Advances in the preclinical characterization of the antimicrobial peptide AS-48

Antimicrobial resistance is a natural and inevitable phenomenon that constitutes a severe threat to global public health and economy. Innovative products, active against new targets and with no cross- or co-resistance with existing antibiotic classes, novel mechanisms of action, or multiple therapeutic targets are urgently required. For these reasons, antimicrobial peptides such as bacteriocins constitute a promising class of new antimicrobial drugs under investigation for clinical development. Here, we review the potential therapeutic use of AS-48, a head-to-tail cyclized cationic bacteriocin produced by Enterococcus faecalis. In the last few years, its potential against a wide range of human pathogens, including relevant bacterial pathogens and trypanosomatids, has been reported using in vitro tests and the mechanism of action has been investigated. AS-48 can create pores in the membrane of bacterial cells without the mediation of any specific receptor. However, this mechanism of action is different when susceptible parasites are studied and involves intracellular targets. Due to these novel mechanisms of action, AS-48 remains active against the antibiotic resistant strains tested. Remarkably, the effect of AS-48 against eukaryotic cell lines and in several animal models show little effect at the doses needed to inhibit susceptible species. The characteristics of this molecule such as low toxicity, microbicide activity, blood stability and activity, high stability at a wide range of temperatures or pH, resistance to proteases, and the receptor-independent effect make AS-48 unique to fight a broad range of microbial infections, including bacteria and some important parasites.

Beneficial Shifts in the Gut Bacterial Community of Gilthead Seabream (Sparus aurata) Juveniles Supplemented with Allium-Derived Compound Propyl Propane Thiosulfonate (PTSO)

Simple Summary

Aquaculture plays an important role in supplying global food demand and protein sources. The increasing restriction of drugs in fish production has forced this sector to carry out changes in the management of farms. Functional feed additives such as probiotics, prebiotics, and phytogenics have been proposed in order to maintain or improve productive levels and general health status of fish. In this study, we explore the effects of Allium-derived food additives in the bacterial community and growth of gilthead seabream (Sparus aurata) juveniles. We found that this additive produced significant changes in bacterial community of the hindgut. In this sense, this shift occurred towards a more diverse microbiota. Especially relevant is the decrease in the populations of potential pathogenic bacteria as Vibrio and Pseudomonas, while this additive enhanced Lactobacillus, a well-known beneficial genus. Our work shows that the addition of PTSO has beneficial effects on bacterial communities while keeping productive parameters on fish growth.

Abstract

This study analyzes the potential use of an Allium-derived compound, propyl propane thiosulfonate (PTSO), as a functional feed additive in aquaculture. Gilthead seabream (Sparus aurata) juveniles had their diet supplemented with this Allium-derived compound (150 mg/kg of PTSO) and were compared with control fish. The effects of this organosulfur compound were tested by measuring the body weight and analyzing the gut microbiota after 12 weeks. The relative abundance of potentially pathogenic Vibrio and Pseudomonas in the foregut and hindgut of supplemented fish significantly decreased, while potentially beneficial Lactobacillus increased compared to in the control fish. Shannon’s alpha diversity index significantly increased in both gut regions of fish fed with a PTSO-supplemented diet. Regarding beta diversity, significant differences between treatments only appeared in the hindgut when minority ASVs were taken into account. No differences occurred in body weight during the experiment. These results indicate that supplementing the diet with Allium-derived PTSO produced beneficial changes in the intestinal microbiota while maintaining the productive parameters of gilthead seabream juveniles.

The Role of Key Amino Acids in the Antimicrobial Mechanism of a Bacteriocin Model Revealed by Molecular Simulations

The AS-48 bacteriocin is a potent antimicrobial polypeptide with enhanced stability due to its circular sequence of peptidic bonds. The mechanism of biological action is still not well understood in spite of both the elucidation of the molecular structure some years ago and several experiments performed that yielded valuable information about the AS-48 bacterial membrane poration activity. In this work, we present a computational study at an atomistic scale to analyze the membrane disruption mechanism. The process is based on the two-stage model: (1) peptide binding to the bilayer surface and (2) membrane poration due to the surface tension exerted by the peptide. Indeed, the induced membrane tension mechanism is able to explain stable formation of pores leading to membrane disruption. The atomistic detail obtained from the simulations allows one to envisage the contribution of the different amino acids during the poration process. Clustering of cationic residues and hydrophobic interactions between peptide and lipids seem to be essential ingredients in the process. GLU amino acids have shown to enhance the membrane disrupting ability of the bacteriocin. TRP24–TRP24 interactions make also an important contribution in the initial stages of the poration mechanism. The detailed atomistic information obtained from the simulations can serve to better understand bacteriocin structural characteristics to design more potent antimicrobial therapies.

Antibacterial directed chemotherapy using AS-48 peptide immobilized on biomimetic magnetic nanoparticles combined with magnetic hyperthermia

The design of new strategies to increase the effectiveness of the antibacterial treatments is a main goal in public health. So, the aim of the study was to achieve a local antibacterial directed therapy as novel alternative allowing both, the delivery of the drug at the target, while minimizing undesirable side effects, thus anticipating an enhanced effectiveness. Hence, we have developed an innovative nanoformulation composed by biomimetic magnetic nanoparticles functionalized with the antimicrobial peptide AS-48 and its potential against Gram-positive and Gram-negative bacteria, either by itself or combined with magnetic hyperthermia has been investigated. Besides, the physical properties, binding efficiency, stability and mechanism of action of this nanoassembly are analyzed. Remarkably, the nanoassembly has a strong bactericidal effect on Gram-positive bacteria, but surprisingly also on E. coli and, finally, when combined with magnetic hyperthermia, on P. aeruginosa and K. pneumoniae. The results obtained represent a breakthrough since it allows a local treatment of infections, reducing and concentrating the dose of antimicrobial compounds, avoiding secondary effects, including the resistance generation and particularly because the combination with magnetic hyperthermia helps sensitizing resistant bacteria to the bactericidal effect of AS-48. Thus, this new formulation should be considered a promising tool in the antibacterial fight.

A glutamic acid-based traceless linker to address challenging chemical protein syntheses

Native chemical ligation (NCL) enables the total chemical synthesis of proteins. However, poor peptide segment solubility remains a frequently encountered challenge. Here we introduce a traceless linker that can be temporarily attached to Glu side chains to overcome this problem. This strategy employs a new tool, Fmoc-Glu(AlHx)-OH, which can be directly installed using standard Fmoc-based solid-phase peptide synthesis. The incorporated residue, Glu(AlHx), is stable to a wide range of chemical protein synthesis conditions and is removed through palladium-catalyzed transfer under aqueous conditions. General handling characteristics, such as efficient incorporation, stability and rapid removal were demonstrated through a model peptide modified with Glu(AlHx) and a Lys6 solubilizing tag. Glu(AlHx) was incorporated into a highly insoluble peptide segment during the total synthesis of the bacteriocin AS-48. This challenging peptide was successfully synthesized and folded, and it has comparable antimicrobial activity to the native AS-48. We anticipate widespread use of this easy-to-use, robust linker for the preparation of challenging synthetic peptides and proteins.

