Molecular and phenotypic characterization of strains of Photobacterium damselae subsp piscicida isolated from hybrid striped bass cultured in Louisiana, USA

Photobacteriosis, which is caused by the bacterium Photobacterium damselae subsp. piscicida (formerly Pasteurella piscicida) was found to be a primary cause of mortality in hybrid striped bass (striped bass Morone saxatilis X white bass M. chrysops) cultured in brackish water in southern Louisiana. The disease was diagnosed on 50 occasions from 1990 through 2000 by the Louisiana Aquatic Diagnostic Laboratory, and mortality ranged from 5% to 90% in individual culture units. Strains of the bacterium from the Louisiana coast of the Gulf of Mexico were identified by means of biochemical tests, guanine-cytosine ratio, and Aquarapid Pp kit. Strains were characterized according to their biochemical phenotype, enzyme activities, plasmid profile, genotypic profile as determined by randomly amplified polymorphic DNA (RAPD) analysis, and antimicrobial susceptibility. When compared with representative isolates from the Chesapeake Bay, Greece, Japan, and Israel, Louisiana Gulf Coast isolates of P. damselae subsp. piscicida were found to be almost identical in biochemical phenotype and enzyme activity. The isolates differed in their plasmid profiles and antimicrobic susceptibilities. Louisiana isolates were found to possess a unique plasmid banding profile relative to strains from other geographic locations. The Louisiana isolates typically produced two large plasmid bands greater than 30 kilo-base pairs (kb) and two smaller bands 8.0 kb and 5.0 kb in size. Isolates from Israel and Greece exhibited similar banding patterns but were different from Louisiana and Japanese isolates. Resistance to Romet and Terramycin by some Louisiana strains was the result of acquisition of an R plasmid. When analyzed by RAPD, the Louisiana Gulf Coast strains were found to belong to clonal lineage group 2, which displays a fingerprint similar to that of the Japanese strains.