Bacteriocin production, plasmid content and plasmid location of enterocin P structural gene in enterococci isolated from food sources

Targeting Enterococci with Antimicrobial Activity against Clostridium perfringens from Poultry

Necrotic enteritis (NE), caused by Clostridium perfringens, is an emerging issue in poultry farming. New approaches, other than antibiotics, are necessary to prevent NE development and the emergence of multidrug-resistant bacteria. Enterococci are commensal microorganisms that can produce enterocins, antimicrobial peptides with activities against pathogens, and could be excellent candidates for protective cultures. This study aimed to screen and characterize Enterococcus strains of poultry origin for their inhibitory activity against C. perfringens. In total, 251 Enterococcus strains of poultry origin plus five bacteriocin-producing (BP+) E. durans strains of other origins were screened for antimicrobial activity against the indicator C. perfringens X2967 strain using the "spot on the lawn" method. We detected thirty-two BP+ strains (eleven Enterococcus faecium, nine E. gallinarum, eight E. faecalis, three E. durans, and one E. casseliflavus). We further studied the antimicrobial activity of the supernatants of these 32 BP+ strains using agar well diffusion and microtitration against a collection of 20 C. perfringens strains. Twelve BP+ enterococci that were found to exhibit antimicrobial activity against C. perfringens were characterized using whole genome sequencing. Among these, E. faecium X2893 and X2906 were the most promising candidates for further studies as protective cultures for poultry farming. Both strains belong to the sequence type ST722, harbor the genes encoding for enterocin A and enterocin B, do not possess acquired resistance genes, do not carry plasmids, and present the acm gene, which is implicated in host colonization. Further research is needed to determine the utility of these strains as protective cultures.

New insights into the role of plasmids from probiotic Lactobacillus pentosus MP-10 in Aloreña table olive brine fermentation

In silico analysis of Lactobacillus pentosus MP-10 plasmids (pLPE-1 to pLPE-5) suggests that plasmid-borne genes mediate the persistence of lactobacilli during olive fermentation and enhance their probiotic properties and their competitiveness in several ecological niches. The role of plasmids in the probiotic activities of L. pentosus MP-10 was investigated by plasmid-curing process which showed that plasmids contribute in increased metal tolerance and the biosequestration of several metals such as iron, aluminium, cobalt, copper, zinc, cadmium and mercury. Statistically significant differences in mucin adhesion were detected between the uncured and the cured L. pentosus MP-10, which possibly relied on a serine-rich adhesin (sraP) gene detected on the pLPE-2 plasmid. However, plasmid curing did not affect their tolerance to gastro-intestinal conditions, neither their growth ability under pre-determined conditions, nor auto-aggregation and pathogen co-aggregation were changed among the cured and uncured L. pentosus MP-10. These findings suggest that L. pentosus MP-10 plasmids play an important role in gastro-intestinal protection due to their attachment to mucin and, thus, preventing several diseases. Furthermore, L. pentosus MP-10 could be used as a bioquencher of metals in the gut, reducing the amount of these potentially toxic elements in humans and animals, food matrices, and environmental bioremediation.

Characterization and Application of Antilisterial Enterocins on Model Fresh Cheese

Enterococcus faecalis strains isolated from an artisanal cheese were selected based on enterocin production against Listeria monocytogenes. The strains formed biofilms and presented high hydrophobic character and good autoaggregation and coaggregation capacity with L. monocytogenes. Strains L3A21M3 and L3B1K3 presented high survival under gastrointestinal conditions, were able to adhere to human intestinal cells (Caco-2 and HT-29), and blocked the adhesion and invasion of L. monocytogenes. The antilisterial activity of enterocins was not affected by pH (2 to 12), heating (100°C), and chemical and surfactant agents. However, strains L3A21M3 and L3A21M8 produced thermolabile enterocins, which were also sensible to extreme pH values. Enterocins exhibited a bacteriostatic mode of action against L. monocytogenes, and maximum production was observed during the stationary phase. Common enterocin structural genes were not detected by PCR amplification with specific primers, although an exhaustive screening was not performed. The enterocin produced by the L3B1K3 strain was purified and applied to model cheeses contaminated with L. monocytogenes. This enterocin reduced survival of L. monocytogenes on fresh cheeses in a dose-dependent manner. The highest dose tested (2,048 arbitrary units per g of cheese) was effective in reducing the pathogen counts to undetectable values throughout storage (6 to 72 h). These results suggest that these strains have great potential to be used as biopreservatives in the food industry and also as probiotics, with the potential to prevent L. monocytogenes gastrointestinal infection.