Antimicrobial Activity of the Circular Bacteriocin AS-48 against Clinical Multidrug-Resistant Staphylococcus aureus

The treatment and hospital-spread-control of methicillin-resistant Staphylococcus aureus (MRSA) is an important challenge since these bacteria are involved in a considerable number of nosocomial infections that are difficult to treat and produce prolonged hospitalization, thus also increasing the risk of death. In fact, MRSA strains are frequently resistant to all β-lactam antibiotics, and co-resistances with other drugs such as macrolides, aminoglycosides, and lincosamides are usually reported, limiting the therapeutical options. To this must be added that the ability of these bacteria to form biofilms on hospital surfaces and devices confer high antibiotic resistance and favors horizontal gene transfer of genetic-resistant mobile elements, the spreading of infections, and relapses. Here, we genotypically and phenotypically characterized 100 clinically isolated S. aureus for their resistance to 18 antibiotics (33% of them were OXA resistant MRSA) and ability to form biofilms. From them, we selected 48 strains on the basis on genotype group, antimicrobial-resistance profile, and existing OXA resistance to be assayed against bacteriocin AS-48. The results showed that AS-48 was active against all strains, regardless of their clinical source, genotype, antimicrobial resistance profile, or biofilm formation capacity, and this activity was enhanced in the presence of the antimicrobial peptide lysozyme. Finally, we explored the effect of AS-48 on formed S. aureus biofilms, observing a reduction in S. aureus S-33 viability. Changes in the matrix structure of the biofilms as well as in the cell division process were observed with scanning electron microscopy in both S-33 and S-48 S. aureus strains.

Enterocin Cross-Resistance Mediated by ABC Transport Systems

In their struggle for life, bacteria frequently produce antagonistic substances against competitors. Antimicrobial peptides produced by bacteria (known as bacteriocins) are active against other bacteria, but harmless to their producer due to an associated immunity gene that prevents self-inhibition. However, knowledge of cross-resistance between different types of bacteriocin producer remains very limited. The immune function of certain bacteriocins produced by the Enterococcus genus (known as enterocins) is mediated by an ABC transporter. This is the case for enterocin AS-48, a gene cluster that includes two ABC transporter-like systems (Transporter-1 and 2) and an immunity protein. Transporter-2 in this cluster shows a high similarity to the ABC transporter-like system in MR10A and MR10B enterocin gene clusters. The aim of our study was to determine the possible role of this ABC transporter in cross-resistance between these two different types of enterocin. To this end, we designed different mutants (Tn5 derivative and deletion mutants) of the as-48 gene cluster in Enterococcus faecalis and cloned them into the pAM401 shuttle vector. Antimicrobial activity assays showed that enterocin AS-48 Transporter-2 is responsible for cross-resistance between AS-48 and MR10A/B enterocin producers and allowed identification of the MR10A/B immunity gene system. These findings open the way to the investigation of resistance beyond homologous bacteriocins.

Assessing the effectiveness of AS-48 in experimental mice models of Chagas' disease

OBJECTIVES

We report the in vivo trypanocidal activity of the bacteriocin AS-48 (lacking toxicity), which is produced by Enterococcus faecalis, against the flagellated protozoan Trypanosoma cruzi, the aetiological agent of Chagas' disease.

METHODS

We determined the in vivo activity of AS-48 against the T. cruzi Arequipa strain in BALB/c mice (in both acute and chronic phases of Chagas' disease). We evaluated the parasitaemia, the reactivation of parasitaemia after immunosuppression and the nested parasites in the chronic phase by PCR in target tissues.

RESULTS

AS-48 reduced the parasitaemia profile in acute infection and showed a noteworthy reduction in the parasitic load in chronic infection after immunosuppression according to the results obtained by PCR (double-checking to demonstrate cure).

CONCLUSIONS

AS-48 is a promising alternative that provides a step forward in the development of a new therapy against Chagas' disease.

Allium Extract Implements Weaned Piglet's Productive Parameters by Modulating Distal Gut Microbiota

Antimicrobial resistance (AMR) has risen as a global threat for human health. One of the leading factors for this emergence has been the massive use of antibiotics growth-promoter (AGPs) in livestock, enhancing the spread of AMR among human pathogenic bacteria. Thus, several alternatives such as probiotics, prebiotics, or phytobiotics have been proposed for using in animal feeding to maintain or improve productive levels while diminishing the negative effects of AGPs. Reducing the use of antibiotics is a key aspect in the pig rearing for production reasons, as well as for the production of high-quality pork, acceptable to consumers. Here we analyze the potential use of Allium extract as an alternative. In this study, weaned piglets were fed with Allium extract supplementation and compared with control and antibiotic (colistin and zinc oxide) treated piglets. The effects of Allium extract were tested by analyzing the gut microbiome and measuring different productive parameters. Alpha diversity indices decreased significantly in Allium extract group in caecum and colon. Regarding beta diversity, significant differences between treatments appeared only in caecum and colon. Allium extract and antibiotic piglets showed better values of body weight (BW), average daily weight gain (ADG), and feed conversion ratio (FCR) than control group. These results indicate that productive parameters can be implemented by modifying the gut microbiota through phytobiotics such as Allium extract, which will drive to drop the use of antibiotics in piglet diet.

Allium-Based Phytobiotic Enhances Egg Production in Laying Hens through Microbial Composition Changes in Ileum and Cecum

Phytobiotics (bioactive compounds extracted from plants) are one of the explored alternatives to antibiotics in poultry and livestock due to their antimicrobial activity and its positive effects on gut microbiota and productive properties. In this study, we supplemented a product based on garlic and onion compounds in the diet to laying hens at the beginning of their productive life (from 16 to 20 weeks post-hatching). The experimental group showed a significant increase in the number of eggs laid and in their size, produced in one month compared to the control. This increase in production was accompanied by microbiota changes in the ileum and cecum by means of high throughput sequencing analyses. These bacterial shifts in the ileum were mainly the result of compositional changes in the rare biosphere (unweighted UniFrac), while in the cecum, treatment affected both majority and minority bacterial groups (weighted and unweighted UniFrac). These changes in the microbiota suggest an improvement in food digestibility. The relative abundance of Lactococcus in the ileum and Lactobacillus in the cecum increased significantly in the experimental group. The relative abundance of these bacterial genera are known to have positive effects on the hosts. These results are very promising for the use of these compounds in poultry for short periods.

