Test of the virulence and live-vaccine efficacy of auxotrophic and galE derivatives of Salmonella choleraesuis

Sugar-Phosphate Toxicities

Accumulation of phosphorylated intermediates during cellular metabolism can have wide-ranging toxic effects on many organisms, including humans and the pathogens that infect them. These toxicities can be induced by feeding an upstream metabolite (a sugar, for instance) while simultaneously blocking the appropriate metabolic pathway with either a mutation or an enzyme inhibitor. Here, we survey the toxicities that can arise in the metabolism of glucose, galactose, fructose, fructose-asparagine, glycerol, trehalose, maltose, mannose, mannitol, arabinose, and rhamnose. Select enzymes in these metabolic pathways may serve as novel therapeutic targets. Some are conserved broadly among prokaryotes and eukaryotes (e.g., glucose and galactose) and are therefore unlikely to be viable drug targets. However, others are found only in bacteria (e.g., fructose-asparagine, rhamnose, and arabinose), and one is found in fungi but not in humans (trehalose). We discuss what is known about the mechanisms of toxicity and how resistance is achieved in order to identify the prospects and challenges associated with targeted exploitation of these pervasive metabolic vulnerabilities.

Refinement of a Live Attenuated Salmonella enterica Serovar Newport Vaccine with Improved Safety

Non-typhoidal Salmonella (NTS) is a major cause of gastroenteritis and is responsible for approximately 93 million cases annually. In healthy individuals, gastroenteritis caused by NTS is usually self-limiting, however, NTS can cause severe invasive disease in immunocompromised patients. Very little research has been directed towards development of vaccines against Salmonella serogroups O:6,7 or O:8. We have constructed a live attenuated serogroup O:8 vaccine, CVD 1979, by deleting guaBA, htrA, and aroA from the genome of S. Newport. We have shown that the candidate vaccine is well tolerated in mice and elicits serum immunoglobulin G (IgG) antibodies against core O-polysaccharide (COPS) when administered orally. Immunized mice were challenged intraperitoneally with wild-type S. Newport and bacterial burden in the liver and spleen was found to be significantly reduced in the livers of immunized mice compared to control mice. We also observed moderate vaccine efficacy (45%) against lethal challenge with the serogroup O:8 serovar, S. Muenchen, but low vaccine efficacy (28%) following lethal challenge with a serogroup O:6,7 serovar, S. Virchow. In vitro, we have shown that antibodies generated by CVD 1979 only recognize lipopolysaccharide (LPS) from serogroup O:8 but not serogroup O:6,7 serovars, and that they mediate opsonophagocytic antibody (OPA) activity against serogroup O:8 but not serogroup O:6,7 serovars. We also showed that OPA activity can be blocked by pre-incubating the antisera with serogroup O:8 lipopolysaccharide. Taken together, our data demonstrate that we have constructed a well-tolerated, effective live attenuated S. Newport vaccine which elicits functional antibodies against serogroup O:8 but not O:6,7 serovars.

Attenuated Salmonella enterica Serovar Typhimurium, Strain NC983, Is Immunogenic, and Protective against Virulent Typhimurium Challenges in Mice

Non-typhoidal Salmonella (NTS) serovars are significant health burden worldwide. Although much effort has been devoted to developing typhoid-based vaccines for humans, currently there is no NTS vaccine available. Presented here is the efficacy of a live attenuated serovar Typhimurium strain (NC983). Oral delivery of strain NC983 was capable of fully protecting C57BL/6 and BALB/c mice against challenge with virulent Typhimurium. Strain NC983 was found to elicit an anti-Typhimurium IgG response following administration of vaccine and boosting doses. Furthermore, in competition experiments with virulent S. Typhimurium (ATCC 14028), NC983 was highly defective in colonization of the murine liver and spleen. Collectively, these results indicate that strain NC983 is a potential live attenuated vaccine strain that warrants further development.

