Divercin V41 from gene characterization to food applications: 1998-2008, a decade of solved and unsolved questions

Synergistic inhibition mechanism of pediocin PA-1 and L-lactic acid against Aeromonas hydrophila

Pediocin PA-1 (PA-1) is a membrane-targeting bacteriocin from lactic acid bacteria, which shows antimicrobial activity against a wide range of Gram-positive pathogens. However, the outer membrane of Gram-negative bacteria does not allow pediocin access to its target. In this work, the synergistic inhibitory mechanism of PA-1 with L-lactic acid against Gram-negative aquaculture and food pathogen Aeromonas hydrophila (A. hydrophila) was analyzed. The combined treatment of 3.5 mmol/L L-lactic acid and 50 μmol/L (or 30 μmol/L) PA-1 had strong bacteriostatic and bactericidal activity against A. hydrophila. Full wavelength scanning and ELISA assay revealed the release of lipopolysaccharide (LPS) from the outer membrane of A. hydrophila caused by L-lactic acid treatment. Laser confocal microscopic imaging of A. hydrophila with FITC-labeled pediocin PA-1 proved the accumulation of PA-1 on lactic acid-treated bacterial cells. PA-1 then caused a rapid dissipation of membrane potential (Δψ) and a proton gradient difference (ΔpH) in lactic acid-treated A. hydrophila. Pediocin PA-1 also caused an increase in the extracellular ATP level. Morphology revealed by SEM and TEM showed that combined treating with lactic acid and PA-1 induced vesicles on the cell surface, the outer and inner membrane disruption, and even cytoplasm leakage and cell lysis. The results proved a potential mechanism of the synergistic inhibition of lactic acid and PA-1 against A. hydrophila, by which L-lactic acid released the outer membrane LPS, making it possible for PA-1 to contact the plasma membrane of A. hydrophila, resulting in the dissipation of proton-motive force in the inner membrane and cell death.

Scientific Opinion on the update of the list of QPS-recommended biological agents intentionally added to food or feed as notified to EFSA

EFSA is requested to assess the safety of a broad range of biological agents in the context of notification for market authorisation as sources of food and feed additives, food enzymes and plant protection products. The qualified presumption of safety (QPS) assessment was developed to provide a harmonised generic pre-assessment to support safety risk assessments performed by EFSA's scientific Panels. The safety of unambiguously defined biological agents (at the highest taxonomic unit appropriate for the purpose for which an application is intended), and the completeness of the body of knowledge are assessed. Identified safety concerns for a taxonomic unit are, where possible and reasonable in number, reflected as 'qualifications' in connection with a recommendation for a QPS status. The list of QPS recommended biological agents was reviewed and updated in the current opinion and therefore becomes the valid list. The 2016 update reviews previously assessed microorganisms including bacteria, yeasts and viruses used for plant protection purposes following an Extensive Literature Search strategy. The taxonomic units related to the new notifications received since the 2013 QPS opinion, were periodically evaluated for a QPS status and the results published as Statements of the BIOHAZ Panel. Carnobacterium divergens, Lactobacillus diolivorans, Microbacterium imperiale, Pasteuria nishizawae, Pediococcus parvulus, Bacillus flexus, Bacillus smithii, Xanthomonas campestris and Candida cylindracea were recommended for the QPS list. All taxonomic units previously recommended for the 2013 QPS list had their status reconfirmed as well their qualifications with the exception of Pasteuria nishizawae for which the qualification was removed. The exclusion of filamentous fungi and enterococci from the QPS evaluations was reconsidered but monitoring will be maintained and the status will be re-evaluated in the next QPS Opinion update. Evaluation of bacteriophages should remain as a case-by-case procedure and should not be considered for QPS status.

Monitoring Salmonella, Campylobacter, Escherichia coli and Staphylococcus aureus in traditional free-range 'Label Rouge' broiler production: a 23-year survey programme

AIM

'Label Rouge' broiler free-range carcasses have been monitored since 1991, and broiler flocks since 2010, for contamination by the main foodborne zoonotic bacteria.

METHODS AND RESULTS

Initially, the monitoring plan mainly focused on the surveillance of Salmonella, and on indicators of the overall microbiological quality of free-range broiler carcasses such as Staphylococcus aureus and coliforms, but was extended in 2007 to include Campylobacter enumeration on carcasses and in 2010, to Salmonella in the environment of live birds. Salmonella contamination of free-range broiler carcasses rose to a peak of 16% in 1994 but less than 1% of carcasses are now regularly found to be positive. Indicators of the overall microbiological quality of carcasses are also improving. These results correlate with the low prevalence of Salmonella in free-range broiler breeding and production flocks, and with the continuous improvement of hazard analysis and critical control points in slaughterhouses, the implementation of a good manufacturing practice guide since 1997 and the application of EU regulations on Salmonella since 1998 in France. Regarding Campylobacter counts on carcasses, the situation has been improving continuously over the last few years, even if 2·5% of the carcasses are still contaminated by more than 1000 Campylobacter per g of skin.