Synergy of the Bacteriocin AS-48 and Antibiotics against Uropathogenic Enterococci

The genus Enterococcus comprises a ubiquitous group of Gram-positive bacteria that can cause diverse health care-associated infections. Their genome plasticity enables easy acquisition of virulence factors as well as antibiotic resistances. Urinary tract infections (UTIs) and catheter-associated UTIs are common diseases caused by enterococci. In this study, Enterococcus strains isolated from UTIs were characterized, showing that the majority were E. faecalis and contained several virulence factors associated to a better colonization of the urinary tract. Their susceptibility against the bacteriocin AS-48 and several antibiotics was tested. AS-48 is a potent circular bacteriocin that causes bacterial death by pore formation in the cell membrane. The interest of this bacteriocin is based on the potent inhibitory activity, the high stability against environmental conditions, and the low toxicity. AS-48 was active at concentrations below 10 mg/L even against antibiotic-resistant strains, whereas these strains showed resistance to, at least, seven of the 20 antibiotics tested. Moreover, the effect of AS-48 combined with antibiotics commonly used to treat UTIs was largely synergistic (with up to 100-fold MIC reduction) and only occasionally additive. These data suggest AS-48 as a potential novel drug to deal with or prevent enterococcal infections.

Subchronic toxicity study in BALBc mice of enterocin AS-48, an anti-microbial peptide produced by Enterococcus faecalis UGRA10

Few studies have examined the use of animal models to evaluate the in-vivo toxicity of antimicrobial peptides, but such research is essential to their safe use in foods. This study was performed to evaluate any adverse effects of enterocin AS-48, a circular bacteriocin produced by Enterococcus strains, when administered to BALB/c mice at concentrations of 50, 100, and 200 mg/kg in the diet for 90 days. Animals dosed with nisin at a dietary concentration of 200 mg/kg served as a reference treated group. There were no deaths in any of the animal groups, and the AS-48 treatment produced no abnormalities or clinical signs on body weights, food consumption, urinalysis, haematology, or blood biochemistry. Furthermore, there were no significant differences in the weights of liver, spleen, heart, kidneys, and intestines between control mice and those treated with AS-48 or nisin. The histopathological study showed moderate vacuolar degeneration in hepatocytes of some animals fed 100 or 200 mg/kg AS-48 (3/10 and 2/10 respectively). However, this anomaly was lower than in the group treated with nisin (5/10). Conclusively, no toxicologically significant changes were associated in BALB/c mice fed with 50, 100, and 200 mg/kg enterocin AS-48 for 90 days.

Preclinical studies of toxicity and safety of the AS-48 bacteriocin

The in vitro antimicrobial potency of the bacteriocin AS-48 is well documented, but its clinical application requires investigation, as its toxicity could be different in in vitro (haemolytic and antibacterial activity in blood and cytotoxicity towards normal human cell lines) and in vivo (e.g. mice and zebrafish embryos) models. Overall, the results obtained are promising. They reveal the negligible propensity of AS-48 to cause cell death or impede cell growth at therapeutic concentrations (up to 27 μM) and support the suitability of this peptide as a potential therapeutic agent against several microbial infections, due to its selectivity and potency at low concentrations (in the range of 0.3-8.9 μM). In addition, AS-48 exhibits low haemolytic activity in whole blood and does not induce nitrite accumulation in non-stimulated RAW macrophages, indicating a lack of pro-inflammatory effects. The unexpected heightened sensitivity of zebrafish embryos to AS-48 could be due to the low differentiation state of these cells. The low cytotoxicity of AS-48, the absence of lymphocyte proliferation in vivo after skin sensitization in mice, and the lack of toxicity in a murine model support the consideration of the broad spectrum antimicrobial peptide AS-48 as a promising therapeutic agent for the control of a vast array of microbial infections, in particular, those involved in skin and soft tissue diseases.

Egg Production in Poultry Farming Is Improved by Probiotic Bacteria

Antimicrobial resistance (AMR) is one of the most serious threats for human health in the near future. Livestock has played an important role in the appearance of antibiotic-resistant bacteria, intestinal dysbiosis in farming animals, or the spread of AMR among pathogenic bacteria of human concern. The development of alternatives like probiotics is focused on maintaining or improving production levels while diminishing these negative effects of antibiotics. To this end, we supplied the potential probiotic Enterococcus faecalis UGRA10 in the diet of laying hens at a final concentration of 10⁸ Colony Forming Units per gram (CFU/g) of fodder. Its effects have been analyzed by: (i) investigating the response of the ileum and caecum microbiome; and (ii) analyzing the outcome on eggs production. During the second half of the experimental period (40 to 76 days), hens fed E. faecalis UGRA10 maintained egg production, while control animals dropped egg production. Supplementation diet with E. faecalis UGRA10 significantly increased ileum and caecum bacterial diversity (higher bacterial operational taxonomic unit richness and Faith’s diversity index) of laying hens, with animals fed the same diet showing a higher similarity in microbial composition. These results point out to the beneficial effects of E. faecalis UGRA10 in egg production. Future experiments are necessary to unveil the underlying mechanisms that mediate the positive response of animals to this treatment.

Insights into Chagas treatment based on the potential of bacteriocin AS-48

Chagas disease caused by the protozoan parasite Trypanosoma cruzi represents a significant public health problem in Latin America, affecting around 8 million cases worldwide. Nowadays is urgent the identification of new antichagasic agents as the only therapeutic options available, Nifurtimox and Benznidazole, are in use for >40 years, and present high toxicity, limited efficacy and frequent treatment failures in the chronic phase of the disease. Recently, it has been described the antiparasitic effect of AS-48, a bacteriocin produced by Enterococcus faecalis, against Trypanosoma brucei and Leishmania spp. In this work, we have demonstrated the in vitro potential of the AS-48 bacteriocin against T. cruzi. Interesting, AS-48 was more effective against the three morphological forms of different T. cruzi strains, and displayed lower cytotoxicity than the reference drug Benznidazole. In addition, AS-48 combines the criteria established as a potential antichagasic agent, resulting in a promising therapeutic alternative. According to the action mechanism, AS-48 trypanocidal activity could be explained in a mitochondrion-dependent manner through a reactive oxygen species production and mitochondrial depolarization, causing a fast and severe bioenergetic collapse.

