Expression and purification of recombinant Lactobacillus casei bacteriocin and analysis of its antibacterial activity

Bacteriocin diversity, function, discovery and application as antimicrobials

Bacteriocins are potent antimicrobial peptides that are produced by bacteria. Since their discovery almost a century ago, diverse peptides have been discovered and described, and some are currently used as commercial food preservatives. Many bacteriocins exhibit extensively post-translationally modified structures encoded on complex gene clusters, whereas others have simple linear structures. The molecular structures, mechanisms of action and resistance have been determined for a number of bacteriocins, but most remain incompletely characterized. These gene-encoded peptides are amenable to bioengineering strategies and heterologous expression, enabling metagenomic mining and modification of novel antimicrobials. The ongoing global antimicrobial resistance crisis demands that novel therapeutics be developed to combat infectious pathogens. New compounds that are target-specific and compatible with the resident microbiota would be valuable alternatives to current antimicrobials. As bacteriocins can be broad or narrow spectrum in nature, they are promising tools for this purpose. However, few bacteriocins have gone beyond preclinical trials and none is currently used therapeutically in humans. In this Review, we explore the broad diversity in bacteriocin structure and function, describe identification and optimization methods and discuss the reasons behind the lack of translation beyond the laboratory of these potentially valuable antimicrobials.

Site Mutation Improves the Expression and Antimicrobial Properties of Fungal Defense

Although antimicrobial peptides (AMPs) have highly desirable intrinsic characteristics in their commercial product development as new antimicrobials, the limitations of AMPs from experimental to scale development include the low oral bioavailability, and high production costs due to inadequate in vitro/in vivo gene expression- and low scale. Plectasin has good bactericidal activity against Staphylococcus and Streptococcus, and the selective bactericidal activity greatly reduces the damage to the micro-ecosystem when applied in vivo. However, its expression level was relatively low (748.63 mg/L). In view of these situations, this study will optimize and modify the structure of Plectasin, hoping to obtain candidates with high expression, no/low toxicity, and maintain desirable antibacterial activity. Through sequence alignment, Plectasin was used as a template to introduce the degenerate bases, and the screening library was constructed. After three different levels of screening, the candidate sequence PN7 was obtained, and its total protein yield in the supernatant was 5.53 g/L, with the highest value so far for the variants or constructs from the same ancestor source. PN7 had strong activity against several species of Gram-positive bacteria (MIC value range 1~16 μg/mL). It was relatively stable in various conditions in vitro; in addition, the peptide showed no toxicity to mice for 1 week after intraperitoneal injection. Meanwhile, PN7 kills Staphylococcus aureus ATCC 43300 with a mode of a quicker (>99% S. aureus was killed within 2 h, whereas vancomycin at 2× MIC was 8 h.) and longer PAE period. The findings indicate that PN7 may be a novel promising antimicrobial agent, and this study also provides a model or an example for the design, modification, or reconstruction of novel AMPs and their derivatives.

Strategies for improving antimicrobial peptide production

Antimicrobial peptides (AMPs) found in a wide range of animal, insect, and plant species are host defense peptides forming an integral part of their innate immunity. Although the exact mode of action of some AMPs is yet to be deciphered, many exhibit membrane lytic activity or interact with intracellular targets. The ever-growing threat of antibiotic resistance has brought attention to research on AMPs to enhance their clinical use as a therapeutic alternative. AMPs have several advantages over antibiotics such as broad range of antimicrobial activities including anti-fungal, anti-viral and anti-bacterial, and have not reported to contribute to resistance development. Despite the numerous studies to develop efficient production methods for AMPs, limitations including low yield, degradation, and loss of activity persists in many recombinant approaches. In this review, we outline available approaches for AMP production and various expression systems used to achieve higher yield and quality. In addition, recent advances in recombinant strategies, suitable fusion protein partners, and other molecular engineering strategies for improved AMP production are surveyed.

Membrane Permeabilization and Antimicrobial Activity of Recombinant Defensin-d2 and Actifensin against Multidrug-Resistant Pseudomonas aeruginosa and Candida albicans

Antimicrobial resistance requires urgent efforts towards the discovery of active antimicrobials, and the development of strategies to sustainably produce them. Defensin and defensin-like antimicrobial peptides (AMPs) are increasingly gaining pharmacological interest because of their potency against pathogens. In this study, we expressed two AMPs: defensin-d2 derived from spinach, and defensin-like actifensin from Actinomyces ruminicola. Recombinant pTXB1 plasmids carrying the target genes encoding defensin-d2 and actifensin were generated by the MEGAWHOP cloning strategy. Each AMP was first expressed as a fusion protein in Escherichia coli, purified by affinity chromatography, and was thereafter assayed for antimicrobial activity against multidrug-resistant (MDR) pathogens. Approximately 985 µg/mL and 2895 µg/mL of recombinant defensin-d2 and actifensin, respectively, were recovered with high purity. An analysis by MALDI-TOF MS showed distinct peaks corresponding to molecular weights of approximately 4.1 kDa for actifensin and 5.8 kDa for defensin-d2. An in vitro antimicrobial assay showed that MDR Pseudomonas aeruginosa and Candida albicans were inhibited at minimum concentrations of 7.5 µg/mL and 23 µg/mL for recombinant defensin-d2 and actifensin, respectively. The inhibitory kinetics of the peptides revealed cidal activity within 4 h of the contact time. Furthermore, both peptides exhibited an antagonistic interaction, which could be attributed to their affinities for similar ligands, as deduced by peptide–ligand profiling. Moreover, both peptides inhibited biofilm formation, and they exhibited no resistance potential and low hemolytic activity. The peptides also possess the ability to permeate and disrupt the cell membranes of MDR P. aeruginosa and C. albicans. Therefore, recombinant actifensin and defensin-d2 exhibit broad-spectrum antimicrobial activity and have the potential to be used as therapy against MDR pathogens.

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.

Mining, heterologous expression, purification, antibactericidal mechanism, and application of bacteriocins: A review

Bacteriocins are generally considered as low-molecular-weight ribosomal peptides or proteins synthesized by G⁺ and G^- bacteria that inhibit or kill other related or unrelated microorganisms. However, low yield is an important factor restricting the application of bacteriocins. This paper reviews mining methods, heterologous expression in different systems, the purification technologies applied to bacteriocins, and identification methods, as well as the antibacterial mechanism and applications in three different food systems. Bioinformatics improves the efficiency of bacteriocins mining. Bacteriocins can be heterologously expressed in different expression systems (e.g., Escherichia coli, Lactobacillus, and yeast). Ammonium sulfate precipitation, dialysis membrane, pH-mediated cell adsorption/desorption, solvent extraction, macroporous resin column, and chromatography are always used as purification methods for bacteriocins. The bacteriocins are identified through electrophoresis and mass spectrum. Cell envelope (e.g., cell permeabilization and pore formation) and inhibition of gene expression are common antibacterial mechanisms of bacteriocins. Bacteriocins can be added to protect meat products (e.g., beef and sausages), dairy products (e.g., cheese, milk, and yogurt), and vegetables and fruits (e.g., salad, apple juice, and soybean sprouts). The future research directions are also prospected.