Serratia-based toxin cluster elements are associated with a type I fimbria

A soil bacterium in the Serratia genus, carrying a 153 kb conjugative amber disease-associated plasmid (pADAP), is commercially exploited for population control of the New Zealand endemic pest beetle Costelytra giveni (Coleoptera: Scarabaeidae). The main insecticidal elements are an anti-feeding prophage and the Sep ABC toxin complex (Tc). Homologs of pADAP, encoding variant Tcs, convey different beetle disease phenotypes. To investigate the correlation between variable bioactivity and the Tc variant, 76 Serratia plasmids were sequenced, resulting in the identification of four additional tc variants. All Serratia tc variants were found to be colocated with a conserved type 1 sef fimbrial-like operon, indicating a conserved sef-tc genetic island not observed outside of the Serratia genus. The conserved co-location of the fimbrial and tc genes suggests the fimbriae somehow contribute to the lifestyle of Tc-producing cells. Expression of the sef operon in a fim-null Escherichia coli strain revealed fimbriae presence while a constructed sef-deficient mutant showed no reduction of virulence or host colonization. Although no detectable contribution of Sef to amber disease in C. giveni was observed, the Sef adhesin sequences clustered similarly to the Serratia species encoding it, suggesting Sef has a species-specific function.