Strain differences in expression of virulence by the 90 kilobase pair virulence plasmid of Salmonella serovar Typhimurium

Salmonella virulence plasmid: pathogenesis and ecology

A current view on the role of the Salmonella virulence plasmid in the pathogenesis of animal and human hosts is discussed; including the possible relevance in secondary ecological niches. Various strategies towards further studies in this respect are proposed within the One Health Concept.

One Health and Food-Borne Disease: Salmonella Transmission between Humans, Animals, and Plants

There are >2,600 recognized serovars of Salmonella enterica. Many of these Salmonella serovars have a broad host range and can infect a wide variety of animals, including mammals, birds, reptiles, amphibians, fish, and insects. In addition, Salmonella can grow in plants and can survive in protozoa, soil, and water. Hence, broad-host-range Salmonella can be transmitted via feces from wild animals, farm animals, and pets or by consumption of a wide variety of common foods: poultry, beef, pork, eggs, milk, fruit, vegetables, spices, and nuts. Broad-host-range Salmonella pathogens typically cause gastroenteritis in humans. Some Salmonella serovars have a more restricted host range that is associated with changes in the virulence plasmid pSV, accumulation of pseudogenes, and chromosome rearrangements. These changes in host-restricted Salmonella alter pathogen-host interactions such that host-restricted Salmonella organisms commonly cause systemic infections and are transmitted between host populations by asymptomatic carriers. The secondary consequences of efforts to eliminate host-restricted Salmonella serovars demonstrate that basic ecological principles govern the environmental niches occupied by these pathogens, making it impossible to thwart Salmonella infections without a clear understanding of the human, animal, and environmental reservoirs of these pathogens. Thus, transmission of S. enterica provides a compelling example of the One Health paradigm because reducing human infections will require the reduction of Salmonella in animals and limitation of transmission from the environment.

Reduction of Salmonella enterica serovar Choleraesuis carrying large virulence plasmids after the foot and mouth disease outbreak in swine in southern Taiwan, and their independent evolution in human and pig

BACKGROUND/PURPOSE

Salmonella enterica serovar Choleraesuis (S. Choleraesuis) is a highly invasive zoonotic pathogen that causes bacteremia in humans and pigs. The prevalence of S. Choleraesuis in man has gradually decreased since the outbreak of foot and mouth disease in pigs in 1997 in southern Taiwan. The goal of this study was to investigate the change in prevalence of S. Choleraesuis carrying the virulence plasmid (pSCV) in human and swine isolates collected in 1995-2005 and characterize these.

METHODS

380 isolates were collected from human and swine blood samples. Large pSCVs were determined by PCR and Southern blot analysis. Antimicrobial susceptibility and resistance genes, and the phylogenetic association of these large pSCV were analyzed.

RESULTS

The number of isolates harboring the large pSCV was significantly reduced, and their prevalence differed between human and swine isolates. These large pSCVs were a recombinant of original 50-kb pSCV and R plasmid. In addition, some large pSCVs lacked two pSCV-specific deletion regions from pef to repC and from traT to samA. These large pSCVs carried the resistance genes bla(TEM,)aadA2, and sulI, as well as class I integrons of 0.65 and/or 1.9 kb in size, but were inconjugatible. Phylogenetic analysis demonstrated that the large pSCV evolves independently in human and swine isolates.

CONCLUSION

S. Choleraesuis with large pSCV was significantly reduced after the foot and mouth disease outbreak and may evolve in human and swine specific isolates.

Association of virulence plasmid and antibiotic resistance determinants with chromosomal multilocus genotypes in Mexican Salmonella enterica serovar Typhimurium strains

Background

Bacterial genomes are mosaic structures composed of genes present in every strain of the same species (core genome), and genes present in some but not all strains of a species (accessory genome). The aim of this study was to compare the genetic diversity of core and accessory genes of a Salmonella enterica subspecies enterica serovar Typhimurium (Typhimurium) population isolated from food-animal and human sources in four regions of Mexico. Multilocus sequence typing (MLST) and macrorestriction fingerprints by pulsed-field gel electrophoresis (PFGE) were used to address the core genetic variation, and genes involved in pathogenesis and antibiotic resistance were selected to evaluate the accessory genome.

