Production of histamine and tyramine by lactic acid bacteria isolated from vacuum-packed sugar-salted fish

Effect of Physicochemical Characteristics and Storage Atmosphere on Microbiological Stability and Shelf-Life of Minimally Processed European Sea Bass (Dicentrarchus labrax) Fillets

The aim of the present work was to evaluate the effect of various hurdles such as aw and pH as well as the storage atmosphere on the microbiological and sensory changes of minimally processed (lightly brined or marinated with acetic or citric acid) European sea bass (Dicentrarchus labrax) fillets. The results indicated that the shelf-life of brined fillets stored aerobically was 6 d while that of vacuum and MAP-stored was 12 d, since a reduced growth rate of spoilage bacteria was recorded. The physicochemical characteristics such as aw and water phase salt (WPS) were not considerably changed, while the oxygen levels into the packages ensure the microbiological safety of the product. The fillets marinated with acetic acid exhibited a longer shelf-life at 30 and 40 d under aerobic and reduced oxygen conditions, respectively, while the products marinated with citric acid were at 25 and 35 d respectively. A low pH resulted in reduced or even limited microbial levels, especially for the fillets marinated with acetic acid; something that ensures microbiological safety as well. The low or limited microbial levels in conjugation with the sensory attributes indicated that spoilage may be due to other mechanisms such as autolysis rather than microbial activity. Overall, the present work highlights the potential for further research and development of minimally processed, microbiologically safe and stable with extended shelf-life value added seafood to satisfy the corresponding consumer demands.

Effect of the Manufacturing Process on the Microbiota, Organoleptic Properties and Volatilome of Three Salmon-Based Products

Lightly preserved seafood products, such as cold-smoked fish and fish gravlax, are traditionally consumed in Europe and are of considerable economic importance. This work aimed to compare three products that were obtained from the same batch of fish: cold-smoked salmon (CSS) stored under vacuum packaging (VP) or a modified atmosphere packaging (MAP) and VP salmon dill gravlax (SG). Classical microbiological analyses and 16S rRNA metabarcoding, biochemical analyses (trimethylamine, total volatile basic nitrogen (TVBN), biogenic amines, pH, volatile organic compounds (VOCs)) and sensory analyses (quantitative descriptive analysis) were performed on each product throughout their storage at a chilled temperature. The three products shared the same initial microbiota, which were mainly dominated by Photobacterium, Lactococcus and Lactobacillus genera. On day 28, the VP CSS ecosystem was mainly composed of Photobacterium and, to a lesser extent, Lactococcus and Lactobacillus genera, while Lactobacillus was dominant in the MAP CSS. The diversity was higher in the SG, which was mainly dominated by Enterobacteriaceae, Photobacterium, Lactobacillus and Lactococcus. Although the sensory spoilage was generally weak, gravlax was the most perishable product (slight increase in amine and acidic off-odors and flavors, fatty appearance, slight discoloration and drop in firmness), followed by the VP CSS, while the MAP CSS did not spoil. Spoilage was associated with an increase in the TVBN, biogenic amines and spoilage associated VOCs, such as decanal, nonanal, hexadecanal, benzaldehyde, benzeneacetaldehyde, ethanol, 3-methyl-1-butanol, 2,3-butanediol, 1-octen-3-ol, 2-butanone and 1-octen-3-one. This study showed that the processing and packaging conditions both had an effect on the microbial composition and the quality of the final product.

The relationship between flavor formation, lipid metabolism, and microorganisms in fermented fish products

Traditional fermented fish products are favored due to their unique flavors. The fermentation process of fish is accompanied by the formation of flavor substances through a complex metabolic reaction of microorganisms, especially lipolysis and lipid oxidation. However, it is difficult to precisely control the reaction of microorganisms during the fermentation process in modern industrial production, and fermented fish products have lost their traditional characteristic flavors. The purpose of this review is to summarize the different kinds of fermented fish, core microorganisms in it, and flavor formation mechanisms, providing guidance for industrial cultural starters. Future research on the flavor formation mechanism is necessary to confirm the relationship between flavor formation, lipid metabolism, and microorganisms to ensure stable flavor and safety, and to elucidate the mechanism directly toward industrial application.

Salmon Gravlax Biopreservation With Lactic Acid Bacteria: A Polyphasic Approach to Assessing the Impact on Organoleptic Properties, Microbial Ecosystem and Volatilome Composition

