Submitted to: Plasmid Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2006
Publication Date: 1/12/2007
Citation: Chen, C., Nace, G., Solow, B., Fratamico, P.M. 2007. Complete nucleotide sequences of 84.5-kb and 3.2-kb plasmids in the multi-antibiotic resistant salmonella enterica serovar typhimurium u302 strain g8430. Plasmid. 57:29-53. Interpretive Summary: The presence of antibiotic-resistant bacterial pathogens in foods poses serious public health concerns. To curb the spread of the multi-resistant (MR) pathogens, it is important to understand the mechanism of how the antibiotic resistance genes are transferred among bacteria and how the multi-resistant pathogens evolve. The antibiotic resistance genes are often transferred from bacterium to bacterium on mobile genetic elements such as plasmids (extrachromosomal circular DNA molecules), bacteriophages (viruses that infect bacteria), and transposable elements (DNA fragments that are able to move from one DNA site to another). To identify the molecular origin of the antibiotic resistance of one MR Salmonella Typhimurium strain G8430 (originally isolated from a case of human illness), two plasmids from this strain were isolated and the DNA sequence was determined. The smaller plasmid contained only one resistance gene coding for resistance to kanamycin, while the large plasmid contained resistance genes to seven antibiotics (ampicillin, streptomycin, kanamycin, gentamicin, tetracycline, erythromycin, and sulfonamides) and to mercury. Analysis of the large plasmid sequence suggested that it obtained DNA fragments and resistance genes from a variety of other pathogenic bacteria such as Salmonella Typhi, Shigella flexneri, Escherichia coli and Citrobacter freundii. This study provides insight into our understanding of antibiotic resistance gene transfer mechanisms. The information will help us to develop strategies to prevent the dissemination of these genes.
Technical Abstract: Multi-antibiotic resistant (MR) strains of Salmonella enterica serovar Typhimurium DT104 and related phage type U302 have been linked to outbreaks and sporadic illnesses in humans and animals worldwide. S. Typhimurium MR U302 strain G8430 is resistant to carbenicillin, erythromycin, kanamycin and gentamicin, in addition to the penta-resistant ACSSuT-phenotype (ampicillin, chlorampheicol, streptomycin, sulfonamides and tetracycline). Two plasmids, 3.2-kb and 84.5-kb in size, containing antibiotic resistance genes were isolated from this strain and the nucleotide sequences were determined and analyzed. The 3.2-kb plasmid, pU302S, belongs to the ColE1 family and carries the aph(3')-I gene (Kan-resistant). Unlike the aph(3')-I gene on the large pU302L plasmid, this copy is not flanked by IS elements. The 84.5-kb plasmid, pU302L, is an F-like plasmid and contains 14 complete IS elements and multiple resistance genes including aac3, aph(3')-I, sulII, tetA/R, strA/B, blaTEM-1, mph, and the mer operon. Sequence analyses of pU302L revealed extensive regions of homology to various plasmids or transposons from Salmonella spp., Shigella flexneri, Escherichia coli O157:H7 and Citrobacter freundii, including the F, R100, pHCM1, pO157, and pCTX-M3 plasmids, in regions involved in the plasmid replication/ maintenance functions and/or in antibiotic resistance gene clusters. Though similar to the conjugative plasmids F and R100 in the plasmid replication regions, pU302L does not contain the oriT (origin of transfer) and the tra genes necessary for conjugal transfer; therefore, it is unable to self-transfer. This mosaic pattern of sequence similarity suggests that pU302L acquired the resistance genes from a variety of different enteric bacteria and underscores the importance of a further understanding of horizontal gene transfer among the enteric bacteria.