Identification of Specific Gene Sequences Conserved in Contemporary Epidemic Strains of Salmonella enterica

Genetic elements specific to recent and contemporary epidemic strains of Salmonella enterica were identified using comparative genomic analysis. Two epidemic multidrug-resistant (MDR) strains, MDR Salmonella enterica serovar Typhimurium definitive phage type 104 (DT104) and cephalosporin-resistant MDR Salmonella enterica serovar Newport, and an epidemic pansusceptible strain, Salmonella serovar Typhimurium DT160, were subjected to Salmonella gene microarray and suppression subtractive hybridization analyses. Their genome contents were compared with those of coexisting sporadic strains matched by serotype, geographic and temporal distribution, and host species origin. These paired comparisons revealed that epidemic strains of S. enterica had specific genes and gene regions that were shared by isolates of the same subtype. Most of these gene sequences are related to mobile genetic elements, including phages, plasmids, and plasmid-like and transposable elements, and some genes may encode proteins conferring growth or survival advantages. The emergence of epidemic MDR strains may therefore be associated with the presence of fitness-associated genetic factors in addition to their antimicrobial resistance genes.     

Genomic relationship of Salmonella enterica serovar typhimurium DT104 isolates from Korea and the United States

Salmonella enterica serovar Typhimurium DT104 (Salmonella Typhimurium DT104 or DT104) has been emerging as a common pathogen for human in Korea since 1997. In order to compare the genomic relationship and to search for the dominant strains in Korea, we conducted pulsed-field gel electrophoresis (PFGE) and IS200 fingerprinting of 25 epidemiological unrelated isolates from human and animals from Korea and cattle from America. Two Salmonella Typhimurium DT104 isolates from human in Korea and all 8 isolates from American cattle had indistinguishable patterns from the PFGE and IS200 fingerprinting but multidrug-resistant Salmonella Typhimurium isolates, including DT104, from Korean animals had diverse genetic patterns. The data suggest that a dominant DT104 strain might have circulated between Korean and American cattle and that it had a high level of clonality.     

Identification of novel Salmonella enterica serovar Typhimurium DT104-specific prophage and nonprophage chromosomal sequences among serovar Typhimurium isolates by genomic subtractive hybridization

Genomic subtractive hybridization was performed between Salmonella enterica serovar Typhimurium LT2 and DT104 to search for novel Salmonella serovar Typhimurium DT104-specific sequences. The subtraction resulted mainly in the isolation of DNA fragments with sequence similarity to phages. Two fragments identified were associated with possible virulence factors. One fragment was identical to irsA of Salmonella serovar Typhimurium ATCC 14028, which is suggested to be involved in macrophage survival. The other fragment was homologous to HldD, an Escherichia coli O157:H7 lipopolysaccharide assembly-related protein. Five selected DNA fragments-irsA, the HldD homologue, and three fragments with sequence similarity to prophages-were tested for their presence in 17 Salmonella serovar Typhimurium DT104 isolates and 27 non-DT104 isolates by PCR. All five selected DNA fragments were Salmonella serovar Typhimurium DT104 specific among the serovar Typhimurium isolates tested. These DNA fragments can be useful for better detection and typing of Salmonella serovar Typhimurium DT104.     

Prevalence and genetic properties of Salmonella enterica serovar typhimurium definitive phage type 104 isolated from Rattus norvegicus and Rattus rattus house rats in Yokohama City, Japan

Salmonella enterica serovar Typhimurium was isolated from the intestinal contents of Rattus rattus and Rattus norvegicus house rats captured at two buildings, designated buildings J and YS, in Yokohama City, Japan. From October 1997 to September 1998, 52 of 339 (15.3%) house rats were found to carry Salmonella serovar Typhimurium definitive phage type 104 (DT104). In building J, 26 of 161 (16.1%) house rats carried DT104 over the 1-year study period, compared to 26 of 178 (14.6%) rats in building YS. The isolation rates of DT104 from R. rattus and R. norvegicus were similar in the two buildings. Most DT104 strains from building J (24 of 26) showed resistance to ampicillin, chloramphenicol, streptomycin, sulfisoxazole, and tetracycline and contained both the 1.0- and 1.2-kbp integrons, carrying genes pse1, pasppflo-like, aadA2, sulI, and tet(G). All DT104 strains from building YS were resistant to ampicillin and sulfisoxazole, and had the 1.2-kbp integron carrying pse1 and sulI. Cluster analysis of pulsed-field gel electrophoresis patterns of BlnI-digested DT104 DNAs showed that 22 of 26 DT104 strains from building J and 24 of 26 strains from building YS could be grouped into separate clusters each specific for the building origin. These results indicated that DT104 strains were prevalent in house rat colonies in each building and suggest that house rats may play an important role in the epidemiology of DT104.     