LAB Bacteriocins Controlling the Food Isolated (Drug-Resistant) Staphylococci

Staphylococci are a group of microorganisms that can be often found in processed food and they might pose a risk for human health. In this study we have determined the content of staphylococci in 7 different fresh goat-milk cheeses. These bacteria were present in all of them, ranging from 103 to 106 CFU/g based on growth on selective media. Thus, a set of 97 colonies was randomly picked for phenotypic and genotypic identification. They could be clustered by RAPD-PCR in 10 genotypes, which were assigned by 16S rDNA sequencing to four Staphylococcus species: Staphylococcus aureus, Staphylococcus chromogenes, S. simulans, and S. xylosus. Representative strains of these species (n = 25) were tested for antibiotic sensitivity, and 11 of them were resistant to at least one of the antibiotics tested, including erythromycin, amoxicillin-clavulanic acid and oxacillin. We also tested two bacteriocins produced by lactic acid bacteria (LAB), namely the circular bacteriocin AS-48 and the lantibiotic nisin. These peptides have different mechanism of action at the membrane level. Nevertheless, both were able to inhibit staphylococci growth at low concentrations ranging between 0.16-0.73 μM for AS-48 and 0.02-0.23 μM for nisin, including the strains that displayed antibiotic resistance. The combined effect of these bacteriocins were tested and the fractional inhibitory concentration index (FICI) was calculated. Remarkably, upon combination, they were active at the low micromolar range with a significant reduction of the minimal inhibitory concentration. Our data confirms synergistic effect, either total or partial, between AS-48 and nisin for the control of staphylococci and including antibiotic resistant strains. Collectively, these results indicate that the combined use of AS-48 and nisin could help controlling (pathogenic) staphylococci in food processing and preventing antibiotic-resistant strains reaching the consumer in the final products.

Autophagic-related cell death of Trypanosoma brucei induced by bacteriocin AS-48

The parasitic protozoan Trypanosoma brucei is the causative agent of human African trypanosomiasis (sleeping sickness) and nagana. Current drug therapies have limited efficacy, high toxicity and/or are continually hampered by the appearance of resistance. Antimicrobial peptides have recently attracted attention as potential parasiticidal compounds. Here, we explore circular bacteriocin AS-48's ability to kill clinically relevant bloodstream forms of T. brucei gambiense, T. brucei rhodesiense and T. brucei brucei. AS-48 exhibited excellent anti-trypanosomal activity in vitro (EC₅₀ = 1–3 nM) against the three T. brucei subspecies, but it was innocuous to human cells at 10⁴-fold higher concentrations. In contrast to its antibacterial action, AS-48 does not kill the parasite through plasma membrane permeabilization but by targeting intracellular compartments. This was evidenced by the fact that vital dye internalization-prohibiting concentrations of AS-48 could kill the parasite at 37 °C but not at 4 °C. Furthermore, AS-48 interacted with the surface of the parasite, at least in part via VSG, its uptake was temperature-dependent and clathrin-depleted cells were less permissive to the action of AS-48. The bacteriocin also caused the appearance of myelin-like structures and double-membrane autophagic vacuoles. These changes in the parasite's ultrastructure were confirmed by fluorescence microscopy as AS-48 induced the production of EGFP-ATG8.2-labeled autophagosomes. Collectively, these results indicate AS-48 kills the parasite through a mechanism involving clathrin-mediated endocytosis of VSG-bound AS-48 and the induction of autophagic-like cell death. As AS-48 has greater in vitro activity than the drugs currently used to treat T. brucei infection and does not present any signs of toxicity in mammalian cells, it could be an attractive lead compound for the treatment of sleeping sickness and nagana.

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AS-48 kills Trypanosoma brucei efficiently and is innocuous in mammalian cells.

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It has greater in vitro activity than drugs currently in use.

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AS-48 must be internalized by the parasite in order to exert its trypanocidal effect.

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AS-48 uptake involves VSG binding and clathrin-mediated endocytosis.

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AS-48 induces an autophagic-related cell death.

The bacteriocin AS-48 requires dimer dissociation followed by hydrophobic interactions with the membrane for antibacterial activity

The molecular mechanism underlining the antibacterial activity of the bacteriocin AS-48 is not known, and two different and opposite alternatives have been proposed. Available data suggested that the interaction of positively charged amino acids of AS-48 with the membrane would produce membrane destabilization and disruption. Alternatively, it has been proposed that AS-48 activity could rely on the effective insertion of the bacteriocin into the membrane. The biological and structural properties of the AS-48G13K/L40K double mutant were investigated to shed light on this subject. Compared with the wild type, the mutant protein suffered an important reduction in the antibacterial activity. Biochemical and structural studies of AS-48G13K/L40K mutant suggest the basis of its decreased antimicrobial activity. Lipid cosedimentation assays showed that the membrane affinity of AS-48G13K/L40K is 12-fold lower than that observed for the wild type. L40K mutation is responsible for this reduced membrane affinity and thus, hydrophobic interactions are involved in membrane association. Furthermore, the high-resolution crystal structure of AS-48G13K/L40K, together with the study of its dimeric character in solution showed that G13K stabilizes the inactive water-soluble dimer, which displays a reduced dipole moment. Our data suggest that the cumulative effect of these three affected properties reduces AS-48 activity, and point out that the bactericidal effect is achieved by the electrostatically driven approach of the inactive water-soluble dimer towards the membrane, followed by the dissociation and insertion of the protein into the lipid bilayer.

The impact of enterocin AS-48 on the shelf-life and safety of sardines (Sardina pilchardus) under different storage conditions

The purpose of this study was to determine the effect of enterocin AS-48, packaged under normal atmosphere (NA), vacuum (VP) or modified atmosphere (MAP) on the shelf life and safety of fresh sardines (Sardina pilchardus) stored at 5 °C. We studied the effect of these hurdles, alone or combined, on the relevant autochthonous bacterial populations. Total volatile basic nitrogen (TVB-N) content was used as indicative of freshness. Levels of biogenic amines cadaverine, putrescine, tyramine, and histamine were also determined. The application of AS-48 did not reduce the mesophilic, psychrotrophic, or Gram negative bacteria viable cell counts under any of the storage conditions tested. AS-48 did cause significant reductions in viable staphylococci counts, especially under VP. In sardines under NA treated with AS-48, the populations of histamine- and tyramine-forming total and lactic acid bacteria (LAB) showed no significant reductions. MAP or VP with AS-48 allowed reductions (significant at some storage times) in histamine- and tyramine-forming LAB. The TVB-N content was also reduced under normal atmosphere and, especially, in sardines stored under MAP. The most interesting results are those concerning the decrease (by several fold) in the levels of the biogenic amines cadaverine, putrescine, tyramine, and histamine determined after treatment with AS-48.

