Invasive African nontyphoidal Salmonella requires high levels of complement for cell-free antibody-dependent killing

Vaccine value profile for invasive non-typhoidal Salmonella disease

Invasive non-typhoidal Salmonella (iNTS) disease is an under-recognized high-burden disease causing major health and socioeconomic issues in sub-Saharan Africa (sSA), predominantly among immune-naïve infants and young children, including those with recognized comorbidities such as HIV infection. iNTS disease is primarily caused by Salmonella enterica serovar Typhimurium sequence type (ST) 313 and 'African-restricted clades' of Salmonella Enteritidis ST11 that have emerged across the African continent as a series of epidemics associated with acquisition of new antimicrobial resistance. Due to genotypes with a high prevalence of antimicrobial resistance and scarcity of therapeutic options, these NTS serovars are designated by the World Health Organization as a priority pathogen for research and development of interventions, including vaccines, to address and reduce NTS associated bacteremia and meningitis in sSA. Novel and traditional vaccine technologies are being applied to develop vaccines against iNTS disease, and the results of the first clinical trials in the infant target population should become available in the near future. The "Vaccine Value Profile" (VVP) addresses information related predominantly to invasive disease caused by Salmonella Enteritidis and Salmonella Typhimurium prevalent in sSA. Information is included on stand-alone iNTS disease candidate vaccines and candidate vaccines targeting iNTS disease combined with another invasive serotype, Salmonella Typhi, that is also common across sSA. Out of scope for the first version of this VVP is a wider discussion on either diarrheagenic NTS disease (dNTS) also associated with Salmonella Enteritidis and Salmonella Typhimurium or the development of a multivalent Salmonella vaccines targeting key serovars for use globally. This VVP for vaccines to prevent iNTS disease is intended to provide a high-level, holistic assessment of the information and data that are currently available to inform the potential public health, economic, and societal value of pipeline vaccines and vaccine-like products. Future versions of this VVP will be updated to reflect ongoing activities such as vaccine development strategies and a "Full Vaccine Value Assessment" that will inform the value proposition of an iNTS disease vaccine. This VVP was developed by a working group of subject matter experts from academia, non-profit organizations, public private partnerships, and multi-lateral organizations, and in collaboration with stakeholders from the World Health Organization African Region. All contributors have extensive expertise on various elements of the iNTS disease VVP and collectively aimed to identify current research and knowledge gaps. The VVP was developed using only existing and publicly available information.

The Prolonged Treatment of Salmonella enterica Strains with Human Serum Effects in Phenotype Related to Virulence

Salmonella enterica as common pathogens of humans and animals are good model organisms to conduct research on bacterial biology. Because these bacteria can multiply in both the external environments and in the living hosts, they prove their wide adaptability. It has been previously demonstrated that prolonged exposition of Salmonella serotype O48 cells to normal human serum led to an increase in resistance to sera in connection with the synthesis of very long O-antigen. In this work, we have studied the phenotype connected to virulence of Salmonella enterica strains that were subjected to consecutive passages in 50% human serum from platelet-poor plasma (SPPP). We found that eight passages in SPPP may not be enough for the bacteria to become serum-resistant (S. Typhimurium ATCC 14028, S. Senftenberg). Moreover, C1q and C3c complement components bound to Salmonellae (S. Typhimurium ATCC 14028, S. Hammonia) membrane proteins, which composition has been changed after passaging in sera. Interestingly, passages in SPPP generated genetic changes within gene fljB, which translated to cells’ motility (S. Typhimurium ATCC 14028, S. Erlangen). One strain, S. Hammonia exposed to a serum developed a multi-drug resistance (MDR) phenotype and two S. Isaszeg and S. Erlangen tolerance to disinfectants containing quaternary ammonium salts (QAS). Furthermore, colonial morphotypes of the serum adaptants were similar to those produced by starter cultures. These observations suggest that overcoming stressful conditions is manifested on many levels. Despite great phenotypic diversity occurring after prolonged exposition to SPPP, morphotypes of colonies remained unchanged in basic media. This work is an example in which stable morphotypes distinguished by altered virulence can be confusing during laboratory work with life-threatening strains.

Complementary measurement of nontyphoidal Salmonella-specific IgG and IgA antibodies in oral fluid and serum

Objectives

Immuno-epidemiological studies of orally acquired, enteric pathogens such as nontyphoidal Salmonella (NTS) often focus on serological measures of immunity, ignoring potentially relevant oral mucosal responses. In this study we sought to assess the levels and detectability of both oral fluid and serum IgG and IgA to NTS antigens, in endemic and non-endemic populations.

Methods

IgG and IgA antibodies specific for Salmonella Typhimurium and Salmonella Enteritidis O antigen and phase 1 flagellin were assessed using Enzyme Linked Immunosorbent Assay (ELISA). Paired oral fluid and serum samples were collected from groups of 50 UK adults, Kenyan adults and Kenyan infants. Additionally, oral fluid alone was collected from 304 Kenyan individuals across a range of ages.

