Salmonella genomic island 1 : a successful antimicrobial resistance genes vehicle

The Salmonella genomic island 1 is an integrative mobilizable element (IME) originally identified in epidemic multidrug-resistant (MDR) Salmonella enterica serovar Typhimurium DT104.1 SGI1 contains a complex integron, which confers various multidrug resistance phenotypes due to its great genetic plasticity. Recently, SGI1 was also identified in the emerging MDR serovar Kentucky ST198 displaying high-level resistance to ciprofloxacin. The occurrence of SGI1 in several S. enterica serovars and recently in Proteus mirabilis has led to demonstrate its horizontal transfer capacity.1 SGI1 excises from the donor chromosome to form a circular extrachromosomal intermediate that can be mobilized in trans to the recipient in a specific manner only by the IncA/C multidrug resistance plasmid family.1,3 In the recipient, SGI1 integrates site-specifically into the chromosome at the 3’ end of the trmE gene. As other IMEs, SGI1 possesses its own mobilization functions. Based on mobilization assays of a non mobilizable vector containing SGI1 fragments, the SGI1 transfer origin (oriT) has been located and identified on a 135 bp DNA fragment. The drastic decrease of transfer frequency of a SGI1 lacking this putative oriT confirmed this location. In addition, a putative SGI1-encoded integrase and an additional small open reading frame were also involved and probably represented the SGI1 mob proteins required for its mobilization. All other transfer functions are provided by the IncA/C helper plasmid. The identification and characterization of the different molecular components involved in SGI1 mobilization are ongoing and represent an important step for understanding the spread of the SGI1 genomic island among bacterial pathogens