O-Serotype Conversion in Salmonella Typhimurium Induces Protective Immune Responses against Invasive Non-Typhoidal Salmonella Infections

Salmonella infections remain a big problem worldwide, causing enteric fever by Salmonella Typhi (or Paratyphi) or self-limiting gastroenteritis by non-typhoidal Salmonella (NTS) in healthy individuals. NTS may become invasive and cause septicemia in elderly or immuno-compromised individuals, leading to high mortality and morbidity. No vaccines are currently available for preventing NTS infection in human. As these invasive NTS are restricted to several O-antigen serogroups including B1, D1, C1, and C2, O-antigen polysaccharide is believed to be a good target for vaccine development. In this study, a strategy of O-serotype conversion was investigated to develop live attenuated S. Typhimurium vaccines against the major serovars of NTS infections. The immunodominant O4 serotype of S. Typhimurium was converted into O9, O7, and O8 serotypes through unmarked chromosomal deletion–insertion mutations. O-serotype conversion was confirmed by LPS silver staining and western blotting. All O-serotype conversion mutations were successfully introduced into the live attenuated S. Typhimurium vaccine S738 (Δcrp Δcya) to evaluate their immunogenicity in mice model. The vaccine candidates induced high amounts of heterologous O-polysaccharide-specific functional IgG responses. Vaccinated mice survived a challenge of 100 times the 50% lethality dose (LD₅₀) of wild-type S. Typhimurium. Protective efficacy against heterologous virulent Salmonella challenges was highly O-serotype related. Furthermore, broad-spectrum protection against S. Typhimurium, S. Enteritidis, and S. Choleraesuis was observed by co-vaccination of O9 and O7 O-serotype-converted vaccine candidates. This study highlights the strategy of expressing heterologous O-polysaccharides via genetic engineering in developing live attenuated S. Typhimurium vaccines against NTS infections.

Maternal vaccination as a Salmonella Typhimurium reduction strategy on pig farms

AIMS

The control of Salmonella in pig production is necessary for public and animal health, and vaccination was evaluated as a strategy to decrease pig prevalence.

METHODS AND RESULTS

The study examined the efficacy of a live Salmonella Typhimurium vaccine, administered to sows on eight commercial farrow-to-finish herds experiencing clinical salmonellosis or Salmonella carriage associated with S. Typhimurium or its monophasic variants. Results of longitudinal Salmonella sampling were compared against eight similarly selected and studied control farms. At the last visit (~14 months after the start of vaccination), when all finishing stock had been born to vaccinated sows, both faecal shedding and environmental prevalence of Salmonella substantially declined on the majority of vaccinated farms in comparison to the controls. A higher proportion of vaccine farms resolved clinical salmonellosis than controls. However, Salmonella counts in positive faeces samples were similar between nonvaccinated and vaccinated herds.

CONCLUSIONS

The results suggest that maternal vaccination is a suitable option for a Salmonella Typhimurium reduction strategy in farrow-to-finish pig herds.

SIGNIFICANCE AND IMPACT OF THE STUDY

Salmonella vaccines have the potential to reduce the prevalence of Salmonella in pigs and result in a reduction of human cases attributed to pork.

