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Isolation and characterization of an Enterococcus-like bacterium causing muscle necrosis and mortality in
Abstract:
A Gram-positive, ovoid, diplococoid bacterium tentatively identified as Enterococcus-like was isolated from diseased Macrobrachium rosenbergii in Taiwanese aquaculture ponds. The diseased prawns displayed poor growth, anorexia, inactivity, opaque and whitish musculature, and mortality. In histological preparations, melanized hemocytic granulomas were seen in the connective tissue around hemal sinuses together with hemocytic aggregation in necrotic musculature. Five isolates of diplococci were collected from diseased prawns at 4 farms and these were evaluated for 93 characteristics including morphology, physiology, biochemistry and sensitivity to antibiotics. The results indicated that the isolates belonged to a single species. They grew in 0.5 to 6.0% NaCl, at 10 to 40 degrees C, at pH 9.6 and on bile esculin medium, gave positive pyrrolidonylarylamidase, arginine dehydrolase and Voges-Proskauer tests, were resistant to bacitracin and SXT, and were CAMP-negative and non-hemolytic on sheep blood agar. These findings indicated an Enterococcus-like bacterium closely related to Enterococcus seriolicida (recently reduced to synonymy with Lactococcus garvieae). Experimental injection of 3 x 10(5) cells of strain KM002 of this Enterococcus-like bacterium into the ventral sinus of the prawn cephalothorax caused 100% mortality in 11 d, and induced muscular necrosis and hepatopancreatitis, gross signs and histopathology similar to those observed in the naturally infected prawns. It was concluded that this Enterococcus-like bacterium was the etiological agent associated with mortality of the farmed, diseased prawns.
Insights
A novel Enterococcus-like bacterium caused significant mortality in farmed giant freshwater prawns (Macrobrachium rosenbergii) in Taiwan. This Gram-positive diplococcus induced disease signs and mortality upon experimental injection, confirming its role as the etiological agent.
Area of Science:
- Aquatic microbiology
- Fish and shellfish pathology
- Bacterial disease diagnostics
Background:
- Diseased giant freshwater prawns (Macrobrachium rosenbergii) in Taiwanese aquaculture exhibited poor growth, anorexia, opaque musculature, and mortality.
- Histopathology revealed melanized hemocytic granulomas and necrotic musculature in affected prawns.
Purpose of the Study:
- To isolate and identify the bacterial pathogen responsible for mass mortality in farmed Macrobrachium rosenbergii.
- To characterize the etiological agent and confirm its pathogenicity through experimental infection.
Main Methods:
- Isolation and characterization of five Gram-positive, ovoid, diplococoid bacterial isolates from diseased prawns.
- Evaluation of 93 characteristics including morphology, physiology, biochemistry, and antibiotic sensitivity.
- Experimental injection of a representative isolate (KM002) into healthy prawns to assess pathogenicity.
Main Results:
- The isolates belonged to a single species, exhibiting characteristics consistent with an Enterococcus-like bacterium related to Lactococcus garvieae.
- The bacterium demonstrated specific growth requirements (0.5-6.0% NaCl, 10-40°C, pH 9.6) and biochemical profiles (positive pyrrolidonylarylamidase, arginine dehydrolase, Voges-Proskauer tests).
- Experimental infection with strain KM002 resulted in 100% mortality within 11 days, inducing muscular necrosis and hepatopancreatitis.
Conclusions:
- The isolated Enterococcus-like bacterium is identified as the etiological agent responsible for mortality in farmed Macrobrachium rosenbergii in Taiwan.
- This finding highlights the importance of bacterial disease management in intensive prawn aquaculture.
- Further research into control strategies for this specific pathogen is warranted.
