Real-time PCR method for Salmonella spp. targeting the stn gene

AIM

To develop a real-time PCR assay for Salmonella spp. targeting the stn gene.

METHODS AND RESULTS

The presence of stn in the Salmonella bongori genome was found by a BLAST with Salmonella enterica stn sequence. Manual alignment of stn sequences showed that Salm. bongori had 88% sequence identity with Salm. enterica. Two primers (stnL-433 and stnR-561) and a probe (stnP-452) were designed to target conserved regions in stn and meet the requirements of a 5'-nuclease assay. The primers and probe were evaluated against 353 isolates, including 255 Salm. enterica representing 158 serotypes, 14 Salm. bongori representing 12 serotypes and 84 non-Salmonella representing 56 species from 31 genera. All isolates were correctly identified, with the exception of three isolates of Citrobacter amalonaticus, which gave false positives. The limit of detection with cultured Salmonella was 3 CFU per reaction.

CONCLUSIONS

The stn real-time PCR method had 100% inclusivity, 96.4% exclusivity and a level of detection of 3 CFU per reaction for cultured Salmonella spp.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study showed that stn is present in Salm. bongori and is a valid target for both species of Salmonella. The Salmonella s tn real-time PCR is a useful method for identifying Salmonella spp.

Comparison of tissue-selective proinflammatory gene induction in mice infected with wild-type, DNA adenine methylase-deficient, and flagellin-deficient Salmonella enterica

Mutants of Salmonella enterica serovar Typhimurium deficient in DNA adenine methylase (Dam) are attenuated for virulence in mice and confer heightened immunity in vaccinated animals. In contrast, infection of mice with wild-type (WT) strains or flagellin-deficient mutants of Salmonella causes typhoid fever. Here we examined the bacterial load and spatiotemporal kinetics of expression of several classes of host genes in Peyer's patches, the liver, and the spleen following oral infection of mice with WT, dam mutant, or flagellin-deficient (flhC) Salmonella. The genes evaluated included inflammatory (interleukin-1beta [IL-1beta], tumor necrosis factor alpha), chemokine (macrophage inflammatory protein 2), Th1/Th2 indicator (IL-12p40, IL-4), and interferon system (beta interferon [IFN-beta], IFN-gamma, protein Mx1 GTPase, RNA-dependent protein kinase, inducible nitric oxide synthase, suppressor of cytokine signaling 1) beacons. We showed that maximal interferon system and proinflammatory gene induction occurred by 5 days after infection and that the levels were comparable for the WT and flhC strains but were significantly lower for the dam mutant. Additionally, host gene expression in systemic tissues of individual animals was dependent on the bacterial load in the Peyer's patches for mice infected with WT, dam mutant, or flhC mutant Salmonella as early as 8 h after infection. Moreover, a bacterial load threshold in the Peyer's patches was necessary to stimulate the host gene induction in the liver and spleen. Taken together, these results suggest that bacterial load and the accompanying strain-specific cytokine signature are important determinants of the host innate immune response and associated disease manifestations observed in dam mutant Salmonella-infected animals compared to the immune response and disease manifestations observed in WT and flhC mutant Salmonella-infected animals.

Plasmid-mediated quinolone resistance conferred by qnrS1 in Salmonella enterica serovar Virchow isolated from Turkish food of avian origin

OBJECTIVES

To study the molecular characteristics of the quinolone and associated ampicillin resistance mechanisms present in Salmonella enterica serovar Virchow isolated from Turkish foods.

METHODS

Nine epidemiologically unrelated Salmonella Virchow strains isolated from foods (chicken and minced meat) sold in different markets in Ankara were analysed for their susceptibility to 17 antimicrobials. The strains were typed by PFGE and plasmid profiling and investigated by molecular methods (PCR/sequencing) for the presence of several resistance genes, class 1 integrons and mutations in the quinolone resistance-determining regions. Plasmids conferring quinolone resistance were analysed by restriction fragment length polymorphism (RFLP) analysis, DNA hybridization, sequencing, replicon-typing PCR and mating experiments.

