Identification and drug susceptibility of a pathogenic Achlya klebsiana strain from the eggs of yellow catfish (Pelteobagrus fulvidraco)
黄颡鱼卵致病性绵霉的分离鉴定与药敏特性

Objective] The aim of the present study was to isolate and identify the pathogen of Pelteobagrus fulvidraco egg saprolegniasis, and study the drug susceptibility characteristics of the pathogen. Methods] Filamentous fungal strains were first isolated using the traditional method from the Pelteobagrus fulvidraco eggs suffering from saprolegniasis, and the pathogenic strain was further confirmed through artificial infection experiment, identified using morphological observation and phylogenetic analysis based on its ITS rDNA sequence. In addition, its drug susceptibility was studied using the doubling dilution method. Results] Eight filamentous fungal strains were isolated form Pelteobagrus fulvidraco eggs with saprolegniasis, and strain YC was proved to be pathogenic to Pelteobagrus fulvidraco eggs by artificial infection. Thus, morphological characterizations of strain YC were studied, the phylogenetic analysis based on its ITS rDNA sequence and drug susceptibility assay were also further conducted. The experimental results showed that the hyphae of strain YC were aseptate and transparent, its zoosporangia were cylindrical, clavate or fusiform, its zoospores were discharged from the tip of zoosporangia, clumped together and fell off or directly swam into the water after a period of time. The zoosporangia were renewed in basipetalous succession, infrequently in sympodial arrangement. Its oogonia were spherical or obpyriform with frequently diclinous and occasionally monoclinous antheridial branches, and produced one to fifteen oospores, which were centric or subcentric, and laterally generated a big oil body. The ITS rDNA sequences of strain YC was naturally clustered with ITS rDNA sequences of Achlya sp. submitted to GenBank with the homology of 99%, and had closest relationship with Achlya klebsiana strain CBS101.49 (GenBank accession number: AF119579). Combined the morphological characterization with phylogenetic analysis based on ITS rDNA sequence, strain YC was identified as Achlya klebsiana. In addition, strain YC was well inhibited by Coptis chinensis and isothiazolinone, whose minimum inhibitory concentrations to strain YC were respectively 256 mg/L and 2 mg/L. Conclusion] A pathogenic Achlya klebsiana strain YC was isolated for the first time from Pelteobagrus fulvidraco eggs suffering from saprolegniasis, and its drug susceptibility was also determined, which could serve as a foundation for its infection in Pelteobagrus fulvidraco eggs.