A rapid turbidometric microplate bioassay for accurate quantification of lactic acid bacteria bacteriocins

Effect of LuxS/AI-2-mediated quorum sensing system on bacteriocin production of Lactobacillus plantarum NMD-17

Lactobacillus plantarum NMD-17 separated from koumiss could produce a bacteriocin named plantaricin MX against Gram-positive bacteria and Gram-negative bacteria. The bacteriocin synthesis of L. plantarum NMD-17 was remarkably induced in co-cultivation with Lactobacillus reuteri NMD-86 as the increase of cell numbers and AI-2 activity, and the expressions of luxS encoding signal AI-2 synthetase, plnB encoding histidine protein kinase, plnD encoding response regulator, and plnE and plnF encoding structural genes of bacteriocin were significantly upregulated in co-cultivation, showing that the bacteriocin synthesis of L. plantarum NMD-17 in co-cultivation may be regulated by LuxS/AI-2-mediated quorum sensing system. In order to further demonstrate the role of LuxS/AI-2-mediated quorum sensing system in the bacteriocin synthesis of L. plantarum NMD-17, plasmids pUC18 and pMD18-T simple were used as the skeleton to construct the suicide plasmids pUC18-UF-tet-DF and pMD18-T simple-plnB-tet-plnD for luxS and plnB-plnD gene deletion, respectively. luxS and plnB-plnD gene knockout mutants were successfully obtained by homologous recombination. luxS gene knockout mutant lost its AI-2 synthesis ability, suggesting that LuxS protein encoded by luxS gene is key enzyme for AI-2 synthesis. plnB-plnD gene knockout mutant lost the ability to synthesize bacteriocin against Salmonella typhimurium ATCC14028, indicating that plnB-plnD gene was a necessary gene for bacteriocin synthesis of L. plantarum NMD-17. Bacteriocin synthesis, cell numbers, and AI-2 activity of luxS or plnB-plnD gene knockout mutants in co-cultivation with L. reuteri NMD-86 were obviously lower than those of wild-type strain in co-cultivation at 6-9 h (P < 0.01). The results showed that LuxS/AI-2-mediated quorum sensing system played an important role in the bacteriocin synthesis of L. plantarum NMD-17 in co-cultivation.

In vitro investigation of gastrointestinal stability and toxicity of 3-hyrdoxypropionaldehyde (reuterin) produced by Lactobacillus reuteri

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Reuterin (3-hyrdoxypropionaldehyde (3-HPA)) is a highly potent metabolite of Lactobacillus reuteri.

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Reuterin is highly stable in gastrointestinal condition.

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Human colorectal adenocarcinoma cells’ viability and membrane integrity remained unaltered by reuterin.

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No significant hemolytic activity was detected.

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Reuterin is a promising therapeutic and/or food preservative.

Reuterin (3-hyrdoxypropionaldehyde (3-HPA)) is a highly potent metabolite of L. reuteri, which has applications in food, health, and veterinary sectors. Similar to other natural antimicrobial compounds, the approval of reuterin as a bio-preservative or therapeutic agent by regulatory agencies relies on sufficient data on its cytotoxicity and behavior in the gastrointestinal environment. Although the antimicrobial activity of reuterin has been broadly studied, its safety and toxicity are yet to be explored in detail. In this study, the stability and activity of reuterin were investigated in the gastrointestinal tract using in vitro models simulating gastrointestinal conditions. In addition, hemolytic activity and in vitro cytotoxicity of reuterin were evaluated by neutral red assay and lactate dehydrogenase (LDH) colorimetric assay using the same cell line. Activity of reuterin was observed to be stable during gastrointestinal transit. Viability and membrane integrity of cells remained unaltered by reuterin up to 1080 mM concentration. Furthermore, no hemolysis was observed in blood cells exposed to 270 mM reuterin. This study provides unique and highly relevant in vitro data regarding gastrointestinal behavior and toxicity of reuterin. In conclusion, the current study indicates that within a certain concentration range, reuterin can be safely used in bio-preservation and therapeutics applications. However, further in vivo studies are required to confirm these findings.

Factors affecting the production and measurement of hydrogen peroxide in honey samples

Many Australian native honeys possess significant antimicrobial properties due to the production of hydrogen peroxide (H₂O₂) by glucose oxidase, an enzyme derived from the honeybee. The level of H₂O₂ produced in different honey samples is highly variable, and factors governing its production and stability are not well understood. In this study, highly active Australian honeys that had been stored for >10 years lost up to 54 % of their antibacterial activity, although almost all retained sufficient activity to be considered potentially therapeutically useful. We used a simple colourimetric assay to quantify H₂O₂ production. Although we found a significant correlation between H₂O₂ production and antibacterial activity across diverse honey samples, variation in H₂O₂ only explained 47 % of the variation observed in activity, limiting the assay as a screening tool and highlighting the complexity of the relationship between H₂O₂ and the killing power of honey. To further examine this, we tested whether H₂O₂ detection in honey was being inhibited by pigmented compounds and if H₂O₂ might be directly degraded in some honey samples. We found no correlation between H₂O₂ detection and honey colour. Some honey samples rapidly lost endogenous and spiked H₂O₂, suggesting that components in honey, such as catalase or antioxidant polyphenols, may degrade or quench H₂O₂. Despite this rapid loss of H₂O₂, these honeys had significant peroxide-based antibacterial activity, indicating a complex relationship between H₂O₂ and other honey components that may act synergistically to augment activity.

