Atmospheric plasma processes for microbial inactivation: food applications and stress response in Listeria monocytogenes

This PhD thesis is focused on cold atmospheric plasma treatments (GP) for microbial inactivation in food applications. In fact GP represents a promising emerging technology alternative to the traditional methods for the decontamination of foods. The objectives of this work were to evaluate: - the effects of GP treatments on microbial inactivation in model systems and in real foods; - the stress response in L. monocytogenes following exposure to different GP treatments. As far as the first aspect, inactivation curves were obtained for some target pathogens, i.e. Listeria monocytogenes and Escherichia coli, by exposing microbial cells to GP generated with two different DBD equipments and processing conditions (exposure time, material of the electrodes). Concerning food applications, the effects of different GP treatments on the inactivation of natural microflora and Listeria monocytogenes, Salmonella Enteritidis and Escherichia coli on the surface of Fuji apples, soya sprouts and black pepper were evaluated. In particular the efficacy of the exposure to gas plasma was assessed immediately after treatments and during storage. Moreover, also possible changes in quality parameters such as colour, pH, Aw, moisture content, oxidation, polyphenol-oxidase activity, antioxidant activity were investigated. Since the lack of knowledge of cell targets of GP may limit its application, the possible mechanism of action of GP was studied against 2 strains of Listeria monocytogenes by evaluating modifications in the fatty acids of the cytoplasmic membrane (through GC/MS analysis) and metabolites detected by SPME-GC/MS and 1H-NMR analyses. Moreover, changes induced by different treatments on the expression of selected genes related to general stress response, virulence or to the metabolism were detected with Reverse Transcription-qPCR. In collaboration with the Scripps Research Institute (La Jolla, CA, USA) also proteomic profiles following gas plasma exposure were analysed through Multidimensional Protein Identification Technology (MudPIT) to evaluate possible changes in metabolic processes.

Study of apoptotic deletion mediated by Bifidobacterium longum with construction of recombinant strains for Serpin encoding gene and phenotypes comparison in a pig cell model