Results

Antigen-specific IgG and IgA was detectable in the oral fluid of both adults and infants. Oral fluid antibody increased with age, peaking in adulthood for both IgG and IgA but a separate peak was also observed for IgA in infants. Oral fluid and serum responses correlated for IgG but not IgA. Despite standardised collection the relationship between oral fluid volume and antibody levels varied with age and country of origin.

Conclusions

Measurement of NTS-specific oral fluid antibody can be used to complement measurement of serum antibody. For IgA in particular, oral fluid may offer insights into how protective immunity to NTS changes as individuals transition with age, from maternal to acquired systemic and mucosal immunity. This may prove useful in helping to guide future vaccine design.

An adsorption method to prepare specific antibody-depleted normal human serum as a source of complement for human serum bactericidal assays for Salmonella

Serum bactericidal assays (SBA) are valuable for assessing the functional activity of natural and vaccine-induced antibodies against many Gram-negative bacteria, such as meningococcus and Salmonella. However, SBA often require an exogenous source of complement and the presence of pre-existing naturally acquired antibodies limits the use of human complement for this purpose. To remove pre-existing Salmonella-specific antibodies, in the context of SBA for Salmonella vaccine research, we incubated human sera with preparations of Salmonella. By incubating at 4 °C, pre-existing antibodies were adsorbed onto the Salmonella bacteria with only minimal complement deposition. We assessed the effects of adsorption on specific antibody levels, complement activity and the bactericidal activity of sera using flow cytometry, SBA and haemolytic assays. Adsorption removed Salmonella-specific antibodies and bactericidal activity against Salmonella from whole serum but was not detrimental to serum complement activity, even after five adsorption cycles. Bactericidal activity could be reconstituted in the adsorbed serum by the addition of exogenous specific antibodies. Sera preadsorbed with Salmonella are suitable as a source of human complement to measure the bactericidal activity of Salmonella antibodies. The adsorption method can be used to deplete, simply and rapidly, specific antibodies from serum to prepare a source of human complement for use in SBA for vaccine research and assessment.

The Role of Maternally Acquired Antibody in Providing Protective Immunity Against Nontyphoidal Salmonella in Urban Vietnamese Infants: A Birth Cohort Study

Background

Nontyphoidal Salmonella (NTS) organisms are a major cause of gastroenteritis and bacteremia, but little is known about maternally acquired immunity and natural exposure in infant populations residing in areas where NTS disease is highly endemic.

Methods

We recruited 503 pregnant mothers and their infants (following delivery) from urban areas in Vietnam and followed infants until they were 1 year old. Exposure to the dominant NTS serovars, Salmonella enterica serovars Typhimurium and Enteritidis, were assessed using lipopolysaccharide (LPS) O antigen–specific antibodies. Antibody dynamics, the role of maternally acquired antibodies, and NTS seroincidence rates were modeled using multivariate linear risk factor models and generalized additive mixed-effect models.

Results

Transplacental transfer of NTS LPS–specific maternal antibodies to infants was highly efficient. Waning of transplacentally acquired NTS LPS–specific antibodies at 4 months of age left infants susceptible to Salmonella organisms, after which they began to seroconvert. High seroincidences of S. Typhimurium and S. Enteritidis LPS were observed, and infants born with higher anti-LPS titers had greater plasma bactericidal activity and longer protection from seroconversion.

Conclusions

Although Vietnamese infants have extensive exposure to NTS, maternally acquired antibodies appear to play a protective role against NTS infections during early infancy. These findings suggest that prenatal immunization may be an appropriate strategy to protect vulnerable infants from NTS disease.

Rapid transcriptional responses to serum exposure are associated with sensitivity and resistance to antibody-mediated complement killing in invasive Salmonella Typhimurium ST313

Background: Salmonella Typhimurium ST313 exhibits signatures of adaptation to invasive human infection, including higher resistance to humoral immune responses than gastrointestinal isolates. Full resistance to antibody-mediated complement killing (serum resistance) among nontyphoidal Salmonellae is uncommon, but selection of highly resistant strains could compromise vaccine-induced antibody immunity. Here, we address the hypothesis that serum resistance is due to a distinct genotype or transcriptome response in S. Typhimurium ST313.

Methods: Six S. Typhimurium ST313 bloodstream isolates, three of which were antibody resistant, were studied. Genomic content (single nucleotide polymorphisms and larger chromosomal modifications) of the strains was determined by Illumina and PACBIO sequencing, and functionally characterized using RNA-seq, transposon directed insertion site sequencing (TraDIS), targeted gene deletion and transfer of selected point mutations in an attempt to identify features associated with serum resistance.