RESULTS

All strains showed nalidixic acid resistance (MIC >or= 128 mg/L) together with a decreased susceptibility to ciprofloxacin (three strains with an MIC of 1 mg/L and six with an MIC of 0.25 mg/L), associated with mutations within the gyrA gene (Asp-87 --> Tyr-87). In three strains, qnrS1 genes were detected. Ampicillin resistance encoded by a bla(CTX-M3) gene and/or bla(TEM-1-like) gene was found in four strains. Three of these strains carried an approximately 45 kb conjugative plasmid, designated pRQ2006, harbouring qnrS1 and a Tn3-like transposon. Partial sequencing and RFLP of pRQ2006 indicated its similarity to the qnrS1 plasmid pAH03786 found in a Japanese Shigella flexneri 2b isolate.

CONCLUSIONS

This is the first study describing the presence of qnrS1 genes in bacterial isolates from Turkey. The pRQ2006 plasmid seems to be more related to the S. flexneri 2b qnrS1 plasmid pAH0376 than to the Salmonella qnrS1-carrying plasmids pINF5 and TPqnrS-2.

Microarray analysis of Mu transposition in Salmonella enterica, serovar Typhimurium: transposon exclusion by high-density DNA binding proteins

All organisms contain transposons with the potential to disrupt and rearrange genes. Despite the presence of these destabilizing sequences, some genomes show remarkable stability over evolutionary time. Do bacteria defend the genome against disruption by transposons? Phage Mu replicates by transposition and virtually all genes are potential insertion targets. To test whether bacteria limit Mu transposition to specific parts of the chromosome, DNA arrays of Salmonella enterica were used to quantitatively measure target site preference and compare the data with Escherichia coli. Essential genes were as susceptible to transposon disruption as non-essential ones in both organisms, but the correlation of transposition hot spots among homologous genes was poor. Genes in highly transcribed operons were insulated from transposon mutagenesis in both organisms. A 10 kb cold spot on the pSLT plasmid was near parS, a site to which the ParB protein binds and spreads along DNA. Deleting ParB erased the plasmid cold spot, and an ectopic parS site placed in the Salmonella chromosome created a new cold spot in the presence of ParB. Our data show that competition between cellular proteins and transposition proteins on plasmids and the chromosome is a dominant factor controlling the genetic footprint of transposons in living cells.

Temporal fragmentation of speciation in bacteria

Because bacterial recombination involves the occasional transfer of small DNA fragments between strains, different sets of niche-specific genes may be maintained in populations that freely recombine at other loci. Therefore, genetic isolation may be established at different times for different chromosomal regions during speciation as recombination at niche-specific genes is curtailed. To test this model, we separated sequence divergence into rate and time components, revealing that different regions of the Escherichia coli and Salmonella enterica chromosomes diverged over a approximately 70-million-year period. Genetic isolation first occurred at regions carrying species-specific genes, indicating that physiological distinctiveness between the nascent Escherichia and Salmonella lineages was maintained for tens of millions of years before the complete genetic isolation of their chromosomes.

The CbiB protein of Salmonella enterica is an integral membrane protein involved in the last step of the de novo corrin ring biosynthetic pathway

We report results of studies of the conversion of adenosylcobyric acid (AdoCby) to adenosylcobinamide-phosphate, the last step of the de novo corrin ring biosynthetic branch of the adenosylcobalamin (coenzyme B12) pathway of Salmonella enterica serovar Typhimurium LT2. Previous reports have implicated the CbiB protein in this step of the pathway. Hydropathy analysis predicted that CbiB would be an integral membrane protein. We used a computer-generated topology model of the primary sequence of CbiB to guide the construction of CbiB-LacZ and CbiB-PhoA protein fusions, which were used to explore the general topology of CbiB in the cell membrane. A refined model of CbiB as an integral membrane protein is presented. In vivo analyses of the effect of single-amino-acid changes showed that periplasm- and cytosol-exposed residues are critical for CbiB function. Results of in vivo studies also show that ethanolamine-phosphate (EA-P) is a substrate of CbiB, but l-Thr-P is not, and that CbiB likely activates AdoCby by phosphorylation. The latter observation leads us to suggest that CbiB is a synthetase not a synthase enzyme. Results from mass spectrometry and bioassay experiments indicate that serovar Typhimurium synthesizes norcobalamin (cobalamin lacking the methyl group at C176) when EA-P is the substrate of CbiB.

