Fimbriae Expression by Edwardsiella tarda in High-salt Culture Conditions

Abstract

Edwardsiella tarda causes a serious disease known as edwardsiellosis in farmed freshwater and marine fishes. We previously reported that the hemagglutination and cell adherence activities of E. tarda increase in high-salt culture conditions (3% NaCl). This paper describes the induction of fimbriae expression in E. tarda in such high-salt (3% NaCl) culture conditions, using two different phenotypic strains of E. tarda, FK1051 (motile) and MEE0309 (non-motile), isolated from diseased fish. Both strains exhibited faster growth in liquid medium supplemented with 0% to 2% NaCl and slower growth in the 3% NaCl conditions. Hemagglutination activity against guinea pig erythrocytes was detected only in the 2% NaCl and/or 3% NaCl cultures. Electron microscopy revealed two types of fimbriae. The first type was thick (ca. 9 nm) and appeared only in the 0% NaCl culture of FK1051, and the second type was thin (ca. 4 nm) and appeared in the 3% cultures of both strains. The expressions of the major fimbrial subunit gene (etfA) in both strains were significantly higher in the 3% NaCl cultures than in the 0% NaCl cultures. The present results suggest that the thin type of fimbriae is involved in the cell adherence of E. tarda.