High-quality genome sequence assembly of R.A73 Enterococcus faecium isolated from freshwater fish mucus

BACKGROUND

Whole-genome sequencing using high throughput technologies has revolutionized and speeded up the scientific investigation of bacterial genetics, biochemistry, and molecular biology. Lactic acid bacteria (LABs) have been extensively used in fermentation and more recently as probiotics in food products that promote health. Genome sequencing and functional genomics investigations of LABs varieties provide rapid and important information about their diversity and their evolution, revealing a significant molecular basis. This study investigated the whole genome sequences of the Enterococcus faecium strain (HG937697), isolated from the mucus of freshwater fish in Tunisian dams. Genomic DNA was extracted using the Quick-GDNA kit and sequenced using the Illumina HiSeq2500 system. Sequences quality assessment was performed using FastQC software. The complete genome annotation was carried out with the Rapid Annotation using Subsystem Technology (RAST) web server then NCBI PGAAP.

RESULTS

The Enterococcus faecium R.A73 assembled in 28 contigs consisting of 2,935,283 bps. The genome annotation revealed 2884 genes in total including 2834 coding sequences and 50 RNAs containing 3 rRNAs (one rRNA 16 s, one rRNA 23 s and one rRNA 5 s) and 47 tRNAs. Twenty-two genes implicated in bacteriocin production are identified within the Enterococcus faecium R.A73 strain.

CONCLUSION

Data obtained provide insights to further investigate the effective strategy for testing this Enterococcus faecium R.A73 strain in the industrial manufacturing process. Studying their metabolism with bioinformatics tools represents the future challenge and contribution to improving the utilization of the multi-purpose bacteria in food.

Characterization and Quantitative Determination of a Diverse Group of Bacillus subtilis subsp. subtilis NCIB 3610 Antibacterial Peptides

Five antibacterial peptides produced by Bacillus subtilis NCIB 3610 were purified, quantified, characterized, and identified in the present study. Cell-free extracts were subjected to three purification protocols employing ammonium sulfate or organic solvent precipitation and their combination, followed by ion-exchange chromatography, solid-phase extraction, and preparative high-performance liquid chromatography (HPLC). The combined ammonium sulfate and organic solvent precipitation extraction protocol presented the best results for peptide purification. In the five fractions that presented antimicrobial activity, antibacterial peptides were quantified by the turbidometric method and by HPLC using nisin for external calibration, with the second providing more accurate results. All peptides were pH- and temperature-resistant and their sensitivity to proteases treatment indicated their proteinic nature. The five peptides were subjected to microwave-assisted acid hydrolysis (MAAH) and following derivatization were analyzed using norleucine as the internal standard, to determine their amino acid content. The identification of the isolated peptides using the UniProt and PubChem databases indicated that the four peptides correspond to UniProt entries of the bacteriocins Subtilosin-A (Q1W152) Subtilosin-SbOX (H6D9P4), Ericin B (Q93GH3), Subtilin (P10946), and the fifth to the non-ribosomal antibacterial lipopeptide surfactin (CID:443592). The amino acid content determination and computational analyses, applied in the present work on the antimicrobial peptides of B. subtilis, proved an efficient screening and quantification method of bacteriocins that could potentially be applied in other bacterial strains. The constructed phylogenetic trees heterogeneity observed across the five peptides investigated might be indicative of competitive advantage of the strain.

Antibacterial and antifungal activity of crude and freeze-dried bacteriocin-like inhibitory substance produced by Pediococcus pentosaceus

Pediococcus pentosaceus LBM 18 has shown potential as producer of an antibacterial and antifungal bacteriocin-like inhibitory substance (BLIS). BLIS inhibited the growth of spoilage bacteria belonging to Lactobacillus, Enterococcus and Listeria genera with higher activity than Nisaplin used as control. It gave rise to inhibition halos with diameters from 9.70 to 20.00 mm, with Lactobacillus sakei being the most sensitive strain (13.50-20.00 mm). It also effectively suppressed the growth of fungi isolated from corn grain silage for up to 25 days and impaired morphology of colonies by likely affecting fungal membranes. These results point out that P. pentosaceus BLIS may be used as a new promising alternative to conventional antibacterial and antifungal substances, with potential applications in agriculture and food industry as a natural bio-controlling agent. Moreover, cytotoxicity and cell death induction tests demonstrated cytotoxicity and toxicity of BLIS to human colon adenocarcinoma Caco-2cells but not to peripheral blood mononuclear cells, with suggests possible applications of BLIS also in medical-pharmaceutical applications.

Methods for quantification of growth and productivity in anaerobic microbiology and biotechnology

Anaerobic microorganisms (anaerobes) possess a fascinating metabolic versatility. This characteristic makes anaerobes interesting candidates for physiological studies and utilizable as microbial cell factories. To investigate the physiological characteristics of an anaerobic microbial population, yield, productivity, specific growth rate, biomass production, substrate uptake, and product formation are regarded as essential variables. The determination of those variables in distinct cultivation systems may be achieved by using different techniques for sampling, measuring of growth, substrate uptake, and product formation kinetics. In this review, a comprehensive overview of methods is presented, and the applicability is discussed in the frame of anaerobic microbiology and biotechnology.

Isolation and Selection of Potential Probiotic Bacteria from the Pig Gastrointestinal Tract

The present study aimed to isolate bacterial strains from the pig gastrointestinal tract that have antagonistic activity against potential pathogens and are able to produce antimicrobial compounds. That ability would be a first requirement for the strains' possible use as probiotics. Samples obtained from pig intestinal mucosa and contents were screened for the presence of antagonistic activity against pathogenic indicator strains of Escherichia coli, Salmonella, and Listeria by means of the double-layer technique. Samples displaying the largest inhibitory halos were further studied for the production of inhibitory substances using the agar diffusion and microtitration methods. The three most promising isolates were identified by sequencing of the 16S rRNA gene and showed highest affiliation to Lactobacillus salivarius. Optimal growth conditions and bacteriocin production were recorded in de Man, Rogosa, and Sharpe broth under anaerobic conditions at 37 °C. The antimicrobial substances were found to be sensitive to proteolytic enzymes but showed good stability at pH values below 6. Our findings suggest that these three intestinal strains are able to produce antimicrobial substances capable of inhibiting the growth of potential enteric pathogens and might have potential as probiotic feed additives for the prevention of gastrointestinal diseases.