Detailed structure of integrons and transposons carried by large conjugative plasmids responsible for multidrug resistance in diverse genomic types of Salmonella enterica serovar Brandenburg

OBJECTIVES

To evaluate the incidence, molecular basis and distribution among genomic types of antimicrobial drug resistance in Salmonella enterica (S.) serovar Brandenburg isolates recorded in the Principality of Asturias, Spain.

METHODS

Thirty-seven S. Brandenburg isolates were tested for susceptibility to antimicrobial agents and typed by random amplified polymorphic DNA (RAPD) and pulsed-field gel electrophoresis (PFGE). PCR amplifications, together with DNA cloning and sequencing, were used to identify resistance genes, integrons and transposons and to establish the structure and physical associations between them. Conjugation experiments were applied to establish the location of the identified elements.

RESULTS

Twenty-one isolates were resistant to one or more unrelated drugs. Resistances to streptomycin, tetracycline, kanamycin, chloramphenicol, ampicillin and trimethoprim-sulfamethoxazole, encoded by aadA1, tet(A) or tet(B), aphA1, catA1, bla(TEM) and dfrA1-sul1-sul3, respectively, were most frequently observed. Multidrug resistance (32.4%) was mainly mediated by mobile genetic elements. These included: (i) class 1 integrons (with dfrA1-aadA1 gene cassettes in their variable region), which were part of Tn21-related transposons associated with Tn9; (ii) a Tn1721-derivative containing tet(A); (iii) a defective Tn10 that carried tet(B), and was linked to an integron; and (iv) large conjugative plasmids carrying a class 1 integron-Tn21-Tn9-like structure, together with the Tn1721- or the Tn10-related element. Two-way-RAPD and XbaI-PFGE discriminated the isolates into 15 and 12 profiles, respectively.

CONCLUSIONS

Complex genetic elements have apparently been responsible for the recruitment, assembly and dispersion of resistance genes among the highly diverse genomic types of S. Brandenburg, identified as causal agents of human salmonellosis in the Principality of Asturias, over recent years.

Feasibility of a molecular screening method for detection of Salmonella enterica and Campylobacter jejuni in a routine community-based clinical microbiology laboratory

Conventional diagnostic methods for the detection of Salmonella enterica and Campylobacter jejuni are laborious and time-consuming procedures, resulting in final results, for the majority of specimens, only after 3 to 4 days. Molecular detection can improve the time to reporting of the final results from several days to the next day. However, molecular assays for the detection of gastrointestinal pathogens directly from stool specimens have not made it into the routine clinical microbiology laboratory. In this study we have assessed the feasibility of a real-time PCR-based molecular screening method (MSM), aimed at S. enterica and C. jejuni, in the daily practice of a routine clinical microbiology laboratory. We have prospectively analyzed 2,067 stool specimens submitted for routine detection of gastrointestinal bacterial pathogens over a 7-month period. The MSM showed 98 to 100% sensitivity but routine culture showed only 77.8 to 86.8% sensitivity when an extended "gold standard" that included all culture-positive and all MSM-positive specimens, as confirmed by an independent secondary PCR of a different target gene, was used. An overall improvement in the rate of detection of both pathogens of 15 to 18% was observed. Both approaches performed nearly identically with regard to the specificity, positive predictive value, and negative predictive value, with the values for MSM being 99.7%, 93.1 to 96.6%, and 99.8 to 100%, respectively, and those for routine culture being 100%, 100%, and 97.6 to 99.5%, respectively. Finally, the final results were reported between 3 and 4 days earlier for negative specimens compared to the time of reporting of the results of routine culture. Positive specimens, on the other hand, required an additional 2 days to obtain a final result compared to the time required for routine culture, although preliminary MSM PCR-positive results were reported, on average, 2.9 to 3.8 days before the final routine culture results were reported. In conclusion, MSM can be incorporated into the daily practice of a routine clinical microbiology laboratory with ease. Furthermore, it provides an improvement in the screening for S. enterica and C. jejuni and substantially improves the time to the reporting of negative results.

