Insecticidal genes of Yersinia spp.: taxonomical distribution, contribution to toxicity towards Manduca sexta and Galleria mellonella, and evolution

BACKGROUND: Toxin complex (Tc) proteins termed TcaABC, TcdAB, and TccABC with insecticidal activity are present in a variety of bacteria including the yersiniae. RESULTS: The tc gene sequences of thirteen Yersinia strains were compared, revealing a high degree of gene order conservation, but also remarkable differences with respect to pseudogenes, sequence variability and gene duplications. Outside the tc pathogenicity island (tc-PAI(Ye)) of Y. enterocolitica strain W22703, a pseudogene (tccC2'/3') encoding proteins with homology to TccC and similarity to tyrosine phosphatases at its C-terminus was identified. PCR analysis revealed the presence of the tc-PAI(Ye )and of tccC2'/3'-homologues in all biotype 2–5 strains tested, and their absence in most representatives of biotypes 1A and 1B. Phylogenetic analysis of 39 TccC sequences indicates the presence of the tc-PAI(Ye )in an ancestor of Yersinia. Oral uptake experiments with Manduca sexta revealed a higher larvae lethality of Yersinia strains harbouring the tc-PAI(Ye )in comparison to strains lacking this island. Following subcutaneous infection of Galleria mellonella larvae with five non-human pathogenic Yersinia spp. and four Y. enterocolitica strains, we observed a remarkable variability of their insecticidal activity ranging from 20% (Y. kristensenii) to 90% (Y. enterocolitica strain 2594) dead larvae after five days. Strain W22703 and its tcaA deletion mutant did not exhibit a significantly different toxicity towards G. mellonella. These data confirm a role of TcaA upon oral uptake only, and suggest the presence of further insecticidal determinants in Yersinia strains formerly unknown to kill insects. CONCLUSION: This study investigated the tc gene distribution among yersiniae and the phylogenetic relationship between TccC proteins, thus contributing novel aspects to the current discussion about the evolution of insecticidal toxins in the genus Yersinia. The toxic potential of several Yersinia spp. towards M. sexta and G. mellonella demonstrated here for the first time points to insects as a natural reservoir for yersiniae.