Analysis of the promoters involved in enterocin AS-48 expression

The enterocin AS-48 is the best characterized antibacterial circular protein in prokaryotes. It is a hydrophobic and cationic bacteriocin, which is ribosomally synthesized by enterococcal cells and post-translationally cyclized by a head-to-tail peptide bond. The production of and immunity towards AS-48 depend upon the coordinated expression of ten genes organized in two operons, as-48ABC (where genes encoding enzymes with processing, secretion, and immunity functions are adjacent to the structural as-48A gene) and as-48C1DD1EFGH. The current study describes the identification of the promoters involved in AS-48 expression. Seven putative promoters have been here amplified, and separately inserted into the promoter-probe vector pTLR1, to create transcriptional fusions with the mCherry gene used as a reporter. The activity of these promoter regions was assessed measuring the expression of the fluorescent mCherry protein using the constitutive pneumococcal promoter PX as a reference. Our results revealed that only three promoters PA, P2(2) and PD1 were recognized in Enterococcus faecalis, Lactococcus lactis and Escherichia coli, in the conditions tested. The maximal fluorescence was obtained with PX in all the strains, followed by the P2(2) promoter, which level of fluorescence was 2-fold compared to PA and 4-fold compared to PD1. Analysis of putative factors influencing the promoter activity in single and double transformants in E. faecalis JH2-2 demonstrated that, in general, a better expression was achieved in presence of pAM401-81. In addition, the P2(2) promoter could be regulated in a negative fashion by genes existing in the native pMB-2 plasmid other than those of the as-48 cluster, while the pH seems to affect differently the as-48 promoter expression.

Discovering the bacterial circular proteins: bacteriocins, cyanobactins, and pilins

Over recent years, several examples of natural ribosomally synthesized circular proteins and peptides from diverse organisms have been described. They are a group of proteins for which the precursors must be post-translationally modified to join the N and C termini with a peptide bond. This feature appears to confer a range of potential advantages because these proteins show increased resistance to proteases and higher thermodynamic stability, both of which improve their biological activity. They are produced by prokaryotic and eukaryotic organisms and show diverse biological activities, related mostly to a self-defense or competition mechanism of the producer organisms, with the only exception being the circular pilins. This minireview highlights ribosomally synthesized circular proteins produced by members of the domain Bacteria: circular bacteriocins, cyanobactins, and circular pilins. We pay special attention to the genetic organization of the biosynthetic machinery of these molecules, the role of circularization, and the differences in the possible circularization mechanisms.

Antimicrobial characterization and safety aspects of the bacteriocinogenic Enterococcus hirae F420 isolated from Moroccan raw goat milk

The F420 strain, isolated from raw goat milk and identified as Enterococcus hirae, was selected because of its strong activity against gram-positive bacteria, including Listeria monocytogenes. Interestingly, the F420 strain lacks the virulence genes and decarboxylase activity of histidine, lysine, and ornithine, and it is susceptible to 11 of 14 tested antibiotics, including vancomycin. The antimicrobial compounds produced by E. hirae F420 strain showed high resistance to heat treatment and to acidic and basic pHs. The MALDI-TOF mass spectrometry analysis coupled with the sequence of peptide and structural gene analysis of one of the purified enterocins showed 100% identity with enterocin P (EntP), previously described in E. faecium strains. The structural gene for EntP is located on a plasmid of 65 kb. Other enterocins with molecular mass higher than 7 kDa were also detected. This is the first report of the production of EntP by E. hirae species naturally occurring in foods. The biotechnological characteristics of the F420 strain and its enterocins indicate their potential for application in the control of L. monocytogenes and other undesirable bacteria in food systems.

Characterization of functional, safety, and probiotic properties of Enterococcus faecalis UGRA10, a new AS-48-producer strain

Enterococcus faecalis UGRA10, a new AS-48-producer strain, has been isolated from a Spanish sheep's cheese. The inhibitory substance produced by E. faecalis UGRA10 was purified and characterized using matrix-assisted laser desorption ionization-time of flight mass spectrometry, confirming its identity with AS-48 enterocin (7.150 Da). Subsequent genetic analysis showed the existence of the as-48 gene cluster on a plasmid of approximately 70-kb. The UGRA10 strain was examined for safety properties such as enterococci virulence genes, biogenic amine production, and antibiotic resistance. As for most E. faecalis strains, PCR amplification revealed the existence of gene encoding for GelE, Asa1, Esp, EfaA, and Ace antigens and for tyrosine decarboxylase. This strain was sensitive to most of the antibiotics tested, being resistant only to aminoglycosides, lincosamide, and pristinamicins. In addition, UGRA10 developed an ability to form biofilms and to adhere to Caco 2 and HeLa 229 cells. More interestingly, this strain shows a high ability to interfere with the adhesion of Listeria monocytogenes to Caco 2 cells. Altogether, the results suggest that this broad-spectrum bacteriocin-producing strain has biotechnological potential to be developed as a protective agent in food preservation and as a probiotic.

Expression of linear permutated variants from circular enterocin AS-48

To confirm whether the head-to-tail circularization could be involved in the stability and activity of the circular bacteriocin AS-48, two permutated linear structural as-48A genes have been constructed by circular permutation. The absence of the leaderless linear AS(23/24) and AS(48/49) proteins in Escherichia coli, under all the conditions investigated, supports the idea that the circular backbone is important to stabilize their structure and also indicates the significance of a leader peptide. In fact, the approach taken in this study to generate linear permutated proteins fused to an appropriate partner was sufficient to prevent cellular proteolysis. In this case, the high expression levels found favour their intracellular accumulations as inclusion bodies, which after solubilization showed a propensity to aggregate, thus hindering the specific EK cleavage. This could explain the presence of active hybrid tagged proteins identified in this work. The conserved distribution of hydrophobic and hydrophilic surfaces in the hybrid proteins is responsible for the antibacterial activity. In addition, the opening of the AS-48 molecule between the residues G(23) W(24) connecting the α1/α2 helices, confers greater stability, suggesting that the sequence and/or the free amino acid in the polypeptide chain are critical aspects in the design of new variants.

