| Summary: | 碩士 === 國立屏東科技大學 === 獸醫學系 === 89 === Abstract
The giant freshwater prawn Macrobrachium rosenbergii that is intensive aquaculture in the southern Taiwan. However, an epizootic bacterial infection in the prawns displayed anorexia, poor growth and mortality. Gross pathological showed opaque and whitish muscles and swollen hepatopancreas. Histopathologically, the prawns showed marked exudative fluid, necrotic lesions and nodules in the muscles and hepatopancreas. Physiological and biochemical characteristics of isolated strains from diseased prawns were Gram-positive cocci. Almost strains were able to grow at high temperature and media in high salt and pH 9.6. The conventional tests and API 20 STREP system results of the isolated strains were similar to those of the reference strains (Enterococcus seriolicida and Lactococcus garvieae). Identitfication of seventeen strains was comfirmed by a PCR assay for L. garvieae that gave the expected amplicon of 1100 bp and 709 bp in size, respectively. Experimental infections with these isolates gave gross signs and histopathological changes similar to those seen in the naturally infected prawns. The isolated strain, MR10 showed the highest pathogenicity and the LD50 value was 3.0×104 colony forming units/prawn. Seventeen strains of L. garvieae isolated from diseased prawns and 2 reference strains were selected for drugs minimum inhibitory concentration (MIC) and disinfectants susceptibility tests in vitro. The results are as follow : Erythromycin showed the highest sensitivity to all the tested organisms, the MIC 90 value was under 0.5μg/ml, the followed by Ampicillin and Cephalothin, the MIC 90 value were both under 4μg/ml. The bactericidal activity of the 4 disinfectants in decreasing order is copper sulphate, benzalkonium chloride after 6h exposure and benzalkonium chloride, copper sulphate after 24h exposure. The 16S rDNA of L. garvieae were digested with restriction enzyme of SpeⅠcan be used to differentiate between L. garvieae and other bacteria. The technique also promises rapid diagnosis of L. garvieae. In addition, MR1 16S rDNA sequence (GenBank accession number AF283499) gave 97% sequence identity to E. seriolicida (synonym L. garvieae; GenBank accession number AF061005). The MR1 formed a clade with E. seriolicida and L. garvieae by phylogenetic tree analysis.
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