Evidence of horizontal transfer as origin of strain to strain variation of the tyramine production trait in Lactobacillus brevis

Lactobacillus brevis strains with the ability to decarboxylate tyrosine to tyramine have been described and the involvement of several genes constituting a tyrdc operon at the chromosomal level has been demonstrated in this species. In this study, the existence of Lb. brevis strains unable to form tyramine was observed. In order to evaluate if the tyramine-producing ability was strain-dependent or if it could be correlated to the existence of a new species or subspecies, different isolates were analysed. Analysis by M13-RAPD and sequencing of 16S rDNA, 16S–23S ISR and house-keeping gene recA confirmed that all the isolates belonged to the Lb. brevis species. Analysis of the TyrDC pathway encoding operon region in representative strains indicated the existence of a polymorphism. The genetic differences observed showed that the tyrosine decarboxylating ability is not a Lb. brevis species trait but that it is strain-dependent within this species and suggest that the genes encoding the tyramine-producing pathway constitute a genomic island.