Structural characterisation of the natively unfolded enterocin EJ97

Bacteriocins belong to the wide variety of antimicrobial ribosomal peptides synthesised by bacteria. Enterococci are Gram-positive, catalase-negative bacteria that produce lactic acid as the major end product of glucose fermentation. Many enterococcal strains produce bacteriocins, named enterocins. We describe in this work, the structural characterisation of the 44 residues-long enterocin EJ97, produced by Enterococcus faecalis EJ97. To this end, we have used a combined theoretical and experimental approach. First, we have characterised experimentally the conformational properties of EJ97 in solution under different conditions by using a number of spectroscopic techniques, namely fluorescence, CD, FTIR and NMR. Then, we have used several bioinformatic tools as an aid to complement the experimental information about the conformational properties of EJ97. We have shown that EJ97 is monomeric in aqueous solution and that it appears to be chiefly unfolded, save some flickering helical- or turn-like structures, probably stabilised by hydrophobic clustering. Accordingly, EJ97 does not show a cooperative sigmoidal transition when heated or upon addition of GdmCl. These conformational features are essentially pH-independent, as shown by NMR assignments at pHs 5.9 and 7.0. The computational results were puzzling, since some algorithms revealed the natively unfolded character of EJ97 (FoldIndex, the mean scaled hydropathy), whereas some others suggested the presence of ordered structure in its central region (PONDR, RONN and IUPRED). A future challenge is to produce much more experimental results to aid the development of accurate software tools for predicting disorder in proteins.

Evaluation of an enterocin AS-48 enriched bioactive powder obtained by spray drying

Enterocin AS-48 is a cationic cyclic bacteriocin produced by Enterococcus faecalis with broad bactericidal activity. Currently we are assaying the efficacy of AS-48 as biopreservative in foods. In this work we have applied the spray drying process to different AS-48 liquid samples to obtain active dried preparations. We have also assayed different methods, heat, UV irradiation and filtration, to inactivate/remove the AS-48 producer cells from the samples. Best results were obtained for the sample from CM-25 cation exchange, for which it was also possible to completely eliminate/inactivate the producer cells by heat or UV irradiation without loss of activity. When added at 0.016% or 5% to Brain Heart Infusion broth or to skim milk, respectively, the AS-48 powder caused early and complete inactivation of Listeria monocytogenes. A partial inhibition of Staphylococcus aureus was achieved in broth and in skim milk supplemented with 2.5% and 10% AS-48 powder, respectively.

Characterization of linear forms of the circular enterocin AS-48 obtained by limited proteolysis

AS-48 is a 70-residue circular peptide from Enterococcus faecalis with a broad antibacterial activity. Here, we produced by limited proteolysis a protein species carrying a single nicking and fragments of 55 and 38 residues. Nicked AS-48 showed a lower helicity by far-ultraviolet circular dichroism and a reduced stability to thermal denaturation, but it was active against the sensitive bacteria assayed. The fragments also partly retained the biological activity of the intact protein. These results indicate that circularization is not required for the bactericidal activity, but it is important to stabilize the native structure. Moreover, it is possible to reduce the sequence to a minimal AS-48 domain without causing inactivation of this bacteriocin.

Bactericidal synergism through enterocin AS-48 and chemical preservatives against Staphylococcus aureus

AIMS

To determine the effectiveness of enterocin AS-48 on Staphylococcus aureus CECT 976 in combination with chemical preservatives at acidic and neutral pH.

METHODS AND RESULTS

At pH 4.5, the activity of AS-48 increases in the presence of lactic acid (1.0%), acetic acid (0.5% and 1.0%), and citric acid (0.3% and 0.6%). This synergistic effect has also been observed during the first 8 h of incubation with benzoate (0.06% and 0.12%) and sorbate (2% and 3%). Interestingly, at pH 7, lactate (1%) increases the inhibitory effect of AS-48, reducing the S. aureus population by 6 log units compared with the control culture. At neutral pH, combinations of AS-48 and sodium tripolyphosphate, STPP (0.3% and 0.5%) also eliminate this pathogen after 24 h.

CONCLUSIONS

These results indicate that enterocin AS-48 could be applied in combination with a range of chemical preservatives in order to increase its efficacy in inhibiting S. aureus.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study supports the potential use of enterocin AS-48 as a biopreservative to control S. aureus in combination with other food-grade chemical hurdles.

Heterologous expression of enterocin AS-48 in several strains of lactic acid bacteria

AIMS

Enterococcus faecalis produces a cationic and circular enterocin, AS-48, of 7149 Da, the genetic determinants of which are located within the pMB2 plasmid. We have compared enterocin AS-48 production by different enterococci species with that of other 'safe' lactic acid bacteris (LAB) (GRAS status) and looked into the subsequent application of this enterocin in food production.

METHODS AND RESULTS

In an effort to exploit this system for the heterologous expression of enterocin AS-48, a number of vectors containing the as-48 cluster were constructed and used to transform several LAB strains (genera Enterococcus, Lactococcus and Lactobacillus)

CONCLUSION

Heterologous production of enterocin AS-48 failed when bacteria other than those belonging to the genus Enterococcus were used as hosts, although expression of a partial level of resistance against AS-48 were always detected, ruling out the possibility of a lack of recognition of the enterococcal promoters.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our results reveal the special capacity of species from the genus Enterococcus to produce AS-48, an enterocin that requires a post-transcriptional modification to generate a circular peptide with a wide range of inhibitory activity against pathogenic and spoilage bacteria. Preliminary experiments in foodstuffs using nonvirulent enterococci with interesting functional properties reveal the possibility of a biotechnological application of these transformants.

Inhibition of Bacillus licheniformis LMG 19409 from ropy cider by enterocin AS-48

AIMS

To determine the activity of enterocin AS-48 against ropy-forming Bacillus licheniformis from cider.

METHODS AND RESULTS

Enterocin AS-48 was tested on B. licheniformis LMG 19409 from ropy cider in MRS-G broth, fresh-made apple juice and in two commercial apple ciders (A and B). Bacillus licheniformis was rapidly inactivated in MRS-G by 0.5 microg ml(-1)AS-48 and in fresh-made apple juice by 3 microg ml(-1). Concentration-dependent inactivation of this bacterium in two commercial apple ciders (A and B) stored at 4, 15 and 30 degrees C for 15 days was also demonstrated. Counts from heat-activated endospores in cider A plus AS-48 decreased very slowly. Application of combined treatments of heat (95 degrees C) and enterocin AS-48 reduced the time required to achieved complete inactivation of intact spores in cider A to 4 min for 6 microg ml(-1) and to 1 min for 12 microg ml(-1). D and z values also decreased as the bacteriocin concentration increased.

CONCLUSION

Enterocin AS-48 can inhibit ropy-forming B. licheniformis in apple cider and increase the heat sensitivity of spores.

SIGNIFICANCE AND IMPACT OF THE STUDY

Results from this study support the potential use of enterocin AS-48 to control B. licheniformis in apple cider.

