Edwardsiella tarda is a pathogen with a broad host range that infects both animals and humans. Resistance to phagocytic killing may be involved in the pathogenicity of this bacterium. Here we show that intracellular replication of E. tarda in murine macrophages is dependent on the type III secretion system and induces an anti-apoptotic effect by up-regulating anti-apoptotic NF-kappaB target genes. The wild-type strain replicates within the phagosomal membrane of macrophages; whereas the type III mutant does not. Microarray analysis shows the mRNA expression level of NF-kappaB target genes (e.g. pro-inflammatory cytokines and anti-apoptotic genes) in macrophages infected with the wild-type strain were up-regulated compared to macrophages infected with the type III mutant. Up-regulation of Bcl2a1a, Bcl2a1b, cIAP-2, and TRAF1 genes induced expression of anti-apoptotic proteins to protect macrophages from apoptosis induced by staurosporine. Further, this protection was inhibited by adding kamebakaurin, an inhibitor of NF-kappaB activation and was confirmed using an NF-kappaB reporter gene assay. Up-regulation of anti-apoptotic NF-kappaB target genes is responsible for the anti-apoptotic activity of E. tarda and is required for intracellular replication in murine macrophages.

ABSTRACT-The hemagglutinating activity (HA) of Edwardsiella tarda, which had been isolated from cultured fish and culture environments, was investigated in relation to NaCl concentration of the growth medium. E. tarda cells were cultured in a peptone-yeast extract broth supplemented with 3% NaCl (3%-NaCl culture) and without NaCl (0%-NaCl culture). Hemagglutination assays with guinea pig erythrocytes classified the strains into three HA patterns. Seventeen strains exhibited HA only with the 3%-NaCl culture (type A). A more frequent type (35 strains) displayed HA in both 0%-and 3%-NaCl cultures but the 3%-NaCl culture showed higher HA activity than the 0%-NaCl culture (type B). No HA was detected in both cultures of the other three strains (type C). The NaCl-induced HA was well correlated with the expression of a 19.3 kDa protein, a fimbrial major subunit (FimA). Infection experiments with a selected strain (type A) of E. tarda revealed that the 3%-NaCl culture was more virulent to Japanese flounder Paralichthys olivaceus than the 0%-NaCl culture, when fish were challenged by an oral route. This induction of the fimbrial protein under higher NaCl concentration may play an important role in the virulence of E. tarda in marine environments.