  Results: Sequence polymorphisms in genes from strains with atypical serum susceptibility when transferred from strains that were highly resistant or susceptible to a strain that exhibited intermediate susceptibility did not significantly alter serum killing phenotype. No large chromosomal modifications typified serum resistance or susceptibility. Genes required for resistance to serum identified by TraDIS and RNA-seq included those involved in exopolysaccharide synthesis, iron scavenging and metabolism. Most of the down-regulated genes were associated with membrane proteins. Resistant and susceptible strains had distinct transcriptional responses to serum, particularly related to genes responsible for polysaccharide biosynthesis. There was higher upregulation of wca locus genes, involved in the biosynthesis of colanic acid exopolysaccharide, in susceptible strains and increased expression of fepE, a regulator of very long-chain lipopolysaccharide in resistant strains.

Conclusion: Clinical isolates of S. Typhimurium ST313 exhibit distinct antibody susceptibility phenotypes that may be associated with changes in gene expression on exposure to serum.

Adding function to the genome of African Salmonella Typhimurium ST313 strain D23580

Salmonella Typhimurium sequence type (ST) 313 causes invasive nontyphoidal Salmonella (iNTS) disease in sub-Saharan Africa, targeting susceptible HIV+, malarial, or malnourished individuals. An in-depth genomic comparison between the ST313 isolate D23580 and the well-characterized ST19 isolate 4/74 that causes gastroenteritis across the globe revealed extensive synteny. To understand how the 856 nucleotide variations generated phenotypic differences, we devised a large-scale experimental approach that involved the global gene expression analysis of strains D23580 and 4/74 grown in 16 infection-relevant growth conditions. Comparison of transcriptional patterns identified virulence and metabolic genes that were differentially expressed between D23580 versus 4/74, many of which were validated by proteomics. We also uncovered the S. Typhimurium D23580 and 4/74 genes that showed expression differences during infection of murine macrophages. Our comparative transcriptomic data are presented in a new enhanced version of the Salmonella expression compendium, SalComD23580: http://bioinf.gen.tcd.ie/cgi-bin/salcom_v2.pl. We discovered that the ablation of melibiose utilization was caused by three independent SNP mutations in D23580 that are shared across ST313 lineage 2, suggesting that the ability to catabolize this carbon source has been negatively selected during ST313 evolution. The data revealed a novel, to our knowledge, plasmid maintenance system involving a plasmid-encoded CysS cysteinyl-tRNA synthetase, highlighting the power of large-scale comparative multicondition analyses to pinpoint key phenotypic differences between bacterial pathovariants.

Age-related variations in the in vitro bactericidal activity of human sera against Pseudomonas aeruginosa

The human serum is a vital component of the innate immunity of the host that acts as the first line of defence against invading pathogens. A key player in serum-mediated innate immune defence is a system of more than 35 proteins, collectively named as the complement system. After exposure of the pathogen, these proteins are activated in a cascade manner, ultimately forming a membrane attack complex (MAC) on the surface of the pathogen that directly lyses the bacterial cell. Formation of the MAC can be demonstrated in vitro by using serum bactericidal assay (SBA) that works in the absence of cellular components of blood after incubating the serum along with bacteria. Here, we describe the age-related differences in the bactericidal activity of human serum against Pseudomonas aeruginosa, an opportunistic human pathogen causing an array of hospital and community-acquired infections.

We demonstrate that adult sera were highly effective in the in vitro killing of Pseudomonas aeruginosa as compared to children and the elderly (p < 0.0001). Sera from children were seriously compromised in the killing P. aeruginosa, whereas elderly sera showed a reduced level of killing. Data revealed a positive correlation between age and serum-killing with higher coefficient of determination values of 0.34, 0.27, and 0.58 and p values of < 0.0001, < 0.001, and < 0.0001, respectively, after 60, 90, and 120 minutes of incubation. Hence, our study highlights the age-related difference in the bactericidal activity of human sera. We conclude that sera of children are totally compromised, whereas elderly sera are only partially compromised, in the killing of P. aeruginosa.

Role of a single noncoding nucleotide in the evolution of an epidemic African clade of Salmonella

Invasive nontyphoidal Salmonella disease is a major and previously neglected tropical disease responsible for an estimated ∼390,000 deaths per year in Africa, largely caused by a variant of Salmonella Typhimurium called ST313. Despite the availability of >100,000 Salmonella genomes, it has proven challenging to associate individual SNPs with pathogenic traits of this dangerous bacterium. Here, we used a transcriptomic strategy to identify a single-nucleotide change in a promoter region responsible for crucial phenotypic differences of African S. Typhimurium. Our findings show that a noncoding nucleotide of the bacterial genome can have a profound effect upon the pathogenesis of infectious disease.