Investigation of antimicrobial activity and entA and entB genes in Enterococcus faecium and Enterococcus faecalis strains isolated from naturally fermented Turkish white cheeses

In this research, the antagonistic effects of Enterococcus faecalis and E. faecium bacteria isolated from naturally fermented Turkish white cheeses, produced without starter culture, against Listeria monocytogenes, L. innocua, L. ivanovii, Staphylococcus aureus, and E. faecalis were evaluated. The presence of entA and entB genes was also detected in the isolates, which had antimicrobial activity. Total 71 strains of E. faecalis and 7 strains of E. faecium were tested; 20 of E. faecalis and none of E. faecium strains showed antimicrobial activity against the tested bacteria using agar spot method. Among E. faecalis strains, which had antimicrobial activity, three strains contained both entA and entB genes, two strains carried only entA gene, and five strains had only entB gene. These cheese-sourced enterococcal strains or their enterocins should be considered to be used for food preservation especially in the dairy industry.

Intraguild predation provides a selection mechanism for bacterial antagonistic compounds

Bacteriocins are bacterial proteinaceous toxins with bacteriostatic or bacteriocidal activity towards other bacteria. The current theory on their biological role concerns especially colicins, with underlying social interactions described as an example of spite. This leads to a rock-paper-scissors game between colicin producers and sensitive and resistant variants. The generality of this type of selection mechanism has previously been challenged with lactic acid bacterial (LAB) bacteriocins as an example. In the natural environment of LAB, batch cultures are the norm opposed to the natural habitats of Escherichia coli where continuous cultures are prevailing. This implies that fitness for LAB, to a large degree, is related to survival rates (bottleneck situations) rather than to growth rates. We suggest that the biological role of LAB bacteriocins is to enhance survival in the stationary growth phase by securing a supply of nutrients from lysed target cells. Thus, this social interaction is an example of selfishness rather than of spite. Specifically, it fits into an ecological model known as intraguild predation (IGP), which is a combination of competition and predation where the predator (LAB bacteriocin producer) and prey (bacteriocin susceptible bacteria) share similar and often limited resources. We hypothesize that IGP may be a common phenomenon promoting microbial production of antagonistic compounds.

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 an anti-listerial enterocin from wheat silage based Enterococcus faecium

Two Enterococcus faecium and one E. faecalis strains isolated and identified from wheat silage were characterized based on plasmid content, hemolytic activity, antibiotic resistance patterns, bacteriocin production potential, and presence of enterocin structural genes (entA, entB, entP, entL50B). Among the isolates, only the E. faecium U7 strain exhibited bacteriocin activity against Listeria monocytogenes ATCC 7644, and vancomycin resistant Enterococcus spp. (VRE). A combination of three structural genes (entA, entB, and entP) was detected in E. faecium U7. A relationship between the presence of enterocin structural genes, and bacteriocin activity was detected in E. faecium U7; therefore partially purified enterocin (PPE) was further investigated from the isolate. Several bands of different molecular weights were expressed from PPE extracts following tricine SDS-PAGE analysis. However, the only band showing bacteriocin activity was in an approximate 4-kDa region. PPE treatment with proteinase K, lysozyme, and α -amylase caused complete loss of bacteriocin activity. PPE heat treatment at various temperatures resulted in a notable reduction in bacteriocin expression. Enterocin U7 was relatively heat stable, and presumably exhibits a glucoprotein nature with distinct inhibitory properties. Specific bacterial inhibitory activity of enterocin U7, and the producer strain absence of β -hemolysis and vancomycin susceptibility features deserves further investigation to evaluate its potential application in silage inoculation and food preservation.