Fate and Biological Activity of the Antimicrobial Lasso Peptide Microcin J25 Under Gastrointestinal Tract Conditions

The bacteriocin microcin J25 (MccJ25) inhibits the growth of Gram-negative pathogens including Salmonella and Shigella species, and Escherichia coli. This 21-amino acid peptide has remarkable stability to heat and extreme pH values and resistance to many proteases, thanks to a characteristic lasso structure. In this study, we used the dynamic simulator TIM-1 as gastro-intestinal tract model to evaluate the stability and antibacterial activity of MccJ25 during passage through the proximal portion of the human gastrointestinal tract. MccJ25 concentration was measured in the different simulator sections by HPLC, and inhibition of Salmonella enterica serotype Enteritidis was evaluated using qualitative and quantitative assays. LC-MS/MS analysis and subsequent molecular networking analysis on the Global Natural Product Social Molecular Networking platform (GNPS) and analysis of the peptide degradation in the presence of proteolytic enzymes mimicking the gastro-intestinal conditions permitted to delineate the fate of MccJ25 through identification of the main degradation products. MccJ25 was relatively stable under gastric conditions, but degraded rapidly in the compartment mimicking the duodenum, notably in the presence of pancreatin. Among pancreatin components, elastase I appeared primarily responsible for MccJ25 breakdown, while α-chymotrypsin was less efficient.

Monitoring of multiple bacteriocins through a developed dual extraction protocol and comparison of HPLC-DAD with turbidometry as their quantification system

The present study describes the development of a simple and efficient screening system that allows identification and quantification of nine bacteriocins produced by Lactococcus lactis. Cell-free L. lactis extracts presented a broad spectrum of antibacterial activity, including Gram-negative bacteria, Gram-positive bacteria, and fungi. The characterization of their sensitivity to pH, and heat, showed that the extracts retained their antibacterial activity at extreme pH values and in a wide temperature range. The loss of antibacterial activity following treatment of the extracts with lipase or protease suggests a lipoproteinaceous nature of the produced antimicrobials. The extracts were subjected to a purification protocol that employs a two phase extraction using ammonium sulfate precipitation and organic solvent precipitation, followed by ion exchange chromatography, solid phase extraction and HPLC. In the nine fractions that presented antimicrobial activity, bacteriocins were quantified by the turbidometric method using a standard curve of nisin and by the HPLC method with nisin as the external standard, with both methods producing comparable results. Turbidometry appears to be unique in the qualitative determination of bacteriocins but the only method suitable to both separate and quantify the bacteriocins providing increased sensitivity, accuracy, and precision is HPLC.

Identification and Partial Characterization of Antilisterial Compounds Produced by Dairy Yeasts

Food-grade yeasts make significant contributions to flavor development in fermented foods. Some yeast species also inhibit undesirable bacteria, yeasts and molds, apparently by producing antimicrobial compounds called mycocins. The aim of this study was to evaluate the ability of wild yeasts, isolated from raw milk and cheese in the Quebec province area, to produce antilisterial compounds. Based on an agar-membrane screening test, 22 of 95 isolates, namely one Candida catenulata, one Candida parapsilosis, five Candida tropicalis, four Debaryomyces hansenii, one Geotrichum candidum, nine Pichia fermentans and one Pichia anomala, exhibited a significant inhibitory effect against Listeria ivanovii HPB28. Four in particular, namely C. tropicalis LMA-693, D. hansenii LMA-916, P. fermentans LMA-256 and P. anomala LMA-827, produced substances extractable from culture supernatant and capable of decreasing 18-h growth of L. ivanovii by, respectively, 97, 92, 84 and 78 %. Heating the extracted material (100 °C for 10 min) decreased these values to 72, 62, 58 and 31 %, respectively, while treatment with trypsin or pronase E decreased them to as little as 27 %. The extracts reduced the numbers of viable Listeria monocytogenes by as much as four log cycles within an hour. Transmission electron microscopy revealed a high proportion of lysis among the cells, apparently due to pore formation. This study clearly shows the potential of these four yeast isolates for use as bio-preservatives in a variety of dairy products.

Assessing the antimicrobial activities of Ocins

The generation of a zone of inhibition on a solid substrate indicates the bioactivity of antimicrobial peptides such as bacteriocin and enterocin. The indicator strain plays a significant role in bacteriocin assays. Other characteristics of bacteriocins, such as their dispersal ability and the different zymogram components, also affect bacteriocin assays. However, universal well diffusion assays for antimicrobials, irrespective of their ability to diffuse (bacteriocin and enterocin), do not exist. The ability of different zymography components to generate non-specific activities have rarely been explored in the literature. The purpose of the present work was to evaluate the impact of major factors (diffusion and rate of diffusion) in a solid substrate bioassay, and to document the adverse effects of sodium dodecyl sulfate in zymograms used to estimate the approximate molecular weight of bacteriocins.

Study of the Efficacy of Real Time-PCR Method for Amikacin Determination Using Microbial Assay

PURPOSE

Microbial assay is used to determine the potency of antibiotics and vitamins. In spite of its advantages like simplicity and easiness, and to reveal the slight changes in the molecules, the microbial assay suffers from significant limitations; these methods are of lower specificity, accuracy and sensitivity. The objective of the present study is to evaluate the efficacy of real time-PCR technique in comparison with turbidimetric method for microbial assay of amikacin.

METHODS

Microbial determination of amikacin by turbidimetric method was performed according to USP. Also amikacin concentrations were determined by microbial assay using taq-man quantitative PCR method. Standard curves in different concentration for both methods were plotted and method validation parameters of linearity, precision and accuracy were calculated using statistical procedures.

