- 學位論文
臺灣南部地區養殖午仔魚無乳鏈球菌之表現型、基因型與致病性研究
Phenotype, genotype and pathogenicity profiling of Streptococcus agalactiae originated from cultured threadfin fish in southern Taiwan
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摘要
午仔魚為馬鮁科魚種(Eleutheronema spp.)的俗稱，因為胸鰭有部分特化為四條絲狀的軟鰭條，故也俗稱為四絲馬鮁。午仔魚因其快速的生長能力、細膩的肉質及豐厚的脂肪而廣受養殖戶及市場的歡迎，臺灣養殖品種則幾為四指馬鮁(Eleutheronema tetradactylum)及多鱗四指馬鮁(Eleutheronema rhadinum)。本研究搜集民國106年至111年間，由高屏地區防治所送檢之午仔魚無乳鏈球菌分離株，利用單管多引子之聚合酶鏈鎖反應進行血清型的鑑定；選用特異性引子對進行各項毒力基因及抗藥性基因的偵測；以脈衝式電泳凝膠的方法進行基因型的相似度分析；最後綜合基因型資料選取菌株進行致病性試驗。結果顯示不同年份間的血清型皆為Ia，即臺灣魚類之鏈球菌報告最多的種類。溶血性的表現則完全溶血及不溶血皆有出現，以不溶血的菌株（佔比約66%）為大宗。毒力基因的檢測結果顯示主要的種類為侵犯型基因，包含cylE與hylB，兩者檢出率分別為47.7%與84.1%。抗藥性基因的檢測在結果則表示近三年所得的菌株有100%的ermA檢出率，然而此基因的有無對於實際藥物感受性並沒有絕對的關聯。致病性試驗的過程中也發現午仔魚相比其他魚種感染無乳鏈球菌的反應較輕微，無明顯特徵性的症狀出現。而近年出現於午仔魚的基因型相較更早的基因型有較低毒力的表現，造成較少量的傷亡，或許是近年送檢數量明顯較低的緣由。
並列摘要
Eleutheronema species, given the common name threadfin fish, is distinguished by its filamentous soft rays that form part of its specialized pectoral fins. This fish is highly prized in the market due to its rapid growth and delicious taste. This study collected S. agalactiae isolates from threadfin fish in the Kaohsiung-Pingtung area from 2017 to 2022, all isolated by governmental animal disease control center. These isolates underwent multiplex PCR to identify serotypes, PCR screenings of different virulence factor (VF) genes and antimicrobial resistance (AMR) genes, pulse-field gel electrophoresis to reveal genotypes, and the genetic profile is comprehended to select isolated for pathogenicity test. The serotype remained consistent, with only serotype Ia reported in cultured fish species in Taiwan. Both β-hemolysis and ɣ-hemolysis were observed, while non-hemolytic strains are the majority (66%). The screening results for VF genes showed that the main category is the invasion genes, including cylE and hylB, showing 47.7% and 84.1% in positivity rate, respectively. As for AMR genes, isolates from the latest three years showed a hundred percent positive result in ermA gene detection, yet the obtain of this gene does not have exact relation to actual drug sensibility. It is found that S. agalactiae has mild affection on the gross lesions when infecting threadfin fish, and that the emerging pulsotypes among this species in recent years has lower pathogenicity than the ones in early years.
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