Rapid,Sensitive Recovery of Recombinant Attenuated Salmonella enterica Serovar Typhi Vaccine Strains from Human Blood

Prior to initiating a phase 1 dose escalation trial of the safety and immunogenicity of live, oral, recombinant, attenuated Salmonella enterica serovar Typhi vaccine strains in human subjects, the suitability of conventional blood culture procedures to rapidly and reliably detect the organisms in human blood was investigated. Blood culture specimens, with and without added growth supplements, were inoculated with study organism concentrations ranging from approximately 300 to as few as 1 to 2 CFU/10 ml culture and processed in a Bactec 9240 fluorescent series aerobic blood culture system. All cultures seeded with >6 CFU and 93% of cultures seeded with ∼1 to 2 CFU were identified as positive for microbial growth within 44 h of incubation. The results were within the performance standard of ≤5 days to detection that is expected for Gram-negative cultures seeded at 10 to 50 CFU/vial. Recovery of test organisms from blood culture was not improved by the addition of supplements, but cultures with added supplements were identified positive an average of 5 h sooner than those without added supplements. Reliable detection of the investigational vaccine strains at <1 CFU/ml of blood within 2 days in conventional blood culture without added supplements allowed for shortened confinement time of study volunteers without compromising subject safety.     

Physical and Chemical Characterization and Immunologic Properties of Salmonella enterica Serovar Typhi Capsular Polysaccharide-Diphtheria Toxoid Conjugates

Typhoid fever remains a serious public health problem in developing countries, especially among young children. Recent studies showed more than 50% of typhoid cases are in children under 5 years old. Licensed vaccines, such as Salmonella enterica serovar Typhi capsular Vi, did not confer protection against typhoid fever for this age group. Vi conjugate, prepared by binding Vi to Pseudomonas aeruginosa recombinant exoprotein A (rEPA), induces protective levels of antibody at as young as 2 years old. Because of the lack of regulatory precedent for rEPA in licensing vaccines, we employed diphtheria toxoid (DT) as the carrier protein to accommodate accessibility in developing countries. Five lots of Vi-DT conjugates were prepared using adipic acid dihydrazide (ADH) as the linker. All 5 lots showed consistency in their physical and chemical characteristics and final yields. These Vi-DT conjugates elicited levels of IgG anti-Vi in young mice significantly higher than those in mice injected with Vi alone and induced a booster response upon reinjection. This booster effect was absent if the Vi replaced one of the two conjugate injections. Vi-DT was stable under repeated freeze-thaw (20 cycles). We plan to perform clinical evaluation of the safety and immunogenicity of Vi-DT when added to the infant combination vaccines.Typhoid fever, a serious systemic infection caused by Salmonella enterica serovar Typhi, remains a major public health problem in Central Asia, Southeast Asia, Africa, and Latin America (11, 52, 53). It was estimated that more than 21 million cases of typhoid fever and >200,000 deaths occurred in 2000 (10). The treatment of patients and management of asymptomatic carriers are becoming more difficult due to the worldwide emergence of multidrug-resistant (MDR) strains (2, 15, 29, 42, 43). Vaccination is considered the most promising strategy for the control of typhoid fever in developing countries (11, 19, 52, 53).Typhoid fever in children younger than 5 years old has often been unrecognized due to atypical clinical symptoms, difficulties in the number and volume of blood drawings, and use of less than optimal culture media (35, 46). Several studies have shown that the incidence of typhoid fever among children less than 5 years old is similar to that in school age children and young adults (14, 27, 34, 50, 51).The 3 licensed typhoid vaccines have limited efficacy, and none are suitable for young children under 5 years old. The use of heat-inactivated whole-cell vaccine was suspended in many countries because of its reactogenicity. The parenteral Vi polysaccharide and the live attenuated oral Ty21a vaccine were introduced in the late 1980s; both vaccines are well accepted and confer moderate protection (50 to 70%) in older children and adults. However, neither vaccine is licensed for routine immunization of infants (52).The Vi capsular polysaccharide is both an essential virulence factor and a protective antigen for S. Typhi (36, 38, 39). The concentration of serum IgG anti-Vi is correlated with immunity to the pathogen (22, 25, 26, 28, 36, 38, 49). However, Vi is not suitable for routine immunization of infants and young children because of its age-related immunogenicity and T-cell independence. As was shown for other capsular polysaccharides, such as Haemophilus influenzae type b (8, 37); meningococcus groups A, C, and W135; and Streptococcus pneumoniae (12, 20), Vi covalently bound with protein conferred T-cell dependence and increased immunogenicity (48-50). To date, diphtheria toxoid (DT), tetanus toxoid (TT), cholera toxins (CT), the B subunit of the heat-labile toxin (LT-B) of Escherichia coli, recombinant outer membrane protein of Klebsiella pneumoniae (rP40), and iron-regulated outer-membrane proteins (IROMPs) of S. Typhi have served as carriers for Vi polysaccharide in laboratory studies (16, 17, 32, 48-50; personal communications). An improved method was developed (24), utilizing adipic acid dihydrazide (ADH) as the linker and Pseudomonas aeruginosa recombinant exoprotein A (rEPA) as the carrier. Clinical trials of Vi-rEPA conjugates conferred 89% protection in Vietnamese children 2 to 5 years old for 46 months (23, 26, 28). The level of serum IgG anti-Vi induced by Vi-rEPA conjugates was correlated with prevention of typhoid fever in these studies (7, 21-23, 26, 28).One limitation of using rEPA as the carrier protein is the lack of regulatory precedent in licensing vaccines. In this report, five lots of Vi conjugates using DT manufactured by pharmaceutical companies in China and India were prepared (24, 48, 49). Modifications of conjugation procedures were made for the purposes of easy adoption and scale up by manufacturers. The stability of Vi-DT was studied for the feasibility of stockpiling in disaster relief.Another important aspect of conjugate vaccine implementation is the optimum immunization formulation and schedule using alternating injections of polysaccharide and conjugate. Priming or boosting effects of polysaccharide on its conjugate vaccine have been observed in infants injected with pneumococcal and meningococcal vaccines (3, 4, 31, 40). There was no consistent conclusion about various types of polysaccharides studied (6, 9, 31, 40, 41). Here, we compared the immune response of Vi polysaccharide injected before or after the administration of Vi-DT with the responses of those receiving 2 injections of Vi-DT. We also investigated the dosage effect for the purpose of better formulation.     