Seafood and fishery products are very perishable commodities with short shelf-lives owing to rapid deterioration of their organoleptic and microbiological quality. Microbial growth and activity are responsible for up to 25% of food losses in the fishery industry. In this context and to meet consumer demand for minimally processed food, developing mild preservation technologies such as biopreservation represents a major challenge. In this work, we studied the use of six lactic acid bacteria (LAB), previously selected for their properties as bioprotective agents, for salmon dill gravlax biopreservation. Naturally contaminated salmon dill gravlax slices, with a commercial shelf-life of 21 days, were purchased from a French industrial company and inoculated by spraying with the protective cultures (PCs) to reach an initial concentration of 106 log CFU/g. PC impact on gravlax microbial ecosystem (cultural and acultural methods), sensory properties (sensory profiling test), biochemical parameters (pH, TMA, TVBN, biogenic amines) and volatilome was followed for 25 days of storage at 8°C in vacuum packaging. PC antimicrobial activity was also assessed in situ against Listeria monocytogenes. This polyphasic approach underlined two scenarios depending on the protective strain. Carnobacterium maltaromaticum SF1944, Lactococcus piscium EU2229 and Leuconostoc gelidum EU2249, were very competitive in the product, dominated the microbial ecosystem, and displayed antimicrobial activity against the spoilage microbiota and L. monocytogenes. The strains also expressed their own sensory and volatilome signatures. However, of these three strains, C. maltaromaticum SF1944 did not induce strong spoilage and was the most efficient for L. monocytogenes growth control. By contrast, Vagococcus fluvialis CD264, Carnobacterium inhibens MIP2551 and Aerococcus viridans SF1044 were not competitive, did not express strong antimicrobial activity and produced only few organic volatile compounds (VOCs). However, V. fluvialis CD264 was the only strain to extend the sensory quality, even beyond 25 days. This study shows that C. maltaromaticum SF1944 and V. fluvialis CD264 both have a promising potential as bioprotective cultures to ensure salmon gravlax microbial safety and sensorial quality, respectively.

Effect of In Vitro Human Digestion on Biogenic Amine (Tyramine) Formation in Various Fermented Sausages

Biogenic amines are formed in various fermented foods by microbial amino acid decarboxylation activities, and ingestion of these amines may cause human illness. However, the effect of digestion on the biogenic amines in fermented sausages has not been studied. This study was conducted to determine the effect of in vitro human digestion with the enterobacteria Escherichia coli and Lactobacillus casei on concentrations of the biogenic amine tyramine in six types of fermented sausages. Tyramine concentration was not significantly changed until simulated digestion in the small intestine. However, tyramine concentration for all sausage samples was increased after simulated digestion in the large intestine. Addition of E. coli and L. casei dramatically increased the tyramine concentrations ( P < 0.05). This result indicates that enterobacteria increase biogenic amine concentrations during human digestion.

Isolation and Characterisation of L. plantarum O1 Producer of Plantaricin as Potential Starter Culture for the Biopreservation of Aquatic Food Products

Lactobacillus plantarum O1 was isolated from the gut of sea bream (Sparus aurata) and identified with the API biochemical test and MALDI-TOF MS. This strain was further characterised according to the selection criteria for lactic acid bacteria as starter cultures for aquatic food production. L. plantarum O1 showed good antimicrobial activity against pathogenic test microorganisms. Further investigation confirmed it as the producer of the bacteriocin plantaricin. This strain also showed good growth at a wide range of temperatures (from 4 to 45 °C) and a wide range of pH (2–12), even in the presence of 3.5% NaCl. Its viability was also good after lyophilisation and in simulated gastric and small intestinal juice. The strain is a promising probiotic, and our further research will focus on its application in the biopreservation of fresh fish and shellfish.

Biogenic Amine Formation and Microbiological Quality of Anchovy (Engraulis encrasicolus) Treated with Lavender and Lemon Balm Ethanol Extracts

Biogenic amine formation and bacterial growth of vacuum-packed European anchovy treated with 1% lemon balm (LB) or lavender (LD) ethanol extracts was investigated. The treated samples were stored at 2 ± 1 °C for 11 d and analyzed for biogenic amines, total viable count, fecal coliform count, and lactic acid bacterial count. The analyses were done on 0, 4, 7, 9, and 11 d of storage. Both LB and LD extracts proved effective in reducing most biogenic amines, with histamine reduced by 3-fold compared to control. The calculated biogenic amine index showed that LB extract was the most effective in inhibiting the biogenic amine formation. The reduction in total viable count was low and the bacterial contamination reached more than 7 log cfu/g at the end of the storage period for all the samples. Moreover, the reduction in fecal coliform and lactic acid bacterial count by LB and LD extracts was below 1 log cfu/g. Treatment of European anchovy with LB and LD extracts proved to be a very effective preservation technique to reduce the biogenic amine formation and lower the risk of histamine fish poisoning but is not effective in reduction of bacterial load. Consequently, LB and LD extracts could be used to reduce the biogenic amine formation which provides additional safety prior to spoilage for histamine sensitive fish species.