Complete Nucleotide Sequence of a 43-Kilobase Genomic Island Associated with the Multidrug Resistance Region of Salmonella enterica Serovar Typhimurium DT104 and Its Identification in Phage Type DT120 and Serovar Agona

This study describes the characterization of the recently described Salmonella genomic island 1 (SGI1) (D. A. Boyd, G. A. Peters, L.-K. Ng, and M. R. Mulvey, FEMS Microbiol. Lett. 189:285-291, 2000), which harbors the genes associated with the ACSSuT phenotype in a Canadian isolate of Salmonella enterica serovar Typhimurium DT104. A 43-kb region has been completely sequenced and found to contain 44 predicted open reading frames (ORFs) which comprised approximately 87% of the total sequence. Fifteen ORFs did not show any significant homology to known gene sequences. A number of ORFs show significant homology to plasmid-related genes, suggesting, at least in part, a plasmid origin for the SGI1, although some with homology to phage-related genes were identified. The SGI1 was identified in a number of multidrug-resistant DT120 and S. enterica serovar Agona strains with similar antibiotic-resistant phenotypes. The G+C content suggests a potential mosaic structure for the SGI1. Emergence of the SGI1 in serovar Agona strains is discussed.     

The SopEPhi phage integrates into the ssrA gene of Salmonella enterica serovar Typhimurium A36 and is closely related to the Fels-2 prophage

Salmonella spp. are enteropathogenic gram-negative bacteria that use a large array of virulence factors to colonize the host, manipulate host cells, and resist the host's defense mechanisms. Even closely related Salmonella strains have different repertoires of virulence factors. Bacteriophages contribute substantially to this diversity. There is increasing evidence that the reassortment of virulence factor repertoires by converting phages like the GIFSY phages and SopEPhi may represent an important mechanism in the adaptation of Salmonella spp. to specific hosts and to the emergence of new epidemic strains. Here, we have analyzed in more detail SopEPhi, a P2-like phage from Salmonella enterica serovar Typhimurium DT204 that encodes the virulence factor SopE. We have cloned and characterized the attachment site (att) of SopEPhi and found that its 47-bp core sequence overlaps the 3' terminus of the ssrA gene of serovar Typhimurium. Furthermore, we have demonstrated integration of SopEPhi into the cloned attB site of serovar Typhimurium A36. Sequence analysis of the plasmid-borne prophage revealed that SopEPhi is closely related to (60 to 100% identity over 80% of the genome) but clearly distinct from the Fels-2 prophage of serovar Typhimurium LT2 and from P2-like phages in the serovar Typhi CT18 genome. Our results demonstrate that there is considerable variation among the P2-like phages present in closely related Salmonella spp.     

Molecular characterization of the multidrug-resistant phage types Salmonella enterica serovar Typhimurium DT104, DT20A and DT120 strains in the Slovakia

An increase in the number of multidrug-resistant Salmonella enterica serovar Typhimurium strains (definitive phage type DT20a and DT120) as well as the occurrence of DT104 strains during 2003-2005 in Slovakia was documented. Based on the results of the molecular analysis we suggest that multidrug-resistant DT20a and DT120 phage types are more closely related to multidrug-resistant phage type, and that the occurrence is probably due to changes in the phage susceptibility of DT104. Continued surveillance and molecular analysis should be maintained to follow the spread of these new multidrug-resistant DT104 variants in animals and humans.     

