WO2007027936A2

WO2007027936A2 - Methods and compositions for the diagnosis of a salmonella spp. infection

Info

Publication number: WO2007027936A2
Application number: PCT/US2006/034097
Authority: WO
Inventors: Edward T. Ryan; Stephen B. Calderwood; Jason Harris
Original assignee: The General Hospital Corporation
Priority date: 2005-09-01
Filing date: 2006-08-30
Publication date: 2007-03-08
Also published as: WO2007027936A3

Abstract

The present invention provides methods and compositions for diagnosing Salmonella spp. Infections.

Description

METHODS AND COMPOSITIONS FOR THE DIAGNOSIS OF A SALMONELLA spp. INFECTION

BACKGROUND OF THE INVENTION

There are many different kinds of Salmonella bacteria. Salmonella serotype typhimurium and Salmonella serotype enteritidis are the most common in the United States. People infected with Salmonella often develop diarrhea, fever, and abdominal cramps 12 to 72 hours after infection. The illness usually lasts 4 to 7 days, and most people recover without treatment. In most cases however, diarrhea may be so severe that the patient needs to be hospitalized. In these patients, the Salmonella infection may spread from the intestines to the blood stream, and then to other body sites and can cause death unless the person is treated promptly with antibiotics. The elderly, infants, and those with impaired immune systems are more likely to have a severe illness. Typhoid fever is a life-threatening illness caused by the bacterium Salmonella typhi

(a.k.a., Salmonella enterica serotype typhi). In the United States, about 400 cases occur each year, and 70% of these are acquired while traveling internationally. Typhoid fever is still common in the developing world, where it affects about 12.5 million persons each year.

SUMMARY OF THE INVENTION

The present invention provides a different approach for the diagnosis of an infection by a Salmonella spp., particularly S. typhi. The invention is based upon the discovery that antibodies that specifically bind to any one of the polypeptides that is differentially expressed in a Salmonella spp. infection (see FIGURES 1-7) are detected in biological samples isolated from individuals infected with Salmonella spp. The detection of the presence of Salmonella spp. in a sample of biological material taken from an individual thought to be infected therewith is important in determining the course of therapy and the agents to be used.

Thus, the invention provides a method of detecting a Salmonella sp. (e.g., S. typhi and S. typhimurium) infection in a subject that involves the steps of: (a) providing a biological sample from a subject; and (b) determining whether the biological sample contains an antibody that specifically binds to any one of the following polypeptides: pagC, oppA, sap A, sinl, shdA, stbD, tcffi, topB, yfeN, yijO, yhjD, or yhjE. The presence of such antibodies in the biological sample identifies the subject as having a Salmonella spp. infection or having been exposed to the infectious agent. The subject is characterized as having a persistent (e.g., 5-7 days) fever. The methods are useful to rapidly distinguishing whether the subject is suffering from a Salmonella infection or a malarial infection.

The methods detect an ongoing or recent exposure to Salmonella. For example, a subject is identified as having a Salmonella spp. infection as follows: a) providing a biological sample from the subject is provided; and b) contacting the biological sample with an antigenic peptide under conditions sufficient to form an antigen-antibody complex. The antigenic peptide contains an amino acid sequence that is substantially identical to an epitope of a pagC, oppA, sapA, sinl, shdA, stbD, tcfB, topB, yfeN, yijO, yhjD, or yhjE polypeptide. Detection of antibody binding in the biological sample to the antigenic peptide indicates that the subject has a Salmonella spp. infection or has been exposed to the bacteria.

Alternatively, an infection with a Salmonella spp. is determined by a method that involves the steps of: a) providing a pagC, oppA, sapA, sinl, shdA, stbD, tcfB, topB, yfeN, yijO, yhjD, or yhjE polypeptide, which is immobilized on a substrate; b) contacting the immobilized polypeptide with a biological sample from a subject; and c) detecting the presence of a bound antibody, which is indicative of a. Salmonella spp. infection in the subject. The biological sample is blood, serum, cervical secretions, tracheal-bronchial secretions, pharyngeal secretions, sweat, tears, cerebral spinal system fluid, serum, urine, synovial fluid, and saliva. The invention also encompasses a method for evaluating the infection status of an individual and/or the progress of therapy in an individual receiving anti -bacterial treatment. The levels of antibodies that specifically bind to pagC, oppA, sapA, sinl, shdA, stbD, tcfB, topB, yfeN, yijO, yhjD, or yhjE in a biological sample from the subject are determined over time, such that the difference in antibody levels over time is indicative of the progress of the therapy. The course of therapy may therefore be monitored.