The first part of the research project of the Co-Advisorship Ph.D Thesis was aimed to select the best Bifidobacterium longum strains suitable to set the basis of our study. We were looking for strains with the abilities to colonize the intestinal mucosa and with good adhesion capacities, so that we can test these strains to investigate their ability to induce apoptosis in “damaged” intestinal cells. Adhesion and apoptosis are the two process that we want to study to better understand the role of an adhesion protein that we have previously identified and that have top scores homologies with the recent serpin encoding gene identified in B. longum by Nestlè researchers. Bifidobacterium longum is a probiotic, known for its beneficial effects to the human gut and even for its immunomodulatory and antitumor activities. Recently, many studies have stressed out the intimate relation between probiotic bacteria and the GIT mucosa and their influence on human cellular homeostasis. We focused on the apoptotic deletion of cancer cells induced by B. longum. This has been valued in vitro, performing the incubation of three B.longum strains with enterocyte-like Caco- 2 cells, to evidence DNA fragmentation, a cornerstone of apoptosis. The three strains tested were defined for their adhesion properties using adhesion and autoaggregation assays. These features are considered necessary to select a probiotic strain. The three strains named B12, B18 and B2990 resulted respectively: “strong adherent”, “adherent” and “non adherent”. Then, bacteria were incubated with Caco-2 cells to investigate apoptotic deletion. Cocultures of Caco-2 cells with B. longum resulted positive in DNA fragmentation test, only when adherent strains were used (B12 and B18). These results indicate that the interaction with adherent B. longum can induce apoptotic deletion of Caco-2 cells, suggesting a role in cellular homeostasis of the gastrointestinal tract and in restoring the ecology of damaged colon tissues. These results were used to keep on researching and the strains tested were used as recipient of recombinant techniques aimed to originate new B.longum strains with enhanced capacity of apoptotic induction in “damaged” intestinal cells. To achieve this new goal it was decided to clone the serpin encoding gene of B. longum, so that we can understand its role in adhesion and apoptosis induction. Bifidobacterium longum has immunostimulant activity that in vitro can lead to apoptotic response of Caco-2 cell line. It secretes a hypothetical eukaryotic type serpin protein, which could be involved in this kind of deletion of damaged cells. We had previously characterised a protein that has homologies with the hypothetical serpin of B. longum (DD087853). In order to create Bifidobacterium serpin transformants, a B. longum cosmid library was screened with a PCR protocol using specific primers for serpin gene. After fragment extraction, the insert named S1 was sub-cloned into pRM2, an Escherichia coli - Bifidobacterium shuttle vector, to construct pRM3. Several protocols for B. longum transformation were performed and the best efficiency was obtained using MRS medium and raffinose. Finally bacterial cell supernatants were tested in a dotblot assay to detect antigens presence against anti-antitrypsin polyclonal antibody. The best signal was produced by one starin that has been renamed B. longum BLKS 7. Our research study was aimed to generate transformants able to over express serpin encoding gene, so that we can have the tools for a further study on bacterial apoptotic induction of Caco-2 cell line. After that we have originated new trasformants the next step to do was to test transformants abilities when exposed to an intestinal cell model. In fact, this part of the project was achieved in the Department of Biochemistry of the Medical Faculty of the University of Maribor, guest of the abroad supervisor of the Co-Advisorship Doctoral Thesis: Prof. Avrelija Cencic. In this study we examined the probiotic ability of some bacterial strains using intestinal cells from a 6 years old pig. The use of intestinal mammalian cells is essential to study this symbiosis and a functional cell model mimics a polarised epithelium in which enterocytes are separated by tight junctions. In this list of strains we have included the Bifidobacterium longum BKS7 transformant strain that we have previously originated; in order to compare its abilities. B. longum B12 wild type and B. longum BKS7 transformant and eight Lactobacillus strains of different sources were co-cultured with porcine small intestine epithelial cells (PSI C1) and porcine blood monocytes (PoM2) in Transwell filter inserts. The strains, including Lb. gasseri, Lb. fermentum, Lb. reuterii, Lb. plantarum and unidentified Lactobacillus from kenyan maasai milk and tanzanian coffee, were assayed for activation of cell lines, measuring nitric oxide by Griess reaction, H202 by tetramethylbenzidine reaction and O2 - by cytochrome C reduction. Cytotoxic effect by crystal violet staining and induction on metabolic activity by MTT cell proliferation assay were tested too. Transepithelial electrical resistance (TER) of polarised PSI C1 was measured during 48 hours co-culture. TER, used to observe epithelium permeability, decrease during pathogenesis and tissue becomes permeable to ion passive flow lowering epithelial barrier function. Probiotics can prevent or restore increased permeability. Lastly, dot-blot was achieved against Interleukin-6 of treated cells supernatants. The metabolic activity of PoM2 and PSI C1 increased slightly after co-culture not affecting mitochondrial functions. No strain was cytotoxic over PSI C1 and PoM2 and no cell activation was observed, as measured by the release of NO2, H202 and O2 - by PoM2 and PSI C1. During coculture TER of polarised PSI C1 was two-fold higher comparing with constant TER (~3000 ) of untreated cells. TER raise generated by bacteria maintains a low permeability of the epithelium. During treatment Interleukin-6 was detected in cell supernatants at several time points, confirming immunostimulant activity. All results were obtained using Lactobacillus paracasei Shirota e Carnobacterium divergens as controls. In conclusion we can state that both the list of putative probiotic bacteria and our new transformant strain of B. longum are not harmful when exposed to intestinal cells and could be selected as probiotics, because can strengthen epithelial barrier function and stimulate nonspecific immunity of intestinal cells on a pig cell model. Indeed, we have found out that none of the strains tested that have good adhesion abilities presents citotoxicity to the intestinal cells and that non of the strains tested can induce cell lines to produce high level of ROS, neither NO2. Moreover we have assayed even the capacity of producing certain citokynes that are correlated with immune response. The detection of Interleukin-6 was assayed in all our samples, including B.longum transformant BKS 7 strain, this result indicates that these bacteria can induce a non specific immune response in the intestinal cells. In fact, when we assayed the presence of Interferon-gamma in cells supernatant after bacterial exposure, we have no positive signals, that means that there is no activation of a specific immune response, thus confirming that these bacteria are not recognize as pathogen by the intestinal cells and are certainly not harmful for intestinal cells. The most important result is the measure of Trans Epithelial Electric Resistance that have shown how the intestinal barrier function get strengthen when cells are exposed to bacteria, due to a reduction of the epithelium permeability. We have now a new strain of B. longum that will be used for further studies above the mechanism of apoptotic induction to “damaged cells” and above the process of “restoring ecology”. This strain will be the basis to originate new transformant strains for Serpin encoding gene that must have better performance and shall be used one day even in clinical cases as in “gene therapy” for cancer treatment and prevention.