RESULTS

The RT-PCR method was linear in the wider concentration range (5.12 - 38.08 for RT-PCR versus 8.00 - 30.47 for turbidimetric method) with a better correlation coefficient (0.976 for RT-PCR versus 0.958 for turbidimetric method). RT-PCR method with LOQ of 5.12 ng/ml was more sensitive than turbidimetric method with LOQ of 8.00 ng/ml and the former could detect and quantify low concentrations of amikacin. The results of accuracy and precision evaluation showed that the RT-PCR method was accurate and precise in all of the tested concentration.

CONCLUSION

The RT-PCR method described here provided an accurate and precise technique for measurement of amikacin potency and it can be a candidate for microbial determination of the antibiotics with the same test organism.

A Microplate Growth Inhibition Assay for Screening Bacteriocins against Listeria monocytogenes to Differentiate Their Mode-of-Action

Lactic acid bacteria (LAB) have historically been used in food fermentations to preserve foods and are generally-recognized-as-safe (GRAS) by the FDA for use as food ingredients. In addition to lactic acid; some strains also produce bacteriocins that have been proposed for use as food preservatives. In this study we examined the inhibition of Listeria monocytogenes 39-2 by neutralized and non-neutralized bacteriocin preparations (Bac+ preps) produced by Lactobacillus curvatus FS47; Lb. curvatus Beef3; Pediococcus acidilactici Bac3; Lactococcus lactis FLS1; Enterococcus faecium FS56-1; and Enterococcus thailandicus FS92. Activity differences between non-neutralized and neutralized Bac+ preps in agar spot assays could not readily be attributed to acid because a bacteriocin-negative control strain was not inhibitory to Listeria in these assays. When neutralized and non-neutralized Bac+ preps were used in microplate growth inhibition assays against L. monocytogenes 39-2 we observed some differences attributed to acid inhibition. A microplate growth inhibition assay was used to compare inhibitory reactions of wild-type and bacteriocin-resistant variants of L. monocytogenes to differentiate bacteriocins with different modes-of-action (MOA) whereby curvaticins FS47 and Beef3, and pediocin Bac3 were categorized to be in MOA1; enterocins FS92 and FS56-1 in MOA2; and lacticin FLS1 in MOA3. The microplate bacteriocin MOA assay establishes a platform to evaluate the best combination of bacteriocin preparations for use in food applications as biopreservatives against L. monocytogenes.

Inhibition of Staphylococcus aureus by crude and fractionated extract from lactic acid bacteria

Increasing levels of antibiotic resistance by Staphyloccocus aureus have posed a need to search for non-antibiotic alternatives. This study aimed to assess the inhibitory effects of crude and fractionated cell-free supernatants (CFS) of locally isolated lactic acid bacteria (LAB) against a clinical strain of S. aureus. A total of 42 LAB strains were isolated and identified from fresh vegetables, fresh fruits and fermented products prior to evaluation of inhibitory activities. CFS of LAB strains exhibiting a stronger inhibitive effect against S. aureus were fractionated into crude protein, polysaccharide and lipid fractions. Crude protein fractions showed greater inhibition against S. aureus compared to polysaccharide and lipid fractions, with a more prevalent effect from Lactobacillus plantarum 8513 and L. plantarum BT8513. Crude protein, polysaccharide and lipid fractions were also characterised with glycine, mannose and oleic acid being detected as the major component of each fraction, respectively. Scanning electron microscopy revealed roughed and wrinkled membrane morphology of S. aureus upon treatment with crude protein fractions of LAB, suggesting an inhibitory effect via the destruction of cellular membrane. This research illustrated the potential application of fractionated extracts from LAB to inhibit S. aureus for use in the food and health industry.

Optimal methods for evaluating antimicrobial activities from plant extracts

The search for antimicrobial agents from plants has been a growing interest in the last few decades. However, results generated from many of these studies cannot be directly compared due to the absence of standardization in particular antimicrobial methods employed. The need for established methods with consistent results for the evaluation of antimicrobial activities from plant extracts has been proposed by many researchers. Nevertheless, there are still many studies reported in the literature describing different methodologies. The aim of this study was to find optimal methods to give consistent quantitative antimicrobial results for studying plant extracts. Three different agar-based assays (pour plate disc diffusion (PPDD), streak plate disc diffusion (SPDD) and well-in agar (WA)) and one broth-based (turbidometric (TB)) assay were used in this study. Extracts from two plant species (Duabanga grandiflora and Acalypha wilkesiana) were tested on two bacterial species, namely Escherichia coli and Staphylococcus aureus. Amongst the agar-based assays, PPDD produced the most reproducible results. TB was able to show the inhibitory effects of the test samples on the growth kinetic of the bacteria including plant extracts with low polarity. We propose that both agar- (i.e PPDD) and broth-based assays should be employed when assessing the antimicrobial activity of plant crude extracts.

Study of the physicochemical and biological stability of pediocin PA-1 in the upper gastrointestinal tract conditions using a dynamic in vitro model

AIMS

To evaluate the survival of Pediococcus acidilactici UL5 and its ability to produce pediocin PA-1 during transit in an artificial gastrointestinal tract (GIT). To investigate the physicochemical and biological stability of purified pediocin PA-1 under GIT conditions.

METHODS AND RESULTS

Skim milk culture of Ped. acidilactici UL5 was fed to a dynamic gastrointestinal (GI) model known as TIM-1, comprising four compartments connected by computer-controlled peristaltic valves and simulating the human stomach, duodenum, jejunum and ileum. This strain tolerated a pH of 2·7 in the gastric compartment, while lower pH reduced its viability. Bile salts in the duodenal compartment brought a further 4-log reduction after 180 min of digestion, while high viable counts (up to 5 × 10(7) CFU ml(-1) fermented milk) of Ped. acidilactici were found in both the jejunal and ileal compartments. Pediococcus acidilactici recovered from all four compartments was able to produce pediocin at the same level as unstressed cells. The activity of the purified pediocin in the gastric compartment was slightly reduced after 90 min of gastric digestion, while no detectable activity was found in the duodenal, jejunal and ileal compartments during 5 h of digestion. HPLC analysis showed partial degradation of the pediocin peptide in the duodenal compartment and massive breakdown in the jejunal and ileal compartments.