In all foregoing aspects of the invention, the subject may have recently acquired the Salmonella spp. infection. Accordingly, the presence of antibodies that bind specifically to a pagC, oppA, sapA, sinl, shdA, stbD, tcfB, topB, yfeN, yijO, yhjD, or yhjE polypeptide may indicate that the subject was exposed to a Salmonella spp. within 2 weeks of undergoing the diagnostic test.

The invention further pertains to a diagnostic kit or pack containing an assembly of materials and reagents that would be necessary to perform any of the methods described herein. Diagnosis of typhoid has been problematic. The Widal assay, for example, was developed almost 100 years ago and is associated with low sensitivity and specificity. Bone marrow biopsy and culture is the most sensitive assay (sensitivity approaching 70-90%) but is painful, invasive, and therefore, rarely used. The methods described herein represent a significant advance over previously used methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a table that lists genes that were identified by in vivo induced antigen technology (IVIAT). FIGURE 2 is a table that lists genes that were identified by in vivo induced antigen technology (IVIAT).

FIGURE 3 is a table that lists genes that were identified by in vivo induced antigen technology (IVIAT).

FIGURE 4 is a table that lists genes that were identified by in vivo induced antigen technology (IVIAT).

FIGURE 5 is a table that lists genes that were identified by in vivo induced antigen technology (IVIAT).

FIGURE 6 is a table that lists genes that were identified by in vivo induced antigen technology (IVIAT). FIGURE 7 is a table that lists genes that were identified by in vivo induced antigen technology (IVIAT).

DETAILED DESCRIPTION OF THE INVENTION

The present invention is based on the identification of Salmonella enterica serovar typhi genes that are immunogenic and uniquely expressed in vivo during typhoid using a technique called in vivo induced antigen technology (IVIAT). Furthermore, because S. enterica serovar typhi and S. enterica serovar typhimurium are closely related pathogens but cause distinct clinical syndromes in humans (typhimurium causes gastroenteritis in humans and a typhoidal illness in mice; typhi causes typhoid in humans, but does not cause a typhoidal illness in mice), the invention also focuses on the identification of genes that are expressed that are expressed in an infection with S. enterica serovar typhi genes but absent in a S. enterica serovar typhimurium infection. Accordingly, the identification of bacterial genes unique to S. enterica serovar typhi and expressed during human infection is useful for the development of improved vaccines and diagnostic methods. Diagnostic and therapeutic methods for the management of Salmonella spp. infections are described in detail below. The diagnostic methods and combination therapies described below are generally applicable for infection caused by any Salmonella species, such as S. typhi and S. typhimurium. Infections in which the causative agent is S. typhi are emphasized. IVIAT methodology was applied to S. typhi, the cause of typhoid fever. Pooled convalescent sera from individuals with documented S. typhi bacteremia or typhoid fever in Bangladesh were adsorbed against in vitro grown S. typhi CTl 8 organisms. Adsorbed sera was then used to probe a 100,000 clone inducible genomic expression library of S. typhi CT 18 in E. coli. Clones reactive to previously adsorbed serum contained immunogenic gene products expressed uniquely in vivo during S. typhi infection. In total, using IVIAT, approximately thirty S. typhi gene products, including fimbrial proteins, transporters, and virulence factors were identified. One of the genes identified by IVIAT was pagC (Accession Numbers AAD13824, YP_217958, YP_216243, NP_457426, NP_456268, CAD02858, CAD02112, CAD08804, A39185, and P23988), a 185 amino-acid, twenty IcDa outer membrane invasion protein that is highly conserved among serovars of Salmonella enterica. Mutations in pagC in other S. enterica serovars have been shown to have attenuated survival within macrophages and decreased virulence in animal models of Salmonellosis.

