Properties and evolution of IncA and IncC plasmids and their interactions with Salmonella genomic island 1

Abstract

IncC plasmids are large conjugative broad host range plasmids important in the spread of resistance genes conferring resistance to a wide range of clinically important antibiotics. In this study, fundamental aspects of the biology of IncC plasmids and the related IncA plasmids were examined. Traditional incompatibility testing confirmed that IncA and IncC plasmids are stable together for more than 100 generations in the absence of antibiotic selection and are therefore compatible. Entry exclusion experiments revealed that an IncC plasmid in the recipient could completely prevent the conjugative transfer of another IncC or IncA plasmid. IncA plasmids were also able to completely exclude IncC plasmids. This level of exclusion was several orders of magnitude higher than observed in other characterised systems. The gene required in the recipient, eexC in IncC plasmids and eexA in IncA plasmids, was predicted based on the position of exclusion genes in known exclusion systems and found to be sufficient to exclude both IncA and IncC plasmids. IncA and IncC plasmids are able to mobilise the integrative mobilisable element (IME) Salmonella genomic island 1 (SGI1) and its variants. These element types interact in several specific ways. The presence of an SGI1 variant in the donor allowed an IncC plasmid to avoid exclusion exerted by EexC in the recipient but not when the complete plasmid when present, indicating there are further interactions preventing entry of IncC plasmids by conjugation. Further exclusion experiments showed that SGI1 in the recipient reduced conjugative transfer of IncC plasmids by 15-240 fold. However, IncC plasmids did not exclude SGI1. IncC plasmids have gained increasing attention with >350 sequences now available in GenBank. Examination of the sequences of IncC plasmids used in this study and sequences available in GenBank led to the identification of several important lineages of type 2 IncC plasmids based on the positions of resistance islands. Some lineages are found in a wide number of species and are globally disseminated. Extensive evolution was observed in the resistance islands of these important lineages. In addition, two novel trimethoprim resistance genes were found on IncC plasmids or an IncC plasmid segment and were characterised.