Results

We found a low genetic diversity for both housekeeping and accessory genes. Sequence type 19 (ST19) was supported as the founder genotype of STs 213, 302 and 429. We found a temporal pattern in which the derived ST213 is replacing the founder ST19 in the four geographic regions analyzed and a geographic trend in the number of resistance determinants. The distribution of the accessory genes was not random among chromosomal genotypes. We detected strong associations among the different accessory genes and the multilocus chromosomal genotypes (STs). First, the Salmonella virulence plasmid (pSTV) was found mostly in ST19 isolates. Second, the plasmid-borne betalactamase cmy-2 was found only in ST213 isolates. Third, the most abundant integron, IP-1 (dfrA12, orfF and aadA2), was found only in ST213 isolates. Fourth, the Salmonella genomic island (SGI1) was found mainly in a subgroup of ST19 isolates carrying pSTV. The mapping of accessory genes and multilocus genotypes on the dendrogram derived from macrorestiction fingerprints allowed the establishment of genetic subgroups within the population.

Conclusion

Despite the low levels of genetic diversity of core and accessory genes, the non-random distribution of the accessory genes across chromosomal backgrounds allowed us to discover genetic subgroups within the population. This study provides information about the importance of the accessory genome in generating genetic variability within a bacterial population.

Evolution of genes on the Salmonella Virulence plasmid phylogeny revealed from sequencing of the virulence plasmids of S. enterica serotype Dublin and comparative analysis

Salmonella enterica serotype Dublin harbors an approximately 80-kb virulence plasmid (pSDV), which mediates systemic infection in cattle. There are two types of pSDV: one is pSDVu (pOU1113) in strain OU7025 and the other pSDVr (pOU1115) in OU7409 (SD Lane) and many clinical isolates. Sequence analysis showed that pSDVr was a recombinant plasmid (co-integrate) of pSDVu and a plasmid similar to a 35-kb indigenous plasmid (pOU1114) of S. Dublin. Most of the F-transfer region in pSDVu was replaced by a DNA segment from the pOU1114-like plasmid containing an extra replicon and a pilX operon encoding for a type IV secretion system to form pSDVr. We reconstructed the particular evolutionary history of the seven virulence plasmids of Salmonella by comparative sequence analysis. The whole evolutionary process might begin with two different F-like plasmids (IncFI and IncFII), which then incorporated the spv operon and fimbriae operon from the chromosome to form the primitive virulence plasmids. Subsequently, these plasmids descended by deletion from a relatively large plasmid to smaller ones, with some recombination events occurring over time. Our results suggest that the phylogeny of virulence plasmids as a result of frequent recombination provides the opportunity for rapid evolution of Salmonella in response to the environmental cues.

Evolution of the virulence plasmids of non-typhoid Salmonella and its association with antimicrobial resistance

Among more than 2,500 serovars, eight contain a virulence plasmid, including medically important Salmonella enterica serovars Choleraesuis, Dublin, Enteritidis, and Typhimurium. These serovar-specific virulence plasmids vary in size, but all contain the spv operon, which plays a role in the expression of the virulence. Genetically, these virulence plasmids are likely derived from a common ancestral plasmid possessing virulence-related genes and loci. Based on the analysis of the available DNA sequences of the plasmids, the phylogenetic path may be split into two: pSPV (virulence plasmid of S. Gallinarum-Pullorum) acquires an incompatibility-related locus that differs from that of the others. At some point, pSCV (virulence plasmid of S. Choleraesuis) and pSDV (virulence plasmid of S. Dublin) lose oriT by recombination or simply by deletion, making the two unable to be mobilized. On the other hand, pSEV (virulence plasmid of S. Enteritidis) also loses some DNA by deletion but not as extensively as pSCV, and therefore pSEV is closest to pSTV (virulence plasmid of S. Typhimurium) both genetically and biologically. The pSTV shows the least alternation during the evolution. There are two types of pSDV. pSDVu recombines with non-virulence 36.6-kb plasmid to acquire additional incompatibility trait to form pSDVr. Recent reports indicated that S. Choleraesuis and S. Typhimurium could generate different types of hybrid plasmids, which consisted of the serovar-specific virulence plasmid and an array of resistance gene cassettes. The recombination gives Salmonella a survival advantage in an unfavorable drug environment. The integration of resistance genes and additional replicons into a Salmonella virulence plasmid constitutes a new and interesting example of plasmid evolution and poses a serious threat to public health.