Characterization of the genomes of a diverse collection of Salmonella enterica serovar Typhimurium definitive phage type 104

Salmonella enterica serovar Typhimurium definitive phage type 104 (DT104) has caused significant morbidity and mortality in humans and animals for almost three decades. We completed the full DNA sequence of one DT104 strain, NCTC13348, and showed that significant differences between the genome of this isolate and the genome of the previously sequenced strain Salmonella serovar Typhimurium LT2 are due to integrated prophage elements and Salmonella genomic island 1 encoding antibiotic resistance genes. Thirteen isolates of Salmonella serovar Typhimurium DT104 with different pulsed-field gel electrophoresis (PFGE) profiles were analyzed by using multilocus sequence typing (MLST), plasmid profiling, hybridization to a pan-Salmonella DNA microarray, and prophage-based multiplex PCR. All the isolates belonged to a single MLST type, sequence type ST19. Microarray data demonstrated that the gene contents of the 13 DT104 isolates were remarkably conserved. The PFGE DNA fragment size differences in these isolates could be explained to a great extent by differences in the prophage and plasmid contents. Thus, here the nature of variation in different Salmonella serovar Typhimurium DT104 isolates is further defined at the gene and whole-genome levels, illustrating how this phage type evolves over time.     

Induction and resuscitation of viable but nonculturable Salmonella enterica serovar typhimurium DT104

Salmonella enterica serovar Typhimurium DT104 11601 was tested for its ability to maintain viability in minimal, chemically defined solutions. Periodic monitoring of growth and survival in microcosms of different ion concentrations, maintained at various temperatures, showed a gradual decline in culturable organisms ( approximately 235 days) at 5 degrees C. Organisms maintained at a higher temperature (21 degrees C) showed continuous, equivalent CFU per milliliter ( approximately 10(6)) up to 400 days after inoculation. Fluorescence microscopy with Baclight revealed that nonculturable cells were actually viable, while observations with scanning electron microscopy showed that the cells had retained their structural integrity. Temperature upshift (56 degrees C +/- 0.5, 15 s) of the nonculturable organisms (5 degrees C) in Trypticase soy broth followed by immediate inoculation onto Trypticase soy agar (TSA) gave evidence of resuscitation. Interestingly, S. enterica serovar Typhimurium DT104 from the microcosms at either 5 degrees C (1 to 200 days) or 21 degrees C (1 to 250 days) did not show enhanced growth after intermittent inoculation onto catalase-supplemented TSA. Furthermore, cells from 21 degrees C microcosms exposed to oxidative and osmotic stress showed greater resistance to stresses over increasing times of exposure than did recently grown cells. It is possible that the exceptional survivability and resilience of this particular strain may in part reflect the growing importance of this multidrug-resistant organism, in general, as a cause of intestinal disease in humans. The fact that S. enterica serovar Typhimurium DT104 11601 is capable of modifying its physiological characteristics, including entry into and recovery from the viable but nonculturable state, suggests the overall possibility that S. enterica serovar Typhimurium DT104 may be able to respond uniquely to various adverse environmental conditions.     

The genomic island SGI1, containing the multiple antibiotic resistance region of Salmonella enterica serovar Typhimurium DT104 or variants of it, is widely distributed in other S. enterica serovars

The global dissemination of the multiply-antibiotic-resistant Salmonella enterica serovar Typhimurium DT104 clone with the resistance genes located in a class 1 integron, here designated In104, within genomic island SGI1 is a significant public health issue. Here, we have shown that SGI1 and variants of it carrying different combinations of resistance genes are found in several Salmonella enterica serovars. These are serovars Cerro, Derby, Dusseldorf, Infantis, Kiambu, and Paratyphi B dT(+) isolated from human infections and serovar Emek from sewage effluent. Two new variants, SGI1-I and SGI1-J, both of which include the dfrA1-orfC cassette array, were identified.     

The genetics of Salmonella genomic island 1

Multidrug-resistant Salmonella enterica serovar Typhimurium phage type DT104, resistant to ampicillin, chloramphenicol/florfenicol, streptomycin, sulfonamides, and tetracycline, has disseminated worldwide. The resistance genes reside on the 43-kb Salmonella genomic island 1 (SGI1), which is transferable. Drug-resistant variants of SGI1 have been identified in numerous serotypes. Strains harboring SGI1 may be more virulent and have a tendency to rapidly disseminate.     

