A Mutant of AB (+) That Carries Binary Toxin Genes but Does Not Cause Acute Hepatopancreatic Necrosis Disease.

carrying binary toxin genes, AB, is one of the etiological agents causing acute hepatopancreatic necrosis disease (AHPND) in shrimp. This disease has emerged recently as a major threat to shrimp aquaculture worldwide. During a routine PCR screening of AHPND-causing strains, an isolate tested PCR positive for (R13) and another isolate tested positive for both the A and B (R14) genes. To evaluate the pathogenicity of these isolates, specific pathogen-free (SPF) were experimentally challenged. For both R13 and R14 isolates, the final survival rate was 100% at termination of the challenge, whereas the final survival with the AHPND-causing was 0%. The nucleotide sequence of the plasmid DNA carrying the binary toxin genes revealed that R13 contains a deletion of the entire gene whereas R14 contains the entire coding regions of both A and B genes. However, R14 possesses an insertion upstream of the A gene. In R14, mRNA for both A and B genes could be detected but no cognate proteins. This shows that the genome of AHPND-causing is highly plastic and, therefore, detection of the A and B genes alone by DNA-PCR is insufficient as a diagnostic test for AHPND.