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The ability to accumulate tyramine and 2-phenylethylamine by two strains of  Enterococcus faecalis  and two strains  Enterococcus faecium  was evaluated in two cultural media added or not with tyrosine. All the enterococcal strains possessed a tyrosine decarboxylase (tyrDC) which determined tyramine accumulation in all the conditions tested, independently on the addition of high concentration of free tyrosine. Enterococci differed in rate and level of biogenic amines accumulation.  E. faecalis  EF37 and  E. faecium  FC12 produced tyramine in high amount since the exponential growth phase, while 2-phenylethylamine was accumulated when tyrosine was depleted.  E. faecium  FC12 and  E. faecalis  ATCC 29212 showed a slower tyraminogenic activity which took place mainly in the stationary phase up to 72 h of incubation. Moreover,  E. faecalis  ATCC 29212 produced 2-phenylethylamine only in the media without tyrosine added. In BHI added or not with tyrosine the  tyrDC  gene expression level differed considerably depending on the strains and the growth phase. In particular, the  tyrDC  gene expression was high during the exponential phase in rich medium for all the strains and subsequently decreased except for  E. faecium  FC12. Even if tyrDC presence is common among enterococci, this study underlines the extremely variable decarboxylating potential of strains belonging to the same species, suggesting strain-dependent implications in food safety.

frontiers in microbiology

Tyrosine decarboxylase activity of enterococci grown in media with different nutritional potential: tyramine and 2-phenylethylamine accumulation and tyrDC gene expression