Differences in the carriage and the ability to utilize the serotype associated virulence plasmid in strains of Salmonella enterica serotype Typhimurium investigated by use of a self-transferable virulence plasmid, pOG669

Most strains of Salmonella enterica subspecies enterica serotype typhimurium (S. typhimurium) naturally harbour a virulence plasmid which carries the salmonella plasmid virulence (spv) genes. However, isolates belonging to certain phage types are generally found without the plasmid. We have utilized a self-transferable virulence plasmid, pOG669 to investigate the effect of introduction of spv genes into strains of such phage types. The use of the co-integrate plasmid, pOG669, was validated on a diverse collection of strains. pOG669 was transferred into strains of serotypes that are normally associated with the possession of virulence plasmids. All strains maintained the wild type level of virulence in a mouse model, except that introduction of pOG669 restored normal virulence levels in an avirulent, plasmid free strain of S. dublin and resulted in a decrease in virulence in a strain of S. dublin from clonal line Du3. S. gallinarum did not become virulent in mice, but pOG669 was functionally interchangeable with the wild type plasmid when strains were tested in a chicken model. Strains of serotypes not normally associated with the carriage of a virulence plasmid did not increase in virulence upon the introduction of pOG669. An IncX plasmid pOG670 that was included as control was incompatible with the virulence plasmid in a strain of S. dublin, demonstrating that the common virulence plasmid of this serotype is of a different incompatibility group than other virulence plasmids. Strains of S. typhimurium from phage types that do not normally carry a virulence plasmid responded differently to attempts to introduce pOG669. No transconjugants were observed with the strains of DT5 and DT21. The introduction of pOG669 did not alter the virulence of JEO3942(DT10), DT35 and JEO3949(DT66) significantly, while DT1 and DT27 became more virulent. DT27 became as virulent as wild type C5, while logVC(10) of DT1 only increased from 4.1 to 5.7. The ability to express spv-genes was measured by use of an spvRAB'-cat fusion. Expression in S. enteritidis was found to be higher than in other serotypes tested. Only serotypes that naturally carry a virulence plasmid expressed spv-genes. The strain of DT1 expressed spv at a very low level, while expression in the strains of DT10 and DT35 was approximately 2-fold lower than in a control strain of S. typhimurium, while the level in the DT66 strain corresponded to the control strain. The plasmid pSTF9, which carried the fusion gene could not be introduced into the strains of DT5, DT21 and DT27. The RpoS level in the strains was measured indirectly by use of a katE-lacZ fusion. In the DT5 strain the level of expression was low, while the strains JEO3942(DT10), DT21, DT27 and DT35 expressed 4-5 fold the level in this strain. An internal fragment of the rpoS gene was sequenced in three strains. These all showed an identical sequence to a published S. typhimurium rpoS gene.

In vivo acquisition of ceftriaxone resistance in Salmonella enterica serotype anatum

The emergence of resistance to antimicrobial agents within the salmonellas is a worldwide and severe problem. A case of treatment failure due to the emergence of resistance to ceftriaxone in Salmonella enterica serotype Anatum was studied. S. enterica serotype Anatum and Escherichia coli, both of which are susceptible to ceftriaxone, were initially isolated from a diabetic patient hospitalized for the treatment of wound and urinary tract infections. Resistant S. enterica serotype Anatum and E. coli strains were isolated concomitantly 2 weeks after the initiation of ceftriaxone therapy. The patient eventually died of a sepsis caused by the ceftriaxone-resistant salmonella. PCR, nucleotide sequence analysis, and DNA-DNA hybridization identified a bla(CTX-M-3) gene located on a 95.1-kb plasmid from the ceftriaxone-resistant isolates of S. enterica serotype Anatum and E. coli. The plasmid was proved to be conjugative. Molecular fingerprinting showed that the susceptible and resistant strains were genetically indistinguishable. The emergence of resistance to ceftriaxone in S. enterica serotype Anatum was due to the in vivo acquisition of a plasmid containing the bla(CTX-M-3) gene and was the cause for treatment failure in this patient.

Nucleotide and amino acid sequences of oriT-traM-traJ-traY-traA-traL regions and mobilization of virulence plasmids of Salmonella enterica serovars enteritidis, gallinarum-pullorum, and typhimurium

