Abstract
SummaryFew information is available about the tyraminogenic potential of the species Enterococcus mundtii. In this study, two plant‐derived strains of E. mundtii were selected and investigated to better understand the phenotypic behaviour and the genetic mechanisms involved in tyramine accumulation. Both the strains accumulated tyramine from the beginning of exponential phase of growth, independently on the addition of tyrosine to the medium. The strains accumulated also 2‐phenylethylamine, although with lower efficiency and in greater extent when tyrosine was not added. Accordingly, the tyrosine decarboxylase (tyrDC) gene expression level increased during the exponential phase with tyrosine added, while it remained constant and high without precursor. The genetic organization as well as sequence identity levels of tyrDC and tyrosine permease (tyrP) genes indicated a correlation with those of phylogenetically closer enterococcal species, such as E. faecium, E. hirae and E. durans; however, the gene Na+/H+ antiporter (nhaC) that usually follow tyrP is missing. In addition, BLAST analysis revealed the presence of additional genes encoding for decarboxylase and permease in the genome of several E. mundtii strains. It is speculated the occurrence of a duplication event and the acquisition of different specificity for these enzymes that deserves further investigations.
Highlights
Tyramine is a biogenic amine (BA) deriving from tyrosine decarboxylation and can have severe acute effects if ingested in excessive amounts with food, consisting in peripheral vasoconstriction, increased cardiac output, accelerated respiration, elevated blood glucose and release of norepinephrine, symptoms known as ‘cheese reaction’ (Shalaby, 1994; McCabe-Sellers et al, 2006; Marcobal et al, 2012)
Microbial Biotechnology published by John Wiley & Sons Ltd and Society for Applied Microbiology, Microbial Biotechnology, 9, 801–813
The two strains considered here produced greater amounts of tyramine than those accumulated by other strains belonging to E. faecium and E. faecalis previously studied in the same conditions (Bargossi et al, 2015b)
Summary
Tyramine is a biogenic amine (BA) deriving from tyrosine decarboxylation and can have severe acute effects if ingested in excessive amounts with food, consisting in peripheral vasoconstriction, increased cardiac output, accelerated respiration, elevated blood glucose and release of norepinephrine, symptoms known as ‘cheese reaction’ (Shalaby, 1994; McCabe-Sellers et al, 2006; Marcobal et al, 2012). Among LAB, species belonging to the genus Enterococcus are recognized as the most frequent and intensive tyramine producers (Leuschner et al, 1999; Suzzi and Gardini, 2003; Ladero et al, 2012) Due to their salt and pH tolerance, and to their ability to grow over a wide temperature range, enterococci are isolated from different habitats and are often contaminants in food of animal origin, such as cheese and sausages (Giraffa, 2003; Franz et al, 2011). Resistance to other microorganisms (Giraffa, 2002; Klein, 2003; Rossi et al, 2014) They are strong tyramine producers and this ability has been deeply exploited in Enterococcus faecalis (in which tyramine production is considered a species trait), Enterococcus faecium and Enterococcus durans (Linares et al, 2009; Ladero et al, 2012; Bargossi et al, 2015a,b). Information on the genetic basis of the tyraminogenic potential of E. mundtii were obtained analysing the expression of the tyrosine decarboxylase (tyrDC) gene, the sequence of tyrDC and tyrosine permease (tyrP) genes, and the genetic organization of the TDC operon region
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