Levels of Expression and Immunogenicity of Attenuated Salmonella enterica Serovar Typhimurium Strains Expressing Escherichia coli Mutant Heat-Labile Enterotoxin

The effects of heterologous gene dosage as well as Salmonella typhimurium strain variability on immune response toward both the heterologous antigen, the nontoxic mutant of the Escherichia coli heat-labile enterotoxin LTK63, and the carrier Salmonella strain have been analyzed. Effects of a single integration into the host DNA and different-copy-number episomal vectors were compared in S. typhimurium Δcya Δcrp Δasd strains of two different serotypes, UK-1 and SR-11. Expression of the enterotoxin in the different Salmonella isolates in vitro was found to vary considerably and, for the episomal vectors, to correlate with the plasmid copy number. LTK63-specific serum immunoglobulin G (IgG) and mucosal immunoglobulin A (IgA) antibodies were highest in mice immunized with the high-level-expression strain. High anti-LTK63 IgG and IgA titers were found to correspond to higher anti-Salmonella immunity, suggesting that LTK63 exerts an adjuvant effect on response to the carrier. Statistically significant differences in anti-LTK63 immune response were observed between groups of mice immunized with the attenuated Δcya Δcrp UK-1 and SR-11 derivatives producing the antigen at the same rate. These data indicate that the same attenuation in S. typhimurium strains of different genetic backgrounds can influence significantly the immune response toward the heterologous antigen. Moreover, delivery of the LTK63 enterotoxin to the immune system by attenuated S. typhimurium strains is effective only when synthesis of the antigen is very high during the initial phase of invasion, while persistence of the S. typhimurium strain in deep tissues has only marginal influence.