Salmonella enterica serovar Typhimurium ST313 is a relatively newly emerged sequence type that is causing a devastating epidemic of bloodstream infections across sub-Saharan Africa. Analysis of hundreds of Salmonella genomes has revealed that ST313 is closely related to the ST19 group of S. Typhimurium that cause gastroenteritis across the world. The core genomes of ST313 and ST19 vary by only ∼1,000 SNPs. We hypothesized that the phenotypic differences that distinguish African Salmonella from ST19 are caused by certain SNPs that directly modulate the transcription of virulence genes. Here we identified 3,597 transcriptional start sites of the ST313 strain D23580, and searched for a gene-expression signature linked to pathogenesis of Salmonella. We identified a SNP in the promoter of the pgtE gene that caused high expression of the PgtE virulence factor in African S. Typhimurium, increased the degradation of the factor B component of human complement, contributed to serum resistance, and modulated virulence in the chicken infection model. We propose that high levels of PgtE expression by African S. Typhimurium ST313 promote bacterial survival and dissemination during human infection. Our finding of a functional role for an extragenic SNP shows that approaches used to deduce the evolution of virulence in bacterial pathogens should include a focus on noncoding regions of the genome.

Role of sapA and yfgA in Susceptibility to Antibody-Mediated Complement-Dependent Killing and Virulence of Salmonella enterica Serovar Typhimurium

The ST313 pathovar of Salmonella enterica serovar Typhimurium contributes to a high burden of invasive disease among African infants and HIV-infected adults. It is characterized by genome degradation (loss of coding capacity) and has increased resistance to antibody-dependent complement-mediated killing compared with enterocolitis-causing strains of S. Typhimurium. Vaccination is an attractive disease-prevention strategy, and leading candidates focus on the induction of bactericidal antibodies. Antibody-resistant strains arising through further gene deletion could compromise such a strategy. Exposing a saturating transposon insertion mutant library of S. Typhimurium to immune serum identified a repertoire of S. Typhimurium genes that, when interrupted, result in increased resistance to serum killing. These genes included several involved in bacterial envelope biogenesis, protein translocation, and metabolism. We generated defined mutant derivatives using S. Typhimurium SL1344 as the host. Based on their initial levels of enhanced resistance to killing, yfgA and sapA mutants were selected for further characterization. The S. Typhimurium yfgA mutant lost the characteristic Salmonella rod-shaped appearance, exhibited increased sensitivity to osmotic and detergent stress, lacked very long lipopolysaccharide, was unable to invade enterocytes, and demonstrated decreased ability to infect mice. In contrast, the S. Typhimurium sapA mutants had similar sensitivity to osmotic and detergent stress and lipopolysaccharide profile and an increased ability to infect enterocytes compared with the wild type, but it had no increased ability to cause in vivo infection. These findings indicate that increased resistance to antibody-dependent complement-mediated killing secondary to genetic deletion is not necessarily accompanied by increased virulence and suggest the presence of different mechanisms of antibody resistance.

Invasive Infections with Nontyphoidal Salmonella in Sub-Saharan Africa

Invasive nontyphoidal Salmonella (NTS) infections in Africa cause an enormous burden of illness. These infections are often devastating, with mortality estimated at 20%, even with appropriate antimicrobial therapy. Two major groups-young children and HIV-infected adults-suffer the great majority of these infections. In children, younger age itself, as well as malaria, malnutrition, and HIV infection, are prominent risk factors. In adults, HIV infection is by far the most important risk factor. The most common serotypes in invasive infections are Salmonella enterica serotypes Typhimurium and Enteritidis. In recent years, a specific strain of Salmonella Typhimurium, multilocus sequence type 313, has caused epidemics of invasive disease. Little is known about risk factors for exposure to NTS, making the design of rational interventions to decrease exposure difficult. Antimicrobial therapy is critically important for treatment of invasive NTS infections. Thus, the emergence and spread of resistance to agents commonly used for treatment of invasive NTS infection, now including third-generation cephalosporins, is an ominous development. Already, many invasive NTS infections are essentially untreatable in many health care facilities in sub-Saharan Africa. Several candidate vaccines are in early development and, if safe and effective, could be promising. Interventions to prevent exposure to NTS (e.g., improved sanitation), to prevent the occurrence of disease if exposure does occur (e.g., vaccination, malaria control), and to prevent severe disease and death in those who become ill (e.g., preserving antimicrobial effectiveness) are all important in reducing the toll of invasive NTS disease in sub-Saharan Africa.

Development of a high-throughput method to evaluate serum bactericidal activity using bacterial ATP measurement as survival readout

Serum Bactericidal Activity (SBA) assay is the method of choice to evaluate the complement-mediated functional activity of both infection- and vaccine-induced antibodies. To perform a typical SBA assay, serial dilutions of sera are incubated with target bacterial strains and complement. The conventional SBA assay is based on plating on agar the SBA reaction mix and counting the surviving bacterial colony forming units (CFU) at each serum dilution. Even with automated colony counting, it is labor-intensive, time-consuming and not amenable for large-scale studies. Here, we have developed a luminescence-based SBA (L-SBA) method able to detect surviving bacteria by measuring their ATP. At the end of the SBA reaction, a single commercially available reagent is added to each well of the SBA plate, and the resulting luminescence signal is measured in a microplate reader. The signal obtained is proportional to the ATP present, which is directly proportional to the number of viable bacteria. Bactericidal activity is subsequently calculated. We demonstrated the applicability of L-SBA with multiple bacterial serovars, from 5 species: Citrobacter freundii, Salmonella enterica serovars Typhimurium and Enteritidis, Shigella flexneri serovars 2a and 3a, Shigella sonnei and Neisseria meningitidis. Serum bactericidal titers obtained by the luminescence readout method strongly correlate with the data obtained by the conventional agar plate-based assay, and the new assay is highly reproducible. L-SBA considerably shortens assay time, facilitates data acquisition and analysis and reduces the operator dependency, avoiding the plating and counting of CFUs. Our results demonstrate that L-SBA is a useful high-throughput bactericidal assay.