Significance of Biogenic Amines in Cold-Smoked Fish and Their Relation to Microbiological Characteristics of Products Available in Portuguese Retail Markets

Studies on microbial characterization of cold-smoked salmon and salmon trout during cold storage were performed on samples available in the Portuguese market. Samples were also classified microbiologically according to guidelines for ready-to-eat (RTE) products. Further investigations on sample variability and microbial abilities to produce tyramine and histamine were also performed. The coefficient of variation for viable counts of different groups of microorganisms of samples collected at retail market point was high in the first 2 wk of storage, mainly in the Enterobacteriaceae group and aerobic plate count (APC), suggesting that microbiological characteristics of samples were different in numbers, even within the same batch from the same producer. This variation seemed to be decreased when storage and temperature were controlled under lab conditions. The numbers of Enterobacteriaceae were influenced by storage temperature, as indicated by low microbial numbers in samples from controlled refrigeration. Lactic acid bacteria (LAB) and Enterobacteriaceae were predominant in commercial products, a significant percentage of which were tyramine and less histamine producers. These results might be influenced by (1) the technological processes in the early stages of production, (2) contamination during the smoking process, and (3) conditions and temperature fluctuations during cold storage at retail market point of sale.

Lactic acid bacteria associated with a heat-processed pork product and sources of variation affecting chemical indices of spoilage and sensory characteristics

AIMS

To evaluate the potential for developing a quality index for a Danish modified atmosphere packaged (MAP) heat-processed and naturally contaminated pork meat product stored at 5 degrees C.

METHODS AND RESULTS

The composition of the predominating microflora and changes in contents of tyramine, arginine, organic acids and sensory characteristics were analysed. The microflora was predominated by Lactobacillus sakei, Leuconostoc carnosum and Carnobacterium divergens. The presence of each species varied between products and batches resulting in limited usefulness of the concentrations of these bacteria or their metabolites as indices of quality. Furthermore, the three species differed in their metabolic activities as shown by use of a model meat extract. However, when MAP storage of the processed pork product was followed by aerobic storage then acetic acid showed some potential as a chemical indicator of sensory quality.

CONCLUSION

Variation in processing parameters and spoilage microbiota limited the usefulness of concentrations of micro-organisms and their metabolites as indices of spoilage for the studied processed MAP pork product.

SIGNIFICANCE AND IMPACT OF THE STUDY

The present study contributes to an understanding of the difficulties experienced in developing quality indices to be used in the control of microbial spoilage of processed MAP meat products.

Selection and evaluation of seafood-borne psychrotrophic lactic acid bacteria as inhibitors of pathogenic and spoilage bacteria

In this study, inhibitory psychrotrophic lactic acid bacteria were isolated and investigated for future use in biopreservation of seafood products. Screening of 5575 colonies isolated from various seafood products resulted in the selection of 132 colonies presenting inhibitory properties. Among them, 52 isolates had characteristics of LAB and showed growth at 15 degrees C but not at 30 degrees C. The inhibition spectrum of these 52 isolates against 14 target strains (Gram-positive and -negative) showed inhibition of typical seafood spoiling and pathogenic bacteria and enabled the formation of seven interesting clusters. Sequencing of the 16S rRNA gene of a representative isolate from each cluster identified three Leuconostoc gelidum, two Lactococcus piscium, one Lactobacillus fuchuensis and one Carnobacterium alterfunditum. Theses strains did not produce histamine nor tyramine, and showed no particular antibiotic resistance profile. Growth rate as a function of temperature was tested for one L. piscium and one L. gelidum isolate and confirmed their psychrotrophic behavior. One out of seven isolates showed bacteriocin-like activity. The inhibition mechanisms of the other isolates are still unknown but may be due to competition for substrate. Absence of a bacteriocin-like component could be a positive point to gain rapid authorization for food application in France. This collection of LAB is now ready for testing on products.

Updated molecular knowledge about histamine biosynthesis by bacteria

Histamine poisoning is caused by the ingestion of food containing high levels of histamine, a biogenic amine. Histamine could be expected in virtually all foods that contain proteins or free histidine and that are subject to conditions enabling microbial activity. In most histamine-containing foods the majority of the histamine is generated by decarboxylation of the histidine through histidine decarboxylase enzymes derived from the bacteria present in food. Bacterial histidine decarboxylases have been extensively studied and characterized in different organisms and two different enzymes groups have been distinguished, pyridoxal phosphate- and the pyruvoyl-dependent. Pyridoxal phosphate-dependent histidine decarboxylases are encountered in gram-negative bacteria belonging to various species. Pyruvoyl-dependent histidine decarboxylases are found in gram-positive bacteria and specially in lactic acid bacteria implicated in food fermentation or spoilage. The molecular organization of the genes involved in histamine production have been elucidated in several histamine-producer bacteria. This molecular knowledge has led to the development of molecular methods for the rapid detection of bacteria possessing the ability to produce histamine. The detection of histamine-producer bacteria is of great importance for its potential health hazard as well as from an economic point of view since products exceeding recommended limits can be refused in commercial transactions.

alpha-Chitinase activity among lactic acid bacteria

Chitin is a polysaccharide widely distributed in nature. Among 115 strains from 29 species of lactic acid bacteria only strains belonging to Carnobacterium divergens and Carnobacterium maltaromaticum hydrolyzed alpha-chitin. This activity was not affected by temperature (10 degrees C versus 30 degrees C) and in most cases not subject to glucose catabolite repression.