Enterotoxigenic Escherichia coli strains produce a plasmid-encoded heat-labile enterotoxin (LT) (15, 34) related to cholera toxin (CT) (9, 35). LT is composed of two subunits, A and B, which are exported to the periplasmic space, where they assemble into an AB₅ multimeric complex (16). Several mutants of LT-A have been constructed, and in particular, a nontoxic mutant which contains a substitution of serine 63 with lysine (LTK63) has been shown to maintain the structural and immunogenic properties of wild-type LT (21, 27, 28). LTK63 has also been found to display the strong mucosal adjuvant activity pertaining to wild-type LT. Efficient induction of mucosal immune response, specifically in the mouse vagina, has been achieved via the intranasal route of immunization (10). For the development of oral vaccines, however, it would be desirable to exploit the properties of LTK63 for enhancing antigen-specific immune response in the intestinal mucosa by means of oral delivery of the potent mucosal adjuvant.Oral delivery of antigens by live vaccines is known to lead to a more effective production of antigen-specific antibodies in mucosal secretions than oral administration of the soluble antigen (36, 39). Several antigen delivery systems which use as carriers mutant intracellular pathogens that have lost the ability to persist and produce the disease while retaining limited growth in vivo have been developed. In particular, attenuated Salmonella mutants are suitable immunological carriers for virulence determinants from other enteric bacteria in that they can induce humoral immune response selectively at the site of colonization, the gut mucosa. Vaccine strains of Salmonella have been successfully attenuated by introducing different types of mutations (5, 8, 23, 26). Notably, Salmonella strains with a galactose epimerase (galE) mutation (18) or deletions in genes for the biosynthesis of aromatic compounds (aro mutants) (11, 12, 17, 19) or in the adenylate cyclase (cya) and cyclic AMP receptor protein (crp) genes (6) are the most extensively characterized.Delivery of the B subunit of the E. coli enterotoxin (LT-B) by a galE mutant of Salmonella typhimurium has been shown to elicit low levels of anti-LT-B serum and mucosal antibodies. Since the vector used for expression of LT-B was rapidly lost in vivo, i.e., in the absence of the antibiotic required for selection of the plasmid, the level of immune response could be correlated only with the amount of antigen expressed during the initial phase of invasion (3).Recently, direct comparison between the aroA aroD/pnirB and the Δcya Δcrp Δasd/asd⁺ delivery systems for the ability to induce humoral and cellular immunity after a single immunization showed that the former vaccine strain had greater potential as a carrier for antigen delivery (20). However, the balanced lethal asd system for in vivo selection of plasmids expressing heterologous antigens in the attenuated Δcya Δcrp Δasd strains is still very attractive in that asd⁺ plasmids do not require antibiotic resistance markers for selection while stably maintained in vivo (24). In addition, the Δcya Δcrp Δasd/asd⁺ delivery system has been reported to induce protective immunity against several pathogens (25, 29, 40). Most of these studies have restricted analysis of the immune response to antigens expressed from the same asd⁺ plasmid carried by Δcya Δcrp Δasd mutants usually of the same S. typhimurium serotype. In this work, we have analyzed the influence of heterologous gene dosage, and thus level of expression, as well as S. typhimurium strain variability on immune response toward both the heterologous antigen, a nontoxic mutant of E. coli LT, and the carrier Salmonella strain. Effects of a single integration into the host DNA and episomal vectors at different copy numbers were compared in S. typhimurium strains of two different Δcya Δcrp Δasd serotypes, UK-1 and SR-11.     

Attenuation and Immunogenicity of Δcya Δcrp Derivatives of Salmonella choleraesuis in Pigs

Six different isogenic Deltacya Deltacrp derivatives of a strain of Salmonella choleraesuis var. kunzendorf-chi3246 virulent for pigs were constructed by transposon-mediated deletion mutagenesis. These strains were evaluated for virulence and ability to elicit a protective immune response in young weaned pigs after oral administration and were compared to a commercially available vaccine which lacks the 50-kb virulence plasmid (vpl(-)). These derivatives were Deltacya Deltacrp vpl(+), Deltacya Deltacrp vpl(-), Deltacya Delta(crp-cdt) vpl(+), Deltacya Delta(crp-cdt) vpl(-), Deltacya Deltacrp pmi-3834 vpl(+), and Deltacya Delta(crp-cdt) pmi-3834. In experiments to evaluate safety, no significant adverse effects of any of the vaccine constructs were observed, except that two of the strains which carried the virulence plasmid (vpl(+)) caused a small, short-term elevation in maximum temperature compared to pretreatment temperature values. Orally immunized animals, except for those vaccinated with the Deltacya Deltacrp pmi-3834 vpl(+) strain or SC-54, developed significant serum antibody responses 21 days postvaccination as measured by enzyme-linked immunosorbent assay. No cell-mediated immune responses to heat-killed S. choleraesuis were noted at the same time point as measured with heat-killed bacteria as antigen in a lymphocyte proliferation assay. In an oral challenge exposure model with a highly virulent heterologous strain of S. choleraesuis, the Deltacya Deltacrp strains with deletions in pmi were not protective. As measured by morbidity scores, the responses to challenge of the pigs vaccinated with the other four Deltacya Deltacrp derivatives were significantly better than those of the nonvaccinated, challenged group. With the exception of temperature elevation and slight differences in diarrhea scores postchallenge, none of these strains differed significantly from each other in the other clinical parameters analyzed. While the commercial vaccine was protective by most of the parameters measured, it was not fully protective against challenge with virulent S. choleraesuis as judged by diarrhea scores and temperature elevation. Collectively, these data demonstrate that Deltacya Deltacrp derivatives, with or without the virulence plasmid but not with deletions in the pmi gene, are candidates for vaccines for protection against salmonellosis in pigs.     

