Impairment of the class IIa bacteriocin receptor function and membrane structural changes are associated to enterocin CRL35 high resistance in Listeria monocytogenes

Understanding bacteriocin heterologous expression: A review

Bacteriocins are a diverse group of ribosomally synthesised antimicrobial peptides/proteins that play an important role in self-defence. They are widely used as bio-preservatives and effective substitutes for disease eradication. They can be used in conjunction with or as an alternative to antibiotics to minimize the risk of resistance development. There are remarkably few reports indicating resistance to bacteriocins. Although there are many research reports that emphasise heterologous expression of bacteriocin, there are no convincing reports on the significant role that intrinsic and extrinsic factors play in overexpression. A coordinated and cooperative expression system works in concert with multiple genetic elements encoding native proteins, immunoproteins, exporters, transporters and enzymes involved in the post-translational modification of bacteriocins. The simplest way could be to utilise the existing E. coli expression system, which is conventional, widely used for heterologous expression and has been further extended for bacteriocin expression. In this article, we will review the intrinsic and extrinsic factors, advantages, disadvantages and major problems associated with bacteriocin overexpression in E. coli. Finally, we recommend the most effective strategies as well as numerous bacteriocin expression systems from E. coli, Lactococcus, Kluveromyces lactis, Saccharomyces cerevisiae and Pichia pastoris for their suitability for successful overexpression.

Antibacterial efficacy of Enterococcus microencapsulated bacteriocin on Listeria monocytogenes, Listeria innocua and Listeria ivanovi

This study focused on the microencapsulation of enterocin from Enterococcus durans (E. durans MF5) in whey powder (WP) using a spray-drying technique followed by the evaluation of how complexation can preserve the enterocin structure and antimicrobial activity against food-borne pathogens. Crude enterocin samples (1 and 5%) were microencapsulated in 10% WP. The antimicrobial activity of unencapsulated (crude) enterocin and microencapsulated enterocin was tested against the target bacteria Salmonella Typhimurium, Escherichia coli, Listeria monocytogenes, Listeria innocua, and Listeria ivanovi. The microencapsulation yields were 31.66% and 34.16% for concentrations of 1 and 5% enterocin, respectively. There was no significant difference between these concentrations. Microencapsulated enterocin was efficient for up to 12 h of cocultivation with Listeria sp., and the concentration required to inhibit the growth of target bacteria presented values of 6400 AU/mL (arbitrary unit). Microencapsulated enterocin demonstrated enhanced efficacy against Listeria species and E. coli when compared with crude enterocin (p < 0.05). Fourier transform-infrared spectroscopy and differential scanning calorimetry results confirmed the presence of enterocin in the microparticles. Scanning electron microscopy showed cell damage of the target bacteria. The results showed that complexation with WP preserved enterocin antimicrobial activity during spray-drying, indicating its potential use as a food preservative.

Response of sensitive and resistant Listeria monocytogenes strains against bacteriocins produced by different Enterococcus spp. strains

Listeria monocytogenes is a relevant foodborne pathogen causing invasive listeriosis in humans, a disease with high mortality rates. Its ubiquity and growth characteristics enable this pathogen to survive harsh food processing environments. The addition of bacteriocins, antimicrobial peptides ribosomally synthesized by certain bacteria, appears as a natural alternative to control this pathogen in food. However, the emergence of L. monocytogenes strains resistant to the inhibitory action of bacteriocins has been detected. In order to analyse the development of this resistance, different properties of L. monocytogenes strains susceptible to bacteriocins (strains 01/155, 99/287 and 99/267) and their respective resistant isolates (strains 01/155B6R, 99/287B6R, 99/286C1R, 99/287 Mo1R, 99/287 M1bR, 99/287 M2dR, 99/267B6R), were compared in this work. Differences were analysed in: a) growth of the pathogen strains in direct contact with bacteriocin solution, in co-cultures with the producing strain, or with different sugars; b) response to antibiotics typically used against listeriosis; c) changes in cell morphology, observed by transmission or scanning electron microscopy; d) expression of mobility and haemolysin activity, two of L. monocytogenes main virulence factors; and e) biofilm formation ability. For all the isolates, the acquired resistance was permanent and crossed between the different bacteriocins under study. An inhibitory effect was observed for resistant strains only when they were grown in mixed culture with any of the bacteriocin-producing strains, with an acidified medium as additional growth stress. In all cases, the decrease in viability was lower for resistant strains and followed a particular profile for each strain. The variation of sugar substrate influenced resistant variants growth ability, with a more pronounced difference in the medium supplemented with glucose. Susceptibility to antibiotics was similar or higher for resistant variants, while neither the mobility nor the haemolytic activity presented differences among resistant or susceptible strains. Finally, the resistant variants showed a greater capacity to form biofilms, although this effect was reversed when grown in the presence of bacteriocins. Each resistant isolate had a particular behaviour pattern, and the acquisition of resistance appeared to be strain and bacteriocin dependent. These results contribute to the knowledge of L. monocytogenes bacteriocin-resistance development, which is essential to favour the use of these peptides as biopreservatives.

