The Salmonella genomic island 1 is an integrative mobilizable element

Abstract

SummarySalmonella genomic island 1 (SGI1) is a genomic island containing an antibiotic resistance gene cluster identified in several Salmonella enterica serovars. The SGI1 antibiotic resistance gene cluster, which is a complex class 1 integron, confers the common multidrug resistance phenotype of epidemic S. enterica Typhimurium DT104. The SGI1 occurrence in S. enterica serovars Typhimurium, Agona, Paratyphi B, Albany, Meleagridis and Newport indicates the horizontal transfer potential of SGI1. Here, we report that 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. First, an extrachromosomal circular form of SGI1 was identified by PCR which forms through a specific recombination of the left and right ends of the integrated SGI1. Chromosomal excision of SGI1 was found to require SGI1-encoded integrase which presents similarities to the lambdoid integrase family. Second, the conjugal transfer of SGI1 required the presence of a helper plasmid. The conjugative IncC plasmid R55 could thus mobilize in trans SGI1 which was transferred from the donor to the recipient strains. By this way, the conjugal transfer of SGI1 occurred at a frequency of 10----6 transconjugants per donor. No transconjugants could be obtained for the SGI1 donor lacking the int integrase gene. Third, chromosomal integration of SGI1 occurred via a site-specific recombination between a 18 bp sequence found in the circular form of SGI1 and a similar 18 bp sequence at the 3 ¢ ¢ ¢ ¢ end of thdF gene in the S. enterica and E. coli chromosome. SGI1 appeared to be transmissible only in the presence of additional conjugative functions provided in trans . SGI1 can thus be classified within the group of integrative mobilizable elements (IMEs).

SGI1-K, a Variant of the SGI1 Genomic Island Carrying a Mercury Resistance Region, in Salmonella enterica Serovar Kentucky

“…Though others have reported sequencing the boundaries between SGI1 and the chromosome for serovars Agona, Paratyphi B, and Albany (4,11,13), the sequences have not been released, nor were any differences between serovar Typhimurium and other serovars in the intergenic region between thdF and yidY mentioned. The sequence reported here for the chromosomal segments adjacent to the site of integration of SGI1-K matches those of serovars Typhi, Paratyphi A, and Choleraesuis in this region and SGI1-K is inserted within the end of the thdF gene.…”

“…These five genes are responsible for the multiple antibiotic resistance phenotype conferred by SGI1, namely resistance to chloramphenicol and florfenicol (floR), ampicillin and carbenicillin (blaP1), streptomycin and spectinomycin (aadA2), sulfonamides (sul1), and tetracycline [tet(G) determinant]. SGI1 or variants of it have also been found in several other S. enterica serovars, consistent with horizontal gene transfer and mobilization of SGI1 by an IncC plasmid, as recently demonstrated (11). In all cases, SGI1 is found in the bacterial chromosome between the thdF and yidY genes (3, 9, 12-14, 16, 18, 22).…”

A quantitative approach to analyse linkages between antimicrobial resistance properties inSalmonellaTyphimurium isolates

“…gyrA and parC (fluoroquinolone resistance)], or resistance genes whose products specify a wide variety of resistance mechanisms [10,11]. Many of the known resistance genes in Salmonella are located on mobile genetic elements, such as plasmids, transposons, gene cassettes or mobile genomic islands [12][13][14] and are easily exchanged between bacteria living in the same habitat, even in the absence of direct selective pressure [15,16]. The latter aspect plays an important role when resistance genes are physically linked, co-located on the same mobile genetic element, and/or are present in gene clusters.…”

Molecular analysis of an integrative conjugative element, ICEH, present in the chromosome of different strains of Mycoplasma hyopneumoniae

“…A number of phenotypes has been associated with this type of dissemination during conjugation of chromosomal regions: the sucrose metabolism in Salmonella enterica Seftemberg via cTnscr94 (Hochhut et al, 1997); the symbiosis traits associated with the 500 kb symbiosis island in Mesorhizobium loti (Sullivan and Ronson, 1998); the degradation of chlorocatechol mediated by gene products resident on the 105 kb clc element of Pseudomonas putida (Ravatn et al, 1998); the tetracycline resistant element Tn916 of Bacteroides (Salyers et al, 1995); the large STX unit of Vibrio cholerae (Hochhut and Waldor, 1999); and the antibiotic resistance gene cluster from Salmonella (Doublet et al, 2005). The term constin has been recently coined to describe this diverse group of conjugative, selftransmissible, integrating elements (Hochhut et al, 2001).…”