A Disease In Arctic Char Caused By Serratia liquefaciens Clifford E. Starliper   United States Gelogical Survey, National Fish Health Research Laboratory, 1700 Leetown Road, Kearneysville, West Virginia 25430 Studies have indicated Arctic char to be an excellent candidate species for aquaculture at (warmer) water temperatures of 10-13ºC e.g. the mid-Atlantic U.S. With this increased interest it will become important to identify pathogens and ways to avoid and treat diseases. Serratia liquefaciens was a cause of morbidity and mortality in Nauyuk and Labrador strains of Arctic char that were being maintained at our lab. (S. liquefaciens has not previously been associated with epizootics in Arctic char, but epizootics have been described in Atlantic salmon and turbot) Mortality commenced at various times, in various sizes, of both strains of char, but did not exceed 2% per week in any lot. Except for redness and slight swelling around the anus, no external lesions were evident. Internal tissues were severely hemorrhagic with copious bloody ascites. S. liquefaciens was isolated from kidney, brain, spleen, swim bladder, intestine, liver and ascites. Isolates were Gram-negative rods, produced gas from glucose, were motile at 21 and 37 ºC, oxidase negative, A/AG in TSI, and reduced nitrates to nitrites. They were positive for gelatin, citrate, esculin, lysine, ornithine, DNAse and grew on 1% m-tartrate and 1% D-malate. Isolates were negative for indole, H2S, malonate, phenylalanine, and arginine. Groups of fish (34 g each) were exposed to viable S. liquefaciens by immersion in 3.48×109 cfu/mL for 2 min; the only mortality was in Atlantic salmon (27%); dead fish displayed severe internal hemorrhaging, but no external signs. Upon IP injection, LD50 values ranged 1.87×108 to 1.41×106 cfu per fish; the two char strains and Atlantic salmon were similar, and more susceptible than brook and rainbow trout. IM LD50 values were lower, 1.11×106 to 1.50×103 cfu per fish; Nauyuk and Labrador char were similar, and were more susceptible than Atlantic salmon, brook and rainbow trout. Additionally: 1) there was relatively minimal mortality among char and salmon injected IP and IM with cell-free supernatant (CFS; 0.03 mg/fish) 2) CFS mortality were hemorrhaged internally and some IM fish had raised areas at the site of injection 3) among IP injected char, mortality (LD50’s: 4.37×105, 1.85×106 cfu/fish) was slightly lessened by washing cells 4) when filtered CFS was recombined with washed cells, mortality was comparable to that of unwashed cells. Isolates were sensitive to nalidixic acid, neomycin, oxolinic acid, Romet, streptomycin, and trimethoprim.  They were resistant to ampicillin, colistin, nitrofurantoin, sulfadiazine, and tetracycline and intermediate to chloramphenicol, oxytetracycline.