CONCLUSIONS

Although the current control system focusing on Salmonella is based on firm epidemiologic data and offers effective means of control (e.g. slaughtering of positive breeder flocks), existing information on Campylobacter makes it more difficult to formulate an effective control plan for free-range broilers, due to their particular exposure to environmental contamination.

SIGNIFICANCE AND IMPACT OF THE STUDY

This long-term surveillance programme provided an extended view of the evolution of the contamination of free-range broilers and a direct measurement of the impact of mandatory and profession-driven interventions on the microbiological quality of carcasses.

Inhibition mechanism of Listeria monocytogenes by a bioprotective bacteria Lactococcus piscium CNCM I-4031

Listeria monocytogenes is a pathogenic Gram positive bacterium and the etiologic agent of listeriosis, a severe food-borne disease. Lactococcus piscium CNCM I-4031 has the capacity to prevent the growth of L. monocytogenes in contaminated peeled and cooked shrimp. To investigate the inhibititory mechanism, a chemically defined medium (MSMA) based on shrimp composition and reproducing the inhibition observed in shrimp was developed. In co-culture at 26 °C, L. monocytogenes was reduced by 3-4 log CFU g(-1) after 24 h. We have demonstrated that the inhibition was not due to secretion of extracellular antimicrobial compounds as bacteriocins, organic acids and hydrogen peroxide. Global metabolomic fingerprints of these strains in pure culture were assessed by liquid chromatography coupled with high resolution mass spectrometry. Consumption of glucose, amino-acids, vitamins, nitrogen bases, iron and magnesium was measured and competition for some molecules could be hypothesized. However, after 24 h of co-culture, when inhibition of L. monocytogenes occurred, supplementation of the medium with these compounds did not restore its growth. The inhibition was observed in co-culture but not in diffusion chamber when species were separated by a filter membrane. Taken together, these data indicate that the inhibition mechanism of L. monocytogenes by L. piscium is cell-to-cell contact-dependent.

Gene cluster responsible for secretion of and immunity to multiple bacteriocins, the NKR-5-3 enterocins

Enterococcus faecium NKR-5-3, isolated from Thai fermented fish, is characterized by the unique ability to produce five bacteriocins, namely, enterocins NKR-5-3A, -B, -C, -D, and -Z (Ent53A, Ent53B, Ent53C, Ent53D, and Ent53Z). Genetic analysis with a genome library revealed that the bacteriocin structural genes (enkA [ent53A], enkC [ent53C], enkD [ent53D], and enkZ [ent53Z]) that encode these peptides (except for Ent53B) are located in close proximity to each other. This NKR-5-3ACDZ (Ent53ACDZ) enterocin gene cluster (approximately 13 kb long) includes certain bacteriocin biosynthetic genes such as an ABC transporter gene (enkT), two immunity genes (enkIaz and enkIc), a response regulator (enkR), and a histidine protein kinase (enkK). Heterologous-expression studies of enkT and ΔenkT mutant strains showed that enkT is responsible for the secretion of Ent53A, Ent53C, Ent53D, and Ent53Z, suggesting that EnkT is a wide-range ABC transporter that contributes to the effective production of these bacteriocins. In addition, EnkIaz and EnkIc were found to confer self-immunity to the respective bacteriocins. Furthermore, bacteriocin induction assays performed with the ΔenkRK mutant strain showed that EnkR and EnkK are regulatory proteins responsible for bacteriocin production and that, together with Ent53D, they constitute a three-component regulatory system. Thus, the Ent53ACDZ gene cluster is essential for the biosynthesis and regulation of NKR-5-3 enterocins, and this is, to our knowledge, the first report that demonstrates the secretion of multiple bacteriocins by an ABC transporter.

Bacteriocin as weapons in the marine animal-associated bacteria warfare: inventory and potential applications as an aquaculture probiotic

As the association of marine animals with bacteria has become more commonly recognized, researchers have increasingly questioned whether these animals actually produce many of the bioactive compounds originally isolated from them. Bacteriocins, ribosomally synthesized antibiotic peptides, constitute one of the most potent weapons to fight against pathogen infections. Indeed, bacteriocinogenic bacteria may prevent pathogen dissemination by occupying the same ecological niche. Bacteriocinogenic strains associated with marine animals are a relevant source for isolation of probiotics. This review draws up an inventory of the marine bacteriocinogenic strains isolated from animal-associated microbial communities, known to date. Bacteriocin-like inhibitory substances (BLIS) and fully-characterized bacteriocins are described. Finally, their applications as probiotics in aquaculture are discussed.