CONCLUSIONS

Pediococcus acidilactici UL5 showed high resistance to GIT conditions, and its ability to produce pediocin was not affected, suggesting its potential as a probiotic candidate. The physicochemical and biological stability of pediocin was significantly poor under GIT conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

Pediococcus acidilactici UL5 appears to be a potential probiotic candidate because its capacity to produce pediocin PA-1 is not affected by the GI conditions as well as the strain shows an acceptable survival rate. Meanwhile, purified pediocin PA-1 losses activity during GIT transit; microcapsules could be used to deliver it to the target site.

Pelagic fish hydrolysates as peptones for bacterial culture media

For several years in the Quebec fisheries’ industry,landings of pelagic fish have been calculated at over 4000 tons. These under-exploited species, rich in lipids and proteins, could be used in valuable new products. In the present study, hydrolysates of mackerel and herring were produced and utilized as sources of peptones in the formulation of new bacterial culture media. The molecular weight distribution analysis showed that molecules present in the hydrolysates were lower than 1300 Da for herring, and lower than 930 Da for mackerel. The formulated media were compared with reference media using 6 bacterial strains (3 lactic acid (LAB) and 3 non-lactic). The absorbance (OD) and carbohydrate measurements revealed that the formulated media possessed similar yields in comparison with the reference media. Finally, the inhibition of Listeria innocua by LAB bacteriocins was evaluated. Results obtained for Pediococcus acidilactici demonstrated high activities for each medium studied. Thus, the medium containing herring peptones generated the highest bacteriocin titre (32768 AU/mL), followed by both the medium containing mackerel peptones and the MRS7 medium (16384 AU/mL). Each medium containing the fish hydrolysates efficiently supported the growth of the bacterial strains. Pelagic fish peptones are promising as a novel bacterial culture media.

Pediocin SA-1, an antimicrobial peptide from Pediococcus acidilactici NRRL B5627: production conditions, purification and characterization

Fermentation broths of Pediococcus acidilactici NRRL B5627 exhibited a certain antimicrobial activity due to a bacteriocin produced during early growth and until the stationary phase of growth was reached (at optimum of 60% dissolved oxygen saturation). Its size was determined by electrospray ionization mass spectrometric analysis as 3.660 kDa. N-terminal sequencing showed that the bacteriocin had 19 amino acid residues in the order KYYGXNGVXTXGKHSXVDX. The purified bacteriocin is similar to pediocins isolated by various Pediococci and therefore, it belongs to the class IIa of bacteriocins and is thus designated pediocin SA-1. Sensitivity of the purified pediocin to various storage temperatures and enzyme treatments was examined. Purified pediocin SA-1 is heat stable for up to 60 min at 121 degrees C. Pediocin SA-1 is inhibitory to several food-borne pathogens and food spoilage bacteria. It appears to be significantly more effective against Listeria spp. compared to pediocin PD-1 produced by P. damnosus. The mode of action of the purified bacteriocin appears to be bactericidal.

Inhibitory activity spectrum of reuterin produced by Lactobacillus reuteri against intestinal bacteria

Background

Reuterin produced from glycerol by Lactobacillus reuteri, a normal inhabitant of the human intestine, is a broad-spectrum antimicrobial agent. It has been postulated that reuterin could play a role in the probiotic effects of Lb. reuteri. Reuterin is active toward enteropathogens, yeasts, fungi, protozoa and viruses, but its effect on commensal intestinal bacteria is unknown. Moreover reuterin's mode of action has not yet been elucidated. Glutathione, a powerful antioxidant, which also plays a key role in detoxifying reactive aldehydes, protects certain bacteria from oxidative stress, and could also be implicated in resistance to reuterin.

The aim of this work was to test the activity of reuterin against a representative panel of intestinal bacteria and to study a possible correlation between intracellular low molecular weight thiols (LMW-SH) such as glutathione, hydrogen peroxide and/or reuterin sensitivity. Reuterin was produced by Lb. reuteri SD2112 in pure glycerol solution, purified and used to test the minimal inhibitory (MIC) and minimal bactericidal concentrations (MBC). Hydrogen peroxide sensitivity and intracellular LMW-SH concentration were also analysed.

Results

Our data showed that most tested intestinal bacteria showed MIC below that for a sensitive indicator Escherichia coli (7.5–15 mM). Lactobacilli and Clostridium clostridioforme were more resistant with MIC ranging from 15 to 50 mM. No correlation between bacterial intracellular concentrations of LMW-SH, including glutathione, and reuterin or hydrogen peroxide sensitivities were found.

Conclusion

Our data showed that intestinal bacteria were very sensitive to reuterin and that their intracellular concentration of LMW-SH was not directly linked to their capacity to resist reuterin or hydrogen peroxide. This suggests that detoxification by LMW-SH such as glutathione is not a general mechanism and that other mechanisms are probably involved in bacterial tolerance to reuterin and hydrogene peroxide.

Bacteriocin detection by liquid chromatography/mass spectrometry for rapid identification

AIMS

To establish a new system to detect and identify bacteriocins in the early stage of screening for novel bacteriocins.

METHODS AND RESULTS

Liquid chromatography/mass spectrometry (LC/MS) was employed for development of a new system for rapid detection and identification of bacteriocins. The system detected and identified bacteriocins such as nisin and lacticin 481 from 25 microl of culture supernatants of their producing strains by accurate mass determination coupled with simultaneous impurity removal within 40 min. Especially, the system clearly distinguished three nisin variants (A, Z, Q) in culture supernatants of their producing strains, although they have similar structures and molecular masses. Each one-step pretreatment by cell adsorption-desorption or acetone precipitation improved bacteriocin detection dramatically, especially for mundticin KS. This system could be applied for detection and molecular mass determination of novel bacteriocins by extracting bacteriocin-related ions.

