Effect of the S008-sgaCD operon on IncC plasmid stability in the presence of SGI1-K or absence of an SGI1 variant

Two birds with one stone: SGI1 can stabilize itself and expel the IncC helper by hijacking the plasmid parABS system

“…Since several previous observations published recently ( 21 , 24 , 36 , 18 , 37 ) indicate that wt SGI1, wt SGI1-C, and especially the replication deficient SGI1 derivatives are unstable in the presence of the IncC helper plasmids without selection, to identify and analyse SGI1-encoded factors responsible for IncC plasmid destabilization, antibiotic selection for SGI1 was applied to avoid SGI1 loss during passaging. Although there have been reports of wt SGI1 being completely stable besides IncC plasmid pRMH760 ( 38 , 39 ), based on our own and others' experience, and due to a consistent experimental setup, all strains were grown without antibiotic selection for R55 ΔTn but with selection for SGI1-C wt and mutants for 5 passages (approx. 20 generations/passage), and the proportion of cells retaining R55 ΔTn was determined at each passage.…”

“…There is strong evidence that SGI1 replication both enhances its persistence in cells where the island coexists with the IncC helper plasmid, and appears is a key factor in plasmid destabilization. Incompatibility between SGI1 and its mobilizing IncC and IncA plasmids has been reported ( 38 , 36 , 18 , 68 , 39 ), and it is proposed that SgaDC, the SGI1-encoded master regulator may have a central role in this phenomenon. Although the exact mechanism of plasmid expulsion is unknown, interference with plasmid maintenance functions (replication, partitioning) has been suggested.…”

“…SgaDC is considered a key element in IncC plasmid destabilization ( 36–38 , 68 , 39 ) and is indispensable for induction of SGI1 replication and maintenance of 7–8 copies per cell. Here we demonstrate that the presence of increased amounts of identified incompatibility factors due to SGI1 replication strengthens its incompatibility with R55.…”

“…Under experimental circumstances, a moderate frequency of SGI1 loss was observed in Salmonella Typhimurium strains when antibiotic selection was applied only for the IncC plasmid, R55 ( 21 ). SGI1 loss was also observed in E. coli under nonselective conditions; however, SGI1 appears to be more stable than the IncC plasmid, which is lost from 45%-85% of the cell population after 20 generations ( 38 , 36 , 18 , 39 ). Since SGI1 behaves like a plasmid in the presence of an IncC plasmid, their mutual destabilization resembles the phenomenon of plasmid incompatibility, which is defined as the failure of stable inheritance of co-resident plasmids without external selection.…”

“…Recent works have indicated that the SGI1-encoded regulator, SgaDC, which has a key role in crosstalk between SGI1 and IncC plasmids, may also be involved in their incompatibility. The SGI1-K variant, lacking the sgaDC operon due to a deletion did not destabilize the IncC plasmid pRMH760 ( 38 , 39 ). Further support for this hypothesis came from stability assays, where the IncC plasmid, R55, appeared to be much more stable under nonselective conditions in the presence of an SGI1 mutant lacking sgaDC than in the presence of a wt SGI1-C ( 18 ).…”