The role of cellulose and O-antigen capsule in the colonization of plants by Salmonella enterica

Numerous salmonellosis outbreaks have been associated with vegetables, in particular sprouted seed. Thin aggregative fimbriae (Tafi), a component of the extracellular matrix responsible for multicellular behavior, are important for Salmonella enterica attachment and colonization of plants. Here, we demonstrate that the other surface polymers composing the extracellular matrix, cellulose, and O-antigen capsule also play a role in colonization of plants. Mutations in bacterial cellulose synthesis (bcsA) and O-antigen capsule assembly and translocation (yihO) reduced the ability to attach to and colonize alfalfa sprouts. A colanic acid mutant was unaffected in plant attachment or colonization. Tafi, cellulose synthesis, and O-antigen capsule, all of which contribute to attachment and colonization of plants, are regulated by AgfD, suggesting that AgfD is a key regulator for survival outside of hosts of Salmonella spp. The cellulose biosynthesis regulator adrA mutant was not affected in the ability to attach to or colonize plants; however, promoter probe assays revealed expression by cells attached to alfalfa sprouts. Furthermore, quantitative reverse-transcriptase polymerase chain reaction revealed differential expression of agfD and adrA between planktonic and plant-attached cells. In addition, there was no correlation among mutants between biofilm formation in culture and attachment to plants. Outside of animal hosts, S. enterica appears to rely on an arsenal of adhesins to persist on plants, which can act as vectors and perpetuate public health concerns.

Occurrence of qacE/qacEDelta1 genes and their correlation with class 1 integrons in salmonella enterica isolates from poultry and swine

In this study, a total of 122 Salmonella enterica isolates from poultry and swine were assessed for susceptibility to clinically important antibiotics and to benzalkonium chloride (BKC). All isolates were examined for the presence of the antiseptic resistance genes qacE and qacEDelta1 and intl1 (class 1 integrase). The intl1-positive strains were further investigated for the presence of the 3' conserved region. The results demonstrated widespread distribution of qacEDelta1 (27%) but no isolate with qacE was observed. The intl1 gene was identified in 23 isolates (70%) with qacEDelta1. All of the intl1-postive strains carried qacEDelta1 in 3' conserved segment, confirming that the qacEDelta1 gene is linked to the integrons. Increased MIC value to BKC was independent of the presence of qacEDelta1, and multiple antibiotic-resistant bacteria were no more tolerant to BKC than the non-multidrugresistant strains, regardless of the presence of qacEDelta1.

HilA gene expression in SCFAs adapted and inorganic acid challenged Salmonella enterica serovar typhimurium

Salmonella enterica serovar typhimurium (S. typhimurium) encounters short chain fatty acids (inorganic acids containing propionate, butyrate and acetate) in the intestine as well as in food preservatives. Short chain fatty acids (SCFAs) exposed organisms have been reported to offer resistance to organic acid resulting into enhanced virulence. However, the role of hilA (hyper invasive loci) gene expression has not been assessed in this context. In the present study, S. typhimurium was grown under SCFAs stress condition simulating the in vivo environment and hilA gene expression was evaluated. The gene expression was measured by beta-galactosidase (beta-gal) assay using a hilA-lacZY fusion strain and calculated as Miller units. hilA gene expression was found to be significantly higher in the SCFAs exposed cells than the unexposed ones, after 2 hrs and 4 hrs of exposure. However, no significant difference was observed between the activities at 2 hrs and 4 hrs. It indicates that hilA gene gets expressed by 2 hrs and persists till 4 hrs at least. The beta-gal activity was also measured in the unadapted / SCFAs adapted organisms followed by acid shock for 1 hr. The gene expression was also found to be higher in the SCFAs adapted--acid (pH 3) challenged as compared to the unadapated acid challenged organisms suggesting that SCFAs adaptation may induce organic acid tolerance by modulating the hilA response. This observation indicates that hilA may be the additional gene contributing to acid resistance and thereby increasing virulence of the organism after SCFAs adaptation.