CONCLUSIONS

The developed system could detect and identify some kinds of bacteriocin from culture supernatants or pretreated samples.

SIGNIFICANCE AND IMPACT OF THE STUDY

The developed system helps us to identify bacteriocins in the early stage of screening without any or with one-step pretreatment. This system is effective on not only detection of known bacteriocins but also identification of novel bacteriocins. Consequently, this system will accelerate discovery of novel bacteriocins.

Production of recombinant bacteriocin divercin V41 by high cell density Escherichia coli batch and fed-batch cultures

To increase the yield of heterologous production of the class II bacteriocin DvnRV41 with Escherichia coli Origami (DE3) (pLysS/pCR03), induction of bacteriocin gene expression was optimized by varying the inducer isopropyl beta-D-thiogalactopyranoside (IPTG) concentration (0-2 mM), and controlled batch and fed-batch cultures were tested on a 2-L scale. A concentration of 0.5 mM IPTG was found to be optimal for cell growth and bacteriocin production. Shake flask cultivation of E. coli Origami (DE3) (pLysS/pCR03) gave biomass and bacteriocin yields of 1.54 +/- 0.06 g cdw/l and 18 +/- 1 mg DvnRV41/l, respectively. Biomass (2.70 +/- 0.06 and 6.8 +/- 0.6 g cdw/l, respectively) and bacteriocin yields (30 and 74 mg DvnRV41 per liter, respectively) were both increased with batch and fed-batch compared to shake flask cultures. Bacteriocin yields reported in this study are among the highest published for other heterologous expression systems in shake flasks.

A rapid and accurate 3-(4,5-dimethyl thiazol-2-yl)-2,5-diphenyl tetrazolium bromide colorimetric assay for quantification of bacteriocins with nisin as an example

The objective of this study is to propose a more accurate and faster MTT [3-(4,5-dimethyl thiazol-2-yl)-2,5-diphenyl tetrazolium bromide] colorimetric assay (MCA) for quantitative measurement of polypeptide bacteriocins in solutions with nisin as an example. After an initial incubation of nisin and indicator bacterium Micrococcus luteus NCIB 8166 in tubes, MTT was added for another incubation period. After that, nisin was quantified by estimating the number of viable bacteria based on measuring the amount of purple formazan produced by cleavage of yellow tetrazolium salt MTT. Then MCA was compared to a standard agar diffusion assay (ADA). The results suggested a high correlation coefficient (r(2)=0.975+/-0.004) between optical density (OD) and the inhibitory effect of nisin on a bacterial strain Micrococcus luteus NCIB 8166 at a range of 0.125-32 IU/ml. The MCA described in this study was very quick. Quantification of nisin took only 7-8 h and the detection limit was at the level of 0.125 IU/ml when compared to 12 IU/ml and 24-28 h for ADA. The MCA provides an accurate and rapid method for quantification of nisin in solutions and is expected to be used for quantification of other antimicrobial substances.

Analysis of hemin effect on lactate reduction in Lactococcus lactis

Lactococcus lactis is a facultative anaerobic microorganism that produces lactate as the major product, and acetate and acetoin as by-products; some strains of this species produce an antimicrobial compound, nisin. Lactate has a strong inhibitory effect on L. lactis growth. On the other hand, hemin has a suppressive effect on lactate production during L. lactis growth under aerobic condition. To achieve the optimum effect of hemin on lactate amount reduction in L. lactis ATCC11454, cultures entailing various conditions were performed with and without hemin. In the culture with hemin, L. lactis growth and lactate reduction improved compared with those in the culture without hemin; that is, lactate production was suppressed by 1.8- and 1.3-fold under batch and fed-batch cultures, respectively. In microaerobic fed-batch culture with hemin, lactate production was sufficiently suppressed. This result suggests that microaerobic fed-batch culture could be applied to the maintenance of the low lactate amount. Under this condition, metabolic shift was observed from lactate to acetoin and acetate. However, no increase in nisin production was observed even though lactate production could significantly decrease in L. lactis ATCC11454.

Extraction of nisin from a 2.5% commercial nisin product using methanol and ethanol solutions

Nisin is a class Ia bacteriocin used widely in the food industry to inhibit a number of gram-positive pathogens. Although this peptide exhibits activity against many gram-positive bacteria, its effectiveness can vary significantly depending upon the food application. Encapsulation is one method that has been investigated for improving the activity of nisin. Improvement of the encapsulation efficiency of nisin requires purification of the compound, which can be accomplished utilizing organic solvents. The objective of this study was to use methanol and ethanol solutions to extract and concentrate nisin from a commercial preparation containing 2.5% nisin. Commercial nisin was extracted with different concentrations of ethanol or methanol in sterile water for up to 8 h. Approximately 75% of the nisin activity was recovered with 10 or 50% ethanol compared with less than 1% recovery with an ethanol concentration higher than 90%. Extraction with 10 or 50% methanol was approximately as effective as that with lower concentrations of ethanol. However, yields were significantly greater for extraction with methanol at concentrations greater than 90%. The solubility of the nisin likely influenced the extraction profiles for the conditions used. Purification for an 8-h extraction using 10 and 50% ethanol was 1.36 and 1.93 times, respectively. Purification was less than 0.1 at higher ethanol concentrations due to poor extraction. For methanol treatments, purification factors were all 1.09 to 5.98, and they increased as methanol concentration increased. This method for extracting and purifying nisin from dairy proteins using organic solvents may provide an alternative means for preparing and concentrating nisin for encapsulation and other applications.

