Abstract
BackgroundThe Salmonella genomic island 1 (SGI1) is a 42.4 kb integrative mobilizable element containing several antibiotic resistance determinants embedded in a complex integron segment In104. The numerous SGI1 variants identified so far, differ mainly in this segment and the explanations of their emergence were mostly based on comparative structure analyses. Here we provide experimental studies on the stability, entrapment and variant formation of this peculiar gene cluster originally found in S. Typhimurium.Methodology/Principal FindingsSegregation and conjugation tests and various molecular techniques were used to detect the emerging SGI1 variants in Salmonella populations of 17 Salmonella enterica serovar Typhimurium DT104 isolates from Hungary. The SGI1s in these isolates proved to be fully competent in excision, conjugal transfer by the IncA/C helper plasmid R55, and integration into the E. coli chromosome. A trap vector has been constructed and successfully applied to capture the island on a plasmid. Monitoring of segregation of SGI1 indicated high stability of the island. SGI1-free segregants did not accumulate during long-term propagation, but several SGI1 variants could be obtained. Most of them appeared to be identical to SGI1-B and SGI1-C, but two new variants caused by deletions via a short-homology-dependent recombination process have also been detected. We have also noticed that the presence of the conjugation helper plasmid increased the formation of these deletion variants considerably.Conclusions/SignificanceDespite that excision of SGI1 from the chromosome was proven in SGI1+ Salmonella populations, its complete loss could not be observed. On the other hand, we demonstrated that several variants, among them two newly identified ones, arose with detectable frequencies in these populations in a short timescale and their formation was promoted by the helper plasmid. This reflects that IncA/C helper plasmids are not only involved in the horizontal spreading of SGI1, but may also contribute to its evolution.
Highlights
Salmonella is one of the most prevalent food-borne zoonotic pathogen
Typhimurium DT104 isolates were tested for their antibiotic resistance and the occurrence of Salmonella genomic island 1 (SGI1)
In this study we have investigated the mobility functions and stability of SGI1 in 17 multidrug resistant S
Summary
Salmonella is one of the most prevalent food-borne zoonotic pathogen. Among more than 2500 serotypes, the majority of infections in humans are caused only by a few serotypes, such as S. enterica serovar Enteritidis and Typhimurium. The left integron located near the IRi has an intact 59 conserved sequence (59-CS) with a functional intI1gene and an aadA2 gene cassette conferring resistance to streptomycin and spectinomycin (StrR and SptR) in the attI1 site and an incomplete 39-CS with the qacED1sulD1 gene fusion. The other two resistance genes floR and tet(G) conferring resistance to chloramphenicol/florfenicol (ChmR/FloR) and tetracycline (TetR) are not parts of the integrons. They are located together with a mobile element ISCR3 [5] in the segment defined by the two integron regions. The Salmonella genomic island 1 (SGI1) is a 42.4 kb integrative mobilizable element containing several antibiotic resistance determinants embedded in a complex integron segment In104.
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