Infertility associated with sub clinical salmonellosis

Subclinical infection of guinea pigs with isogenic wild type and aroA, htrA and aroA-htrA mutants of Salmonella enterica subspecies enterica serovar Abortusequi (S. Abortusequi) induced infertility, while mutants had little or no effect on conception rate in guinea pigs. Conception rate was significantly lower in guinea pigs inoculated with wild type (S-787) and aroA mutant of S. Abortusequi than those inoculated with intracellular survival deficient htrA or aroA-htrA mutants of S. Abortusequi. Chi-test analysis revealed that none of the three mutants could be attributed to low conception rate, but wild type Salmonella inoculation and chronic carriage of the pathogen were significant cause of low conception rate in guinea pigs. Role of S. Abortusequi in causation of infertility was proven from the experiment for the first time.

A new concept to stimulate mucosal as well as systemic immunity by parenteral vaccination as applied to the development of a live attenuated Salmonella enterica serovar Dublin vaccine

A new concept of slow "drip feeding" that enables activation of mucosal as well as systemic immunity following parenteral vaccination was demonstrated using Salmonella Dublin in a mouse model. The live vaccine candidate, N-RM25, generated from a wild S. Dublin strain utilising metabolic-drift (spontaneous chromosomal) mutations had a unique sensitivity to bile and restricted growth in the presence of a very low concentration of bile salts No. 3 (0.075% (w/v)) but also had the ability to survive in a high concentration (19.2%) of the substance. Following intraperitoneal administration with 10(7) cfu, N-RM25 colonised and survived (10(1)-10(3) cfu/g) in the liver and spleen of mice for over 24 days without causing disease. A small number of the mutant organisms also penetrated the gall bladder and gut, most likely via the enterohepatic circulation. N-RM25 induced significant levels of serum IgG, IgA and intestinal secretory IgA. A second metabolic-drift mutant (R-NM29) that was rapidly eliminated from the liver and spleen and highly unlikely to penetrate the gall bladder and gut, stimulated some systemic immunity, but induced no mucosal immunity because it did not reach the immune stimulation sites within the gut. In vaccine trials, N-RM25 was significantly more effective in eliminating the homologous challenge bacteria (S. Dublin wild strain FD436) from the internal organs and intestinal lumen when compared to R-NM29 and the negative control. N-RM25 prevented the development of systemic infection and produced 100% protection.

Comparative analysis of IncHI2 plasmids carrying blaCTX-M-2 or blaCTX-M-9 from Escherichia coli and Salmonella enterica strains isolated from poultry and humans

Salmonella enterica bla(CTX-M-2) and bla(CTX-M-9) plasmid backbones from isolates from Belgium and France were analyzed. The bla(CTX-M-2-)plasmids from both human and poultry isolates were related to the IncHI2 pAPEC-O1-R plasmid, previously identified in the United States in avian Escherichia coli strains; the bla(CTX-M-9) plasmids were closely related to the IncHI2 R478 plasmid.

The thiamine kinase (YcfN) enzyme plays a minor but significant role in cobinamide salvaging in Salmonella enterica

Cobinamide (Cbi) salvaging is impaired, but not abolished, in a Salmonella enterica strain lacking a functional cobU gene. CobU is a bifunctional enzyme (NTP:adenosylcobinamide [NTP:AdoCbi] kinase, GTP:adenosylcobinamide-phosphate [GTP:AdoCbi-P] guanylyltransferase) whose AdoCbi kinase activity is necessary for Cbi salvaging in this bacterium. Inactivation of the ycfN gene in a DeltacobU strain abrogated Cbi salvaging. Introduction of a plasmid carrying the ycfN(+) allele into a DeltacobU DeltaycfN strain substantially restored Cbi salvaging. Mass spectrometry data indicate that when YcfN-enriched cell extracts were incubated with AdoCbi and ATP, the product of the reaction was AdoCbi-P. Results from bioassays confirmed that YcfN converted AdoCbi to AdoCbi-P in an ATP-dependent manner. YcfN is a good example of enzymes that are used by the cell in multiple pathways to ensure the salvaging of valuable precursors.