Acid resistance variability among isolates of Salmonella enterica serovar Typhimurium DT104

AIMS: Acid resistance could be an indicator of virulence since acid resistant strains are able to better survive the human stomach passage and in macrophages. We studied the acid resistance of several Salmonella Typhimurium DT104 strains isolated from food and humans and identified cellular parameters contributing to the enhanced acid resistance of these isolates. METHODS AND RESULTS: Acid resistance was tested in 37 Salmonella enterica Typhimurium serovar DT104 (S. Typhimurium DT104) strains. Acid adaptation at pH 5 followed by exposure for 2 h at pH 2.5 in the 27 human, nine nonhuman, and in two reference strains, revealed strong variation of acid survival. After 2 h at pH 2.5 six strains of S. Typhimurium DT104 were considered high acid resistant as they displayed a level of survival >10%, 14 strains were considered intermediate acid resistant (level of survival was <10% and >0.01%) and 19 strains were considered low acid resistant (level of survival <0.01%). Six strains were selected for further studies and proteomics revealed a relatively high amount of phase 2 flagellin in an acid-sensitive strain and a relatively high amount of the beta component of the H(+)/ATPase in an acid-resistant strain. Two strains were slightly more heat resistant possibly as the result of increased levels of DnaK or GroEL. CONCLUSIONS: A significant difference could be detected between human and food isolates regarding their acid resistance; all high acid-resistant strains were human isolates. SIGNIFICANCE AND IMPACT OF THE STUDY: S. Typhimurium DT104 is known for two decades and has a great impact on human health causing serious food-borne diseases. Our results suggest the existence of a positive correlation between acid resistance and pathogenicity in S. Typhimurium DT104 as all high acid-resistant strains were isolated from humans.     

Molecular characterization of Irish Salmonella enterica serotype typhimurium: detection of class I integrons and assessment of genetic relationships by DNA amplification fingerprinting

Salmonella enterica is among the principal etiological agents of food-borne illness in humans. Increasing antimicrobial resistance in S. enterica is a cause for worldwide concern. There is concern at present in relation to the increasing incidence of human infection with antimicrobial agent-resistant strains of S. enterica serotype Typhimurium, in particular of phage type DT104. Integrons appear to play an important role in the dissemination of antimicrobial resistance genes in many Enterobacteriaceae including S. enterica. In this study the antimicrobial susceptibilities and phage types of 74 randomly collected strains of S. enterica serotype Typhimurium from the Cork region of southern Ireland, obtained from human, animal (clinical), and food sources, were determined. Each strain was examined for integrons and typed by DNA amplification fingerprinting (DAF). Phage type DT104 predominated (n = 48). Phage types DT104b (n = 3), -193 (n = 9), -195 (n = 6), -208 (n = 3), -204a (n = 2), PT U302 (n = 1), and two nontypeable strains accounted for the remainder. All S. enterica serotype Typhimurium DT104 strains were resistant to ampicillin, chloramphenicol, streptomycin, Sulfonamide Duplex, and tetracycline, and one strain was additionally resistant to trimethoprim. All DT104 strains but one were of a uniform DAF type (designated DAF-I) and showed a uniform pattern of integrons (designated IP-I). The DT104b and PT U302 strains also exhibited the same resistance phenotype, and both had the DAF-I and IP-I patterns. The DAF-I pattern was also observed in a single DT193 strain in which no integrons were detectable. Greater diversity of antibiograms and DAF and IP patterns among non-DT104 phage types was observed. These data indicate a remarkable degree of homogeneity at a molecular level among contemporary isolates of S. enterica serotype Typhimurium DT104 from animal, human, and food sources in this region.     

A novel multiplex PCR assay for the detection of Salmonella enterica serovar Enteritidis in human faeces

AIMS: To develop a multiplex PCR assay for the detection of Salmonella enterica serovar Enteritidis in human faeces. METHODS AND RESULTS: A total of 54 Salmonella strains representing 19 serovars and non-Salmonella strains representing 11 different genera were used. Five primer pairs were employed in the assay. Three of them targeted to the genes hilA, spvA and invA that encode virulence-associated factors. A fourth primer pair amplified a fragment of a unique sequence within S. enterica serovar Enteritidis genomes. An internal amplification control (a fragment of a conservative sequence within the 16S rRNA genes) was targeted by a fifth primer pair. The assay produced two or three amplicons from the invA, hilA and 16S rRNA genes for 19 Salmonella serovars. All Salmonella and non-Salmonella strains yielded a band of an internal amplification control. For S. enterica serovar Typhimurium, four products (the fourth from the spvA gene), and for S. enterica serovar Enteritidis five amplicons (the fifth from the sdf gene) were observed. S. enterica serovar Enteritidis was cultured from three of 71 rectal swabs from diarrhoeal patients. Five specific amplicons were generated with the multiplex PCR assay only from culture-positive faecal samples. CONCLUSION: The multiplex PCR assay specifically detects S. enterica serovar Enteritidis. SIGNIFICANCE AND IMPACT OF THE STUDY: This is a novel multiplex PCR assay, which contains an internal amplification control and enables concurrent survey for Salmonella virulence genes.     

