Intracellular Replication of <i>Edwardsiella ictaluri</i> in Channel Catfish Macrophages

Abstract

Abstract The ability of Edwardsiella ictaluri to enter, survive, and replicate in head kidney−derived macrophages from channel catfish Ictalurus punctatus was investigated in vitro. Primary macrophage cultures were exposed to E. ictaluri and were sampled after 0 (90 min postinfection), 4, 8, and 12 h. The data indicated that bacterial numbers for E. ictaluri increased 2.6-, 5.1-, and 7.1-fold after 4, 8, and 12 h of incubation, respectively. Microscopic examination of infected cell cultures revealed numerous intracellular bacteria that were contained within clear, spacious vacuoles. Opsonization with normal serum resulted in significantly greater internalization of E. ictaluri at 0 h compared with pretreatment with heat-inactivated serum or no serum at the same time point, and bacteria pretreated with normal serum maintained significantly greater numbers than those in the other treatments after 4 and 8 h. Opsonization, however, did not affect the ability of E. ictaluri to replicate once the cell was entered, as there were statistically significant increases in bacterial numbers at all time points regardless of the serum pretreatment.