Bactericidal Immunity to Salmonella in Africans and Mechanisms Causing Its Failure in HIV Infection

Background

Nontyphoidal strains of Salmonella are a leading cause of death among HIV-infected Africans. Antibody-induced complement-mediated killing protects healthy Africans against Salmonella, but increased levels of anti-lipopolysaccharide (LPS) antibodies in some HIV-infected African adults block this killing. The objective was to understand how these high levels of anti-LPS antibodies interfere with the killing of Salmonella.

Methodology/Principal Findings

Sera and affinity-purified antibodies from African HIV-infected adults that failed to kill invasive S. Typhimurium D23580 were compared to sera from HIV-uninfected and HIV-infected subjects with bactericidal activity. The failure of sera from certain HIV-infected subjects to kill Salmonella was found to be due to an inherent inhibitory effect of anti-LPS antibodies. This inhibition was concentration-dependent and strongly associated with IgA and IgG2 anti-LPS antibodies (p<0.0001 for both). IgG anti-LPS antibodies, from sera of HIV-infected individuals that inhibit killing at high concentration, induced killing when diluted. Conversely, IgG, from sera of HIV-uninfected adults that induce killing, inhibited killing when concentrated. IgM anti-LPS antibodies from all subjects also induced Salmonella killing. Finally, the inhibitory effect of high concentrations of anti-LPS antibodies is seen with IgM as well as IgG and IgA. No correlation was found between affinity or avidity, or complement deposition or consumption, and inhibition of killing.

Conclusion/Significance

IgG and IgM classes of anti-S. Typhimurium LPS antibodies from HIV-infected and HIV-uninfected individuals are bactericidal, while at very high concentrations, anti-LPS antibodies of all classes inhibit in vitro killing of Salmonella. This could be due to a variety of mechanisms relating to the poor ability of IgA and IgG2 to activate complement, and deposition of complement at sites where it cannot insert in the bacterial membrane. Vaccine trials are required to understand the significance of lack of in vitro killing by anti-LPS antibodies from a minority of HIV-infected individuals with impaired immune homeostasis.

Bacteremia caused by nontyphoidal Salmonellae are a major health burden in Africa. While antibody-induced complement-mediated killing protects healthy Africans against Salmonella, increased levels of anti-LPS antibodies in some HIV-infected Africans block this killing. Little is known about the mechanism of the interference of killing by these antibodies. Here, we compared sera and affinity-purified antibodies from African HIV-infected adults that are unable to kill invasive S. Typhimurium D23580, with sera from HIV-uninfected and HIV-infected subjects with bactericidal activity. We found that the blocking effect of anti-LPS antibodies is a factor of antibody concentration, rather than antibody structure or specificity. While all three isotypes (IgG, IgA and IgM) can inhibit killing of Salmonella at grossly high concentrations, the IgG and IgM isotypes of the anti-LPS antibodies have in vitro bactericidal activity against invasive African S. Typhimurium. Inhibition of killing did not associate with antibody affinity or avidity, or complement deposition or consumption. It is possible that a LPS-based vaccine would induce antibodies at bactericidal rather than inhibitory concentrations in HIV-uninfected individuals. In HIV-infected individuals, it is uncertain whether vaccination will induce a protective response or a dysregulated excess of anti-LPS antibodies that impairs serum killing of Salmonella.

Differential Killing of Salmonella enterica Serovar Typhi by Antibodies Targeting Vi and Lipopolysaccharide O:9 Antigen

Salmonella enterica serovar Typhi expresses a capsule of Vi polysaccharide, while most Salmonella serovars, including S. Enteritidis and S. Typhimurium, do not. Both S. Typhi and S. Enteritidis express the lipopolysaccharide O:9 antigen, yet there is little evidence of cross-protection from anti-O:9 antibodies. Vaccines based on Vi polysaccharide have efficacy against typhoid fever, indicating that antibodies against Vi confer protection. Here we investigate the role of Vi capsule and antibodies against Vi and O:9 in antibody-dependent complement- and phagocyte-mediated killing of Salmonella. Using isogenic Vi-expressing and non-Vi-expressing derivatives of S. Typhi and S. Typhimurium, we show that S. Typhi is inherently more sensitive to serum and blood than S. Typhimurium. Vi expression confers increased resistance to both complement- and phagocyte-mediated modalities of antibody-dependent killing in human blood. The Vi capsule is associated with reduced C3 and C5b-9 deposition, and decreased overall antibody binding to S. Typhi. However, purified human anti-Vi antibodies in the presence of complement are able to kill Vi-expressing Salmonella, while killing by anti-O:9 antibodies is inversely related to Vi expression. Human serum depleted of antibodies to antigens other than Vi retains the ability to kill Vi-expressing bacteria. Our findings support a protective role for Vi capsule in preventing complement and phagocyte killing of Salmonella that can be overcome by specific anti-Vi antibodies, but only to a limited extent by anti-O:9 antibodies.

