SERRATIA-MARCESCENS - A POTENTIAL PATHOGEN FOR FISH

Characterization of a red pigmented enterobacterium isolated from natural populations 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 marcescens. 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 ranging from 5 x 10(3) to 1 x 10(5). Similarly, the extracellular products (ECP) from the three isolates were lethal for fish (LD50 ranging from 0.22 to 4.8-mu-g protein g-1 fish). However, only the 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-degrees-C for 10 min. In contrast, only the two S. marcescens strains which possessed phospholipase activity were pathogenic for mice and produced enterotoxins. None of the Serratia strains displayed dermonecrotic factor in rabbits. All these findings indicate that a direct relationship between cytotoxicity and virulence cannot be established.