Characterisation of probiotic strains for the control and prevention of enteropathogens in the food chain

60 strains (belonging to the genera Lactobacillus, Bifidobacterium, Leuconostoc and Enterococcus) were tested for their capacity to inhibit the growth of 3 strains of Campylobacter jejuni: Lactobacilli and bifidobacteria were left to grow in MRS or TPY broth at 37°C overnight in anaerobic conditions; Campylobacter jejuni was inoculated in blood agar plates at 37°C for 24-48 hours in microaerophilic conditions. The inhibition experiments were carried out in vitro using ”Spot agar test” and “Well diffusion assay” techniques testing both cellular activity and that of the surnatant. 11 strains proved to inhibit the growth of Campylobacter jejuni. These strains were subsequently analised analised in order to evaluate the resistance to particular situations of stress which are found in the gastrointestinal tract and during the industrial transformation processes (Starvation stress, osmotic stress, heat stress, resistance to pH and to bile salts). Resistance to starvation stress: all strains seemed to resist the stress (except one strain). Resistance to osmotic stress: all strains were relatively resistant to the concentrations of 6% w/v of NaCl (except one strain). Resistance to heat stress: only one strain showed little resistance to the 55°C temperature. Resistance to pH: In the presence of a low pH (2.5), many strains rapidly lost their viability after approximately 1 hour. Resistance to bile salts: Except for one strain, all strains seemed to be relatively resistant to the 2% w/v concentration of bile salts. Afterward, strains were identified by using phenotipic and molecular techniques. Phenotipic identification was carried out by using API 50 CHL (bioMérieux) and API 20 STREP identification system (bioMérieux); molecular identification with species-specific PCR: the molecular techniques confirmed the results by phenotipic identification. For testing the antibiotic resistance profile, bacterial strains were subcultured in MRS or TPY broth and incubated for 18 h at 37°C under anaerobic conditions. Antibiotics tested (Tetracycline, Trimethoprim, Cefuroxime, Kanamycin, Chloramphenicol, Vancomycin, Ampycillin, Sterptomycin, Erythromycin) were diluted to the final concentrations of: 2,4,8,16,32,64,128,256 mg/ml. Then, 20 μl fresh bacterial culture (final concentration in the plates approximately 106 cfu/ml) were added to 160 μl MRS or TPY broth and 20 μl antibiotic solution. As positive control the bacterial culture (20 ul) was added to broth (160 ul) and water (20 ul). Test was performed on plates P96, that after the inoculum were incubated for 24 h at 37oC, then the antibiotic resistance was determined by measuring the Optical Density (OD) at 620 nm with Multiscan EX. All strains showed a similar behaviour: resistance to all antibiotic tested. Further studies are needed.

Caratterizzazione biomolecolare di listeria monocytogenes in suini regolarmente macellati

Listeria monocytogenes è un batterio patogeno responsabile di una malattia potenzialmente molto grave per l’uomo. L’infezione avviene soprattutto tramite l’ingestione di alimenti di origine animale contaminati, e può propagarsi per via transplacentare al feto. Il potenziale patogeno di L. monocytogenes è dovuto soprattutto a caratteristici fattori di virulenza con i quali alcuni ceppi sono in grado di attaccare la cellula dell’organismo ospite potendo aderire, invadere, moltiplicare e propagare alle cellule adiacenti. Il presente studio è rivolto al rilevamento tramite reazione polimerasica a catena (PCR) di alcuni fattori di virulenza di ceppi di L. monocytogenes isolati da campioni prelevati presso macelli suini, mediante l’identificazione dei geni responsabili della sintesi delle proteine di superficie che intervengono nel processo patogenetico, allo scopo di valutare la potenziale pericolosità di quelli isolati sia sulle carcasse, sia dal contenuto intestinale.

Use of sub-lethal high pressure homogenization (HPH) treatments to enhance functional properties of lactic acid bacteria probiotic strains

The aim of this PhD thesis was to evaluate the effect of a sub-lethal HPH treatment on some probiotic properties and on cell response mechanisms of already-known functional strains, isolated from Argentinean dairy products. The results achieved showed that HPH treatments, performed at a sub-lethal level of 50 MPa, increased some important functional and technological characteristics of the considered non intestinal probiotic strains. In particular, HPH could modify cell hydrophobicity, autoaggregation and resistance to acid gastric conditions (tested in in vitro model), cell viability and cell production of positive aroma compounds, during a refrigerate storage in a simulated dairy product. In addition, HPH process was able to increase also some probiotic properties exerted in vivo and tested for two of the considered strains. In fact, HPH-treated cells were able to enhance the number of IgA+ cells more than other not treated cells, although this capacity was time dependent. On the other hand, HPH treatment was able to modify some important characteristics that are linked to the cell wall and, consequently, could alter the adhesion capacity in vivo and the interaction with the intestinal cells. These modifications, involving cell outermost structures, were highlighted also by Trasmission Electron Microscopy (TEM) analysis. In fact, the micrographs obtained showed a significant effect of the pressure treatment on the cell morphology and particularly on the cell wall. Moreover, the results achieved showed that composition of plasma membranes and their level of unsaturation are involved in response mechanisms adopted by cells exposed to the sub-lethal HPH treatment. Although the response to the treatment varied according to the characteristics of individual strains, time of storage and suspension media employed, the results of present study, could be exploited to enhance the quality of functional products and to improve their organoleptic properties.