Carnobacterium: positive and negative effects in the environment and in foods

The genus Carnobacterium contains nine species, but only C. divergens and C. maltaromaticum are frequently isolated from natural environments and foods. They are tolerant to freezing/thawing and high pressure and able to grow at low temperatures, anaerobically and with increased CO₂ concentrations. They metabolize arginine and various carbohydrates, including chitin, and this may improve their survival in the environment. Carnobacterium divergens and C. maltaromaticum have been extensively studied as protective cultures in order to inhibit growth of Listeria monocytogenes in fish and meat products. Several carnobacterial bacteriocins are known, and parameters that affect their production have been described. Currently, however, no isolates are commercially applied as protective cultures. Carnobacteria can spoil chilled foods, but spoilage activity shows intraspecies and interspecies variation. The responsible spoilage metabolites are not well characterized, but branched alcohols and aldehydes play a partial role. Their production of tyramine in foods is critical for susceptible individuals, but carnobacteria are not otherwise human pathogens. Carnobacterium maltaromaticum can be a fish pathogen, although carnobacteria are also suggested as probiotic cultures for use in aquaculture. Representative genome sequences are not yet available, but would be valuable to answer questions associated with fundamental and applied aspects of this important genus.

Carnobacterium species: effect of metabolic activity and interaction with Brochothrix thermosphacta on sensory characteristics of modified atmosphere packed shrimp

The importance of carnobacteria as spoilage microorganisms or potential protective cultures in food is not resolved, and little is known about their metabolism during growth in specific products. This study used chromatographic techniques including GC-MS and HPLC to evaluate the spoilage metabolism of Carnobacterium divergens, Carnobacterium maltaromaticum, and Carnobacterium mobile. Metabolic activity was studied in cooked and peeled modified atmosphere packed (MAP) shrimp at 5 degrees C as carnobacteria has been anticipated to contribute to spoilage of shrimp products. C. divergens and C. maltaromaticum caused sensory spoilage of shrimps and generated ammonia, tyramine, and various alcohols, aldehydes, and ketones. The effects of Carnobacterium species on the growth and metabolism of Brochothrix thermosphacta were also evaluated, but metabiosis between the two groups of bacteria was not observed. C. mobile and a specific cluster of C. maltaromaticum isolates (cluster L) did not cause sensory spoilage of shrimp.

Effect of inoculation of Carnobacterium divergens V41, a biopreservative strain against Listeria monocytogenes risk, on the microbiological, chemical and sensory quality of cold-smoked salmon

The aim of this study was to develop a bio-preservation strategy for cold-smoked salmon (CSS) by the use of lactic acid bacteria previously selected for their capability to inhibit the growth of Listeria monocytogenes in the product. The spoiling potential of three Carnobacterium strains (Carnobacterium divergens V41, Carnobacterium piscicola V1 and SF668) was tested in sterile CSS blocks inoculated by 10(4-5) CFU g(-)(1) and stored under vacuum for 9 days at 4 degrees C followed by 19 days at 8 degrees C. C. divergens V41 grew a little faster than the other strains and none of the three carnobacteria showed any adverse effect on quality of the product, i.e. no off-odour detected by a trained panel, no total volatile basic nitrogen (TVBN) production, no acidification and no biogenic amine except a slight production of tyramine. An application on commercial CSS was tested by spraying C. divergens V41 (10(4-5) CFU g(-1)) on slices of four batches freshly processed in different smoke-houses. Microbial, chemical and sensory characteristics were weekly compared to a control during 4 weeks of vacuum storage. When the natural microflora was initially weak (two batches<20 CFU g(-1)), C. divergens V41 quickly reached 10(7-8) CFU g(-1) and a slight inhibition of endogenous Enterobacteriaceae, lactobacilli and yeasts was observed. The presence of C. divergens V41 was slightly detected (odour and flavour) but none of the sample was considered as spoiled by the sensory panel. When the natural microflora was initially high (2 batches>10(4-5) CFU g(-1)), no effect on the microflora, TVBN and biogenic amine production, nor on the sensory characteristics was observed in presence of C. divergens V41. In conclusion, bio-preservation of CSS using lactic acid bacteria such as C. divergens V41 is a promising way to inhibit the growth of pathogenic bacteria such as L. monocytogenes with low effect on the quality of the product.

Leuconostoc durionis sp. nov., a heterofermenter with no detectable gas production from glucose

Three lactic acid bacterial (LAB) strains obtained from a Malaysian acid-fermented condiment, tempoyak (made from pulp of the durian fruit), showed analogous but distinct patterns after screening by SDS-PAGE of whole-cell proteins and comparison with profiles of all recognized LAB species. 16S rRNA gene sequencing of one representative strain showed that the taxon belongs phylogenetically to the genus Leuconostoc, with its nearest neighbour being Leuconostoc fructosum (98 % sequence similarity). Biochemical characteristics and DNA-DNA hybridization experiments demonstrated that the strains differ from Leuconostoc fructosum and represent a single, novel Leuconostoc species for which the name Leuconostoc durionis sp. nov. is proposed. The type strain is LMG 22556(T) (= LAB 1679(T) = D-24(T) = CCUG 49949(T)).