Capsular polysaccharide surrounds smooth and rugose types of Salmonella enterica serovar Typhimurium DT104

The biofilms and rugose colony morphology of Salmonella enterica serovar Typhimurium strains are usually associated with at least two different exopolymeric substances (EPS), curli and cellulose. In this study, another EPS, a capsular polysaccharide (CP) synthesized constitutively in S. enterica serovar Typhimurium strain DT104 at 25 and 37 degrees C, has been recognized as a biofilm matrix component as well. Fluorophore-assisted carbohydrate electrophoresis (FACE) analysis indicated that the CP is comprised principally of glucose and mannose, with galactose as a minor constituent. The composition differs from that of known colanic acid-containing CP that is isolated from cells of Escherichia coli and other enteric bacteria grown at 37 degrees C. The reactivity of carbohydrate-specific lectins conjugated to fluorescein isothiocyanate or gold particles with cellular carbohydrates demonstrated the cell surface localization of CP. Further, lectin binding also correlated with the FACE analysis of CP. Immunoelectron microscopy, using specific antibodies against CP, confirmed that CP surrounds the cells. Confocal microscopy of antibody-labeled cells showed greater biofilm formation at 25 degrees C than at 37 degrees C. Since the CP was shown to be produced at both 37 degrees C and 25 degrees C, it does not appear to be significantly involved in attachment during the early formation of the biofilm matrix. Although the attachment of S. enterica serovar Typhimurium DT104 does not appear to be mediated by its CP, the capsule does contribute to the biofilm matrix and may have a role in other features of this organism, such as virulence, as has been shown previously for the capsules of other gram-negative and gram-positive bacteria.     

Diversity of genome structure in Salmonella enterica serovar Typhi populations

The genomes of most strains of Salmonella and Escherichia coli are highly conserved. In contrast, all 136 wild-type strains of Salmonella enterica serovar Typhi analyzed by partial digestion with I-CeuI (an endonuclease which cuts within the rrn operons) and pulsed-field gel electrophoresis and by PCR have rearrangements due to homologous recombination between the rrn operons leading to inversions and translocations. Recombination between rrn operons in culture is known to be equally frequent in S. enterica serovar Typhi and S. enterica serovar Typhimurium; thus, the recombinants in S. enterica serovar Typhi, but not those in S. enterica serovar Typhimurium, are able to survive in nature. However, even in S. enterica serovar Typhi the need for genome balance and the need for gene dosage impose limits on rearrangements. Of 100 strains of genome types 1 to 6, 72 were only 25.5 kb off genome balance (the relative lengths of the replichores during bidirectional replication from oriC to the termination of replication [Ter]), while 28 strains were less balanced (41 kb off balance), indicating that the survival of the best-balanced strains was greater. In addition, the need for appropriate gene dosage apparently selected against rearrangements which moved genes from their accustomed distance from oriC. Although rearrangements involving the seven rrn operons are very common in S. enterica serovar Typhi, other duplicated regions, such as the 25 IS200 elements, are very rarely involved in rearrangements. Large deletions and insertions in the genome are uncommon, except for deletions of Salmonella pathogenicity island 7 (usually 134 kb) from fragment I-CeuI-G and 40-kb insertions, possibly a prophage, in fragment I-CeuI-E. The phage types were determined, and the origins of the phage types appeared to be independent of the origins of the genome types.     

Survival and filamentation of Salmonella enterica serovar enteritidis PT4 and Salmonella enterica serovar typhimurium DT104 at low water activity