Table 1: PagC homologs in S. enterica

Serovar Protein % Identity typhi (CT-18: drug resistant) PagC typhi (Ty2) PagC 100% paratyphi ATCC9150 PagC: 98.9% typhimurium LT2 PagC 96.8% cholerasuis SC-B67 PagC 95.6%

PagC is also belongs to of a larger family of outer membrane porin proteins (OmpX/OmpA) which have broad homology, sharing 30-40% homology at the amino acid level. Most of the homology to this family of proteins occurs in the transmembrane regions, while the cell surface regions have diverged substantially. Table 2 lists the other outer -membrane proteins with at least 30% homology to PagC that are found in the sequenced genome of S. enterica serovar typhi CT- 18. In S. enterica, there are only 3 homologs with greater than 30% identity (listed below). Many non-Salmonella pathogens also contain homologous proteins (OmpX or OmpA), although homology is commonly <30-40%.

Table 2: Extended list of PagC homologs (>30%) in CT-18

Locus % Identity % Similarity STYl 878 (PagC)

STY0872 (OmpX) 36% 52%

STY0380 (putative omp) 36% 51%

STY3179 (putative omp) 34% 51 %

PagC has 5 transmembrane sections and 4 extracellular domains. Most homology to related molecules is to the transmembrane domains.

Using individual serum samples, differential anti-pagC reactivity was found in acute relative to convalescent sera samples. Specifically, using an in vitro transcription-translation kit, pagC was expressed in vitro, and specific anti-pagC responses were detected in 11 of 14 individuals with documented S. typhi bacteremia/typhoid fever and no responses in 10 individuals recovering from cholera at the International Centre for Diarrhoeal Disease Research in Bangladesh. These date indicate that anti-PagC responses are useful serological markers of systemic S. typhi infection.

Diagnosis of Salmonella Infection

The invention pertains to methods for diagnosing an individual infected with a Salmonella spp. by determining the presence of antibodies that specifically bind to any one of the polypeptides encoded by the genes found in FIGURES 1-7. Thus, an individual is identified as being infected with Salmonella if a biological sample isolated from such an individual contains antibodies that bind, for example, a pagC, oppA, sapA, sinl, shdA, stbD, tcfB, topB, yfeN, yijO, yhjD, or yhjE polypeptide. Such polypeptides may be identified using the methods described herein or described, for example, in U.S. Patent Numbers 6,489,128; 6,660,482; and 6,838,552, all of which are hereby incorporated by reference. For example, peptide-based assays may be used for the detection of one or more immunoglobulins, such as IgG, IgM, IgA and IgE, against antigenic sequences within the full length polypeptides described herein (e.g., pagC, oppA, sapA, sinl, shdA, stbD, tcfB, topB, yfeN, yijO, yhjD, or yhjE). The invention also provides methods to evaluate the progress of a Salmonella infection in an infected subject by determining the serological status over time of an individual undergoing antibacterial therapy. For purposes of this application, "biological sample" includes, but is not limited to, bodily secretions, bodily fluids and tissue specimens. Examples of bodily secretions include cervical secretions, tracheal-bronchial secretions and pharyngeal secretions, bodily fluids include blood, sweat, tears, cerebral spinal system fluid, serum, urine, synovial fluid and saliva. Animals, cells and tissue or cell specimens such as from a variety of biopsies such as bone marrow are embraced by this term. A biological material, such as a sample of tissue and/or fluid, is obtained from an individual and a suitable assay is used to assess the presence or amount of an antibody that specifically binds to any polypeptide that is specifically expressed in Salmonella spp. infections, such as those found in FIGURES 1-7 (e.g., pagC, oppA, sapA, sinl, shdA, stbD, tcfB, topB, yfeN, yijO, yhjD, or yhjE). Suitable assays include immunological methods such as enzyme-linked immunosorbent assays (ELISA), including luminescence assays (e.g., fluorescence and chemiluminescence), radioimmunoassay, and immunohistology. The identification of such antibodies in a biological sample isolated from an individual identifies the individual as being recently (e.g., 5, 10, 15, 20, or 25 days from exposure to the pathogen) infected with a Salmonella spp. Optionally, the techniques described herein may be used to differentiate between a Salmonella infection and a second condition having similar symptoms. Alternatively, the methods described herein may be used to differentiate between two different strains of Salmonella spp. (e.g., S. typhi and S. typhimurium) or to evaluate the course of antibacterial therapy. A patient-derived sample and an antigenic peptide that contains an amino acid sequence that is substantially identical to an antigen found in a polypeptide that is specifically expressed in Salmonella spp. (e.g., pagC) are combined under conditions suitable for the formation of an antibody-protein complex, and the formation of antibody-protein complex is assessed (directly or indirectly). As used herein, an antigenic peptide is any peptide having the ability to induce an immune response (humoral or cellular) in a subject. The antigenic peptide described herein may be at least 70%, 80%, 85%, 90%, 95%, 99%, or even 100% identical to the full-length polypeptide that is specifically expressed during a Salmonella infection. Alternatively, the antigenic peptide contains an amino acid sequence that is substantially identical (at least 70%, 80%, 85%, 90%, 95%, 99%, or even 100%) to an antigenic sequence found within the polypeptide that is expressed specifically during a