Intermediate Susceptibility to Ciprofloxacin among Salmonella enterica Serovar Typhi Isolates in Lima,Peru

Thirty-three Salmonella enterica serovar Typhi blood isolates from Lima, Peru (2008 to 2012), were fully susceptible to trimethoprim-sulfamethoxazole, chloramphenicol, ceftriaxone, and tetracycline; 8/33 (24.2%) showed intermediate susceptibility to ciprofloxacin carrying mutations in the quinolone resistance-determining region of the gyrA gene (Ser83-Phe and Asp87-Asn) and in the gyrB gene (Ser464-Phe).     

Construction of a Stable Attenuated Shigella sonnei ΔvirG Vaccine Strain, WRSS1, and Protective Efficacy and Immunogenicity in the Guinea Pig Keratoconjunctivitis Model

Construction of a stable Shigella sonnei vaccine has been complicated by the instability of the virulence phenotype caused by the spontaneous loss of the invasion plasmid. To select a suitable candidate for vaccine construction, 16 S. sonnei strains were screened for stability of the virulence phenotype. A stable strain, S. sonnei Mosely, was selected for further work. pΔvirG2, a deletion derivative of the virG gene in the sacB suicide vector pCVD442, was used to generate an S. sonnei virG deletion strain, WRSS1, which was invasive in HeLa cells but negative in the Sereny test. WRSS1 was found to be both immunogenic and protective in the guinea pig keratoconjunctivitis model.     

Kinetics of the Natural,Humoral Immune Response to Salmonella enterica Serovar Typhi in Kathmandu,Nepal

Typhoid fever is a major public health problem in developing countries, conservatively estimated to occur in 17 million cases and be responsible for 200,000 deaths annually. We investigated the acquisition of natural immunity to Salmonella enterica serovar Typhi in a region where typhoid is endemic by testing sera from an age-stratified sample of 210 healthy participants in Kathmandu, Nepal, for bactericidal activity toward S. Typhi and for anti-Vi capsular polysaccharide antibodies. Bactericidal titers in children were significantly lower than those in newborns and adults (P < 0.0001). Anti-S. Typhi bactericidal geometric mean titers were age dependent, increasing 10-fold during childhood. Anti-Vi polysaccharide antibody geometric mean concentrations were also lower in children than in adults. Data presented here indicate the possibility of a relationship between low levels of bactericidal activity toward S. Typhi in serum and susceptibility to disease, as observed for other polysaccharide-encapsulated bacteria. Bactericidal antibody may be a marker of protective immunity against S. Typhi.Salmonella enterica subspecies enterica serotype Typhi is responsible for 17 million new cases of typhoid fever and 200,000 deaths annually (5). Since typhoid fever is a food- and waterborne disease transmitted by the fecal-oral route, it is endemic in regions where the quality of the drinking water supply is poor and sewage disposal facilities are inadequate, with annual incidence rates above 100 per 100,000 person years (2). Southern Asia carries a heavy burden of disease, with S. Typhi causing almost 40% of culture-positive bloodstream infections in Nepal (26). Although typhoid was thought to be a disease of school children and young adults, there is increasing evidence of a substantial disease burden in younger children in countries where typhoid is endemic (3, 24, 27).Three vaccines for the prevention of typhoid fever exist. The whole-cell inactivated vaccine is unpopular due to high rates of adverse reactions (19). A live attenuated oral vaccine (Ty21a) and a parenteral plain polysaccharide vaccine have comparable efficacies ranging between 50 and 70% and are suitable for community vaccination programs in areas of hyperendemicity (8, 9). A polysaccharide-protein conjugate vaccine has shown promising results in clinical trials and potentially could be used for children under 2 years of age, for whom the former two vaccines are either ineffective or unlicensed (17, 21). Similarly, a novel, drinkable, single-dose attenuated vaccine has demonstrated promising immunogenicity and tolerability in phase II trials in Vietnam (31). Despite the availability of two moderately efficacious vaccines and the possibility of another two, no country has implemented a systematic typhoid vaccination strategy. This is due partly to the limited data on disease burden and age-specific immunity, which are critical in determining the timing of and need for vaccination (6). Today, typhoid immunization is limited to travelers visiting regions of endemicity or laboratory workers potentially exposed to the pathogen.We investigated anti-S. Typhi bactericidal antibody titers and the concentrations of antibody against the S. Typhi Vi polysaccharide (ViPS) in a cross-sectional study of a population in Kathmandu, Nepal, in order to relate antibody levels to both age and reported disease rates.     

