Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Veterinary Medical Sciences - Pathobiological Sciences

First Advisor

Ronald L. Thune


Photobacterium damsela subsp. piscicida, formerly Pasteurella piscicida, is an important new pathogen of hybrid striped bass cultured in brackish water on Louisiana mariculture farms. Louisiana isolates compared with strains of P. damsela subsp. piscicida from Greece, Japan, Israel and Chesapeake Bay, USA and were found to be identical in biochemical phenotype and enzymic activity. Small plasmids of 8 and 5 kb were unique to Louisiana strains. Sequential daily bacteriology and histopathology on hybrid striped bass experimentally infected by immersion revealed that following high doses (13,000 CFU/ml) the gills and spleen are colonized first as early as 24 hours PI, followed by the blood at 48 hours and the liver at 72 hours. Bacteria reached levels as high as $5.7\times10\sp9$ CFU/gram in the spleen and $8.6\times10\sp8$ in the blood of moribund fish. The spleen contained a significantly higher number of cells than any other organ at all sampling time periods. Histological examination of the gills using semi-thin sections showed high numbers of bacteria invading and colonizing the gill lamellae at 24 hours PI. Death of the fish appeared to result from respiratory failure due to necrosis of the gill lamellae and blockage of blood flow to the gills by masses of bacteria in the capillaries. Pathogenic isolates of P. damsela subsp. piscicida were shown to produce an iron siderophore. A wild type strain 91-197, was mutagenized by introduction of a transposon on pEIS, a suicide delivery plasmid by electroporation. A stable mutant (LSU-P1) deficient in siderophore biosynthesis was generated using this system. LSU-P1 evaluated in an immersion infection model for hybrid striped bass and also by IP injection was found to be non-virulent. The insertion of the transposon into the genome of P. damsela subsp. piscicida in LSU-P1 was confirmed by PCR analysis using primers to the kan gene in the transposon.