In vitro growth control of selected pathogens by Lactobacillus acidophilus- and Lactobacillus casei-fermented milk

AIMS

Food-borne pathogen inhibition was tested in the presence of a mixture of Lactobacillus acidophilus and Lactobacillus casei during fermentation under controlled pH conditions.

METHODS AND RESULTS

The growth of Escherichia coli O157:H7, Salmonella serotype Typhimurium, Staphylococcus aureus, Listeria innocua, Enterococcus faecium and Enterococcus faecalis was evaluated for 48 h at 37 degrees C. In the presence of the lactic acid bacteria (LAB), an increase of the generation time was observed for all the gram-positive bacteria evaluated. Staphylococcus aureus was the most sensitive strain showing an increase of the generation time by 210%. However, for all the gram-negative bacteria evaluated, no inhibition occurred after 8 h of fermentation. The soluble portion of Lact. acidophilus- and Lact. casei-fermented milk was recuperated and tested for its antimicrobial activity. Listeria innocua and Staph. aureus were the most sensitive to the presence of fermented milk supernatant showing an inhibition of 85.9% and 84.7%, respectively. This soluble fraction was neutralized to eliminate the antimicrobial effect of the organic acids produced; the most sensitive strains were L. innocua and E. coli O157:H7 showing an inhibition of 65.9% and 61.9%, respectively. Finally, the soluble fraction was neutralized and irradiated at 45 kGy using a (60)Co source to eliminate the possible antimicrobial effect of both organic acids and bacteriocin-like substances. Enterococcus faecalis, E. coli O157:H7 and Staph. aureus were the most affected bacteria by this fraction, showing 39.1, 32 and 31.2% inhibition, respectively.

CONCLUSIONS

The results obtained in this study suggest the implication of both organic acids and bacteriocin-like inhibitory substances in the antimicrobial activity observed in the soluble fraction of the probiotic preparation.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study revealed the antimicrobial mechanisms of action of Lact. acidophilus- and Lact. casei-fermented milk used to prevent antibiotic-associated diarrhoea.

Modification of the data-processing method for the turbidimetric bioassay of nisin

The data processing method of the turbidimetric bioassay of nisin was modified to facilitate its industrial application. The influence of the initial indicator concentration was minimized by a redefined specific dose of the bacteriocin as the quotient between the titer of the added bacteriocin and the initial population density of the indicator in the suspension. It was found that dc = 0.125 microg ml(-1) was the critical dose of nisin that can cause a complete inhibition of the indicator, Pediococcus acidilactici UL5, with an initial OD of 0.135. To eliminate the interference of the cell debris, an equation, epsilonI = ODi(1-di)/OD0 = epsilonA(1-di/dc), exploiting dc, was formulated to obtain the intrinsic survival proportion. The use of the specific dose of the bacteriocin and the intrinsic survival proportion as parameters of the dose/response curve greatly enhanced its repeatability and feasibility. A dual-dosage approach was developed to further simplify the conventional standard dose/response curve method.

Determination of bacteriocin activity with bioassays carried out on solid and liquid substrates: assessing the factor "indicator microorganism"

BACKGROUND

Successful application of growth inhibition techniques for quantitative determination of bacteriocins relies on the sensitivity of the applied indicator microorganism to the bacteriocin to which is exposed. However, information on indicator microorganisms' performance and comparisons in bacteriocin determination with bioassays is almost non-existing in the literature. The aim of the present work was to evaluate the parameter "indicator microorganism" in bioassays carried out on solid -agar diffusion assay- and liquid -turbidometric assay- substrates, applied in the quantification of the most studied bacteriocin nisin.

RESULTS

The performance of characterized microorganisms of known sources, belonging to the genera of Lactobacillus, Pediococcus, Micrococcus and Leuconostoc, has been assessed in this work in the assays of plate agar diffusion and turbidometry. Dose responses and sensitivities were examined and compared over a range of assay variables in standard bacteriocin solutions, fermentation broth filtrates and processed food samples. Measurements on inhibition zones produced on agar plates were made by means of digital image analysis. The data produced were analyzed statistically using the ANOVA technique and pairwise comparisons tests. Sensitivity limits and linearity of responses to bacteriocin varied significantly among different test-microorganisms in both applied methods, the lower sensitivity limits depending on both the test-microorganism and the applied method. In both methods, however, only two of the nine tested microorganisms (Lactobacillus curvatus ATCC 51436 and Pediococcus acidilactici ATCC 25740) were sensitive to very low concentrations of the bacteriocin and produced a linear-type of response in all kinds of samples used in this work. In all cases, very low bacteriocin concentrations, e.g. 1 IU/ml nisin, were more accurately determined in the turbidometric assay.

CONCLUSION

The present work shows that in growth inhibition techniques used in bacteriocin quantification, the choice of the indicator microorganism is critical. Evaluation of sensitivity levels and type of produced responses showed that they can vary widely among different test-microorganisms and different applied methods, indicating that not all microorganisms can be used successfully as indicators and that measurements of growth inhibition in liquid media produce more reliable results.

Bioluminescence-based bioassays for rapid detection of nisin in food

We have developed a method for determining ultralow amounts of nisin in food samples that is based on luminescent biosensor bacteria. Modified bacterial luciferase operon luxABCDE was placed under control of the nisin-inducible nisA promoter in plasmid pNZ8048, and the construct was transformed into Lactococcus lactis strains NZ9800 and NZ9000. The nisRK genes of these strains allow them to sense nisin and relay the signal to initiate transcription from nisA promoter. The resulting luminescence can be directly measured from living bacteria without the addition of exogenous substrates. Induction leads to detectable luminescence within ten minutes. Lyophilization of the biosensor cells produced viable and inducible sensor elements that can be utilized as freshly cultivated cells for rapid detection of nisin. The linear dose-response relationship perceived in the assay facilitates quantification of nisin in samples. The sensitivity of the nisin bioassay was 0.1 pg/ml in pure solution and 3 pg/ml in milk, exceeding the performance of all previously reported methods.

