Abstract

Bacillus cereus is a common opportunistic human pathogen well known as a food contaminant. In recent years, tiger frog farming has been undertaken in China and many other countries to meet human food habits. However, there is no definitive data to indicate the contamination of potentially pathogenic B. cereus strains to fresh cultured tiger frogs. In this study, a virulent B. cereus strain W10 with hemolytic activity was first isolated from the liver of cultured tiger frogs. It was initially identified based on the spore-forming characterization and phenotypic characterization using API identification kits and its taxonomic position, was further determined by nucleotide blast search in NCBI website and phylogenetic analysis. The constructed phylogenetic tree using neighbor-joining method further showed that strain W10 was closely related to B. cereus strain F46 (GenBank accession no. EF203906), which was isolated from the contaminated food. To further confirm the virulence and potential danger of strain W10, its LD₅₀ value was tested to be 5.22×10⁴ cfu/g, which indicated that strain W10 had strong potential virulence to healthy tiger frogs. Pathological changes were observed in liver and body surface of the challenged tiger frogs. Thus, as a potential pathogen of a zoonotic disease, the distribution of the virulent B. cereus strain, may be a threat to the safety of tiger frog food and more importance should be attached to the virulent B. cereus strain in tiger frog farming.

Key words: Tiger frog, Bacillus cereus, phenotypic characterization, phylogenetic analysis.