IncC plasmids were among the first conjugative plasmids to be linked with antibiotic resistance. These broad host range plasmids contribute substantially to the dissemination of antibiotic resistance. RA1, isolated in 1971, was found to be compatible with all plasmid groups then known, including IncC, and assigned to IncA. Later, based on entry exclusion IncA and IncC plasmids were combined into the IncA/C complex.
The transfer of pRMH760, an IncC plasmid that conjugates at a high frequency (3.12 x 10-3 transconjugants/donor), into a recipient cell containing RA1 was not detectable (<1.35 x 10-8 transconjugants/donor). Similarly, conjugation of RA1 into an E. coli recipient containing pRMH760 was not detected (<1.16 x 10-8 transconjugants/donor). Hence the IncA plasmid RA1 and the IncC plasmid pRMH760 mutually exert virtually complete entry exclusion.
To examine compatibility, RA1 was transformed into E. coli UB1637 containing pRMH760. In the absence of antibiotic selection pRMH760 and RA1 were stable over 5 cycles of overnight cell growth (~22 generations/cycle). Therefore plasmids in the IncA/C complex must be separated into IncA and IncC groups.
Salmonella genomic islands SGI1 and SGI2 are integrative mobilizable elements that can be mobilized by IncA and IncC plasmids. SG1 and SGI2 rapidly destabilize IncA and IncC plasmids in a phenomenon akin to incompatibility. We observed the conjugation frequency of pRMH760 into an E. coli cell containing SGI1-I, SGI1-K or the related SGI2 was 99 fold, 12 fold and 194 fold lower, respectively, than the transfer of pRMH760 into an otherwise isogenic E. coli cell lacking an SGI variant. Hence SGI1 exerts entry exclusion upon IncC plasmids.
Further work is being conducted to determine whether IncC plasmids can exclude SGI1 and also to identify gene(s) responsible for entry exclusion between IncA and IncC plasmids as well as between these plasmids and SGI1.