Tyramine production by Enterococcus faecalis in a milk protein-rich mediumwith variable tyramine concentration

Milk and milk products can contain major amounts of tyramine resulting from microbial enzymatic activity. Enterococci are often involved as faecal con taminants of milk. Experiments were conducted in high milk protein substrat e at 22 degrees C to investigate whether or not Enterococcus faecalis var. liquefaciens is capable of decarboxylating tyrosine. Maximum amine formatio n was noted after 18 h of cultivation with 150 mu g/ml tyramine. After this time interval, the amine concentration in the broth was seen to diminish. With initial tyramine concentrations in the culture medium increasing, the development of microbes was found to slow down and tyramine formation was o bserved to decrease versus the control. These effects were independent of t he pH development. While both tyramine and tyrosine were maintained separat e in the high milk protein medium thin-layer chromatography (TLC) failed to reveal any binding to casein. The decrease in tyramine level in broth coul d not be fully elucidated; still it appears to be specific to substrates co ntaining milk. Occurrence of monoamine oxidase and condensation of several amine molecules, or formation of aldimide are discussed as possible sources of tyramine reduction. It is reasonable to assume that the proteolytic act ivity of enterococci influences the formation of tyramine through liberatio n of tyrosine from casein.