Selenium (Se) is an essential trace element for humans, plants and microorganisms. Inorganic selenium is present in nature in four oxidation states: selenate, selenite, elemental Se and selenide in decreasing order of redox status. These forms are converted by all biological systems into more bioavailable organic forms, mainly as the two seleno-amino acids selenocysteine and selenomethionine. Humans, plants and microorganisms are able to fix twhese amino acids into proteins originating Se-containing proteins by a simple replacement of methionine with selenomethionine, or "true" selenoproteins if the insertion of selenocysteine is genetically encoded by a specific UGA codon. Selenocysteine is usually present in the active site of enzymes, being essential for their catalytic activity. This review will focus on the strategies adopted by the different biological systems for selenium incorporation into proteins and on the importance of this element for the physiological functions of living organisms. The most known selenoproteins of humans and microorganisms will be listed highlighting the importance of this element and the problems connected with its deficiency.

The soluble and membrane proteome of a tyramine producing Enterococcus faecalis, isolated from an Italian goat cheese, was investigated. A detailed analysis revealed that this strain also produces small amounts of beta-phenylethylamine. Kinetics of tyramine and beta-phenylethylamine accumulation, evaluated in tyrosine plus phenylalanine-enriched cultures (stimulated condition), suggest that the same enzyme, the tyrosine decarboxylase (TDC), catalyzes both tyrosine and phenylalanine decarboxylation: tyrosine was recognized as the first substrate and completely converted into tyramine (100% yield) while phenylalanine was decarboxylated to beta-phenylethylamine (10% yield) only when tyrosine was completely depleted. The presence of an aspecific aromatic amino acid decarboxylase is a common feature in eukaryotes, but in bacteria only indirect evidences of a phenylalanine decarboxylating TDC have been presented so far. Comparative proteomic investigations, performed by 2-DE and MALDI-TOF/TOF MS, on bacteria grown in conditions stimulating tyramine and beta-phenylethylamine biosynthesis and in control conditions revealed 49 differentially expressed proteins. Except for aromatic amino acid biosynthetic enzymes, no significant down-regulation of the central metabolic pathways was observed in stimulated conditions, suggesting that tyrosine decarboxylation does not compete with the other energy-supplying routes. The most interesting finding is a membrane-bound TDC highly over-expressed during amine production. This is the first evidence of a true membrane-bound TDC, longly suspected in bacteria on the basis of the gene sequence.

Lactic acid bacteria (LAB) are very ancient organisms that can't obtain metabolic energy by respiration without external heme supplementation. Since the gain in ATP from lactic fermentation is inadequate to support efficient growth, they developed alternative strategies for energy production. Three main energy generating routes are present in LAB: amino acid decarboxylation, malate decarboxylation and arginine deimination (ADI pathway). These routes, apart from supplying energy, also play a role in pH control. Lactic fermentation, which leads to lactic acid accumulation, causes a pH decrease that amino acid decarboxylations, originating basic amines, and the ADI pathway, giving rise to ammonia, may partially contrast. In the present mini-review, the reciprocal relationships among these metabolic pathways are considered, on the basis of proteomic results obtained from four different LAB strains, all of which possess the ADI pathway, but express different amino acid decarboxylases. The strains have been isolated and selected from different habitats and the role of some inducing molecules as well as of the growth phases is discussed. The overall results have revealed that LAB are complex biosystems able to set up a sophisticated metabolic regulation through a complex network of proteins that also include stress responses, as well as protease activation or inhibition.

Selenium (Se), Se-cysteines and selenoproteins have received growing interest in the nutritional field as redox-balance modulating agents. The aim of this study was to establish the Se-concentrating and Se-metabolizing capabilities of the probiotic Lactobacillus reuteri Lb2 BM, for nutraceutical applications. A comparative proteomic approach was employed to study the bacteria grown in a control condition (MRS modified medium) and in a stimulated condition (4.38 mg/L of sodium selenite). The total protein extract was separated into two pI ranges: 4-7 and 6-11; the 25 identified proteins were divided into five functional classes: (i) Se metabolism; (ii) energy metabolism; (iii) stress/adhesion; (iv) cell shape and transport; (v) proteins involved in other functions. All the experimental results indicate that L. reuteri Lb2 BM is able to metabolize Se(IV), incorporating it into selenoproteins, through the action of a selenocysteine lyase, thus enhancing organic Se bioavailability. This involves endo-ergonic reactions balanced by an increase of substrate-level phosphorylation, chiefly through lactic fermentation. Nevertheless, when L. reuteri was grown on Se a certain degree of stress was observed, and this has to be taken into account for future applicative purposes. The proteomic approach has proven to be a powerful tool for the metabolic characterization of potential Se-concentrating probiotics.

Microbial complexity and contamination level of food processing plants heavily impact on the final product fate and are mainly controlled by proper environmental cleaning and sanitizing. Among the emerging disinfection technologies, ozonation is considered an effective strategy to improve the ordinary cleaning and sanitizing of slaughterhouses. However, its effect on contamination levels and environmental microbiota still need to be understood. With this purpose we monitored the change of microbiota composition in different slaughterhouse environments along the phases of cleaning-sanitizing and ozonation at 40, 20 and 4 ppm. Overall, the meat processing plants microbiota differed significantly between secondary processing and deboning rooms, with a greater presence of psychrotrophic taxa in secondary processing rooms in reason of their lower temperatures. Cleaning-sanitizing procedure significantly reduced the contamination level and in parallel increased the number of detectable OTUs, by removing the masking effect of the most abundant human/animal derived OTUs, which belonged to Firmicutes phylum. Afterward, ozonations at 40 and 20 ppm effectively decreased the remaining viable bacterial populations. However, we could observe a selective ozone-mediated inactivation of psychrotrophic bacteria only in the secondary processing rooms. Here, the Brochothrix and Pseudomonas abundances and their viable counts were significantly affected by 40 and 20 ppm of ozone, while more ubiquitous genera like Staphylococcus showed a remarkable resistance to the same treatments. This study showed the effectiveness of highly concentrated gaseous ozone as adjunct sanitizing method that can minimize cross-contamination and so extend the meat shelf-life. IMPORTANCE Our in situ survey demonstrates that RNA-based sequencing of 16S amplicons is a reliable approach to qualitatively probe at high taxonomic resolution the changes triggered by new and existing cleaning-sanitizing strategies on the environmental microbiota in human built environments. This approach could represent soon a fast tool to clearly define which routine sanitizing intervention is more suitable for a specific food processing environment, by limiting therefore the costs for special cleaning intervention and potential product loss.