Through transposon mutagenesis and DNA sequence analysis, the main disease determinant of the entomopathogenic bacterium Yersinia entomophaga MH96 was localized to an ϳ32-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 (ϳ19 kb) of PAI Ye96 was deleted from the Y. entomophaga MH96 genome, and a subsequent bioassay of the ⌬TC derivative toward Costelytra zealandica larvae showed it to be innocuous. Virulence of the ⌬TC 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 ⌬TC strain grown at temperatures of 25°C or less, were able to cause mortality. The 50% lethal doses (LD 50 s) 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.Toxin complexes (TCs) active on insects were first identified in the nematode-associated bacterium Photorhabdus luminescens and termed TCs, as three proteins combined to form a complex with insecticidal activity (5). TC toxins were subsequently identified in the genome of Serratia entomophila where they reside in the designated gene order sepA, sepB, and sepC (33); this toxin complex ABC designation defines the revised nomenclature of the TC proteins (25). The TC toxins derived from P. luminescens reside as multiple but dissimilar orthologues throughout the P. luminescens T011 genome (22), and different insecticidal activities may be attributed to a different TC cluster (32). The S. entomophila sepABC genes are plasmid borne, and their translated products are host specific, only causing amber disease in larvae of the New Zealand grass grub Costelytra zealandica (Coleoptera: Scarabaeidae) (33). TC-like toxins have since been identified in the genome of Xenorhabdus nematophilus (60), Pseudomonas syringae pv. tomato DC3000 (9), and some Yersinia species. The toxin complex A (TCA)-like (tcaB) gene of Yersinia pestis CO92 contains a frameshift mutation, and the toxin complex B (TCB)-like (tcaC) gene contains an internal deletion (51), indicative of a loss of function, while the corresponding TCA-like and TCBlike orthologues in Y. pestis KIM and 91001 do not (18, 62). Tennant et al. (66) showed that mutations in each of the Yersinia enterocolitica biotype 1A T83 genes, TCA-like (tcbA), TCB-like (tcaC), and TCC-like (tccC) gen...

Serratia entomophila and Serratia proteamaculans cause amber disease of the grass grub Costelytra zealandica (Coleoptera: Scarabaeidae). Three genes required for virulence, sepABC, are located on a large plasmid, pADAP. The translated products of the sep genes are members of the toxin complex (Tc) family of insecticidal toxins that reside in the genomes of some Enterobacteriaceae. Each of the sep genes was placed either singly or as various combinations under the control of an inducible arabinose promoter, allowing their inductive expression. Western Immunoblot confirmed that each of the Sep proteins migrated at their predicted size on sodium dodecyl sulphate-polyacrylamide gel electrophoresis gel. Bioassays of sonicated filtrates derived from the various arabinose-induced para-SEP constructs showed that only when sepA, sepB and sepC were coexpressed were amber disease symptoms observed in grass grub larvae. Fourteen days after ingestion of the Sep protein filtrate, approximately 64% of the larvae reverted from a diseased to a healthy phenotype. Redosing the revertents with a fresh Sep protein filtrate reinitiated the amber pathotype, indicating that the Sep proteins are needed to be continuously present to exert an effect.

Strains of Serratia spp. showed a high level of virulence when injected into the hemocoel of larvae Costelytra zealandica, with Serratia entomophila, S. plymuthica, and S. marcescens showing significantly higher virulence than S. proteamaculans. Toxicity was independent of the amber disease-causing plasmid pADAP, suggesting a generalized Serratia toxin.

No abstract