Monoclonal Antibodies of a Diverse Isotype Induced by an O-Antigen Glycoconjugate Vaccine Mediate In Vitro and In Vivo Killing of African Invasive Nontyphoidal Salmonella

Nontyphoidal Salmonella (NTS), particularly Salmonella enterica serovars Typhimurium and Enteritidis, is responsible for a major global burden of invasive disease with high associated case-fatality rates. We recently reported the development of a candidate O-antigen–CRM₁₉₇ glycoconjugate vaccine against S. Typhimurium. Here, using a panel of mouse monoclonal antibodies generated by the vaccine, we examined the relative efficiency of different antibody isotypes specific for the O:4 antigen of S. Typhimurium to effect in vitro and in vivo killing of the invasive African S. Typhimurium strain D23580. All O:4-specific antibody isotypes could mediate cell-free killing and phagocytosis of S. Typhimurium by mouse blood cells. Opsonization of Salmonella with O:4-specific IgA, IgG1, IgG2a, and IgG2b, but not IgM, resulted in cell-dependent bacterial killing. At high concentrations, O:4-specific antibodies inhibited both cell-free complement-mediated and cell-dependent opsonophagocytic killing of S. Typhimurium in vitro. Using passive immunization in mice, the O:4-specific antibodies provided in vivo functional activity by decreasing the bacterial load in the blood and tissues, with IgG2a and IgG2b being the most effective isotypes. In conclusion, an O-antigen–CRM₁₉₇ glycoconjugate vaccine can induce O-antigen-specific antibodies of different isotypes that exert in vitro and in vivo killing of S. Typhimurium.

Characterization of the Invasive, Multidrug Resistant Non-typhoidal Salmonella Strain D23580 in a Murine Model of Infection

A distinct pathovar of Salmonella enterica serovar Typhimurium, ST313, has emerged in sub-Saharan Africa as a major cause of fatal bacteremia in young children and HIV-infected adults. D23580, a multidrug resistant clinical isolate of ST313, was previously shown to have undergone genome reduction in a manner that resembles that of the more human-restricted pathogen, Salmonella enterica serovar Typhi. It has since been shown through tissue distribution studies that D23580 is able to establish an invasive infection in chickens. However, it remains unclear whether ST313 can cause lethal disease in a non-human host following a natural course of infection. Herein we report that D23580 causes lethal and invasive disease in a murine model of infection following peroral challenge. The LD₅₀ of D23580 in female BALB/c mice was 4.7 x 10⁵ CFU. Tissue distribution studies performed 3 and 5 days post-infection confirmed that D23580 was able to more rapidly colonize the spleen, mesenteric lymph nodes and gall bladder in mice when compared to the well-characterized S. Typhimurium strain SL1344. D23580 exhibited enhanced resistance to acid stress relative to SL1344, which may lend towards increased capability to survive passage through the gastrointestinal tract as well as during its intracellular lifecycle. Interestingly, D23580 also displayed higher swimming motility relative to SL1344, S. Typhi strain Ty2, and the ST313 strain A130. Biochemical tests revealed that D23580 shares many similar metabolic features with SL1344, with several notable differences in the Voges-Proskauer and catalase tests, as well alterations in melibiose, and inositol utilization. These results represent the first full duration infection study using an ST313 strain following the entire natural course of disease progression, and serve as a benchmark for ongoing and future studies into the pathogenesis of D23580.

A deadly form of non-typhoidal Salmonella has emerged as a major cause of invasive disease in sub-Saharan Africa. Initial genomic profiling of this novel Salmonella sequence type, ST313, indicated that although it is technically classified as S. Typhimurium (a serovar characterized by a broad host range), it may be evolving towards becoming a more human-specific, ‘typhoid-like’ pathogen. However, it was recently demonstrated that ST313 strains were indeed able to establish an invasive and damaging infection in chickens. Despite these important findings, it remains unclear whether ST313 is able to cause lethal disease in a non-human host, since no study has yet followed the entire natural course of disease progression. As such, there are no data available concerning the median lethal dose (LD₅₀) of any ST313 strain. This is an important metric, as the LD₅₀ value will serve as a benchmark for mechanistic studies focused on understanding the relationship between virulence and the phenotypic and molecular genetic attributes associated with ST313 infections. Here we report that D23580 causes lethal disease in BALB/c mice and determined the LD₅₀ following peroral challenge. Phenotypic characterization revealed distinct differences in tissue distribution, acid stress resistance, and biochemical utilization between D23580 and the ‘classic’ Typhimurium strain SL1344.