Salmonella Serogroup C: Current Status of Vaccines and Why They Are Needed

Nontyphoidal Salmonella (NTS; i.e., Salmonella enterica organisms that do not cause typhoid or paratyphoid) are responsible for 94 million infections and 155,000 deaths worldwide annually, 86% of which are estimated to be foodborne. Although more than 50 serogroups and 2,600 serovars have been described, not all Salmonella serovars cause disease in humans and animals. Efforts are being made to develop NTS vaccines, with most approaches eliciting protection against serovars Typhimurium and Enteritidis (serogroups B [O:4] and D [O:9], respectively), as they are widely considered the most prevalent. Here, we show that serogroup C (O:6,7, O:6,8, or O:8 epitopes) is the most common serogroup in the United States, and the prevalence of serovars from this serogroup has been increasing in Europe and the United States over the last decade. They are also the most commonly isolated serovars from healthy cattle and poultry, indicating the underlying importance of surveillance in animals. Four out of the 10 most lethal serovars in the United States are serogroup C, and reports from African countries suggest that strains within this serogroup are highly antibiotic resistant. Serogroup C consists of highly diverse organisms among which 37 serovars account for the majority of human cases, compared to 17 and 11 serovars for serogroups B and D, respectively. Despite these concerning data, no human vaccines targeting serogroup C NTS are available, and animal vaccines are in limited use. Here, we describe the underestimated burden represented by serogroup C NTS, as well as a discussion of vaccines that target these pathogens.

Live attenuated vaccines for invasive Salmonella infections

Salmonella enterica serovar Typhi produces significant morbidity and mortality worldwide despite the fact that there are licensed Salmonella Typhi vaccines available. This is primarily due to the fact that these vaccines are not used in the countries that most need them. There is growing recognition that an effective invasive Salmonella vaccine formulation must also prevent infection due to other Salmonella serovars. We anticipate that a multivalent vaccine that targets the following serovars will be needed to control invasive Salmonella infections worldwide: Salmonella Typhi, Salmonella Paratyphi A, Salmonella Paratyphi B (currently uncommon but may become dominant again), Salmonella Typhimurium, Salmonella Enteritidis and Salmonella Choleraesuis (as well as other Group C Salmonella). Live attenuated vaccines are an attractive vaccine formulation for use in developing as well as developed countries. Here, we describe the methods of attenuation that have been used to date to create live attenuated Salmonella vaccines and provide an update on the progress that has been made on these vaccines.

Assessment of immunity against avian colibacillosis induced by an aroA mutant containing increased serum survival gene in broilers

Colibacillosis is an important disease in the poultry industry which causes serious economic damages. As it is suggested that vaccination is one of the means to control colibacillosis, we tried to investigate the vaccine potential of a ∆aroA derivative of an O78:K80 avian pathogenic Escherichia coli containing increased serum survival gene. 490 chicks were selected as follows: For assessment of virulence of ∆aroA mutant, 30 chicks were divided into three groups and injected with 0.5ml of PBS or bacterial suspension containing either10⁷ colony forming units (CFU) of mutant or parent strains via subcutaneous route. Macroscopic lesions and mortality rate were recorded in different groups during the week after challenge. For assessment of safety and immunogenicity of the ∆aroA mutant, three groups of 20 chicks were vaccinated by aerosol administration of 250 ml of suspension containing 10⁸ CFU of mutant strain at days 1 and 14, while the two other groups received PBS or wild type strain. Macroscopic lesions and mortality rate were recorded in different groups until day 21. To determine whether the vaccination is protective against challenges or not, the chickens were vaccinated at days 1 and 14 and challenged intramuscularly with either a homologous or heterologous strains at day 21. Macroscopic lesions and mortality rate were recorded in different groups during the week after challenge. The results revealed that the ∆aroA mutant was slightly virulent, however it was safe and did not cause mortality, lesions or weight loss after vaccination. Antibody responses were similar in the control and mutant groups and vaccination did not induce a significant humoral immunity. The mutant could not protect chickens against both homologous and heterologous challenges. This could be due to several factors such as the high amount of maternal antibodies in the first two weeks of life, and the vaccination procedure.