The virulence plasmid of Salmonella enterica serovar Gallinarum-Pullorum (pSPV) but not those of Salmonella enterica serovars Enteritidis (pSEV) and Typhimurium (pSTV) can be readily mobilized by an F or F-like conjugative plasmid. To investigate the reason for the difference, the oriT-traM-traJ-traY-traA-traL regions of the three salmonella virulence plasmids (pSVs) were cloned and their nucleotide and deduced amino acid sequences were examined. The cloned fragments were generally mobilized more readily than the corresponding full-length pSVs, but the recombinant plasmid containing the oriT of pSPV was, as expected, more readily mobilized, with up to 100-fold higher frequency than the recombinant plasmids containing the oriT of the other two pSVs. The nucleotide sequences of the oriT-traM-traJ-traY-traA-traL region of pSEV and pSTV were almost identical (only 4 bp differences), but differed from that of pSPV. Major nucleotide sequence variations were found in traJ, traY, and the Tra protein binding sites sby and sbm. sby of pSPV showed higher similarity than that of pSEV or pSTV to that of the F plasmid. The reverse was true for sbm: similarity was higher with pSEV and pSTV than with pSPV. In the deduced amino acid sequences of the five Tra proteins, major differences were found in TraY: pSEV's TraY was 75 amino acids, pSTV's was 106 amino acids, and pSPV's was 133 amino acids; and there were duplicate consensus betaalphaalpha fragments in the TraY of pSPV and F plasmid, whereas there was only a single betaalphaalpha fragment in that of pSEV and pSTV.

Characterization of a laboratory-derived, high-level ampicillin-resistant Salmonella enterica serovar Typhimurium strain that caused meningitis in an infant

A Salmonella enterica serovar Typhimurium strain that harbored a plasmid carrying a TEM-1-type beta-lactamase gene was isolated from the blood and cerebrospinal fluid of an infant with meningitis. This 3.2-kb plasmid was further characterized to be a nonconjugative pGEM series cloning vector containing a foreign insert. The strain was likely laboratory derived and contaminated the environment before it caused the infection.

Excess production of interleukin-12 subunit p40 stimulated by the virulence plasmid of Salmonella enterica serovar Typhimurium in the early phase of infection in the mouse

The production of interleukin-12 (IL-12) and its subunits in response to Salmonella enterica serovar Typhimurium infection in the BALB/c mouse was examined. Unlike wild-type Typhimurium, a plasmidless strain, isolated by curing of the virulence plasmid (pSTV), did not stimulate excess IL-12p40 production. When a Tn 5 tagged pSTV was transferred back to the plasmidless strain, the ability to stimulate IL-12p40 production was restored. However, a strain harbouring another Tn50pSTV failed to stimulate excess IL-12p40 production. This Tn 5 insertion area, located on fragment H3 of pSTV, was designated spf (stimulation of protein forty). The ability to stimulate IL-12p40 production was restored in a partial diploid that carried a wild-type fragment covering the spf site. There is one known gene, repA, a locus, rsk, and two putative ORFs, in the vicinity of the Tn 5 insertion site; however, these are not spf. The precise location of the spf locus is still unknown.

Lack of evidence of an association between the carriage of virulence plasmid and the bacteremia of Salmonella typhimurium in humans

The involvement of the virulence plasmid (pSTV) of Salmonella typhimurium in human salmonellosis was examined. Most of the 224 clinical strains isolated from the blood (53) and nonblood samples (171) contained a 90 kb or larger plasmid, most of which were pSTV. The rates of pSTV carriage in the isolates showed no statistically significant difference between those derived from the blood and those from other sources (87% vs. 83%; chi2=0.49, 0.1<P<0.9), suggesting that pSTV may not play a critical role in promoting S. typhimurium bacteremia in humans. Nine strains with representative plasmid profiles were tested for the mouse virulence. The result revealed that these clinical isolates contained all three virulent types known: the avirulent, the highly virulent when a pSTV was present, and the moderately virulent regardless of the presence or absence of pSTV. This indicated that mouse virulence of S. typhimurium did not correlate their virulence in humans. Clinical data showed that most patients with primary bacteremia had underlying immunosuppressive diseases, whereas only a few patients with secondary bacteremia had preexisting diseases (87% vs. 13%; chi2=22.73, P<0.005). It is suggested that the contribution of pSTV to S. typhimurium bacteremia in humans is likely to be limited, and both the host factor and the microbial virulence determinants on the chromosome are more important than virulence plasmid in predisposing patients to bacteremia.