Biotechnological use of dairy by-products for the production and microencapsulation of the food preservative enterocin CRL35

Bacteriocins from Gram-positive bacteria have been proposed as natural food preservative and there is a need for large-scale production for commercial purposes. The aim of the present work is to evaluate whey, a cheese industrial by-product, for the production and microencapsulation of enterocin CRL35. Whey proved to be a promising basal medium for bacterial growth although the bacteriocin production was quite low. However, it could be much favored with the addition of yeast extract at concentrations as low as 0.5%. Besides improving bacteriocin production, this peptide was successfully microencapsulated by spray drying using whey protein concentrate and a chitosan derivative as wall materials. Microcapsules averaging 10 ± 5 μm diameter were obtained, with good structural integrity and high antimicrobial activity with a stability of at least 12 weeks at 4°C. In summary, sustainable bacteriocin production and microencapsulation was achieved recycling whey or its derivatives. In addition, the formulation owns high antimicrobial activity with a long shelf life. The development of a food preservative may represent a green solution for handling whey.

Phenotypic comparison and DNA sequencing analysis of a wild-type and a pediocin-resistant mutant of Listeria ivanovii

Listeria ivanovii is one of the two pathogenic species within the genus Listeria, the other being Listeria monocytogenes. In this study, we generated a stable pediocin resistant mutant Liv-r1 of a L. ivanovii strain, compared phenotypic differences between the wild-type and the mutant, localised the pediocin-induced mutations in the chromosome, and analysed the mechanisms behind the bacteriocin resistance. In addition to pediocin resistance, Liv-r1 was also less sensitive to nisin. The growth of Liv-r1 was significantly reduced with glucose and mannose, but less with cellobiose. The cells of Liv-r1 adsorbed less pediocin than the wild-type cells. Consequently, with less pediocin on the cell surface, the mutant was also less leaky, as shown as the release of intracellular lactate dehydrogenase to the supernatant. The surface of the mutant cells was more hydrophobic than that of the wild-type. Whole genome sequencing revealed numerous changes in the Liv-r1 chromosome. The mutations were found e.g., in genes encoding sigma-54-dependent transcription regulator and internalin B, as well as in genes involved in metabolism of carbohydrates such as glucose and cellobiose. Genetic differences observed in the mutant may be responsible for resistance to pediocin but no direct evidence is provided.

Antimicrobial activity of enterocins against Listeria sp. and other food spoilage bacteria

OBJECTIVE

To determine bacteriocin producers and the prevalence of structural enterocin genes and to detect the spectrum of activity against foodborne pathogens, from isolates of Enterococcus faecium and Enterococcus faecalis that were isolated from food and the environment.

RESULTS

The entA, entB, entP, ent1071 and entX genes, which encode enterocins were the most frequently observed. Enterocins were thermostable, proteinaceous, and resistant to catalase. None of the isolates produced hemolysin, and inhibition resulting from bacteriophage lysis was excluded. The bactericidal effect of enterocins against L. innocua 12612 was determined by optical density and colony forming units. For the activity spectrum, elimination of mainly Listeria strains, Bacillus sp. and clinical enterococci, was observed. Imaging with scanning electron microscopy after treatment with enterocin Efm22 showed irregular rod-shaped cells and loss of cellular integrity.

CONCLUSIONS

The isolates evaluated in this study are candidates for the production of enterocins that will be used as food biopreservatives, because they have high anti-listerial activity even after 24 h of experimentation, and used in the pharmaceutical area because they inhibit clinical microorganisms.

Dimerization of Antimicrobial Peptides: A Promising Strategy to Enhance Antimicrobial Peptide Activity

Antimicrobial resistance is a global health problem with strong social and economic impacts. The development of new antimicrobial agents is considered an urgent challenge. In this regard, Antimicrobial Peptides (AMPs) appear to be novel candidates to overcome this problem. The mechanism of action of AMPs involves intracellular targets and membrane disruption. Although the exact mechanism of action of AMPs remains controversial, most AMPs act through membrane disruption of the target cell. Several strategies have been used to improve AMP activity, such as peptide dimerization. In this review, we focus on AMP dimerization, showing many examples of dimerized peptides and their effects on biological activity. Although more studies are necessary to elucidate the relationship between peptide properties and the dimerization effect on antimicrobial activity, dimerization constitutes a promising strategy to improve the effectiveness of AMPs.