Salmonella infection (e.g., such as those found in FIGURES 1-7). Such antigenic peptides may be synthesized using an appropriate expression system, such as in E. coli or Baculovirus. The expressed protein thus serves as the antigen for suitable immunological methods, as discussed herein. In all of the diagnostic methods described herein, the peptides may be directly labeled with an enzyme, fluorophore, radioisotope or luminescent tag. Alternatively, peptides may be covalently linked with a specific scavenger such as biotin. Subsequent detection involves binding avidin or strepavidin labeled with an indicator enzyme, fluorophore, radioisotope, or luminescent tag. Production of a detectable product (e.g., color production, enzyme reaction, fluorescence, radioactivity or luminescence emission) is detected. If desired, the biological sample is contacted with the peptide bound to solid matrix, e.g., microtiter plate, bead, dipstick. For example, the solid matrix is dipped into a patient-derived sample of a bodily fluid, washed, and the solid matrix contacted with a reagent to detect the presence of immune complexes present on the solid matrix. In one example, a peptide that contains an amino acid sequence that is substantially identical to an antigenic sequence in pagC is immobilized on a solid matrix such as a porous strip to form at least one anti-pagC antibody detection site. The measurement or detection region of the porous strip may include a plurality of sites containing the immobilized peptide. A test strip may also contain sites for negative and/or positive controls. Alternatively, control sites are located on a separate strip from the test strip. Optionally, the different detection sites may contain different amounts of immobilized peptides, i.e., a higher amount in the first detection site and lesser amounts in subsequent sites. For example, if 20 nanograms of the peptide captures the equivalent of 1 nmol/min/ml of antibody, then the first detection site of an assay device might contain 50 nanograms of the peptide while the subsequent sites contain 10, 20, 30, etc. nanograms of antibody. Upon the addition of test sample, the number of sites displaying a detectable signal provides a quantitative indication of the amount of anti-pagC antibodies present in the sample. The detection sites may be configured in any suitably detectable shape and are typically in the shape of a bar spanning the width of a test strip.

A multi-capture assay configuration is prepared such that if a threshold amount of anti-pagC antibodies is not present in the biological sample, then substantially all of the antibodies will bind to the peptide in the first capture site and thus become immobilized at that site. If a greater than threshold amount of anti-pagC antibodies is present in the biological sample, the remaining anti-pagC antibodies bind to subsequent detection zones of immobilized peptide along the length of the test strip. The greater the amount of anti-pagC antibodies in the test sample, the greater the number of capture sites that will display a detectable signal due to the presence of such antibodies.

Methods and means for covalently or noncovalently binding proteins to solid matrices are known in the art. The nature of the solid surface may vary depending upon the assay format. For assays carried out in microtiter wells, the solid surface is the wall of the well or cup. For assays using beads, the solid surface is the surface of the bead. In assays using a dipstick (i.e., a solid body made from a porous or fibrous material such as fabric or paper) the surface is the surface of the material from which the dipstick is made. Examples of useful solid supports include nitrocellulose (e.g., in membrane or microtiter well form), polyvinyl chloride (e.g., in sheets or microtiter wells), polystyrene latex (e.g., in beads or microliter plates, polyvinylidine fluoride (known as IMMULON™), diazotized paper, nylon membranes, activated beads, and Protein A beads. Microtiter plates may be activated {e.g., chemically treated or coated) to covalently bind proteins. The solid support containing the antibody is typically washed after contacting it with the test sample, and prior to detection of bound immune complexes.

