A study of lactose metabolism in Lactococcus garvieae reveals a genetic marker for distinguishing between dairy and fish biotypes.

Dairy and fish isolates of Lactococcus garvieae were tested for their ability to utilize lactose and to grow in milk. Fish isolates were unable to assimilate lactose, but unexpectedly, they possessed the ability to grow in milk. Genetic studies, carried out constructing different vectorette libraries, provided evidence that in fish isolates, no genes involved in lactose utilization were present. For L. garvieae dairy isolates, a single system for the catabolism of lactose was found. It consists of a lactose transport and hydrolysis depending on a phosphoenolpyruvate-dependent phosphotransferase system combined with a phospho-beta-galactosidase. The genes involved were highly similar at the nucleotide sequence level to their counterparts in Lactococcus lactis; however, while in many L. lactis strains these genes are plasmid encoded, in L. garvieae they are chromosomally located. Thus, in the species L. garvieae, the phospho-beta-galactosidase gene, detectable in all strains of dairy origin but lacking in fish isolates, can be considered a reliable genetic marker for distinguishing biotypes in the two diverse ecological niches. Moreover, we obtained information regarding the complete nucleotide sequence of the gal operon in L. garvieae, consisting of a galactose permease and the Leloir pathway enzymes. This is one of the first reports concerning the determination of the nucleotide sequences of genes (other than the 16S rDNA gene) in L. garvieae and should be considered a step in a continuous effort to explore the genome of this species, with the aim of determining the real relationship between the presence of L. garvieae in dairy products and food safety.