Relationship between antibody susceptibility and lipopolysaccharide O-antigen characteristics of invasive and gastrointestinal nontyphoidal Salmonellae isolates from Kenya

Background

Nontyphoidal Salmonellae (NTS) cause a large burden of invasive and gastrointestinal disease among young children in sub-Saharan Africa. No vaccine is currently available. Previous reports indicate the importance of the O-antigen of Salmonella lipopolysaccharide for virulence and resistance to antibody-mediated killing. We hypothesised that isolates with more O-antigen have increased resistance to antibody-mediated killing and are more likely to be invasive than gastrointestinal.

Methodology/Principal Findings

We studied 192 NTS isolates (114 Typhimurium, 78 Enteritidis) from blood and stools, mostly from paediatric admissions in Kenya 2000–2011. Isolates were tested for susceptibility to antibody-mediated killing, using whole adult serum. O-antigen structural characteristics, including O-acetylation and glucosylation, were investigated. Overall, isolates were susceptible to antibody-mediated killing, but S. Enteritidis were less susceptible and expressed more O-antigen than Typhimurium (p<0.0001 for both comparisons). For S. Typhimurium, but not Enteritidis, O-antigen expression correlated with reduced sensitivity to killing (r = 0.29, 95% CI = 0.10-0.45, p = 0.002). Both serovars expressed O-antigen populations ranging 21–33 kDa average molecular weight. O-antigen from most Typhimurium were O-acetylated on rhamnose and abequose residues, while Enteritidis O-antigen had low or no O-acetylation. Both Typhimurium and Enteritidis O-antigen were approximately 20%–50% glucosylated. Amount of S. Typhimurium O-antigen and O-antigen glucosylation level were inversely related. There was no clear association between clinical presentation and antibody susceptibility, O-antigen level or other O-antigen features.

Conclusion/Significance

Kenyan S. Typhimurium and Enteritidis clinical isolates are susceptible to antibody-mediated killing, with degree of susceptibility varying with level of O-antigen for S. Typhimurium. This supports the development of an antibody-inducing vaccine against NTS for Africa. No clear differences were found in the phenotype of isolates from blood and stool, suggesting that the same isolates can cause invasive disease and gastroenteritis. Genome studies are required to understand whether invasive and gastrointestinal isolates differ at the genotypic level.

Nontyphoidal Salmonellae (NTS) are an emerging major cause of invasive bacterial disease in African children aged less than 5 years and immunocompromised adults, with an estimated case fatality rate of 20–25%. NTS also cause diarrhoea, a killer of about 1.5 million young children annually, mainly in low- and middle-income countries. No vaccine against NTS is available, but improved understanding of the Salmonella bacteria that cause disease in Africa would help the development of new vaccines. The authors characterized a collection of 192 Kenyan NTS strains (114 S. Typhimurium and 78 S. Enteritidis) from blood and stool specimens. All strains could be killed to differing extents by antibodies present in the blood of healthy HIV-uninfected African adults, supporting the development of a vaccine that will induce protective antibodies when given to African children. Differences in killing by antibody were partly related to the amount of O-antigen on the bacterial surface. There were no clear distinction between stains causing invasive disease and diarrhoea, suggesting that the same strains may be capable of causing both forms of disease. Clarification of this will require genomic analysis.

Serum bactericidal assays to evaluate typhoidal and nontyphoidal Salmonella vaccines

Invasive Salmonella infections for which improved or new vaccines are being developed include enteric fever caused by Salmonella enterica serovars Typhi, Paratyphi A, and Paratyphi B and sepsis and meningitis in young children in sub-Saharan Africa caused by nontyphoidal Salmonella (NTS) serovars, particularly S. enterica serovars Typhimurium and Enteritidis. Assays are needed to measure functional antibodies elicited by the new vaccines to assess their immunogenicities and potential protective capacities. We developed in vitro assays to quantify serum bactericidal antibody (SBA) activity induced by S. Typhi, S. Paratyphi A, S. Typhimurium, and S. Enteritidis vaccines in preclinical studies. Complement from various sources was tested in assays designed to measure antibody-dependent complement-mediated killing. Serum from rabbits 3 to 4 weeks of age provided the best complement source compared to serum from pigs, goats, horses, bovine calves, or rabbits 8 to 12 weeks of age. For S. Enteritidis, S. Typhimurium, and S. Typhi SBA assays to be effective, bacteria had to be harvested at log phase. In contrast, S. Paratyphi A was equally susceptible to killing whether it was grown to the stationary or log phase. The typhoidal serovars were more susceptible to complement-mediated killing than were the nontyphoidal serovars. Lastly, the SBA endpoint titers correlated with serum IgG anti-lipopolysaccharide (LPS) titers in mice immunized with mucosally administered S. Typhimurium, S. Enteritidis, and S. Paratyphi A but not S. Typhi live attenuated vaccines. The SBA assay described here is a useful tool for measuring functional antibodies elicited by Salmonella vaccine candidates.