Role of nucleoid-associated proteins Hha and H-NS in expression of Salmonella enterica activators HilD, HilC, and RtsA required for cell invasion

The coordinate expression of Salmonella enterica invasion genes on Salmonella pathogenicity island 1 is under the control of the complex circuits of regulation that involve the AraC/XylS family transcriptional activators HilD, HilC, and RtsA and nucleoid-associated proteins. Single-copy transcription fusions were used to assess the effects of nucleoid-associated proteins Hha and H-NS on hilD, hilC, and rtsA expression. The data show that all three genes, hilD, hilC, and rtsA, were repressed by H-NS and/or Hha. The repression of rtsA was the highest among tested genes. The level of rtsA-lac was equally elevated in hns and hha mutants and was further enhanced in the hns hha double mutant under low-osmolarity conditions. Electrophoretic mobility shift experiments showed that H-NS and Hha directly bind to the rtsA promoter. In addition to the negative control that was exerted by H-NS/Hha under low-osmolarity conditions, the homologous virulence activators HilD, HilC, and RtsA (Hil activators) induced rtsA-lac expression in a high-salt medium. A DNase footprinting assay of the rtsA promoter revealed one common DNA-binding site for all three Hil activators centered at position -54 relative to the transcriptional start site. In the absence of Hha and H-NS, however, osmoregulation of the rtsA promoter was lost, and Hil activators were not required for rtsA transcription. These results taken together suggest that the HilD, HilC, and RtsA proteins induce the transcription of the rtsA promoter by counteracting H-NS/Hha-mediated repression.

Tricarballylate catabolism in Salmonella enterica. The TcuB protein uses 4Fe-4S clusters and heme to transfer electrons from FADH2 in the tricarballylate dehydrogenase (TcuA) enzyme to electron acceptors in the cell membrane

Tricarballylate, a citrate analogue, is considered the causative agent of grass tetany, a ruminant disease characterized by acute magnesium deficiency. Although the normal rumen flora cannot catabolize tricarballylate, the Gram-negative enterobacterium Salmonella enterica can. An operon dedicated to tricarballylate utilization (tcuABC) present in this organism encodes all functions required for tricarballylate catabolism. Tricarballylate is converted to the cis-aconitate in a single oxidative step catalyzed by the FAD-dependent tricarballylate dehydrogenase (TcuA) enzyme. We hypothesized that the uncharacterized TcuB protein was required to reoxidize the flavin cofactor in vivo. Here, we report the initial biochemical characterization of TcuB. TcuB is associated with the cell membrane and contains two 4Fe-4S clusters and heme. Site-directed mutagenesis of cysteinyl residues putatively required as ligands of the 4Fe-4S clusters completely inactivated TcuB function. TcuB greatly increased the Vmax of the TcuA reaction from 69 +/- 2 to 8200 +/- 470 nmol min-1 mg-1; the Km of TcuA for tricarballylate was unaffected. Inhibition of TcuB activity by an inhibitor of ubiquinone oxidation, 2,5-dibromo-3-methyl-6-isoproylbenzoquinone (DBMIB), implicated the quinone pool as the ultimate acceptor of electrons from FADH2. We propose a model for the electron flow from FADH2, to the 4Fe-4S clusters, to the heme, and finally to the quinone pool.

