Serratia marcescens: a potential pathogen for fish

Abstract. Characterization of a red pigmented enterobacterium isolated from natural population of white perch, Morone americanus (Gmelin), during the course of a bacteriological survey in the Back River (Baltimore, Maryland, USA) indicated that the bacterium belonged to the species Serratia marcescents. The virulence properties of this isolate (RB 469), studied in comparison with the reference strain of S. marcescens ATCC 1800 and S. plymuthica K1R, revealed that all the strains were highly pathogenic for fish with LD₅₀s raging from 5 × 10³ to 1 × 10⁵. Similarly, te extracellular products (ECP) from the three isolates were lethal for fish (LD₅₀ ranging from 0·22 to 4·8 μg protein g^-1 fish). However, only ECP from strains with strong proteolytic activity (the white perch isolate and S. plymuthica ) were cytotoxic for both in fish and homoiothermic cell cultures and both activities were completely destroyed by heating at 100°C for 10min. In contrast, only the two S. marcescens strains which possessed phospholipase active were pathogenic for mice and produced enterotoxins. None of the Serratia strains displayed dermonecrotic factor in rabbits. All these lindings indicate that a direct relationship between eytotoxicity and virulence cannot be established.