The role of the st313-td gene in virulence of Salmonella Typhimurium ST313

Multidrug-resistant Salmonella enterica serovar Typhimurium ST313 has emerged in sub-Saharan Africa causing severe infections in humans. Therefore, it has been speculated that this specific sequence type, ST313, carries factors associated with increased pathogenicity. We assessed the role in virulence of a gene with a yet unknown function, st313-td, detected in ST313 through comparative genomics. Additionally, the structure of the genomic island ST313-GI, harbouring the gene was determined. The gene st313-td was cloned into wild type S. Typhimurium 4/74 (4/74-C) as well as knocked out in S. Typhimurium ST313 02–03/002 (Δst313-td) followed by complementation (02-03/002-C). Δst313-td was less virulent in mice following i.p. challenge than the wild type and this phenotype could be partly complemented in trans, indicating that st313-td plays a role during systemic infection. The gene st313-td was shown not to affect invasion of cultured epithelial cells, while the absence of the gene significantly affects uptake and intracellular survival within macrophages. The gene st313-td was proven to be strongly associated to invasiveness, harboured by 92.5% of S. Typhimurium blood isolates (n = 82) and 100% of S. Dublin strains (n = 50) analysed. On the contrary, S. Typhimurium isolates of animal and food origin (n = 82) did not carry st313-td. Six human, non-blood isolates of S. Typhimurium from Belarus, China and Nepal harboured the gene and belonged to sequence types ST398 and ST19. Our data showed a global presence of the st313-td gene and in other sequence types than ST313. The gene st313-td was shown to be expressed during logarithmic phase of growth in 14 selected Salmonella strains carrying the gene. This study reveals that st313-td plays a role in S. Typhimurium ST313 pathogenesis and adds another chapter to understanding of the virulence of S. Typhimurium and in particular of the emerging sequence type ST313.

Analysis of the contribution of bacteriophage ST64B to in vitro virulence traits of Salmonella enterica serovar Typhimurium

Comparison of the publicly available genomes of the virulent Salmonella enterica serovar Typhimurium (S. Typhimurium) strains SL1344, 14028s and D23580 to that of the virulence-attenuated isolate LT2 revealed the absence of a full sequence of bacteriophage ST64B in the latter. Four selected ST64B regions of unknown function (sb7-sb11, sb46, sb49-sb50 and sb54) were mapped by PCR in two strain collections: (i) 310 isolates of S. Typhimurium from human blood or stool samples, and from food, animal and environmental reservoirs; and (ii) 90 isolates belonging to other serovars. The region sb49-sb50 was found to be unique to S. Typhimurium and was strongly associated with strains isolated from blood samples (100  and 28.4 % of the blood and non-blood isolates, respectively). The region was cloned into LT2 and knocked out in SL1344, and these strains were compared to wild-type isogenic strains in in vitro assays used to predict virulence association. No difference in invasion of the Int407 human cell line was observed between the wild-type and mutated strains, but the isolate carrying the whole ST64B prophage was found to have a slightly better survival in blood. The study showed a high prevalence and a strong association between the prophage ST64B and isolates of S. Typhimurium collected from blood, and may indicate that such strains constitute a selected subpopulation within this serovar. Further studies are indicated to determine whether the slight increase in blood survival observed in the strain carrying ST64B genes is of paramount importance for systemic infections.

Role of antilipopolysaccharide antibodies in serum bactericidal activity against Salmonella enterica serovar Typhimurium in healthy adults and children in the United States

Recent observations from Africa have rekindled interest in the role of serum bactericidal antibodies in protecting against systemic infection with Salmonella enterica serovar Typhimurium. To determine whether the findings are applicable to other populations, we analyzed serum samples collected from healthy individuals in the United States. We found that all but 1 of the 49 adult samples tested had robust bactericidal activity against S. Typhimurium in a standard in vitro assay. The activity was dependent on complement and could be reproduced by immunoglobulin G (IgG) purified from the sera. The bactericidal activity was inhibited by competition with soluble lipopolysaccharide (LPS) from S. Typhimurium but not from Escherichia coli, consistent with recognition of a determinant in the O-antigen polysaccharide. Sera from healthy children aged 10 to 48 months also had bactericidal activity, although it was significantly less than in the adults, correlating with lower levels of LPS-specific IgM and IgG. The lone sample in our collection that lacked bactericidal activity was able to inhibit killing of S. Typhimurium by the other sera. The inhibition correlated with the presence of an LPS-specific IgM and was associated with decreased complement deposition on the bacterial surface. Our results indicate that healthy individuals can have circulating antibodies to LPS that either mediate or inhibit killing of S. Typhimurium. The findings contrast with the observations from Africa, which linked bactericidal activity to antibodies against an S. Typhimurium outer membrane protein and correlated the presence of inhibitory anti-LPS antibodies with human immunodeficiency virus infection.