Treatment of vegetable sauces with enterocin AS-48 alone or in combination with phenolic compounds to inhibit proliferation of Staphylococcus aureus

The antimicrobial activity of enterocin AS-48 against Staphylococcus aureus was tested in vegetable sauces, alone and in combination with phenolic compounds. When added alone at 25 microg/ml, AS-48 inactivated all detectable staphylococci in napoletana and pesto sauces stored at 22 degrees C, but it only caused limited growth inhibition when these sauces were stored at 10 degrees C, as well as in other sauces such as carbonara and green sauce for fish. At 80 microg/ml, AS-48 eliminated all detectable staphylococci in napoletana, pesto, and green sauce for fish regardless of storage temperature, but it still had much more limited effect in carbonara sauce. Antistaphylococcal activity was potentiated significantly when AS-48 was used in combination with the phenolic compounds carvacrol, geraniol, eugenol, terpineol, caffeic acid, p-coumaric acid, citral, and hydrocinnamic acid. The efficacy of the combined treatments depended both on the phenolic compound and the type of sauce. In carbonara sauce stored at 22 degrees C, the combinations of 80 microg/ml AS-48 and 20 mM hydrocinnamic acid or 126 mM carvacrol reduced viable counts of staphylococci below detection limits for up to 30 days.

Control of Listeria monocytogenes in goat's milk and goat's jben by the bacteriocinogenic Enterococcus faecium F58 strain

The bacteriocinogenic Enterococcus faecium F58 strain, a natural goat's jben cheese isolate, lacks decarboxylase activity involved in most biogenic amine formation. It was also sensitive to 13 antibiotics assayed and free of virulence and vancomycin resistance genes. The F58 strain reached the stationary phase after 12 h of growth in sterile goat's milk, and the production of enterocin F-58 (Ent L50) was first detected after 48 h (400 AU/ml), thereafter remaining stable up to 5 days. The effectiveness of the F58 strain in controlling Listeria monocytogenes serovar 4b in reduced fat and whole goat's milk, and in goat's jben has been examined. Coculture experiments of F58-L. monocytogenes in both types of milk demonstrated that listeriae were not eliminated, although reductions by 1 to 4 log units were found. Nevertheless, when the F58 strain was previously inoculated in whole milk and left to grow for 12 h before contamination, the pathogen was completely eliminated after 130 h of coculture. Production of jben cheese contaminated with L. monocytogenes prior to packaging, using preparations of F58-producer strain, caused a significant decrease in the number of viable listeriae, which were undetectable after 1 week of cheese storage at 22 degrees C. Altogether, results from this study suggest that E. faecium F58 strain may be used as an adjunct culture in cheese to control contamination and growth of L. monocytogenes by in situ enterocin production, thus providing an additional hurdle to enhance control of this pathogen.

Application of the broad-spectrum bacteriocin enterocin AS-48 to inhibit Bacillus coagulans in canned fruit and vegetable foods

The enterococcal bacteriocin (enterocin) AS-48 is a broad-spectrum cyclic peptide. Enterocin AS-48 was tested against Bacillus coagulans in three vegetable canned foods: tomato paste (pH 4.64), syrup from canned peaches (pH 3.97), and juice from canned pineapple (pH 3.65). When vegetative cells of B. coagulans CECT (Spanish Type Culture Collection) 12 were inoculated in tomato paste supplemented with 6 microg/ml AS-48 and stored at different temperatures, viable cell counts were reduced by approximately 2.37 (4 degrees C), 4.3 (22 degrees C) and 3.0 (37 degrees C) log units within 24 h storage. After 15-days storage, no viable cells were detected in any sample. Strain B. coagulans CECT 561 showed a poor survival in tomato paste, but surviving cells were also killed by AS-48. The bacteriocin was also very active against B. coagulans CECT 12 vegetative cells in juice from canned pineapple stored at 22 degrees C, and slightly less active in syrup from canned peaches. In food samples supplemented with 1.5% lactic acid, enterocin AS-48 (6 microg/ml) rapidly reduced viable counts of vegetative cells below detection limits within 24 h storage. Addition of glucose and sucrose (10% and 20%) significantly increased bacteriocin activity against vegetative cells of B. coagulans CECT 12. Enterocin AS-48 had no significant effect on B. coagulans CECT 12 spores. However, the combined application of AS-48 and heat (80-95 degrees C for 5 min) significantly increased the effect of thermal treatments on spores.

Characterization of antimicrobial substances produced by Enterococcus faecalis MRR 10-3, isolated from the uropygial gland of the hoopoe (Upupa epops)

The uropygial gland (preen gland) is a holocrine secretory gland situated at the base of the tail in birds which produces a hydrophobic fatty secretion. In certain birds, such as the hoopoe, Upupa epops, the composition of this secretion is influenced by both seasonal and sexual factors, becoming darker and more malodorous in females and in their nestlings during the nesting phase. The secretion is spread throughout the plumage when the bird preens itself, leaving its feathers flexible and waterproof. It is also thought to play a role in defending the bird against predators and parasites. We have isolated from the uropygial secretion of a nestling a bacterium that grows in monospecific culture which we have identified unambiguously by phenotypic and genotypic means as Enterococcus faecalis. The strain in question produces antibacterial substances that are active against all gram-positive bacteria assayed and also against some gram-negative strains. Its peptide nature identifies it as a bacteriocin within the group known as enterocins. Two peptides were purified to homogeneity (MR10A and MR10B), and matrix-assisted laser desorption ionization-time of flight (mass spectrometry) analysis showed masses of 5201.58 and 5207.7 Da, respectively. Amino acid sequencing of both peptides revealed high similarity with enterocin L50A and L50B (L. M. Cintas, P. Casaus, H. Holo, P. E. Hernández, I. F. Nes, and L. S. Håvarstein, J. Bacteriol. 180:1988-1994, 1998). PCR amplification of total DNA from strain MRR10-3 with primers for the L50A/B structural genes and sequencing of the amplified fragment revealed almost identical sequences, except for a single conservative change in residue 38 (Glu-->Asp) in MR10A and two changes in residues 9 (Thr-->Ala) and 15 (Leu-->Phe) in MR10B. This is the first time that the production of bacteriocins by a bacterium isolated from the uropygial gland has been described. The production of these broad-spectrum antibacterial substances by an enterococcal strain living in the uropygial gland may be important to the hygiene of the nest and thus to the health of the eggs and chicks.

Inhibition of toxicogenic Bacillus cereus in rice-based foods by enterocin AS-48

The antimicrobial effect of the broad-spectrum bacteriocin enterocin AS-48 against the toxicogenic psychrotrophic strain Bacillus cereus LWL1 has been investigated in a model food system consisting of boiled rice and in a commercial infant rice-based gruel dissolved in whole milk stored at temperatures of 37 degrees C, 15 degrees C and 6 degrees C. In food samples supplemented with enterocin AS-48 (in a concentration range of 20-35 mug/ml), viable cell counts decreased rapidly over incubation time, depending on the bacteriocin concentration, the temperature of incubation and the food sample. Enterotoxin production at 37 degrees C was also inhibited. Heat sensitivity of endospores increased markedly in food samples supplemented with enterocin AS-48: inactivation of endospores was achieved by heating for 1 min at 90 degrees C in boiled rice or at 95 degrees C in rice-based gruel. Activity of enterocin AS-48 in rice gruel was potentiated by sodium lactate in a concentration-dependent way.