A common feature of all of the assays described herein is that the antigenic peptide containing the amino acid sequence that is substantially identical to an antigenic sequence found in the polypeptides of FIGURES 1-7 {e.g., pagC, oppA, sapA, sinl, shdA, stbD, tcfJB, topB, yfeN, yijO, yhjD, or yhjE) is contacted with a biological sample isolated from a subject suspected being infected with a Salmonella spp. under conditions that permit the peptide to bind to the antibody forming an immune complex, which contains the polypeptide and the subject's antibody that specifically binds to it. Such conditions are typically physiologic temperature, pH, and ionic strength.

Diagnostic Reagents

The invention also provides a diagnostic reagent pack or kit containing one or more containers filled with one or more of the ingredients used in the assays of the invention. Reagents, e.g., peptides containing an antigenic sequence found in pagC, for carrying out the diagnostic or prognostic assay may be packaged together as a kit. For example, the peptide is immobilized on a solid phase and packaged together with other reagents suitable for detecting the peptide-antibody complexes. For example, enzyme-conjugated reagents may be included. Antibodies that bind specifically to the peptide may also be included as a standard or control reagent. The solid phase component of the kit onto which an antibody or antigen is immobilized is preferably an assay plate, an assay well, a nitrocellulose membrane, a bead, a dipstick, or a component of an elution column. The kit may also contain a second antibody or other detectable marker. The second antibody or marker is labeled, e.g., using a radioisotope, fluorochrome, or other means of detection. The pack or kit can be labeled with information regarding the sequence of execution (e.g., obtaining a biological sample, contacting with a peptide containing an antigenic sequence, and detecting the presence or absence of antibodies specific to the peptide in the biological sample), or the like. The pack or kit can be a single unit assay or it can be a plurality of unit assays. For the purpose of this invention, unit assay is intended to mean materials sufficient to perform only a single assay.

The invention is further illustrated by the following non-limiting examples. Example 1: In vivo induced antigen technology (IVIAT)

Although a number of technologies have been developed to identify in vivo expressed bacterial genes (e.g., in vivo expression technology (IVET), signature tagged mutagenesis (STM), differential fluorescence induction (DFI), and gene array), in vivo induced antigen technology (IVIAT) has a number of unique advantages over other technologies in that it directly identifies immunogenic bacterial antigens expressed uniquely during human infection. Although immunogenicity, as detected by IVIAT, is based upon serum antibody responses, such responses correlate with host mucosal and cellular immune responses. During IVIAT, pooled convalescent sera collected from humans infected with a pathogen was adsorbed against bacteria grown in vitro. Adsorbed serum was then used to probe an inducible genomic DNA expression library of the pathogen in an Escherichia coli host system. Reactive clones express an antigen expressed uniquely in vivo and reactive genes and their products were further identified and analyzed using known methods (e.g., Hang L et al, Proc. Natl. Acad. Sci. USA 100:8508-13, 2003).

Blood and stool were collected from individuals with typhoid fever. In particular, acute and convalescent serum samples were obtained from 14 individuals in Kamalapur who presented with fever and were found to be bacteremic with S. enterica serovar typhi. IVIAT was applied to the acute and convalescent serum samples collected from these individuals. S. enterica serovar typhi CTl 8, a sequenced virulent strain, was used as the sequence data facilitates the identification and analysis of gene fragments identified by IVIAT. CT 18 was grown in vitro in routine aerated conditions (LB broth at 37°C). The use of simple in vitro conditions facilitated the identification of genes expressed uniquely in vivo during human infection. A S. enterica serovar typhi CT 18 inducible genomic (chromosomal and plasmid) DNA expression library was next created using partial digestion to recover Sau3A 0.5-1.5 kbp fragments. Fragments were introduced into pET30abc expression vectors to ensure inducible in vitro expression from a heterologous promoter, and plasmids will be electroporated into E. coli BL21(DE3).

To generate adsorbed sera, each 500 μl aliquot of pooled convalescent serum was subjected to five successive direct adsorptions with S. enterica serovar typhi CT 18 cells grown aerobically in LB broth at 37⁰C. Each adsorption involves an overnight incubation with mild agitation at 4°C of pooled serum with approximately 10¹¹ bacteria in 100 μl of PBS containing 0.02% sodium azide. Pooled sera was further adsorbed by exposing it to: 1) a nitrocellulose membrane saturated with extracts of CT 18 prepared by a bead beater apparatus; and 2) a nitrocellulose membrane saturated with a heat-denatured extract of CT 18.