The case for class II bacteriocins: A biophysical approach using "suicide probes" in receptor-free hosts to study their mechanism of action

Class II bacteriocins are unmodified membrane-active peptides that act over a narrow spectrum of target bacteria. They bind a specific receptor protein on the membrane to form a pore, leading to membrane permeabilization and cell death. However, little is known about the molecular events triggering the pore formation after the bacteriocin recognizes the receptor. It is not clear yet if the pore is the same receptor forced into an open conformation or if the pore results from the bacteriocin insertion and oligomeric assembly in the lipid bilayer. In order to reveal which model is more suitable to explain the toxicity mechanism, in this work we use chimeric peptides, resulting from the fusion of the bitopic membrane protein EtpM with different class II bacteriocins: enterocin CRL35, pediocin PA-1 and microcin V. E. coli strains lacking the specific receptors for these bacteriocins were chosen as expression hosts. As these constructs display a lethal effect when they are heterologously expressed, they are called "suicide probes". The results suggest that, indeed, the specific receptor would act as a docking molecule more than as a structural piece of the pore, as long as the bacteriocin is somehow anchored to the membrane. These set of chimeric peptides also represent an in vivo system that allows to study the interaction of the bacteriocins with real bacterial membranes, instead of model membranes. Hence, the effects of these suicide probes in membrane fluidity and transmembrane potential were also assessed, using fluorescence spectroscopy. The data show that the different suicide probes are able to increase phospholipid order and depolarize the membranes of receptor-free bacterial cells.

Pediocin-Like Antimicrobial Peptides of Bacteria

Bacteriocins are bacterial antimicrobial peptides that, unlike classical peptide antibiotics, are products of ribosomal synthesis and usually have a narrow spectrum of antibacterial activity against species closely related to the producers. Pediocin-like bacteriocins (PLBs) belong to the class IIa of the bacteriocins of Gram-positive bacteria. PLBs possess high activity against pathogenic bacteria from Listeria and Enterococcus genera. Molecular target for PLBs is a membrane protein complex - bacterial mannose-phosphotransferase. PLBs can be synthesized by components of symbiotic microflora and participate in the maintenance of homeostasis in various compartments of the digestive tract and on the surface of epithelial tissues contacting the external environment. PLBs could give a rise to a new group of antibiotics of narrow spectrum of activity.

Recent Progress in the Chemical Synthesis of Class II and S-Glycosylated Bacteriocins

A wide variety of antimicrobial peptides produced by lactic acid bacteria (LAB) have been identified and studied in the last decades. Known as bacteriocins, these ribosomally synthesized peptides inhibit the growth of a wide range of bacterial species through numerous mechanisms and show a great variety of spectrum of activity. With their great potential as antimicrobial additives and alternatives to traditional antibiotics in food preservation and handling, animal production and in veterinary and medical medicine, the demand for bacteriocins is rapidly increasing. Bacteriocins are most often produced by fermentation but, in several cases, the low isolated yields and difficulties associated with their purification seriously limit their use on a large scale. Chemical synthesis has been proposed for their production and recent advances in peptide synthesis methodologies have allowed the preparation of several bacteriocins. Moreover, the significant cost reduction for peptide synthesis reagents and building blocks has made chemical synthesis of bacteriocins more attractive and competitive. From a protein engineering point of view, the chemical approach offers many advantages such as the possibility to rapidly perform amino acid substitution, use unnatural or modified residues, and make backbone and side chain modifications to improve potency, modify the activity spectrum or increase the stability of the targeted bacteriocin. This review summarized synthetic approaches that have been developed and used in recent years to allow the preparation of class IIa bacteriocins and S-linked glycopeptides from LAB. Synthetic strategies such as the use of pseudoprolines, backbone protecting groups, microwave irradiations, selective disulfide bridge formation and chemical ligations to prepare class II and S-glycosylsated bacteriocins are discussed.

Pediocin-like bacteriocins: new perspectives on mechanism of action and immunity

This review attempts to analyze the mechanism of action and immunity of class IIa bacteriocins. These peptides are promising alternative food preservatives and they have a great potential application in medical sciences. Class IIa bacteriocins act on the cytoplasmic membrane of Gram-positive cells dissipating the transmembrane electrical potential by forming pores. However, their toxicity and immunity mechanism remains elusive. Here we discuss the role of the mannose phosphotransferase system (man-PTS) as the receptor for class IIa bacteriocins and the influence of the membrane composition on the activity of these antimicrobial peptides. A model that is consistent with experimental results obtained by different researchers involves the non-specific binding of the bacteriocin to the negatively charged membrane of target bacteria. This step would facilitate a specific binding to the receptor protein, altering its functionality and forming an independent pore in which the bacteriocin is inserted in the membrane. An immunity protein could specifically recognize and block the pore. Bacteriocins function in bacterial ecosystems and energetic costs associated with their production are also discussed. Theoretical models based on solid experimental evidence are vital to understand bacteriocins mechanism of action and to promote new technological developments.