Effect of immersion solutions containing enterocin AS-48 on Listeria monocytogenes in vegetable foods

The effect of immersion solutions containing enterocin AS-48 alone or in combination with chemical preservatives on survival and proliferation of Listeria monocytogenes CECT 4032 inoculated on fresh alfalfa sprouts, soybean sprouts, and green asparagus was tested. Immersion treatments (5 min at room temperature) with AS-48 solutions (25 microg/ml) reduced listeria counts of artificially contaminated alfalfa and soybean sprouts by approximately 2.0 to 2.4 log CFU/g compared to a control immersion treatment in distilled water. The same bacteriocin immersion treatment applied on green asparagus had a very limited effect. During storage of vegetable samples treated with immersion solutions of 12.5 and 25 microg of AS-48/ml, viable listeria counts were reduced below detection limits at days 1 to 7 for alfalfa and soybean sprouts at 6 and 15 degrees C, as well as green asparagus at 15 degrees C. Only a limited inhibition of listeria proliferation was detected during storage of bacteriocin-treated alfalfa sprouts and green asparagus at 22 degrees C. Treatment with solutions containing AS-48 plus lactic acid, sodium lactate, sodium nitrite, sodium nitrate, trisodium phosphate, trisodium trimetaphosphate, sodium thiosulphate, n-propyl p-hydroxybenzoate, p-hydoxybenzoic acid methyl ester, hexadecylpyridinium chloride, peracetic acid, or sodium hypochlorite reduced viable counts of listeria below detection limits (by approximately 2.6 to 2.7 log CFU/g) upon application of the immersion treatment and/or further storage for 24 h, depending of the chemical preservative concentration. Significant increases of antimicrobial activity were also detected for AS-48 plus potassium permanganate and in some combinations with acetic acid, citric acid, sodium propionate, and potassium sorbate.

Synergistic effect of enterocin AS-48 in combination with outer membrane permeabilizing treatments against Escherichia coli O157:H7

AIMS

To determine the effects of outer membrane (OM) permeabilizing agents on the antimicrobial activity of enterocin AS-48 against Escherichia coli O157:H7 CECT 4783 strain in buffer and apple juice.

METHODS AND RESULTS

We determined the influence of pH, EDTA, sodium tripolyphosphate (STPP) and heat on E. coli O157:H7 CECT 4783 sensitivity to enterocin AS-48 in buffer and in apple juice. Enterocin AS-48 was not active against intact cells of E. coli O157:H7 CECT 4783 at neutral pH. However, cells sublethally injured by OM permeabilizing agents (EDTA, STPP, pH 5, pH 8.6 and heat) became sensitive to AS-48, decreasing the amount of bacteriocin required for inhibition of E. coli O157:H7 CECT 4783.

CONCLUSIONS

The results presented indicate that enterocin AS-48 could potentially be applied with a considerably wider range of protective agents, such as OM permeabilizing agents, with increased efficacy in inhibiting E. coli O157:H7.

SIGNIFICANCE AND IMPACT OF THE STUDY

Results from this study support the potential use of enterocin AS-48 to control E. coli O157:H7 in combination with other hurdles.

Control of Alicyclobacillus acidoterrestris in fruit juices by enterocin AS-48

Alicyclobacillus acidoterrestris is a spoilage-causing bacterium in fruit juices. Control of this bacterium by enterocin AS-48 from Enterococcus faecalis A-48-32 is described. Enterocin AS-48 was active against one A. acidocaldarius and three strains of A. acidoterrestris tested. In natural orange and apple juices incubated at 37 degrees C, vegetative cells of A. acidoterrestris DSMZ 2,498 were inactivated by enterocin AS-48 (2.5 microg/ml) and no growth was observed in 14 days. In commercial fruit juices added of AS-48 (2.5 microg/ml) and inoculated with vegetative cells or with endospores of strain DSMZ 2,498, no viable cells were detected during 90 days of incubation at temperatures of 37 degrees C, 15 degrees C or 4 degrees C, except for apple, peach and grapefruit juices inoculated with vegetative cells and incubated at 37 degrees C which were protected efficiently for up to 60 days. Remarkably, in all commercial fruit juices tested, no viable cells were detected as early as 15 min after incubation with the bacteriocin. Endospores incubated for a very short time (1 min) with increasing bacteriocin concentrations were inactivated by 2.5 microg/ml AS-48. Electron microscopy examination of vegetative cells and endospores treated with enterocin AS-48 revealed substantial cell damage and bacterial lysis as well as disorganization of endospore structure.

Stability of enterocin AS-48 in fruit and vegetable juices

Enterocin AS-48 is a candidate bacteriocin for food biopreservation. Before addressing application of AS-48 to vegetable-based foods, the interaction between AS-48 and vegetable food components and the stability of AS-48 were studied. Enterocin AS-48 had variable interactions with fruit and vegetable juices, with complete, partial, or negligible loss of activity. For some juices, loss of activity was ameliorated by increasing the bacteriocin concentration, diluting the juice, or applying a heat pretreatment. In juices obtained from cabbage, cauliflower, lettuce, green beans, celery, and avocado, AS-48 was very stable for the first 24 to 48 h of storage under refrigeration, and decay of activity was markedly influenced by storage temperature. In fresh-made fruit juices (orange, apple, grapefruit, pear, pineapple, and kiwi) and juice mixtures, AS-48 was very stable for at least 15 days at 4 degrees C, and bacteriocin activity was still detectable after 30 days of storage. Gradual and variable loss of activity occurred in juices stored at 15 and 28 degrees C; inactivation was faster at higher temperatures. In commercial fruit juices (orange, apple, peach, and pineapple) stored at 4 degrees C, the bacteriocin was completely stable for up to 120 days, and over 60% of initial activity was still present in juices stored at 15 degrees C for the same period. Commercial fruit juices stored at 28 degrees C for 120 days retained between 31.5% (apple) and 67.71% (peach) of their initial bacteriocin activity. Solutions of AS-48 in sterile distilled water were stable (120 days at 4 to 28 degrees C). Limited loss of activity was observed after mixing AS-48 with some food-grade dyes and thickening agents. Enterocin AS-48 added to lettuce juice incubated at 15 degrees C reduced viable counts of Listeria monocytogenes CECT 4032 and Bacillus cereus LWL1 to below detection limits and markedly reduced viable counts of Staphylococcus aureus CECT 976.