Therapeutic microbiology: characterization of Bifidobacterium strains for the treatment of enteric disorders in newborns.

Several studies support the use of probiotics for the treatment of minor gastrointestinal problems in infants. Positive effects on newborn colics have been evidenced after administration of Lactobacillus strains, whereas no studies have been reported regarding the use of bifidobacteria for this purpose. This work was therefore aimed at the characterization of Bifidobacterium strains capable of inhibiting the growth of pathogens typical of the infant gastro-intestinal tract and of coliforms isolated from colic newborns. Among the 46 Bifidobacterium strains considered, 16 showed high antimicrobial activity against potential pathogens; these strains were further characterized from a taxonomic point of view, for the presence and transferability of antibiotic resistances, for citotoxic effects and adhesion to non tumorigenic gut epithelium cell lines. Moreover, their ability to stimulate gut health by increasing the metabolic activity and the immune response of epithelial cells was also studied. The examination of all these features allowed to identify 3 B. breve strains and a B. longum subsp. longum strain as potential probiotics for the treatments of enteric disorders in newborns such as infantile colics. The formulation of a synbiotic product with an appropriate prebiotic fiber capable of supporting the growth of the selected Bifidobacterium strains was also considered in this study. In this respect the ability of the 4 selected Bifidobacterium strains to use as the sole carbon source and energy source different polisaccharide fibers was investigated The last phase of the work has been dedicated to the evaluation of the gut microbial diversity in newborns whose mothers has been subjected to antibiotic therapy a few hours before the delivery because of a Streptococcus type B infection. These newborns can represent a possible target for the probiotic strains selected in this work.

Characterization and exploiment of Microbial Strains to be used in Meat Processing and Fermentation

The principal aim of this research project has been the evaluation of the specific role of yeasts in ripening processes of dry-cured meat products, i.e. speck and in salami produced by adding Lactobacilli starter cultures, i.e. L. sakei, L. casei, L. fermentum, L. rhamnosus, L.sakei + S.xylosus. In particular the contribution of the predominant yeasts to the hydrolytic patterns of meat proteins has been studied both in model system and in real products. In fact, although several papers have been published on the microbial, enzymatic, aromatic and chemical characterization of dry-cured meat e.g. ham over ripening, the specific role of yeasts has been often underestimated. Therefore this research work has been focused on the following aspects: 1. Characterization of the yeasts and lactic acid bacteria in samples of speck produced by different farms and analyzed during the various production and ripening phases 2. Characterization of the superficial or internal yeasts population in salami produced with or without the use of lactobacilli as starter cultures 3. Molecular characterization of different strains of yeasts and detection of the dominant biotypes able to survive despite environmental stress factors (such as smoke, salt) 4. Study of the proteolytic profiles of speck and salami during the ripening process and comparison with the proteolytic profiles produced in meat model systems by a relevant number of yeasts isolated from speck and salami 5. Study of the proteolytic profiles of Lactobacilli starter cultures in meat model systems 6. Comparative statistical analysis of the proteolytic profiles to find possible relationships between specific bands and peptides and specific microorganisms 7. Evaluation of the aromatic characteristics of speck and salami to assess relationships among the metabolites released by the starter cultures or the dominant microflora

Study of thiabendazole resistance and volatile organic compounds production of Penicillium expansum strains