A connector of two-component regulatory systems promotes signal amplification and persistence of expression

Organisms rely on a variety of regulatory architectures to control gene transcription. Whereas the functional characteristics of particular architectures are well understood, the properties of newly discovered regulatory designs cannot be easily predicted. One emerging design depends on small proteins that connect two-component regulatory systems, which constitute the dominant form of bacterial signal transduction. These connectors enable one system to respond to the signal perceived by a different system. To understand the functional properties of such connector-mediated architectures, we investigated the pathway controlled by the PhoP-dependent connector protein PmrD of Salmonella enterica and contrasted it to the circuit in which genes are regulated directly by the transcription factor PhoP. The PmrD-mediated pathway displayed both signal amplification and persistence of expression when compared with the direct pathway. Mathematical modeling of the two pathways allowed us to identify critical factors responsible for signal amplification.

Salmonella pathogenicity islands in host specificity, host pathogen-interactions and antibiotics resistance of Salmonella enterica

Salmonella enterica is a pathogen highly successful in causing a variety of gastrointestinal and systemic diseases in animals and humans. While some serovars of S. enterica are able to infect a broad range of host organisms, other serovars are highly restricted to specific host species. The colonization of hosts by S. enterica depends on the function of a large number of virulence determinants. The molecular analyses of virulence genes demonstrated that most of these loci are clustered within Salmonella Pathogenicity Islands (SPI). SPI1 and SPI2 each encode type III secretion systems (T355) that confer main virulence traits of S. enterica, i.e. invasion, enteropathogenesis and intracellular survival and proliferation. Further SPI encode factors that contribute to intracellular survival, different types of adhesins, or effector proteins of the SPI1-T3SS or SPI2-T3SS. The availability of genome sequences of several serovars of S. enterica also revealed serovar-specific SPI. In this review, the main characteristics of the currently known SPI are summarized with focus on their roles in various animal hosts and putative functions in human infections.

NMR assignment of reduced form of copper, zinc superoxide dismutase from Salmonella enterica

Almost complete assignment (97%) of NMR resonances was obtained for the reduced, Cu(I), form of prokaryotic CuZnSOD from Salmonella enterica. 13C direct detection was used to complement the standard bouquet of 1H detected triple resonance experiments and contributed to the identification of proline backbone resonances and to side chains assignments of Asx, Glx and aromatic rings. This is the only complete assignment available for monomer SOD from prokaryotic organisms.

Acid pH activation of the PmrA/PmrB two-component regulatory system of Salmonella enterica

Acid pH often triggers changes in gene expression. However, little is known about the identity of the gene products that sense fluctuations in extracytoplasmic pH. The Gram-negative pathogen Salmonella enterica serovar Typhimurium experiences a number of acidic environments both inside and outside animal hosts. Growth in mild acid (pH 5.8) promotes transcription of genes activated by the response regulator PmrA, but the signalling pathway(s) that mediates this response has thus far remained unexplored. Here we report that this activation requires both PmrA's cognate sensor kinase PmrB, which had been previously shown to respond to Fe³⁺ and Al³⁺, and PmrA's post-translational activator PmrD. Substitution of a conserved histidine or of either one of four conserved glutamic acid residues in the periplasmic domain of PmrB severely decreased or abolished the mild acid-promoted transcription of PmrA-activated genes. The PmrA/PmrB system controls lipopolysaccharide modifications mediating resistance to the antibiotic polymyxin B. Wild-type Salmonella grown at pH 5.8 were > 100 000-fold more resistant to polymyxin B than organisms grown at pH 7.7. Our results suggest that protonation of the PmrB periplasmic histidine and/or of the glutamic acid residues activate the PmrA protein, and that mild acid promotes cellular changes resulting in polymyxin B resistance.

