Abstract
The multidrug resistance Salmonella Genomic Island 1 (SGI1) is an integrative mobilizable element identified in several enterobacterial pathogens. This chromosomal island requires a conjugative IncA/C plasmid to be excised as a circular extrachromosomal form and conjugally mobilized in trans. Preliminary observations suggest stable maintenance of SGI1 in the host chromosome but paradoxically also incompatibility between SGI1 and IncA/C plasmids. Here, using a Salmonella enterica serovar Agona clonal bacterial population as model, we demonstrate that a Toxin-Antitoxin (TA) system encoded by SGI1 plays a critical role in its stable host maintenance when an IncA/C plasmid is concomitantly present. This system, designated sgiAT for Salmonella genomic island 1 Antitoxin and Toxin respectively, thus seems to play a stabilizing role in a situation where SGI1 is susceptible to be lost through plasmid IncA/C-mediated excision. Moreover and for the first time, the incompatibility between SGI1 and IncA/C plasmids was experimentally confirmed.
Highlights
The Salmonella genomic island 1 (SGI1) is a chromosomally-located island that may carry several antibiotic resistance genes and was firstly identified end of the 1990s in the multidrug-resistant epidemic clone of Salmonella enterica serovar Typhimurium
Among the uncharacterized Salmonella Genomic Island 1 (SGI1) orfs there are two, i.e. S025 and S026, that may encode a TA stabilization system according to similarity with the deduced amino acid sequences of a TA system identified on the tumor-inducing plasmid pTiC58 from Agrobacterium tumefaciens[29]
When expression of the S026-S025 orfs was induced in plasmid pKH04, a slight defective growth of the E. coli host strain could be observed in this Post Segregational Killing (PSK) assay (Fig. 2d)
Summary
The Salmonella genomic island 1 (SGI1) is a chromosomally-located island that may carry several antibiotic resistance genes and was firstly identified end of the 1990s in the multidrug-resistant epidemic clone of Salmonella enterica serovar Typhimurium While several essential functional genes or regulatory genes have been experimentally uncovered in this relationship promoting the transfer of SGI14,5,8,10, some observations raise other questions Among these is the fact that to our knowledge SGI1 and IncA/C plasmids have not been found together in clinical isolates. Among stabilization systems the most classical known to date for genomic islands are the toxin-antitoxin (TA) systems[20,21,22,23,24] They were originally discovered on low-copy-number plasmids, and have been more recently described on integrative conjugative elements[25,26]. As an interplay with plasmids of the IncA/C family is suspected in the SGI1 maintenance, we studied stability of SGI1 and its putative TA system in presence or not of an IncA/C plasmid
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