Effects of sodium chloride on Edwardsiella ictaluri: Studies in channel catfish, fish cell lines, and bacterial cultures

Channel catfish Ictalurus punctatus, either unacclimated or acclimated to different saltwater concentrations (up to 2000 mg/L Cl -), were exposed by immersion or intraperitoneaI injection with Edwardsiella ictaluri, the causative agent of enteric septicemia of catfish. Freshwater-acclimated catfish, exposed to E. ictaluri and then transferred to water with different salinities did not show improved resistance to the bacteria. Edwardsiella ictaluri was transmitted from injected to uninjected freshwater-acclimated catfish in cohabitation, regardless the water salinity. When all catfish in the aquarium were injected with E. ictaluri, recovery of the bacteria after 21 d was successful for 90% of the freshwater-acclimated and for only 50% of the saltwater-acclimated catfish. No conclusion could be drawn on which salinity concentration provided the best protection.;The survival of Edwardsiella ictaluri was studied in distilled water containing different concentrations of sodium chloride and calcium. Survival of the bacteria in waters with 0, 300, 3,000 and 30,000 mg/L NaCl and 0 mg/L calcium showed inconsistent results. The effect of calcium addition (approximately 50, 100 and 200 mg/L as CaCO3) on survival, did not follow a pattern. Lowest survivals where in the 30,000 mg/L NaCl, for all the calcium concentrations.;Survival of E. ictaluri in defined minimal media was negatively affected by increased sodium chloride concentration. Bacteria in media with 10 and 15 a NaCl/L failed to survive for 48 hours, while at 5 g NaCl/L E. ictaluri survived for 84 hours.;Fish cell lines were used to evaluate the cellular invasive ability of E. ictaluri in cell cultures. Adhesion, invasion and replication were tested with 5 types of established cell lines from fish. Photomicrographs demonstrated bacteria inside the cells and confocal microscopy showed bacteria in intracellular vacuoles. The presence of large numbers of bacteria in intracellular vacuoles suggests the replication of E. ictaluri inside the cells. Quantification of the number of bacteria that adhered to and invaded cells, indicated that the invasion mechanism used by E. ictaluri was probably the same regardless of host cell type. In view of this, fish cell lines could be a valuable tool to determine virulence mechanisms of E. ictaluri.