Carnobacterium divergens and Carnobacterium maltaromaticum as spoilers or protective cultures in meat and seafood: phenotypic and genotypic characterization

Carnobacterium, a genus of lactic acid bacteria, frequently dominate the microflora of chilled vacuum- or modified atmosphere-packed meat and seafood. In this study Carnobacterium isolates were characterized by phenotypic and molecular methods in order to investigate the association of species and intra-species groups with distinct kinds of meat and seafood. Of 120 test strains, 50 originated from meat (beef and pork products, including 44 strains isolated during this study and 6 strains obtained from culture collections) and 52 from seafoods (cod, halibut, salmon, shrimps and roe products). In addition, 9 reference strains of Carnobacterium spp from other sources than meat and fish and 9 reference strains of lactic acid bacteria belonging to other genera than Carnobacterium were included. Numerical taxonomy relying on classical biochemical reactions, carbohydrate fermentation and inhibition tests (temperature, salt, pH, chemical preservatives, antibiotics, bacteriocins), SDS-PAGE electrophoresis of whole cell proteins, plasmid profiling, intergenic spacer region (ISR) analysis and examination of amplified-fragment length polymorphism (AFLP) were employed to characterize the strains. The numerical taxonomic approach divided the carnobacteria strains into 24 groups that shared less than 89% similarity. These groups were identified as Carnobacterium divergens with one major cluster (40 strains) and 7 branches of one to four strains, Carnobacterium maltaromaticum (previous C. piscicola) with one major cluster (37 strains) and 9 branches of one to four strains and Carnobacterium mobile (three branches consisting in total of 4 strains). Branches consisting of references strains of the remaining Carnobacterium spp. were separated from clusters and branches of C. divergens, C. maltaromaticum and C. mobile. Isolates from the main clusters of C. divergens and C. maltaromaticum were found both in fresh and lightly preserved meat and seafood products. High phenotypic intra-species variability was observed for C. divergens and C. maltaromaticum but despite this heterogeneity in phenotypic traits a reliable identification to species levels was obtained by SDS-PAGE electrophoresis of whole cell proteins and by ISR based on 16S-23S rDNA intergenic spacer region polymorphism. With AFLP, two distinct clusters were observed for C. divergens but only one for C. maltaromaticum. The two C. divergens clusters were not identical to any of the clusters observed by numerical taxonomy. A limited number of C. divergens and C. maltaromaticum isolates possessed a biopreservative potential due to their production of bacteriocins with a wide inhibition spectrum. This study serves as a base-line for further investigations on the potential role of species of Carnobacterium in foods where they predominate the spoilage microflora.

Molecular characterization, technological properties and safety aspects of enterococci from 'Hussuwa', an African fermented sorghum product

AIMS

To identify enterococci from Hussuwa, a Sudanese fermented sorghum product, and determine their technological properties and safety for possible inclusion in a starter culture preparation.

METHODS AND RESULTS

Twenty-two Enterococcus isolates from Hussuwa were identified as Enterococcus faecium by using phenotypic and genotypic tests such as 16S rDNA gene sequencing, RAPD-PCR and restriction fragment length polymorphism of the 16S/23S intergenic spacer region fingerprinting. Genotyping revealed that strains were not clonally related and exhibited a considerable degree of genomic diversity. Some strains possessed useful technological properties such as production of bacteriocins and H2O2 or utilization of raffinose and stachyose. None produced alpha-amylase or tannase. A safety investigation revealed that all strains were susceptible to the antibiotics ampicillin, gentamicin, chloramphenicol, tetracycline and streptomycin, but some were resistant to ciprofloxacin, erythromycin, penicillin and vancomycin. Production of biogenic amines or presence of genes encoding virulence determinants occurred in some strains.

CONCLUSIONS

Enterococcus faecium strains are associated with fermentation of Sudanese Hussuwa. Some strains exhibited useful technological properties such as production of antimicrobial agents and fermentation of indigestible sugars, which may aid in stabilizing and improving the digestibility of the product respectively.

SIGNIFICANCE AND IMPACT OF THE STUDY

Enterococci were shown to play a role in the fermentation of African foods. While beneficial properties of these bacteria are indicated, their presence in this food may also imply a hygienic risk as a result of antimicrobial resistances or presence of virulence determinants.