Control of Listeria spp. by competitive-exclusion bacteria in floor drains of a poultry processing plant

In previous studies workers determined that two lactic acid bacterium isolates, Lactococcus lactis subsp. lactis C-1-92 and Enterococcus durans 152 (competitive-exclusion bacteria [CE]), which were originally obtained from biofilms in floor drains, are bactericidal to Listeria monocytogenes or inhibit the growth of L. monocytogenes both in vitro and in biofilms at 4 to 37 degrees C. We evaluated the efficacy of these isolates for reducing Listeria spp. contamination of floor drains of a plant in which fresh poultry is processed. Baseline assays revealed that the mean numbers of Listeria sp. cells in floor drains sampled on six different dates (at approximately biweekly intervals) were 7.5 log(10) CFU/100 cm(2) for drain 8, 4.9 log(10) CFU/100 cm(2) for drain 3, 4.4 log(10) CFU/100 cm(2) for drain 2, 4.1 log(10) CFU/100 cm(2) for drain 4, 3.7 log(10) CFU/100 cm(2) for drain 1, and 3.6 log(10) CFU/100 cm(2) for drain 6. The drains were then treated with 10(7) CE/ml in an enzyme-foam-based cleaning agent four times in 1 week and twice a week for the following 3 weeks. In samples collected 1 week after CE treatments were applied Listeria sp. cells were not detectable (samples were negative as determined by selective enrichment culture) for drains 4 and 6 (reductions of 4.1 and 3.6 log(10) CFU/100 cm(2), respectively), and the mean numbers of Listeria sp. cells were 3.7 log(10) CFU/100 cm(2) for drain 8 (a reduction of 3.8 log(10) CFU/100 cm(2)), <1.7 log(10) CFU/100 cm(2) for drain 1 (detectable only by selective enrichment culture; a reduction of 3.3 log(10) CFU/100 cm(2)), and 2.6 log(10) CFU/100 cm(2) for drain 3 (a reduction of 2.3 log(10) CFU/100 cm(2)). However, the aerobic plate counts for samples collected from floor drains before, during, and after CE treatment remained approximately the same. The results indicate that application of the two CE can greatly reduce the number of Listeria sp. cells in floor drains at 3 to 26 degrees C in a facility in which fresh poultry is processed.

Use of a spectrophotometric bioassay for determination of microbial sensitivity to manuka honey

The antimicrobial activity of manuka honey has been well documented (Molan, 1992a,b,c, 1997) [Molan, P.C., 1992. The antibacterial activity of honey. 1: the nature of the antibacterial activity. Bee World 73 (1) 5-28; Molan, P.C., 1992. The antibacterial activity of honey. 2: variation in the potency of the antibacterial activity. Bee World 73 (2) 59-76; Molan, P.C., 1992. Medicinal uses for honey. Beekeepers Quarterly 26; Molan, P.C., 1997. Finding New Zealand honeys with outstanding antibacterial and antifungal activity. New Zealand Beekeeper 4 (10) 20-26]. The current bioassays for determining this antimicrobial effect employ a well diffusion (Ahn and Stiles, 1990) [Ahn, C., Stiles, M.E., 1990. Antibacterial activity of lactic acid bacteria isolated from vacuum-packed meats. Journal of Applied Bacteriology 69, 302-310], (Weston et al., 1999) [Weston, R.J., Mitchell, K.R., Allen, K.L., 1999. Antibacterial phenolic components of New Zealand manuka honey. J. Food Chem. 64, 295-301] or disc diffusion (Taormina et al., 2001) [Taormina, Peter J., Niemira, Brendan A., Beuchat, Larry R., 2001. Inhibitory activity of honey against food borne pathogens as influenced by the presence of hydrogen peroxide and level of antioxidant power. Int. J. Food Microbiol. 69, 217-225] assay using zones of inhibition as indicators of bacterial susceptibility. The development of a 24-h spectrophotometric assay employing 96-well microtiter plates, that is more sensitive and more amenable to statistical analysis than the assays currently employed, was undertaken. This simple and rapid assay permits extensive kinetic studies even in the presence of low honey concentrations, and is capable of detecting inhibitory levels below those recorded for well or disc diffusion assays. In this paper, we compare the assay to both well and disc diffusion assays. The results we obtained for the spectrophotometric method MIC values show that this method has greater sensitivity than the standard well and disc diffusion assays. In addition, inter- and intra-assay variance for this method was investigated, demonstrating the methods reproducibility and repeatability.

Bioactivity quantification of crude bacteriocin solutions

Bacteriocin activity can be quantified by a simple parallel line model based on an agar diffusion assay. The quotient between the inhibition zone area and the sensitivity of the indicator bacterium can be used to compare different bacteriocins. Deferred calculations of relative potencies are possible, once purified compounds are obtained.

Development of a robust microtiter plate-based assay method for assessment of bioactivity

A microtiter plate-based assay was developed for the quantitative monitoring of bioactive compound production in Streptomyces hygroscopicus fermentation samples. The method reported demonstrates the successful application of the theories of disk diffusion based methods of bioactivity assessment, to a microtiter assay for high throughput analysis. The assay method facilitates the generation of the dose-response curve of test organisms (Escherichia coli, Bacillus subtilis and Saccharomyces cerevisiae) to a bioactive compound. Using this dose-response curve, the method facilitates definition of three distinct Minimum Inhibitory Concentration (MIC) values for use in the characterisation of the bioactive attributes of a sample. The assay uses established standard procedures to facilitate adaptation of the assay for use with a wider range of test microorganisms. Errors due to the assumption of a linear relationship between turbidity and biomass concentration are also reduced, due to incorporation of a step to convert turbidity to biomass concentration, for use in the calculation of bioactivity.