Isolation, identification, phylogenetic analysis and related properties of a pathogen in Silurus meridionalis Chen

Abstract:

In October 2005, a large number of adults of Silurus meridionalis Chen died in the mud fish farming of Sichuan province. Later, three predominate strains of bacteria were isolated from the body of moribund fish. By artificial infection tests, strain TWN3 was confirmed to be the pathogen of the disease. Based on the characteristics of morphology, physiology and biochemistry tests, TWN3 was initially identified as Proteus vulgaris, and its G+C content of DNA is 39.1%. After being amplified, the sequence of its 16S rDNA was analyzed in the database of NCBI and it showed that TWN3 had the highest similarity to P. vulgaris, with 99.52% identity. By constructing the molecular phylogenetic dendrogram with Minimum Evolution method in Mega3.1, it was revealed that TWN3 was in the same branch with P. vulgaris. Based on all the results above, TWN3 is identified as P. vulgaris. However, the result of one biochemistry test, growth in KCN, deviates from the description in Bergey′s Manual of Systematic Bacteriology. With reference to the Manual above, Proteus vulgaris is divided into two groups, P. vulgaris BG2 and 3.According to the specific biochemical properties, TWN3 is classified as a member of P. vulgaris BG3. Relevant tests of biological properties were also conducted, which showed that this strain has no haemolysis and is sensitive to four kinds of antibiotic such as gentamicin. Moreover, it can strongly cause diseases to mice. The research on the growth property of strain TWN3 indicated that its growth temperature ranges from 10℃ to 43℃, optimum 37℃; growth pH ranges from 4 to 11, optimum 6.Its optimum salinity varies under different temperatures，and it grows best under 1.5% salinity while 37℃. The aim of these researches is to provide an evidence for the prevention and cure of TWN3. According to the appearance of the diseased Silurus meridionalis Chen and results of artificial infection test on crucian carps, it is considered that Silurus meridionalis Chen is infected through digestive system and it is also recommended that Bdellovibrio should be used in biological control. Although the pathogenicity of P. vulgaris is extensive, there has been no report that P. vulgaris is considered as the pathogen of cultivated Silurus meridionalis Chen so far. In addition，the identification of strain TWN3 has positive effects on the future research on the taxonomy of Proteus.