Characterization of Tyrosine Decarboxylase (tyrDC) Activity in Genus Enterococcus

The presence of biogenic amines in foods and the risks associated to their consumption (especially considered the increase of susceptible consumers) are well known since long time, but systematic studies about their production have been carried out only in the last decades. Fermented foods, in particular, are often characterized by the presence of relevant concentrations of biogenic amines due to the activity of the microbiota responsible for the secondary fermentation. Within this microbiota, enterococci play a controversial role, in fact they contribute to the definition of typical organoleptic profile of some fermented foods, but at the same time they are characterized by different virulence factors and by the ability to produce high concentrations of biogenic amines, in particular tyramine and 2-phenylethylamine. In this perspective, this PhD thesis represents a contribution for a deeper insight of factors, biological mechanisms and genetic characteristics that influence the activity of tyrosine decarboxylase enzymes in strains belonging to the species Enterococcus faecalis, Enterococcus faecium and Enterococcus mundtii. The results of these studies indicated that qualitatively and quantitatively kinetics of tyramine and 2-phenylethylamine production can vary within species, but also strains. These differences can be related to the effects of the specific composition of substrates, the technological variables (temperature, NaCl concentrations, pH) and the specific genetic characteristics of tyrosine decarboxylase cluster and its transcription. Moreover this thesis highlighted the differences in the responses to environmental factors of a pure enzyme respect microbial cells, to better understand the relationships between decarboxylating activity and the integrity of microbial cells. The data obtained indicating that the decarboxylation activity has to be viewed in the light of the overall cell metabolism. Finally, the use of bioprotective cultures producing bacteriocins as antagonists against biogenic amine producing microorganisms has been exploited to reduce the risks of biogenic amine accumulation in fermented foods.