Characterization of Extended-Spectrum β-Lactamase-Producing Salmonella typhimurium by Phenotypic and Genotypic Typing Methods

During 1994, 10 isolates of extended-spectrum beta-lactamase-producing Salmonella typhimurium were recovered from children transferred to our hospital from two different centers. Two additional isolates were recovered from two nurses from one of these centers. The aim of this study was to determine if there is any relationship between these isolates. The characterization was done by phenotypic and genotypic methods: biotyping, phage typing, antibiotic susceptibility pattern determination, plasmid analysis, ribotyping (by the four endonucleases EcoRI, SmaI, BglII, and PvuII), pulsed-field gel electrophoresis (PFGE) of genome macrorestriction patterns with XbaI, and randomly amplified polymorphic DNA (RAPD) pattern determination (with the three primers 217 d2, B1, and A3). The same biotype, the same serotype, and an identical antibiotype were found. All isolates were resistant to oxyimino-beta-lactams, gentamicin, tobramycin, and sulfamethoxazole-trimethoprim. All isolates showed an indistinguishable pattern by ribotyping and very similar patterns by PFGE and RAPD. The overall results indicated the spread of a closely related strain of S. typhimurium in children and nurses.     

Nitric Oxide and Apoptosis Induced in Peyer's Patches by Attenuated Strains of Salmonella enterica Serovar Enteritidis

Nitric oxide (NO) is a toxic molecule of the immune system which contributes to the control of microbial pathogens. Additional functions of NO in innate and adaptive immunity have recently been described; these functions include the modulation of the cytokine response of lymphocytes and the regulation of immune cell apoptosis. In addition to direct microbicidal actions, NO has immunoregulatory effects relevant to the control of infections. In turn, infected macrophages and macrophage-regulating lymphocytes may undergo apoptosis during infection by Salmonella spp. In this work we investigated the ability of attenuated strains of Salmonella enterica serovar Enteritidis with different protective capacities to induce intestinal inducible nitric oxide synthase (iNOS) and apoptosis in Peyer's patches (PP) in mice. Results showed that the intestinal iNOS activity correlated with increased apoptosis in PP. Furthermore, the ability to induce intestinal NO production and apoptosis within the first few hours after immunization seemed to correlate with the protective capacity of mutant E/1/3 of S. enterica serovar Enteritidis. It was found that nonprotective mutant C/2/2, which was unable to induce intestinal NO production, also failed to induce apoptosis in PP. Moreover, aminoguanidine treatment at the time of immunization resulted in inhibition of the NO production and apoptosis induced by protective mutant E/1/3 and completely abolished protection against challenge. These results suggest that the induction of iNOS in the intestinal mucosa by attenuated mutant E/1/3 of S. enterica serovar Enteritidis at the time of immunization is necessary to generate a protective immune response.     