The biogenic amine tyramine modulates the adherence of Escherichia coli O157:H7 to intestinal mucosa

The environmental factors that influence the ability of Escherichia coli O157:H7 to attach to the intestinal mucosa are incompletely understood. In the present study, the ability of one of the most common biogenic amines present in food, tyramine, to influence the ability of E. coli O157:H7 to adhere to murine cecal mucosa was examined. Ex vivo full-thickness sheets of murine cecum were mounted in Ussing chambers, which preserved the enteric nervous system innervation of the luminal epithelia and thereby allowed us to achieve a closer approximation of bacterial adherence than would be encountered in vivo. After exposure of the luminal aspect of the cecum to tyramine, E. coli O157:H7 was added for 90 min. The cecal tissue was then removed and washed, and adhered E. coli O157:H7 was enumerated using a selective medium. Tyramine significantly increased E. coli O157:H7 adherence to cecal mucosa when compared to that of controls. The 50% effective concentration of tyramine was 92.6 microM. Specific adrenergic antagonists were then employed to examine whether the effect of tyramine was mediated through alpha- or beta-adrenergic receptors on the intestinal tissue. Pretreatment of tissues with either the alpha-adrenergic receptor antagonist phentolamine or the beta-adrenergic receptor antagonist propranolol prevented the action of tyramine. Measurement of active transepithelial ion transport and ionic permeability in the cecal sheets before and after the addition of tyramine and E. coli O157:H7 did not show any impairment of tissue viability or transepithelial conductance. Further, tyramine did not influence either the growth of E. coli O157:H7 or the expression of the intimin attachment factor. The present findings suggest that biogenic amines, such as tyramine, present within the food matrix influence host susceptibility to E. coli O157:H7 infection.

Identification of lactic acid bacteria from spoilage associations of cooked and brined shrimps stored under modified atmosphere between 0 degrees C and 25 degrees C

AIMS

To evaluate spoilage and identify lactic acid bacteria (LAB) from spoilage associations of cooked and brined shrimps stored under modified atmosphere packaging (MAP) at 0, 5, 8, 15 and 25 degrees C.

METHODS AND RESULTS

Bacterial isolates (102) from spoilage associations of cooked and brined MAP shrimps were characterized by phenotypic tests and identified as lactic acid bacteria (78 isolates), other Gram-positive bacteria (13 isolates) and Gram-negative bacteria (11 isolates). A selection of 48 LAB isolates were further characterized and identified by phenotypic tests and SDS-PAGE electrophoresis of whole cell proteins. Selected clusters of LAB isolates were analysed by plasmid profiling, pulsed field gel electrophoresis and 16S rRNA gene sequencing. Enterococcus faecalis was identified in spoilage associations at 15 degrees C and 25 degrees C, and its metabolic activity corresponded to chemical changes in spoiled products. Carnobacterium divergens, a non-motile Carnobacterium sp. nov. and Lactobacillus curvatus were the LAB species observed in spoilage associations of products stored at 0 degrees C, 5 degrees C and 8 degrees C.

CONCLUSIONS

Enterococcus spp. and Carnobacterium spp. were the dominant parts of spoilage associations of cooked and brined MAP shrimps stored at high and low temperatures, respectively.

SIGNIFICANCE AND IMPACT OF THE STUDY

The SDS-PAGE technique and simple biochemical keys allowed the majority of LAB isolates from spoilage associations of cooked and brined MAP shrimps to be identified at the species level.

Production of biogenic amines and divercin V41 in cold smoked salmon inoculated with Carnobacterium divergens V41, and specific detection of this strain by multiplex-PCR

AIMS

The objective of this study was to determine the technological behaviour (implantation and biogenic amines production) of Carnobacterium divergens V41, an anti-Listeria bacteriocin producer (divercin V41), after inoculation in cold smoked salmon (CSS).

METHODS AND RESULTS

Implantation of the strain was followed by multiplex-PCR during 27 days of storage at 4 degrees C, and biogenic amines were quantified by HPLC. It was found that the strain was able to develop quite well in CSS among lactic wild flora. Divercin V41 (400 AU ml-1) was produced in CSS, and the biogenic amine content was not modified by inoculation of the bacteria.

CONCLUSIONS

Carnobacterium divergens V41 is a safe, interesting, bioprotective agent.

SIGNIFICANCE AND IMPACT OF THE STUDY

This strain could potentially be used for efficient prevention of L. monocytogenes growth in CSS.

Growth of Carnobacterium divergens V41 and production of biogenic amines and divercin V41 in sterile cold-smoked salmon extract at varying temperatures, NaCl levels, and glucose concentrations

A complete factorial design 2(3) was used to study some aspects of Carnobacterium divergens V41 metabolism (growth, biogenic amine production, and divercin V41 production) in sterile cold-smoked salmon extract (SSE) at varying temperatures (3 to 9 degrees C), NaCl levels (2.5 to 6.5%), and glucose concentrations (2 to 6 g liter(-1)). The results showed that temperature and NaCl content were the most influential factors on growth parameters in SSE. Predictive models are suggested for the assessment of C. divergens lag time (t(lag)) and maximum specific growth rate (micro(max)) Among the biogenic amines studied, only tyramine was found to be produced by C. divergens in SSE. Furthermore, we showed that temperature, NaCl, and glucose variations did not greatly affect tyramine and divercin V41 production by the bacteria under the experimental conditions used. Indeed, divercin V41, a bacteriocin from C. divergens V41 that is highly active against some Listeria strains, was produced in SSE even under harsh culture conditions. Similarly, tyramine production in SSE was delayed at 3 degrees C but reached 35 microg ml(-1) in all experiments after 27 days of storage. However, this final tyramine concentration in SSE is low compared with the threshold values of 100 to 800 microg g(-1) reported as the potentially toxic dose in foods. Thus, we have found that C. divergens V41 is a promising strain for the biopreservation of refrigerated cold-smoked salmon.