Genetic identification of the bacteriocins produced by Enterococcus faecium IT62 and evidence that bacteriocin 32 is identical to enterocin IT

Enterococcus faecium IT62, a strain isolated from ryegrass in Japan, produces three bacteriocins (enterocins L50A, L50B, and IT) that have been previously purified and the primary structures of which have been determined by amino acid sequencing (E. Izquierdo, A. Bednarczyk, C. Schaeffer, Y. Cai, E. Marchioni, A. Van Dorsselaer, and S. Ennahar, Antimicrob. Agents Chemother., 52:1917-1923, 2008). Genetic analysis showed that the bacteriocins of E. faecium IT62 are plasmid encoded, but with the structural genes specifying enterocin L50A and enterocin L50B being carried by a plasmid (pTAB1) that is separate from the one (pTIT1) carrying the structural gene of enterocin IT. Sequencing analysis of a 1,475-bp region from pTAB1 identified two consecutive open reading frames corresponding, with the exception of 2 bp, to the genes entL50A and entL50B, encoding EntL50A and EntL50B, respectively. Both bacteriocins are synthesized without N-terminal leader sequences. Genetic analysis of a sequenced 1,380-bp pTIT1 fragment showed that the genes entIT and entIM, encoding enterocin IT and its immunity protein, respectively, were both found in E. faecium VRE200 for bacteriocin 32. Enterocin IT, a 6,390-Da peptide made up of 54 amino acids, has been previously shown to be identical to the C-terminal part of bacteriocin 32, a 7,998-Da bacteriocin produced by E. faecium VRE200 whose structure was deduced from its structural gene (T. Inoue, H. Tomita, and Y. Ike, Antimicrob. Agents Chemother., 50:1202-1212, 2006). By combining the biochemical and genetic data on enterocin IT, it was concluded that bacteriocin 32 is in fact identical to enterocin IT, both being encoded by the same plasmid-borne gene, and that the N-terminal leader peptide for this bacteriocin is 35 amino acids long and not 19 amino acids long as previously reported.

Functional properties of novel protective lactic acid bacteria and application in raw chicken meat against Listeria monocytogenes and Salmonella enteritidis

In this study 635 lactic acid bacteria of food origin were evaluated for their potential application as protective cultures in foods. A stepwise selection method was used to obtain the most appropriate strains for application as protective cultures in chicken meat. Specifically, all strains were examined for antimicrobial activity against various Gram positive and Gram negative pathogenic and spoilage bacteria. Strains exhibiting anti-bacterial activity were subsequently examined for survival in simulated food processing and gastrointestinal tract conditions, such as high temperatures, low pH, starvation and the presence of NaCl and bile salts. Selected strains where then examined for basic safety properties such as antibiotic resistance and haemolytic potential, while their antimicrobial activity was further investigated by PCR screening for possession of known bacteriocin genes. Two chosen strains were then applied on raw chicken meat to evaluate their protective ability against two common food pathogens, Listeria monocytogenes and Salmonella enteritidis, but also to identify potential spoilage effects by the application of the protective cultures on the food matrix. Antimicrobial activity in vitro was evident against Gram positive indicators, mainly Listeria and Brochothrix spp., while no antibacterial activity was obtained against any of the Gram negative bacteria tested. The antimicrobial activity was of a proteinaceous nature while strains with anti-listerial activity were found to possess one or more bacteriocin genes, mainly enterocins. Strains generally exhibited sensitivity to pH 2.0, but good survival at 45 degrees C, in the presence of bile salts and NaCl as well as during starvation, while variable survival rates were obtained at 55 degrees C. None of the strains was found to be haemolytic while variable antibiotic resistance profiles were obtained. Finally, when the selected strains Enterococcus faecium PCD71 and Lactobacillus fermentum ACA-DC179 were applied as protective cultures in chicken meat against L. monocytogenes and S. enteritidis respectively, a significantly reduced growth of these pathogenic bacteria was observed. In addition, these two strains did not appear to have any detrimental effect on biochemical parameters related to spoilage of the chicken meat.

