TWENTY-FOURTH ANNUAL EASTERN FISH HEALTH WORKSHOP Royal Pavilion Resort, Atlantic Beach, NC MARCH 9-11, 1999 Widespread Nature Of Antimicrobial Resistance In Fish Pathogens, With Particular Reference To R-Plasmids Clifford E. Starliper1, Richard K. Cooper2, John P. Hawke3, Emmett B. Shotts1 and Scott Rodgers1 1USGS/BRD National Fish Health Laboratory, Kearneysville, West Virginia, 25430; 2Department of Veterinary Science, and 3Department of Veterinary Microbiology and Parisitology, Louisiana State University, Baton Rouge, Louisiana, 70803. Currently, three antimicrobials are approved by the US Food and Drug Administration for use in fisheries to combat bacterial diseases: Romet, Terramycin (oxytetracycline) and sulfamerazine. A recurring obstacle to their efficacy is the development of resistance among the targeted pathogens. In fact, sulfa resistance in Aeromonas salmonicida was noted in the late 1950's. Resistance to Romet and oxytetracycline has also been reported in A. salmonicida, as well as in Edwardsiella ictaluri. A Tribrissen-resistant Escherichia coli from an equine cystitis has also been shown to be resistant to Romet. Tribrissen and Romet are similar compounds, both being potentiated sulfa drugs. Furthermore, we report isolation of Romet and oxytetracycline resistant strains of Photobacterium damselae subsp. piscicida (formerly Pasturella piscicida). We have demonstrated the presence of a resistance (R-) plasmid in each of these bacterial species as a molecular basis for their ability to be refractive to therapy. These four bacteria originated from very different hosts, environments and geographic areas yet, possess a similar, if not identical, 55 kilobase R-plasmid that is readily transferrable via simple conjugation. The similarity was demonstrated by plasmid mobility in agarose gels and by comparison of sequenced DNA of the resistance gene. These results suggest a possible scenario; perhaps, a very prevalent R-plasmid exists in environmental bacteria that is readily available (via conjugation?) to the pathogens being targeted with antimicrobial therapy. Return to 24th Annual Eastern Fish Health Workshop Return to Leetown Science Center Home Page