunity, transportation, and regulatory genes. Among class II bacteriocin-producing LAB, some strains/species showed low yield. A multitude of fermentation factors including medium composition, temperature, and pH have a strong influence on bacteriocin production which is usually strain-specific. Consequently, scientists are motivated to develop high-yielding strains through the genetic engineering approach. Thus, this review aims to present and discuss the distribution, sequence characteristics, as well as biosynthesis of class II bacteriocins of LAB. Moreover, the integration of modern biotechnology and genetics with conventional fermentation technology to improve bacteriocin production will also be discussed in this review.

Biophysics involved in the process of tumor immune escape

Much of the current research into immune escape from cancer is focused on molecular and cellular biology, an area of biophysics that is easily overlooked. A large number of immune drugs entering the clinic are not effective for all patients. Apart from the molecular heterogeneity of tumors, the biggest reason for this may be that knowledge of biophysics has not been considered, and therefore an exploration of biophysics may help to address this challenge. To help researchers better investigate the relationship between tumor immune escape and biophysics, this paper provides a brief overview on recent advances and challenges of the biophysical factors and strategies by which tumors acquire immune escape and a comprehensive analysis of the relevant forces acting on tumor cells during immune escape. These include tumor and stromal stiffness, fluid interstitial pressure, shear stress, and viscoelasticity. In addition, advances in biophysics cannot be made without the development of detection tools, and this paper also provides a comprehensive summary of the important detection tools available at this stage in the field of biophysics.

Bacteriocins, A Natural Weapon Against Bacterial Contamination for Greater Safety and Preservation of Food: A Review

Nowadays, consumers have become increasingly attentive to human health and the use of more natural products. Consequently, the demand for natural preservatives in the food industry is more frequent. This has led to intense research to discover new antimicrobial compounds of natural origin that could effectively fight foodborne pathogens. This research aims to safeguard the health of consumers and, above all, to avoid potentially harmful chemical compounds. Lactobacillus is a bacterial genus belonging to the Lactic Acid Bacteria and many strains are defined GRAS, generally recognized as safe. These strains are able to produce substances with antibacterial activity against food spoilage bacteria and contaminating pathogens: the bacteriocins. The aim of this review was to focus on this genus and its capability to produce antibacterial peptides. The review collected all the information from the last few years about bacteriocins produced by Lactobacillus strains, isolated from clinical or food samples, with remarkable antimicrobial activities useful for being exploited in the food field. In addition, the advantages and disadvantages of their use and the possible ways of improvement for industrial applications were described.

The mannose phosphotransferase system (Man-PTS) - Mannose transporter and receptor for bacteriocins and bacteriophages

Mannose transporters constitute a superfamily (Man-PTS) of the Phosphoenolpyruvate Carbohydrate Phosphotransferase System (PTS). The membrane complexes are homotrimers of protomers consisting of two subunits, IIC and IID. The two subunits without recognizable sequence similarity assume the same fold, and in the protomer are structurally related by a two fold pseudosymmetry axis parallel to membrane-plane (Liu et al. (2019) Cell Research 29 680). Two reentrant loops and two transmembrane helices of each subunit together form the N-terminal transport domain. Two three-helix bundles, one of each subunit, form the scaffold domain. The protomer is stabilized by a helix swap between these bundles. The two C-terminal helices of IIC mediate the interprotomer contacts. PTS occur in bacteria and archaea but not in eukaryotes. Man-PTS are abundant in Gram-positive bacteria living on carbohydrate rich mucosal surfaces. A subgroup of IICIID complexes serve as receptors for class IIa bacteriocins and as channel for the penetration of bacteriophage lambda DNA across the inner membrane. Some Man-PTS are associated with host-pathogen and -symbiont processes.

Metabolic Characterization of Supernatants Produced by Lactobacillus spp. With in vitro Anti-Legionella Activity

Legionella pneumophila is an organism of public health interest for its presence in water supply systems and other humid thermal habitats. In this study, ten cell-free supernatants produced by Lactobacillus strains were evaluated for their ability to inhibit L. pneumophila strains isolated from hot tap water. Production of antimicrobial substances by Lactobacillus strains were assessed by agar well diffusion test on BCYE agar plates pre-inoculated with L. pneumophila. Cell-free supernatants (CFS) showed antimicrobial activity against all Legionella strains tested: L. rhamnosus and L. salivarius showed the highest activity. By means of a proton-based nuclear magnetic resonance (¹H-NMR) spectroscopy, we detected and quantified the Lactobacillus metabolites of these CFSs, so to gain information about which metabolic pathway was likely to be connected to the observed inhibition activity. A panel of metabolites with variations in concentration were revealed, but considerable differences among inter-species were not showed as reported in a similar work by Foschi et al. (2018). More than fifty molecules belonging mainly to the groups of amino acids, organic acids, monosaccharides, ketones, and alcohols were identified in the metabolome. Significant differences were recorded comparing the metabolites found in the supernatants of strains grown in MRS with glycerol and the same strains grown in MRS without supplements. Indeed, pathway analysis revealed that glycine, serine and threonine, pyruvate, and sulfur metabolic pathways had a higher impact when strains were grown in MRS medium with a supplement such as glycerol. Among the metabolites identified, many were amino acids, suggesting the possible presence of bacteriocins which could be linked to the anti-Legionella activity shown by cell-free supernatants.

Biological properties and production of bacteriocins-like-inhibitory substances by Lactobacillus sp. strains from human vagina

AIMS

The aim of this study was to characterize Lactobacillus strains for their biological properties and amensalistic activities against genital and nongenital pathogens.

METHODS AND RESULTS

For the purpose, some special characteristics (H₂ O₂ , biofilm and antimicrobial substances production) as well as safety properties of 112 lactobacilli were evaluated. All the strains had good amensalistic characteristics, in particular cell-free supernatants of 10 strains showed antibacterial activity against bacteria, as well as Candida sp. Moreover, these 10 strains were excellent biofilm producers.

CONCLUSIONS

These results provide evidence for the possible use as probiotics for vaginal co-therapy in case of dysbiosis.

SIGNIFICANCE AND IMPACT OF THE STUDY

Recently, the problem of antibiotic resistance is constantly increasing, even though resources and energy are invested in order to increase knowledge on the mechanisms of action. Bacteriocins have a rapid mechanism of action, act at extremely low concentrations, are generally sensitive to proteases and they usually have a narrow killing spectrum; these characteristics reduce the possibility of the bacterium to develop resistance. This study is focused on the feasibility of a high production of antimicrobial substances and their characterization in order to be exploited as a therapeutic alterative or in co-therapy with antibiotics in case of vaginal dysbiosis.