In vitro evaluation of intracellular activity of antibiotics against non-typhoid Salmonella

Non-typhoid salmonellae are the most common causative organisms of bacterial enteritis in children. Clinical studies have failed to show any influence of various antibiotics on the natural course of acute salmonella enteritis. Poor penetration of antibiotics into phagocytic cells that contain intracellular Salmonella spp., and possible intracellular antibiotic inactivation have been considered as possible reasons for this. In this study, we used an in vitro model to assess the intracellular activity of antibiotics against non-typhoid salmonellae. The survival of intracellular Salmonella spp. in P388D1 cells, a mouse macrophage cell line was measured in the presence of various antibiotics. Except for gentamicin, which entered phagocytes poorly, ofloxacin, azithromycin, chloramphenicol and three beta-lactam antibiotics, ampicillin, cefixime and ceftriaxone, exhibited bacteriostatic activity against susceptible intracellular Salmonella spp. at an extracellular concentration equal to the minimal inhibitory concentration (MIC). At a concentration of 10 x MIC, neither chloramphenicol nor the three beta-lactam antibiotics produced a bactericidal response; however, both ofloxacin and azithromycin were bactericidal after 8-24 h of incubation. The results showed that fluoroquinolones and new macrolides were more efficient than the other antibiotics in eradicating intracellular salmonella and might be useful agents for the treatment of non-typhoid salmonella enteritis in children. Clinical trials should be considered.

Comparative physical and genetic maps of the virulence plasmids of Salmonella enterica serovars typhimurium, enteritidis, choleraesuis, and dublin

Using fragment profiling, PCR, and Southern hybridization, we found that Salmonella enterica serovar Choleraesuis harbored virulence plasmids of various sizes, whereas serovars Typhimurium, Enteritidis, and Dublin carried a plasmid of a unique size. Also, the virulence plasmid of Typhimurium contained genes in the same order detected in the other three plasmids, all of which contained deletions.

Intracellular Salmonella typhimurium induce lysis of human polymorphonuclear leukocytes which is not associated with the Salmonella virulence plasmid

The interaction between Salmonella typhimurium and human polymorphonuclear leukocytes (PMNs) was analyzed in vitro. Three S. typhimurium strains, the wild-type strain OU5043, its isogenic virulence plasmid-cured strain OU5048, and LT2, which represented the types that exhibited three mouse virulence levels, respectively, were used in this study. There was no correlation between the recovery of intracellular S. typhimurium from PMNs and the presence or absence of the virulence plasmid, or the strains' mouse virulence level. When the oxygen-dependent response of PMNs upon phagocytosis of S. typhimurium was examined by checking the intracellular reduction of nitroblue tetrazolium (NBT), the fraction of PMNs that reduced NBT on phagocytosis of the three strains was around 80%, whereas it was 58% with Escherichia coli, 95% with phorbol 12-myristate 13-acetate and 15% with a negative control. Thus there were no significant differences among the three Salmonella strains in terms of their ability to induce the oxidative response in PMNs. Microscopic analysis of Salmonella-infected PMNs indicated that the intracellular Salmonella induced lysis of PMNs. Both OU5043 and OU5048 exhibited a significant intracellular cytotoxic effect on PMNs after 24 hr of infection and this effect was not associated with the presence or absence of the virulence plasmid. On the other hand, lysis of PMNs was related to the intracellular survival of Salmnonella, as ofloxacin, an antibiotic, appeared to be able to protect human PMNs from Salmonella-induced cytotoxicity when this agent was added into the medium to inactivate the intracellular organism. The ability to induce lysis of PMNs by either wild-type or plasmid-cured strains of S. typhimurium may play a crucial role in the pathogenesis of non-typhoid Salmonella. The contribution of pSTV to human salmonellosis is likely to be limited. Furthermore, early institution of antibiotics with a high intracellular activity against Salmonella, such as fluoroquinolones, may be useful to prevent the dissemination of Salmonella infection.

Importance of the serovar-specific plasmid for virulence of salmonella strains in calves

To evaluate the influence of serovar-specific plasmids on salmonella virulence in calves, experiments were performed involving infection, by the oral route, with mixtures of strains containing equal counts of a plasmid-carrying and a plasmid-free strain of the same serovar. The concentration ratio between the plasmid-carrying and the plasmid-free strain which had developed in the organs of the infected animals was used for a comparative evaluation of virulence and pathogenetic behaviour of the strains. While in the S. typhimurium strains studied, the presence of the plasmid was accompanied by a significantly increased colonization and multiplication of the agent in the host's body, examination of S. enteritidis and S. dublin revealed that the plasmid-free strains exhibited identical or even significantly higher bacterial counts than the plasmid-carrying strains in organs. The fact that plasmid-free salmonella strains with a high virulence for calves have been found demonstrates that the presence of a serovar-specific plasmid is not an indispensable requirement for the development of salmonellosis in calves.