Abstract
BackgroundThe Salmonella genomic island 1 is an integrative mobilizable element (IME) originally identified in epidemic multidrug-resistant Salmonella enterica serovar Typhimurium (S. Typhimurium) DT104. SGI1 contains a complex integron, which confers various multidrug resistance phenotypes due to its genetic plasticity. Previous studies have shown that SGI1 integrates site-specifically into the S. enterica, Escherichia coli, or Proteus mirabilis chromosome at the 3′ end of thdF gene (attB site).Methodology/Principal FindingsHere, we report the transfer of SGI1 to a ΔthdF mutant of S. Typhimurium LT2. In the absence of thdF, the frequency of transconjugant formation was reduced by around thirty times of magnitude. Through DNA sequencing SGI1 was shown to integrate specifically into a secondary attachment site (2nd attB), which is located in the intergenic region between the chromosomal sodB and purR genes. At this 2nd attB site, we found that a significant fraction of SGI1 transconjugants (43% of wild type and 100% of ΔthdF mutant) contained tandem SGI1 arrays. Moreover, in wild type S. Typhimurium LT2 transconjugants, SGI1 integrated into both attachment sites, i.e., thdF and sodB-purR. The formation of SGI1 tandem arrays occurred in both specific attB sites. There was heterogeneity in the size of the SGI1 tandem arrays detected in single transconjugant colonies. Some arrays consisted as far as six SGI1s arranged in tandem. These tandem arrays were shown to persist during serial passages with or without antibiotic selection pressure.Conclusions/SignificanceThe ability of integration into two distinct chromosomal sites and tandem array formation of SGI1 could contribute to its spread and persistence. The existence of a secondary attachment site in the Salmonella chromosome has potential implications for the mobility of SGI1, which may integrate in other attachment sites of other bacterial pathogens that do not possess the 1st or 2nd specific SGI1 attB sites of Salmonella.
Highlights
Genomic islands are large chromosomal regions that have been acquired by horizontal transfer
In 2005, we reported that Salmonella Genomic island 1 (SGI1) could be conjugally transferred from S. enterica donor strains to non-SGI1 S. enterica and Escherichia coli recipient strains where it integrated into the recipient chromosome in a site-specific manner [13]
Typhimurium LT2 recipient strain To examine whether SGI1 integration is limited to its 1st attB site, i.e. the last 18 bp of thdF, and whether integration in secondary attachment sites may occur, we constructed a DthdF deletion mutant of S
Summary
Genomic islands are large chromosomal regions that have been acquired by horizontal transfer. Genomic islands often carry genes that bring a selective advantage to the host bacterium in a specific environment. They are classified into pathogenicity islands which encode virulence determinants, resistance islands which confer multiple antibiotic resistances, xenobiotic degradation islands, and symbiosis islands [1,2]. They are frequently integrated near or into tRNA genes, flanked by repeat structures, and contain mobility genes coding for integrases or transposases [1]. Previous studies have shown that SGI1 integrates site- into the S. enterica, Escherichia coli, or Proteus mirabilis chromosome at the 39 end of thdF gene (attB site)
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