Identification of lactic acid bacteria from spoiled, vacuum-packaged 'gravad' rainbow trout using ribotyping

A total of 296 lactic acid bacteria (LAB) isolated from spoiled, vacuum-packaged 'gravad' rainbow trout stored at 3 and 8 degrees C were characterised and identified using a molecular approach. The isolates were initially grouped according to their HindIII restriction endonuclease profiles and further identified to species level using an rRNA gene restriction pattern (ribotype) identification database. Lactobacillus sakei, L. curvatus and Carnobacterium piscicola were the three main species detected. Only one isolate was identified as C. divergens. Most of the carnobacteria were found in the samples stored at 3 degrees C. The relative proportion of L. sakei was higher in the samples stored at 8 degrees C.

Characterisation of volatile compounds produced by bacteria isolated from the spoilage flora of cold-smoked salmon

This study investigated the volatile compounds produced by bacteria belonging to nine different bacterial groups: Lactobacillus sake, L. farciminis, L. alimentarius, Carnobacterium piscicola, Aeromonas sp., Shewanella putrefaciens, Brochothrix thermosphacta, Photobacterium phosphoreum and Enterobacteriaceae isolated from cold-smoked salmon. Each bacterial group was represented by several strains. In addition, combinations of the groups were examined as well. Sterile blocks of cold-smoked salmon were inoculated, vacuum-packed and stored at 6 degrees C. After 40 days of storage at 6 degrees C, aerobic viable count and pH were recorded, the volatile fraction of the samples was analysed by gas chromatography-mass spectrometry (GC-MS), and spoilage was assessed by sensory evaluation. Among the 81 volatile compounds identified by GC-MS, 30 appeared to be released as a result of bacterial metabolism. Some of the effects of inoculated bacterial strains on the composition of the volatile fraction seemed to be characteristic of certain bacterial species. Sensory analysis showed relationships between bacteria, the composition of the volatile fraction and the organoleptic quality of smoked salmon.

The effect of biogenic amine production by single bacterial cultures and metabiosis on cold-smoked salmon

AIM

Biogenic amines are important indicators of spoilage in vacuum-packed cold-smoked salmon. It is the aim of this study to identify bacteria responsible for biogenic amine production in cold-smoked salmon.

METHODS AND RESULTS

The present study identified spoilage microflora from cold-smoked salmon and determined biogenic amine production of single and co-cultures growing in cold-smoked salmon. Photobacterium phosphoreum was the only species that produced histamine when inoculated on sterile cold-smoked salmon. Production of putrescine was enhanced 10-15 times when cultures of Serratia liquefaciens or Hafnia alvei were grown with Carnobacterium divergens or Lactobacillus sakei subsp. carnosus. This phenomenon was explained by interspecies microbial metabolism of arginine, i.e., metabiosis.

CONCLUSIONS

The amounts of biogenic amines produced by single and co-cultures corresponded to those observed during spoilage of naturally-contaminated cold-smoked salmon. Photobacterium phosphoreum and Lact. curvatus were identified as the specific spoilage organisms in cold-smoked salmon.

SIGNIFICANCE AND IMPACT OF THE STUDY

Determination of the specific spoilage organism is needed before a model can be developed for shelf-life predictions of cold-smoked salmon.

Improved screening procedure for biogenic amine production by lactic acid bacteria

An improved screening plate method for the detection of amino acid decarboxylase-positive microorganisms (especially lactic acid bacteria) was developed. The suitability and detection level of the designed medium were quantitatively evaluated by confirmation of amine-forming capacity using an HPLC procedure. The potential to produce the biogenic amines (BA) tyramine, histamine, putrescine, and cadaverine, was investigated in a wide number of lactic acid bacteria (LAB) of different origin, including starter cultures, protective cultures, type strains and strains isolated from different food products. Also, several strains of Enterobacteriaceae were examined. Modifications to previously described methods included lowering glucose and sodium chloride concentrations, and increasing the buffer effect with calcium carbonate and potassium phosphate. In addition, pyridoxal-5-phosphate was included as a codecarboxylase factor for its enhancing effect on the amino acid decarboxylase activity. The screening plate method showed a good correlation with the chemical analysis and due to its simplicity it is presented as a suitable and sensitive method to investigate the capacity of biogenic amine production by LAB. Tyramine was the main amine formed by the LAB strains investigated. Enterococci, carnobacteria and some strains of lactobacilli, particularly of Lb. curvatus. Lb. brevis and Lb. buchneri, were the most intensive tyramine formers. Several strains of lactobacilli, Leuconostoc spp., Weissella spp. and pediococci did not show any potential to produce amines. Enterobacteriaceae were associated with cadaverine and putrescine formation. No significant histamine production could be detected for any of the strains tested.