To probe the DNA expression library, BHI agar plates with kanamycin containing approximately 500 colonies per plate of the genomic expression library are replica plated onto BHI plates containing kanamycin and IPTG, to induce expression of genes in the genomic library. Colonies are exposed to chloroform vapors for 15 minutes in a hermetic container. Plates are overlaid with a nitrocellulose membranes and colonies transferred by colony lift. Membranes are reacted with adsorbed sera at a 1:5,000-10,000 dilution. Reactive clones were detected using peroxidase conjugated to goat anti-human immunoglobulin and developed using the ECL chemiluminesence kit and Hyperfilm ECL system (Amersham Limited).

Inserts were identified as follows. Reactive clones are identified by their position on master plates. Reactivity is confirmed with re-analysis using adsorbed sera. Inserted DNA is next sequenced and located on the S. enterica serovar typhi genomic sequence. Inserts are also surveyed for homology, open reading frames, signal sequences, and structural motifs using BLAST and other DNA-protein analytical software.

Primary IVIAT screening identified approximately 140 Salmonella typhi genes. Approximately 60 of these were confirmed by secondary screening. Approximately 50 genes were confirmed by tertiary screening. A preliminary listing of these genes is attached (see FIGURES 1-7). E. coli clones expressing Salmonella typhi genes identified by IVIAT were screened with acute and convalescent sera collected from humans recovering from Salmonella typhi bacteremia and typhoid. At least two clones that were differentially reactive were identified. One of these clones expressed PagC, the other expressed OppA. These are two distinct Salmonella typhi antigens. PagC was expressed in an in vitro transcription-translation assay and acute and convalescent individual sera responses was measured in 14 individuals with documented Salmonella typhi bacteremia/typhoid fever. In eleven of these individuals, there was a clear enhancement of reactivity of anti-pagC antibodies in convalescent sera compared to acute samples. In two individuals, there were prominent responses both in acute and convalescent sera, and in one the acute is more reactive than the convalescent. The acute serum samples of these three individuals may have collected late in their clinical course as well {i.e., that they are actually convalescent samples; many individuals with typhoid present in their 3-4 week of illness). Thus, the identification of anti-pagC antibodies is useful in diagnostic assays for typhoid and Salmonella typhi bacteremia and to distinguish these infections from other pathological conditions that may initially present in a similar fashion, i.e., high persistent fever.

Example 2: Enzyme Linked Immuno Sorbent Assay (ELISA; Diagnostic) The full length pagC gene of S. typhi is directionally cloned into an expression plasmid at specific restriction sites using primers to introduce these unique restriction sites into the pagC ends. The construction of the pagC insert into the expression vector yields, on transformation of a permissive host (e.g., E. coli), a full length pagC peptide which can be used as the antigen for a Salmonella species ELISA. Any expression systems, such as those suitable for expression in E. coli or Baculovirus for example, may be used for synthesis of the pagC peptide as the antigen in ELISA. A standard ELISA assay is performed.

For example, pagC (purified, recombinant) is non-covalently attached to each well of a 96 well microtiter plate in carbonate buffer at pH 9.5 with an overnight incubation at 4⁰C. The plate is washed with PBS, 0.15% Tween 20 and is then blocked with PBS, 1% BSA, 0.15% Tween 20 for 1 hour at RT and then washed with PBS, 0.15% Tween 20. Serum is serially diluted in PBS in a separate plate and 50 μl of each well transferred to corresponding wells of a pagC ligand plate. The plate is next incubated at 37⁰C for 1 hour using a parafilm or other suitable cover to prevent non-uniform evaporation. Each well was next washed with PBS, 1% FCS, 0.05% NaN₃X5, incubated with a predetermined dilution of biotin conjugated anti-human monoclonal IgG or monoclonal IgM, and incubate at 37⁰C for 1 hour with cover. Each well is next washed with PBS, 1% FCS, 0.05% NaN₃ and 50 μl strepavidin-alkaline phosphatase conjugate (1:200 in PBS, 1% BSA, 0.15% Tween20) is next added to each well for 1 hour at 37⁰C with cover. Each well is next washed with PBS, 1% CS (calf serum), 0.05% NaN₃. Color is developed with p-nitrophenyl phosphate in glycine buffer at pH 9.6. The color yield is measured on a microtiter colorimeter using a 405 nm filter. The end point titer is the highest dilution of serum or secretion yielding a color yield>3 SD over background. Analysis is simplified by computer-generated end point antibody titer or other antibody level measure identification and/or quantity of specific antibody (IgG, IgM, or total Ig) in the test sample using appropriate controls. Other strepavidin or avidin enzyme conjugates can be substituted such as strepavidin peroxidase or strepavidin- galactosidase with an approximate substitute yielding a detectable color for quantitation. Equivalents