Evaluation of Regulated Delayed Attenuation Strategies for Salmonella enterica Serovar Typhi Vaccine Vectors in Neonatal and Infant Mice

We developed regulated delayed attenuation strategies for Salmonella vaccine vectors. In this study, we evaluated the combination of these strategies in recombinant attenuated Salmonella enterica serovar Typhi and Salmonella enterica serovar Typhimurium vaccine vectors with similar genetic backgrounds in vitro and in vivo. Our goal is to develop a vaccine to prevent Streptococcus pneumoniae infection in newborns; thus, all strains delivered a pneumococcal antigen PspA and the impact of maternal antibodies was evaluated. The results showed that all strains with the regulated delayed attenuated phenotype (RDAP) displayed an invasive ability stronger than that of the S. Typhi vaccine strain, Ty21a, but weaker than that of their corresponding wild-type parental strains. The survival curves of different RDAP vaccine vectors in vitro and in vivo exhibited diverse regulated delayed attenuation kinetics, which was different from S. Typhi Ty21a and the wild-type parental strains. Under the influence of maternal antibody, the persistence of the S. Typhimurium RDAP strain displayed a regulated delayed attenuation trend in nasal lymphoid tissue (NALT), lung, and Peyer''s patches, while the persistence of S. Typhi RDAP strains followed the curve only in NALT. The bacterial loads of S. Typhi RDAP strains were lower in NALT, lung, and Peyer''s patches in mice born to immune mothers than in those born to naive mothers. In accordance with these results, RDAP vaccine strains induced high titers of IgG antibodies against PspA and against Salmonella lipopolysaccharides. Immunization of mothers with S. Typhi RDAP strains enhanced the level of vaginal mucosal IgA, gamma interferon (IFN-γ), and interleukin 4 (IL-4) and resulted in a higher level of protection against S. pneumoniae challenge.     

Emergence and Characterization of Salmonella enterica Serovar Typhimurium Phage Type DT191a

The emergence of a previously undefined phage type of Salmonella enterica serovar Typhimurium, designated DT191a, occurred in England and Wales in July 2008. The new strain exhibits a number of distinctive phenotypic and genotypic features. This report provides the tools necessary to track S. Typhimurium DT191a globally.Salmonella enterica serovar Typhimurium is the second most prevalent serovar isolated in Europe, exceeded only by Salmonella enterica serovar Enteritidis (6). Provisional data for 2008 to July 2009 from the Health Protection Agency Salmonella data set show that S. Typhimurium accounted for 2,690 (18.8%) of Salmonella infections from humans in England and Wales. Subtyping of S. Typhimurium by phage typing is well established and not only plays a vital role in detecting outbreaks (11) but also enables the appearance of new phage types or phage types with new characteristics to be monitored.We report here on the emergence, in mid-2008, in England and Wales of a previously undefined phage type of S. Typhimurium and on the further characterization of this phage type by pulsed-field gel electrophoresis (PFGE), variable-number tandem repeat (VNTR) typing, and multilocus sequence typing (MLST). This strain was initially identified in our laboratory, as it did not conform to any of the recognized patterns in the current schemes as described by Callow (4) and Anderson et al. (1). Its unique pattern of phage lysis was subsequently defined as DT191a. The full phage reaction pattern with the panel of 32 typing phages is shown in Table ​Table1,1, where the only significant difference is in the reaction for phage 18. To date, over 230 cases of DT191a have been typed by the Laboratory of Gastrointestinal Pathogens (LGP), Health Protection Agency (HPA). In England and Wales, DT191a is currently the most common phage type of S. Typhimurium obtained from cases of human infection. The increased isolation rate of this phage type is compared with the level for DT104 in Fig. ​Fig.11.Open in a separate windowFIG. 1.Relative proportions of Salmonella enterica serovar Typhimurium DT104 and DT191a typed at the Salmonella Reference Laboratory, United Kingdom, between 2008 and 2009.

TABLE 1.

Phage reactions for the new phage type S. Typhimurium DT191a

Molecular and Cellular Characterization of a Salmonella enterica Serovar Paratyphi A Outbreak Strain and the Human Immune Response to Infection