Characteristics of bacteriocin-like inhibitory substances from dochi-isolated Enterococcus faecium D081821 and D081833

AIMS

To characterize bacteriocin-like inhibitory substances (BLIS) from two dochi-isolated Enterococcus faecium.

METHODS AND RESULTS

Enterococcus faecium D081821 and D081833 were isolated from dochi (a traditional fermented food in Taiwan) and found to produce BLIS with inhibitory activities against Listeria monocytogenes, Clostridium perfringens, and Bacillus cereus. Strains D081821 and D081833 showed different growth temperatures and their BLIS showed different sensitivities to heat, proteolytic enzymes, and antibacterial spectra. Both BLIS were collected, partially purified, and analysed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). SDS-PAGE showed that both partially purified BLIS were approximately 3.0 kDa in size.

CONCLUSIONS

These results indicate that E. faecium D081821 and D081833 produce different BLIS with strong antibacterial actions against the tested pathogenic bacteria.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results of this study suggest that two different BLIS from dochi-isolated lactic acid bacteria have potential for use as food preservatives.

Diversity of enterococcal bacteriocins and their grouping in a new classification scheme

Enterococci are lactic acid bacteria of importance in food, public health and medical microbiology. Many strains produce bacteriocins, some of which have been well characterized. This review describes the structural and genetic characteristics of enterocins, the bacteriocins produced by enterococci. Some of these can be grouped with typical bacteriocins produced by lactic acid bacteria according to traditional classification, whereas others are atypical and structurally distinct from the general classes of bacteriocins. These atypical enterocins recently played an important role in and prompted reclassification of the class II bacteriocins into a new scheme. In this review, a more simplified classification scheme for enterocins based on amino acid sequence homologies is proposed. Enterocins are of interest for their diversity and potential for use as food biopreservatives. The emergence of multiple antibiotic-resistant enterococci among agents of nosocomial disease and the presence of virulence factors among food isolates requires a careful safety evaluation of isolates intended for potential biotechnical use. Nevertheless, enterococcal bacteriocins produced by heterologous hosts or added as cell-free preparations may still be attractive for application in food preservation.

Single nucleotide polymorphism analysis of the enterocin P structural gene of Enterococcus faecium strains isolated from nonfermented animal foods

The bacteriocins produced by two lactic acid bacteria isolated from nonfermented fresh meat and fish, respectively, and exhibiting a remarkable antilisterial activity, were characterized. Bacteriocinogenic strains were identified as Enterococcus faecium and the maximum bacteriocin production by both strains was detected in the stationary phase of growth. The activity against Listeria monocytogenes was maintained in pH range of 3-7 and was stable in both strains after heating at 100 or 121 degrees C. The genes coding for enterocin P were detected, isolated, and sequenced in both E. faecium strains. They exhibited DNA/DNA homology in the 87.1-97.2% range with respect to the other four enterocin P genes reported so far. Three single nucleotide polymorphism events, silent at the amino acid level, were detected at nucleotide positions 45 (G/A), 75 (A/G), and 90 (T/C) in E. faecium LHICA 28-4 and may explain the differences reported for those loci in other enterocin P-producing E. faecium strains. This work provides the first description of enterocin P-producing E. faecium strains in nonfermented foodstuffs and, in the case of E. faecium LHICA 51, the first report of an enterocin P-producing strain isolated from fish so far.