LAUSR

Abstract Outbreaks of mass mortality among cultured zoea of M. rosenbergii occurred in different hatcheries of Gaoyou country of China in April 2017. Of the isolates obtained from the diseased zoea, XL3–1 was pathogenic to the zoea and postlarva of M. rosenbergii in the virulence test, with a LD50 value 3.2 × 106 CFU/mL and 3.5 × 106 CFU/mL, respectively. XL3–1 was identified as Enterobacter cloacae by biochemical characteristics and molecular tools. Detection of virulence-associated genes by PCR indicated that XL3–1 was positive for irp2, fhuA, sodB, sltA, flaD and OmpX. According to the antibiotic resistant assay, the isolate was sensitive to quinolones, while they were resistant to cephalosporins and penicillins. This study also investigated the protection of specific chicken egg yolk antibodies (IgY) in E. cloacae-infected M. rosenbergii. The highest titer against E. cloacae of the obtained specific IgY by ELISA was 1: 213. M. rosenbergii treated with specific egg yolk powder could achieve the survival rate of 67% after E. cloacae infection, while M. rosenbergii treated with non-specific egg yolk powder or no egg yolk powder could achieve survival rates of 20% and 17%, respectively. As well, the bacterial burden in hepatopancreas was significantly lower in M. rosenbergii treated with specific egg yolk powder than those treated with non-specific egg yolk powder. This study clarified the virulence characteristics and antibiotic resistance of E. cloacae, and provided a strategy to control the disease by using specific IgY.