The years of excessive use of thiabendazole to control Penicillium expansum has induced the development of resistance. Sensitivity of fourty eight strains collected from orchards and packinghouses in Emilia Romagna to pure and commercial TBZ was determined in vitro on TBZ amended medium (400μg/mL). Out of 48 strains, 35 were thiabendazole-sensitive (S) and 13 were thiabendazole-resistant (R). Microtiter assay adapted to P. expansum, showed EC50 values ranging from 54 to 320 μg/mL for ten TBZ-resistant strains. At the highest dose (50 μg/mL), resistant strains growth was not inhibited and the reported MICs value were >1000 μg/mL. Therefore, preliminary screening combined with microtiter assay, can be a good strategy to test susceptibility to TBZ. Mutations in the β-tubulin gene were studied on amino acid sequences from residue 167 to residue 357 of 10 P. expansum strains. Mutation at codon 198 was associated with TBZ-resistance. However, its absence in 3 resistant strains can be explained by the involvement of other mechanisms. Moreover, a P. expansum strain LB8/99 showed good antifungal effect against some fungal pathogens through double petri dish assay. It inhibited both mycelium growth and conidia germination of B. cinerea, C. acutatum, and M. laxa, and reduced significantly by 53% and 18% respectively P. expansum. Three major VOCS: geosmin, phenethyl alcolhol (PEA) and an unknown substance were identified by GC-MS analysis. Consistent fumigation of fungal pathogens with PEA (1230 mg/mL), inhibited both conidia germination and mycelium growth of all pathogens, except conidia germination of P. expansum that was reduced by 90% with respect to control. While, the concentration of PEA produced naturally by LB8/99 was ineffective in controlling the pathogens and seemed to have a synergic or additive effect with the other VOCS. Investigations to study the biofumigant effect of LB8/99 on other commodities like seeds and seedlings are in progress.

Molecular variability of Meningococcal antigens in carriage and disease strains and in other Neisseria species

This PhD thesis is focused on the study of the molecular variability of some specific proteins, part of the outer membrane of the pathogen Neisseria meningitidis, and described as protective antigens and important virulence factors. These antigens have been employed as components of the vaccine developed by Novartis Vaccines against N. meningitidis of serogroup B, and their variability in the meningococcal population is a key aspect when the effect of the vaccine is evaluated. The PhD project has led to complete three major studies described in three different manuscritps, of which two have been published and the third is in preparation. The thesis is structured in three main chapters, each of them dedicated to the three studies. The first, described in Chapter 1, is specifically dedicated to the analysis of the molecular conservation of meningococcal antigens in the genomes of all species classified in the genus Neisseria (Conservation of Meningococcal Antigens in the Genus Neisseria. A. Muzzi et al.. 2013. mBio 4 (3)). The second study, described in Chapter 2, focuses on the analysis of the presence and conservation of the antigens in a panel of bacterial isolates obtained from cases of the disease and from healthy individuals, and collected in the same year and in the same geographical area (Conservation of fHbp, NadA, and NHBA in carrier and pathogenic isolates of Neisseria meningitidis collected in the Czech Republic in 1993. A. Muzzi et al.. Manuscript in preparation). Finally, Chapter 3 describes the molecular features of the antigens in a panel of bacterial isolates collected over a period of 50 years, and representatives of the epidemiological history of meningococcal disease in the Netherlands (An Analysis of the Sequence Variability of Meningococcal fHbp, NadA and NHBA over a 50-Year Period in the Netherlands. S. Bambini et al.. 2013. PloS one e65043).

Characterization of West Nile virus strains isolated in Italy

West Nile virus (WNV) is a neurotropic flavivirus that is maintained in an enzootic cycle between mosquitoes and birds, but can also infect and cause disease in humans and other vertebrate species. Most of WNV infections in humans are asymptomatic, but approximately 20% of infected people develop clinical symptoms, although severe neurological diseases are observed in less than 1% of them. WNV is the most widely distributed arbovirus in the world and has been recently associated with outbreaks of meningo-encephalitis in Europe, including Italy, caused by different viral strains belonging to distinct lineages 1 and 2. The hypothesis is that genetic divergence among viral strains currently circulating in Italy might reflect on their pathogenic potential and that the rapid spread of WNV with increased pathogenicity within naïve population suggest that epidemic forms of the virus may encode mechanisms to evade host immunity. Infection with WNV triggers a delayed host response that includes a delay in the production of interferon-α (IFN-α). IFNs are a family of immuno-modulatory cytokines that are produced in response to virus infection and serve as integral signal initiators of host intracellular defenses. The increased number of human cases and the lack of data about virulence of European WNV isolates highlight the importance to achieve a better knowledge on this emerging viral infection. In the present study, we investigate the phenotypic and IFN-α-regulatory properties of different WNV lineage 1 and 2 strains that are circulating in Europe/Italy in two cell lines: Vero and 1321N1. We demonstrate that: Vero and 1321N1 cells are capable of supporting WNV replication where different WNV strains show similar growth kinetics; WNV lineage 2 strain replicated in Vero and 1321N1 cells as efficiently as WNV lineage 1 strains; and both lineages 1 and 2 were highly susceptible to the antiviral actions of IFN-α.