Characterisation of lactic acid bacteria from spoiled, vacuum-packaged, cold-smoked rainbow trout using ribotyping

A total of 405 lactic acid bacteria (LAB) isolated from spoiled, vacuum-packaged, salted, sodium nitrite- or potassium nitrate-treated, cold-smoked rainbow trout stored at 4 degrees C or 8 degrees C were characterised and identified using a molecular method. The isolates were initially classified according to their restriction endonuclease profiles using HindIII and EcoRI restriction endonucleases and further characterised by rRNA gene restriction patterns (ribotypes). Numerical analysis of these ribopatterns was performed together with 19 reference LAB strain patterns in order to identify the isolates to species level. The strains were divided with HindIII and EcoRI ribopatterns into ten and nine clusters at the similarity level of 65% and 50%, respectively. The Leuconostoc-clusters and the Lb. sakei/Lb. curvatus-clusters formed the two main groups. Only one isolate was identified as Lactobacillus plantarum and no Carnobacterium strains were discovered. For both enzymes, the 35 isolates possessing six individual ribotypes and forming five clusters could not be identified further with the reference strains used. The relative proportion of Leuconostoc mesenteroides subsp. mesenteroides was higher in all samples stored at 4 degrees C. Most of the Leuconostoc citreum were found in the samples stored at 8 degrees C, and particularly in the nitrite-treated samples.

Identification of lactic acid bacteria from chili bo, a Malaysian food ingredient

Ninety-two strains of lactic acid bacteria (LAB) were isolated from a Malaysian food ingredient, chili bo, stored for up to 25 days at 28 degreesC with no benzoic acid (product A) or with 7,000 mg of benzoic acid kg-1 (product B). The strains were divided into eight groups by traditional phenotypic tests. A total of 43 strains were selected for comparison of their sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) whole-cell protein patterns with a SDS-PAGE database of LAB. Isolates from product A were identified as Lactobacillus plantarum, Lactobacillus fermentum, Lactobacillus farciminis, Pediococcus acidilactici, Enterococcus faecalis, and Weissella confusa. Five strains belonging to clusters which could not be allocated to existing species by SDS-PAGE were further identified by 16S rRNA sequence comparison. One strain was distantly related to the Lactobacillus casei/Pediococcus group. Two strains were related to Weissella at the genus or species level. Two other strains did not belong to any previously described 16S rRNA group of LAB and occupied an intermediate position between the L. casei/Pediococcus group and the Weissella group and species of Carnobacterium. The latter two strains belong to the cluster of LAB that predominated in product B. The incidence of new species and subspecies of LAB in chili bo indicate the high probability of isolation of new LAB from certain Southeast Asian foods. None of the isolates exhibited bacteriocin activity against L. plantarum ATCC 14917 and LMG 17682.

The spoilage flora of vacuum-packaged, sodium nitrite or potassium nitrate treated, cold-smoked rainbow trout stored at 4 degrees C or 8 degrees C

The spoilage flora of vacuum-packaged, salted, cold-smoked rainbow trout fillets, with or without the addition of nitrate or nitrite, stored at 4 degrees C and 8 degrees C, was studied. Of 620 isolates, lactic acid bacteria were the major fraction (76%), predominating in all samples of spoiled product. However, the phenotypical tests used were insufficient to identify the lactic acid bacteria to the species level. Gram-positive, catalase-positive cocci, gram-negative, oxidase-negative rods and gram-negative, oxidase-positive rods were found in 6%, 16% and 2% of the samples, respectively. Of 39 gram-positive, catalase-positive cocci, 29 were identified as staphylococci and 10 as micrococci. Eighty-five isolates were found to belong to the family Enterobacteriaceae, with 45 of those being Serratia plymuthica. Eleven isolates from the nitrate treated samples stored at 8 degrees C were identified as Pseudomonas aeruginosa. The occurrence of P. aeruginosa and staphylococci in the nitrate-containing samples, stored at 8 degrees C, may cause problems with respect to the safety of the product. The types of lactic acid and other bacteria in the spoilage flora were generally reduced by the addition of nitrate or nitrite to fillets.

Characterization of a bacteriophage for Carnobacterium divergens NCFB 2763 by host specificity and electron microscopy

Carnobacterium divergens NCFB 2763 was used to isolate bacteriophage cd1 from minced beef using an enrichment isolation technique. Host specificity testing showed that it exhibited lytic activity only against a limited number of C. divergens strains. No activity was observed against any of the C. gallinarum, C. mobile, C. piscicola, Enterococcus durans, Lactobacillus plantarum and Lact. brevis strains tested. Anaerobic incubation had no effect on the host specificity pattern. Bacteriophage cd1 belongs to Bradley group B and on average the head was 94 nm, the tail 103 x 23 nm and the base plate 48 x 32 nm.