The opportunistic marine pathogenVibrio parahaemolyticusbecomes virulent by acquiring a plasmid that expresses a deadly toxin

Abstract

Acute hepatopancreatic necrosis disease (AHPND) is a severe, newly emergent penaeid shrimp disease caused byVibrio parahaemolyticusthat has already led to tremendous losses in the cultured shrimp industry. Until now, its disease-causing mechanism has remained unclear. Here we show that an AHPND-causing strain ofV. parahaemolyticuscontains a 70-kbp plasmid (pVA1) with a postsegregational killing system, and that the ability to cause disease is abolished by the natural absence or experimental deletion of the plasmid-encoded homologs of thePhotorhabdusinsect-related (Pir) toxins PirA and PirB. We determined the crystal structure of theV. parahaemolyticusPirA and PirB (PirAvpand PirBvp) proteins and found that the overall structural topology of PirAvp/PirBvpis very similar to that of theBacillusCry insecticidal toxin-like proteins, despite the low sequence identity (<10%). This structural similarity suggests that the putative PirABvpheterodimer might emulate the functional domains of the Cry protein, and in particular its pore-forming activity. The gene organization of pVA1 further suggested thatpirABvpmay be lost or acquired by horizontal gene transfer via transposition or homologous recombination.