Antigenic characterization of Enterococcus strains pathogenic for turbot and their relationship with other Gram-positive bacteria

We describe here the serological and antigenic characterization of a group of highly virulent Enterococcus strains that, since 1993, have caused heavy economic losses in turbot Scophthalmus maximus reared in northern and northwestern Spain. Reference Gram-positive strains and the corresponding polyclonal antisera raised against some species of lactic acid bacteria as well as Corynebacterium aquaticurn and Renibacterium salmoninarum were included. Agglutination assays demonstrated that regardless of the farm and year of isolation, all strains from turbot were serologically homogeneous. However, a number of cross-reactions occurred among the various groups of bacteria (malnly among Enterococcus, Carnobacterium and Lactobacillus species) Analysis of the surface proteins and western blot assays supported the antigenic homogeneity of the Enterococcus strains isolated from turbot because they all yielded an essentially identical protein pattern (major common bands were 40, 45, and 50 kDa) and reacted similarly with antisera raised against representative strains of this group (there were at least 7 common bands of 20, 25, 30, 40, 45, 57 and 80 kDa). This serological homogene~ty suggests that the development of vaccines to control enterococcosis in turbot should be feasible. Irrespective of the source of the polyclonal antisera employed, common immunoreactive proteins were present in all the Gram-positive bacilli and cocci tested. This, together with the cross-agglutination reactions noted among the various bacterial taxa tested, suggests that serological identification of the turbot Enterococcus using polyclonal antisera could be problematical.