The Main Virulence Determinant of Yersinia entomophaga MH96 Is a Broad-Host-Range Toxin Complex Active against Insects

Abstract

Through transposon mutagenesis and DNA sequence analysis, the main disease determinant of the entomopathogenic bacterium Yersinia entomophaga MH96 was localized to an ~32-kb pathogenicity island (PAI) designated PAI(Ye₉₆). Residing within PAI(Ye₉₆) are seven open reading frames that encode an insecticidal toxin complex (TC), comprising not only the readily recognized toxin complex A (TCA), TCB, and TCC components but also two chitinase proteins that form a composite TC molecule. The central TC gene-associated region (~19 kb) of PAI(Ye₉₆) was deleted from the Y. entomophaga MH96 genome, and a subsequent bioassay of the ΔTC derivative toward Costelytra zealandica larvae showed it to be innocuous. Virulence of the ΔTC mutant strain could be restored by the introduction of a clone containing the entire PAI(Ye₉₆) TC gene region. As much as 0.5 mg of the TC is released per 100 ml of Luria-Bertani broth at 25°C, while at 30 or 37°C, no TC could be detected in the culture supernatant. Filter-sterilized culture supernatants derived from Y. entomophaga MH96, but not from the ΔTC strain grown at temperatures of 25°C or less, were able to cause mortality. The 50% lethal doses (LD₅₀s) of the TC toward diamondback moth Plutella xylostella and C. zealandica larvae were defined as 30 ng and 50 ng, respectively, at 5 days after ingestion. Histological analysis of the effect of the TC toward P. xylostella larva showed that within 48 h after ingestion of the TC, there was a general dissolution of the larval midgut.