Enteric fever is an invasive life-threatening systemic disease caused by the Salmonella enterica human-adapted serovars Typhi and Paratyphi. Increasing incidence of infections with Salmonella enterica serovar Paratyphi A and the spreading of its antibiotic-resistant derivates pose a significant health concern in some areas of the world. Herein, we describe a molecular and phenotypic characterization of an S. Paratyphi A strain accounted for a recent paratyphoid outbreak in Nepal that affected at least 37 travelers. Pulsed-field gel electrophoresis analysis of the outbreak isolates revealed one genetic clone (pulsotype), confirming a single infecting source. Genetic profiling of the outbreak strain demonstrated the contribution of specific bacteriophages as a prime source of genetic diversity among clinical isolates of S. Paratyphi A. Phenotypic characterization in comparison with the S. Paratyphi A ATCC 9150 reference sequenced strain showed differences in flagellar morphology and increased abilities of the outbreak strain with respect to its motility, invasion into nonphagocytic cells, intracellular multiplication, survival within macrophages, and higher induction of interleukin-8 (IL-8) secreted by host cells. Collectively, these differences suggest an enhanced virulence potential of this strain and demonstrate an interesting phenotypic variation among S. Paratyphi A isolates. In vivo profiling of 16 inflammatory cytokines in patients infected with the outbreak strain revealed a common profile of a remarkable gamma interferon (IFN-γ) induction together with elevated concentrations of tumor necrosis factor alpha (TNF-α), IL-6, IL-8, IL-10, and IL-15, but not IL-12, which was previously demonstrated as elevated in nontyphoidal Salmonella infections. This apparent profile implies a distinct immune response to paratyphoid infections.     

An Inducible and Secreted Eukaryote-Like Serine/Threonine Kinase of Salmonella enterica Serovar Typhi Promotes Intracellular Survival and Pathogenesis

Eukaryote-like serine/threonine kinases (eSTKs) constitute an important family of bacterial virulence factors. Genome analysis had predicted putative eSTKs in Salmonella enterica serovar Typhi, although their functional characterization and the elucidation of their role in pathogenesis are still awaited. We show here that the primary sequence and secondary structure of the t4519 locus of Salmonella Typhi Ty2 have all the signatures of eukaryotic superfamily kinases. t4519 encodes a ∼39-kDa protein (T4519), which shows serine/threonine kinase activities in vitro. Recombinant T4519 (rT4519) is autophosphorylated and phosphorylates the universal substrate myelin basic protein. Infection of macrophages results in decreased viability of the mutant (Ty2Δt4519) strain, which is reversed by gene complementation. Moreover, reactive oxygen species produced by the macrophages signal to the bacteria to induce T4519, which is translocated to the host cell cytoplasm. That T4519 may target a host substrate(s) is further supported by the activation of host cellular signaling pathways and the induction of cytokines/chemokines. Finally, the role of T4519 in the pathogenesis of Salmonella Typhi is underscored by the significantly decreased mortality of mice infected with the Ty2Δt4519 strain and the fact that the competitive index of this strain for causing systemic infection is 0.25% that of the wild-type strain. This study characterizes the first eSTK of Salmonella Typhi and demonstrates its role in promoting phagosomal survival of the bacteria within macrophages, which is a key determinant of pathogenesis. This, to the best of our knowledge, is the first study to describe the essential role of eSTKs in the in vivo pathogenesis of Salmonella spp.     

Evaluation of a Simple Blood Culture Amplification and Antigen Detection Method for Diagnosis of Salmonella enterica Serovar Typhi Bacteremia

In most areas where typhoid is endemic, laboratory diagnosis is not possible due to the lack of appropriate facilities. We investigated whether the combination of blood culture amplification of Salmonella enterica serovar Typhi with an S. Typhi antigen rapid diagnostic test (RDT) could be an accurate and inexpensive tool for the accelerated diagnosis of patients with acute typhoid in Laos. For a panel of 23 Gram-negative reference pathogens, the Standard Diagnostics (catalog no. 15FK20; Kyonggi-do, South Korea) RDT gave positive results for S. Typhi NCTC 8385, S. Typhi NCTC 786 (Vi negative), Salmonella enterica serovar Enteritidis (ATCC 13076), and Salmonella enterica serovar Ndolo NCTC 8700 (all group D). In a prospective study of 6,456 blood culture bottles from 3,028 patients over 15 months, 392 blood culture bottles (6.1%) from 221 (7.3%) patients had Gram-negative rods (GNRs) seen in the blood culture fluid. The sensitivity, negative predictive value, specificity, and positive predictive value were 96.7%, 99.5%, 97.9%, and 87.9%, respectively, for patients with proven S. Typhi bacteremia and 91.2%, 98.4%, 98.9%, and 93.9% for patients with group D Salmonella. The median (range) number of days between diagnosis by RDT and reference assays was 1 (−1 to +2) day for those with confirmed S. Typhi. The use of antigen-based pathogen detection in blood culture fluid may be a useful, relatively rapid, inexpensive, and accurate technique for the identification of important causes of bacteremia in the tropics.     