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described specifically herein. Such equivalents are intended to be encompassed in the scope of the claims.

Claims

What is claimed is:

1. A method of detecting a Salmonella spp. infection in a subject, said method comprising:

(a) providing a biological sample from said subject; and

(b) determining whether said biological sample comprises an antibody that specifically binds to a polypeptide selected from the group consisting of pagC, oppA, sap A, sinl, shdA, stbD, tcfB, topB, yfeN, yijO, yhjD, and yhjE, wherein the presence of said antibody indicates a Salmonella sp. infection in said subject..

2. The method of claim 1, wherein said Salmonella spp. is Salmonella typhi.

3. The method of claim 1, wherein said Salmonella spp. is Salmonella typhimurium.

4. The method of claim 1, wherein said infection is recently acquired.

5. The method of claim 1, wherein said presence indicates exposure of said subject to said Salmonella sp. within 2 weeks of obtaining said biological sample from said subject.

6. The method of claim 1, wherein said polypeptide is a pagC polypeptide.

7. The method of claim 1, wherein said biological sample is blood.

8. The method of claim 1, wherein said biological sample is serum.

9. A method for detecting a Salmonella spp. infection in a subject, said method comprising the steps of: a) providing a biological sample from said subject; and b) contacting said biological sample with an antigenic peptide under conditions sufficient to form an antigen-antibody complex, wherein said antigenic peptide comprises an amino acid sequence that is substantially identical to an antigenic sequence in a polypeptide selected from the group consisting of pagC, oppA, sapA, sinl, shdA, stbD, tcffi, topB, yfeN, yijO, yhjD, and yhjE polypeptide; and b) detecting the binding of an antibody in said biological sample to said antigenic peptide, wherein binding of said antibody to said polypeptide indicates a Salmonella spp. infection in said subject.

10. The method of claim 9, wherein said Salmonella spp. is Salmonella typhi.

11. The method of claim 9, wherein said Salmonella spp. is Salmonella typhimnrium.

12. The method of claim 9, wherein said polypeptide is a pagC polypeptide.

13. The method of claim 9, wherein said infection is recently acquired.

14. The method of claim 9, wherein said presence indicates exposure of said subject to said Salmonella sp. within 2 weeks of obtaining said biological sample from said subject.

15. The method of claim 1, wherein said polypeptide is a pagC polypeptide.

16. The method of claim 9, wherein said biological sample is blood.

17. The method of claim 9, wherein said sample is serum.

18. A method for detecting the presence of Salmonella typhi in a sample, said method comprising the steps of: a) providing an antigenic peptide immobilized on a substrate, said antigenic peptide comprising an amino acid sequence that is substantially identical to an antigenic sequence in a polypeptide selected from the group consisting of pagC, oppA, sapA, sinl, shdA, stbD, tcfB, topB, yfeN, yijO, yhjD, and yhjE polypeptide; c) contacting said immobilized antigenic peptide with a biological sample from a subject; and d) detecting the presence of a bound antibody, wherein the presence of bound antibody indicates the a Salmonella spp. infection in said subject.

19. The method of claim 18, wherein said Salmonella spp. is Salmonella typhi.

20. The method of claim 18, wherein said Salmonella spp. is Salmonella typhimurium.

21. The method of claim 18, wherein said polypeptide is a pagC polypeptide.

22. The method of claim 18, wherein said infection is recently acquired.

23. The method of claim 18, wherein said presence indicates exposure of said subject to said Salmonella sp. within 2 weeks of obtaining said biological sample from said subject.