The SPI-2 type III secretion system restricts motility of Salmonella-containing vacuoles

Intracellular replication of Salmonella enterica occurs in membrane-bound compartments, called Salmonella-containing vacuoles (SCVs). Following invasion of epithelial cells, most SCVs migrate to a perinuclear region and replicate in close association with the Golgi network. The association of SCVs with the Golgi is dependent on the Salmonella-pathogenicity island-2 (SPI-2) type III secretion system (T3SS) effectors SseG, SseF and SifA. However, little is known about the dynamics of SCV movement. Here, we show that in epithelial cells, 2 h were required for migration of the majority of SCVs to within 5 μm from the microtubule organizing centre (MTOC), which is located in the same subcellular region as the Golgi network. This initial SCV migration was saltatory, bidirectional and microtubule-dependent. An intact Golgi, SseG and SPI-2 T3SS were dispensable for SCV migration to the MTOC, but were essential for maintenance of SCVs in that region. Live-cell imaging between 4 and 8 h post invasion revealed that the majority of wild-type SCVs displaced less than 2 μm in 20 min from their initial starting positions. In contrast, between 6 and 8 h post invasion the majority of vacuoles containing sseG, sseF or ssaV mutant bacteria displaced more than 2 μm in 20 min from their initial starting positions, with some undergoing large and dramatic movements. Further analysis of the movement of SCVs revealed that large displacements were a result of increased SCV speed rather than a change in their directionality, and that SseG influences SCV motility by restricting vacuole speed within the MTOC/Golgi region. SseG might function by tethering SCVs to Golgi-associated molecules, or by controlling microtubule motors, for example by inhibiting kinesin recruitment or promoting dynein recruitment.

Comparison of Salmonella enterica serotype Infantis isolates from a veterinary teaching hospital

AIMS

To compare Salmonella enterica serotype Infantis isolates obtained from patients or the environment of a veterinary teaching hospital over a period of 9 years following a nosocomial outbreak to determine whether isolates were epidemiologically related or represented unrelated introductions into the hospital environment.

METHODS AND RESULTS

Fifty-six S. Infantis isolates were compared based on their phenotypic (antimicrobial drug [AMD] susceptibility pattern) and genotypic (pulsed-field gel electrophoresis [PFGE] pattern and presence of integrons) characteristics. Epidemiologically unrelated S. Infantis isolates clustered separately from all but two of the hospital isolates, and several isolates from different years and various sources were indistinguishable from each other in cluster analysis of two-enzyme PFGE results. A high percentage of isolates (80.3%) were resistant to at least one AMD, with 67.8% showing resistance to >5 AMD. The majority (74.1%) of isolates tested contained type 1 integrons.

CONCLUSION

Results strongly suggest that there was nosocomial transmission of S. Infantis during the initial outbreak, and that contamination arising from this outbreak persisted across years despite rigorous hygiene and biosecurity precautions and may have led to subsequent nosocomial infections.

SIGNIFICANCE AND IMPACT OF THE STUDY

Evidence of persistence and transmission of Salmonella clones across years, even in the face of rigorous preventive measures, has important implications for other facilities that have experienced outbreaks of Salmonella infections.

Enhanced detection rate of typhoid fever in children in a periurban slum in Karachi, Pakistan using polymerase chain reaction technology

OBJECTIVE

Yield of blood culture in clinically suspected cases of typhoid fever is low, whereas indirect serological diagnostic tests are unreliable. Hence, polymerase chain reaction (PCR)-based detection of Salmonella enterica Serovar typhi was used as an aid for diagnosis of typhoid fever in addition to other diagnostic tests. Two periurban communities in Karachi were selected for an epidemiological study of typhoid fever. The aim of the study was to assess whether PCR increased the detection rate of typhoid fever in children in the community.

MATERIAL AND METHODS

Children aged 2 to 14 years presenting with fever lasting for three or more days were selected. PCR using Hashimoto's protocol based on ViaB gene sequence was used in addition to blood culture and other serological tests.

RESULTS

Of the 214 children included in the study, blood culture was found positive for S. enterica S. typhi in 26 (12.4%) cases, whereas 24 children (11.7%) were diagnosed as suffering from typhoid fever when the PCR-based method was used. Both tests were positive in only 10 (4.9%) children. The number of children found positive for either test was 40. PCR increased the rate of detection of typhoid fever by 51%.

CONCLUSION

The sensitivity, specificity, +ve and -ve predictive values of PCR in this study were 40%, 93%, 45% and 92%, respectively.