The pathogenesis of experimental Edwardsiella tarda infection in channel catfish (Ictalurus punctatus)

Two experimental trials of channel catfish infection with E. tarda were conducted. In each trial, 80 juvenile channel catfish were infected by waterborne exposure to the bacteria after skin scraping. In the first experiment, the clinical signs, the gross and microscopic pathology, and the colony forming units (CFU) per gram of tissue or milliliter of blood were studied daily for 5 days post infection. In the second experiment, the same parameters (with the exception of CFU) were studied every other day for 12 days post infection. Scattered cutaneous ulcers, depigmentation areas, and petechial hemorrhages were seen on the skin. The livers showed whitish patchy discoloration and petechial hemorrhages. Histological sections had granulomatous inflammation with predominantly macrophages in the liver, spleens, head and trunk kidney. The bacteria in the hepatic lesions and cutaneous lesions at scraping sites were specifically stained by an avidin-biotin conjugate immuno-peroxidase technique using primary antibodies specific for E. tarda. The bacterial growth in the liver, trunk kidney and blood peaked on the 3{dollar}rmsp{lcub}rd{rcub}{dollar} day post infection then started to decline on the 4{dollar}rmsp{lcub}th{rcub}{dollar} and 5{dollar}rmsp{lcub}th{rcub}{dollar} day post infection. There were no clinical signs or gross and microscopic pathology noted on the 8{dollar}rmsp{lcub}th{rcub}{dollar} day post infection.; The outer membrane proteins (OMP) prepared with sodium N-lauroyl sarcocinate from 10 E. tarda fish isolates were examined using sodium dodecyl sulfate-polyacrylamide gradient gel electrophoresis and compared to the OMP of the type stain E. tarda (ATCC 15947). The type isolate OMP profile had four major proteins with molecular masses of 40, 36.5, 34, and 28.5 kDa and a large number of minor protein bands. A comparison of the OMP of 10 isolates of E. tarda revealed significant variations between the isolates indicating non-homogenous isolates. The incubation temperature did not change the OMP profile in 90% of the isolates examined. The change in salinity of growth medium effected the outer membrane protein profile of the isolates. Reactions of E. tarda to salinity levels were placed in three groups. The first major group expressed fewer or more major outer membrane protein bands at 1.5% salinity. The second group lost major bands at 3.0% while the third group had no change in the OMP profile when it was grown at different salinity levels.