Molecular Typing of Multiple-Antibiotic-Resistant Salmonella enterica Serovar Typhi from Vietnam: Application to Acute and Relapse Cases of Typhoid Fever

The rate of multiple-antibiotic resistance is increasing among Salmonella enterica serovar Typhi strains in Southeast Asia. Pulsed-field gel electrophoresis (PFGE) and other typing methods were used to analyze drug-resistant and -susceptible organisms isolated from patients with typhoid fever in several districts in southern Vietnam. Multiple PFGE and phage typing patterns were detected, although individual patients were infected with strains of a single type. The PFGE patterns were stable when the S. enterica serovar Typhi strains were passaged many times in vitro on laboratory medium. Paired S. enterica serovar Typhi isolates recovered from the blood and bone marrow of individual patients exhibited similar PFGE patterns. Typing of S. enterica serovar Typhi isolates from patients with relapses of typhoid indicated that the majority of relapses were caused by the same S. enterica serovar Typhi strain that was isolated during the initial infection. However, some individuals were infected with distinct and presumably newly acquired S. enterica serovar Typhi isolates.     

Genomic Signature of Multidrug-Resistant Salmonella enterica Serovar Typhi Isolates Related to a Massive Outbreak in Zambia between 2010 and 2012

Retrospectively, we investigated the epidemiology of a massive Salmonella enterica serovar Typhi outbreak in Zambia during 2010 to 2012. Ninety-four isolates were susceptibility tested by MIC determinations. Whole-genome sequence typing (WGST) of 33 isolates and bioinformatic analysis identified the multilocus sequence type (MLST), haplotype, plasmid replicon, antimicrobial resistance genes, and genetic relatedness by single nucleotide polymorphism (SNP) analysis and genomic deletions. The outbreak affected 2,040 patients, with a fatality rate of 0.5%. Most (83.0%) isolates were multidrug resistant (MDR). The isolates belonged to MLST ST1 and a new variant of the haplotype, H58B. Most isolates contained a chromosomally translocated region containing seven antimicrobial resistance genes, catA1, bla_TEM-1, dfrA7, sul1, sul2, strA, and strB, and fragments of the incompatibility group Q1 (IncQ1) plasmid replicon, the class 1 integron, and the mer operon. The genomic analysis revealed 415 SNP differences overall and 35 deletions among 33 of the isolates subjected to whole-genome sequencing. In comparison with other genomes of H58, the Zambian isolates separated from genomes from Central Africa and India by 34 and 52 SNPs, respectively. The phylogenetic analysis indicates that 32 of the 33 isolates sequenced belonged to a tight clonal group distinct from other H58 genomes included in the study. The small numbers of SNPs identified within this group are consistent with the short-term transmission that can be expected over a period of 2 years. The phylogenetic analysis and deletions suggest that a single MDR clone was responsible for the outbreak, during which occasional other S. Typhi lineages, including sensitive ones, continued to cocirculate. The common view is that the emerging global S. Typhi haplotype, H58B, containing the MDR IncHI1 plasmid is responsible for the majority of typhoid infections in Asia and sub-Saharan Africa; we found that a new variant of the haplotype harboring a chromosomally translocated region containing the MDR islands of IncHI1 plasmid has emerged in Zambia. This could change the perception of the term “classical MDR typhoid” currently being solely associated with the IncHI1 plasmid. It might be more common than presently thought that S. Typhi haplotype H58B harbors the IncHI1 plasmid or a chromosomally translocated MDR region or both.     

Genetic Analysis of Non-Hydrogen Sulfide-Producing Salmonella enterica Serovar Typhimurium and S. enterica Serovar Infantis Isolates in Japan

Whole-genome sequencing of non-H₂S-producing Salmonella enterica serovar Typhimurium and S. enterica serovar Infantis isolates from poultry meat revealed a nonsense mutation in the phsA thiosulfate reductase gene and carriage of a CMY-2 β-lactamase. The lack of production of H₂S might lead to the incorrect identification of S. enterica isolates carrying antimicrobial resistance genes.