In this study we investigated the long-term survival of and morphological changes in Salmonella strains at low water activity (a(w)). Salmonella enterica serovar Enteritidis PT4 and Salmonella enterica serovar Typhimurium DT104 survived at low a(w) for long periods, but minimum humectant concentrations of 8% NaCl (a(w), 0. 95), 96% sucrose (a(w), 0.94), and 32% glycerol (a(w), 0.92) were bactericidal under most conditions. Salmonella rpoS mutants were usually more sensitive to bactericidal levels of NaCl, sucrose, and glycerol. At a lethal a(w), incubation at 37 degrees C resulted in more rapid loss of viability than incubation at 21 degrees C. At a(w) values of 0.93 to 0.98, strains of S. enterica serovar Enteritidis and S. enterica serovar Typhimurium formed filaments, some of which were at least 200 microm long. Filamentation was independent of rpoS expression. When the preparations were returned to high-a(w) conditions, the filaments formed septa, and division was complete within approximately 2 to 3 h. The variable survival of Salmonella strains at low a(w) highlights the importance of strain choice when researchers produce modelling data to simulate worst-case scenarios or conduct risk assessments based on laboratory data. The continued increase in Salmonella biomass at low a(w) (without a concomitant increase in microbial count) would not have been detected by traditional microbiological enumeration tests if the tests had been performed immediately after low-a(w) storage. If Salmonella strains form filaments in food products that have low a(w) values (0.92 to 0.98), there are significant implications for public health and for designing methods for microbiological monitoring.     

Variation between pathogenic serovars within Salmonella pathogenicity islands

Although four of the five Salmonella pathogenicity islands (SPIs) have been characterized in detail for Salmonella enterica serovar Typhimurium, and the fifth has been characterized for Salmonella enterica serovar Dublin, there have been limited studies to examine them in detail in a range of pathogenic serovars of S. enterica. The aim of this study was to examine these regions, shown to be crucial in virulence, in pathogenic serovars to identify any major deletions or insertions that may explain variation in virulence and provide further understanding of the elements involved in the evolution of these regions. Multiple strains of each of the 13 serovars were compared by Southern blot hybridization using a series of probes that together encompassed the full length of all five SPIs. With the exception of serovar Typhimurium, all strains of the same serovar were identical in all five SPIs. Those serovars that differed from serovar Typhimurium in SPI-1 to SPI-4 and from serovar Dublin in SPI-5 were examined in more detail in the variant regions by PCR, and restriction endonuclease digestion and/or DNA sequencing. While most variation in hybridization patterns was attributable to loss or gain of single restriction endonuclease cleavage sites, three regions, in SPI-1, SPI-3, and SPI-5, had differences due to major insertions or deletions. In SPI-1 the avrA gene was replaced by a 200-base fragment in three serovars, as reported previously. In SPI-5, two serovars had acquired an insertion with similarity to the pagJ and pagK genes between pipC and pipD. In SPI-3 the genes sugR and rhuM were deleted in most serovars and in some were replaced by sequences that were very similar to either the Escherichia coli fimbrial operon, flanked by two distinct insertion sequence elements, or to the E. coli retron phage PhiR73. The distribution of these differences suggests that there have been a number of relatively recent horizontal transfers of genes into S. enterica and that in some cases the same event has occurred in multiple lineages of S. enterica. Thus, it seems that insertion sequences and retron phages are likely to be involved in continuing evolution of the pathogenicity islands of pathogenic Salmonella serovars.     

Multidrug-resistant Salmonella enterica serovar paratyphi A harbors IncHI1 plasmids similar to those found in serovar typhi

Salmonella enterica serovars Typhi and Paratyphi A cause systemic infections in humans which are referred to as enteric fever. Multidrug-resistant (MDR) serovar Typhi isolates emerged in the 1980s, and in recent years MDR serovar Paratyphi A infections have become established as a significant problem across Asia. MDR in serovar Typhi is almost invariably associated with IncHI1 plasmids, but the genetic basis of MDR in serovar Paratyphi A has remained predominantly undefined. The DNA sequence of an IncHI1 plasmid, pAKU_1, encoding MDR in a serovar Paratyphi A strain has been determined. Significantly, this plasmid shares a common IncHI1-associated DNA backbone with the serovar Typhi plasmid pHCM1 and an S. enterica serovar Typhimurium plasmid pR27. Plasmids pAKU_1 and pHCM1 share 14 antibiotic resistance genes encoded within similar mobile elements, which appear to form a 24-kb composite transposon that has transferred as a single unit into different positions into their IncHI1 backbones. Thus, these plasmids have acquired similar antibiotic resistance genes independently via the horizontal transfer of mobile DNA elements. Furthermore, two IncHI1 plasmids from a Vietnamese isolate of serovar Typhi were found to contain features of the backbone sequence of pAKU_1 rather than pHCM1, with the composite transposon inserted in the same location as in the pAKU_1 sequence. Our data show that these serovar Typhi and Paratyphi A IncHI1 plasmids share highly conserved core DNA and have acquired similar mobile elements encoding antibiotic resistance genes in past decades.