Mutations in the Salmonella enterica serovar Choleraesuis cAMP-receptor protein gene lead to functional defects in the SPI-1 Type III secretion system

Salmonella enterica serovar Choleraesuis (Salmonella Choleraesuis) causes a lethal systemic infection (salmonellosis) in swine. Live attenuated Salmonella Choleraesuis vaccines are effective in preventing the disease, and isolates of Salmonella Choleraesuis with mutations in the cAMP-receptor protein (CRP) gene (Salmonella Choleraesuis ∆crp) are the most widely used, although the basis of the attenuation remains unclear. The objective of this study was to determine if the attenuated phenotype of Salmonella Choleraesuis ∆crp was due to alterations in susceptibility to gastrointestinal factors such as pH and bile salts, ability to colonize or invade the intestine, or cytotoxicity for macrophages. Compared with the parental strain, the survival rate of Salmonella Choleraesuis ∆crp at low pH or in the presence of bile salts was higher, while the ability of the mutant to invade intestinal epithelia was significantly decreased. In examining the role of CRP on the secretory function of the Salmonella pathogenicity island 1 (SPI-1) encoded type III secretion system (T3SS), it was shown that Salmonella Choleraesuis ∆crp was unable to secrete the SPI-1 T3SS effector proteins, SopB and SipB, which play a role in Salmonella intestinal invasiveness and macrophage cytotoxicity, respectively. In addition, caspase-1 dependent cytotoxicity for macrophages was significantly reduced in Salmonella Choleraesuis ∆crp. Collectively, this study demonstrates that the CRP affects the secretory function of SPI-1 T3SS and the resulting ability to invade the host intestinal epithelium, which is a critical element in the pathogenesis of Salmonella Choleraesuis.

Heterologous protection in pigs induced by a plasmid-cured and crp gene-deleted Salmonella choleraesuis live vaccine

In this study, we exploited a crp (cAMP receptor protein) gene-deleted, virulence plasmid-cured Salmonella choleraesuis mutant with decreased carbon source utilization, designated S.C.-Deltacrp/vpl(-), as a live vaccine strain. Normal weight gain with no clinical signs was observed in pigs immunized with high doses of S.C.-Deltacrp/vpl(-) live vaccine. Vaccination in pregnant sows induced high maternal antibodies, which could prevent piglets from Salmonella infection. Moreover, serial transmission of the vaccine strain in piglets produced no evidence of reversion to virulence. Furthermore, the peripheral blood mononuclear cells from immunized piglets also developed Salmonella specific T-cell proliferative response in vitro. Our results indicate that immunogenic antigens in S.C.-Deltacrp/vpl(-) can induce adequate immunity to protect pigs against challenge with a heterologous virulent strain. Thus, this mutant holds promise for the development of a new live S. choleraesuis vaccine.

Novel attenuated Salmonella enterica serovar Choleraesuis strains as live vaccine candidates generated by signature-tagged mutagenesis

Salmonella enterica serovar Choleraesuis is a host-adapted pathogen that causes swine paratyphoid. Signature-tagged mutagenesis (STM) was used to understand the pathogenicity of S. enterica serovar Choleraesuis in its natural host and also to develop novel attenuated live vaccine candidates against this disease. A library of 960 signature-tagged mutants of S. enterica serovar Choleraesuis was constructed and screened for attenuation in pigs. Thirty-three mutants were identified by the STM screening, and these mutants were further screened for attenuation by in vivo and in vitro competitive growth. Of these, 20 mutants targeting the outer membrane, type III secretion, transporter, lipopolysaccharide biosynthesis, and other unknown proteins were confirmed for attenuation. Five highly attenuated mutants (SC2D2 [ssaV], SC4A9 [gifsy-1], SC6F9 [dgoT], SC12B12 [ssaJ], and SC10B1[spiA]) were selected and evaluated for safety and protective efficacy in pigs by comparison with a commercially available vaccine strain. STM-attenuated live vaccine strains SC4A9 (gifsy-1) and SC2D2 (ssaV) were superior to commercially available live vaccine because they provided both safety and a protective immune response against challenge in pigs.

Vaccination against fowl cholera with acapsular Pasteurella multocida A:1

We have previously constructed an acapsular Pasteurella multocida X-73 (serogroup A) mutant strain which was attenuated in virulence for chickens (Chung JY, Wilkie IW, Boyce JD, Townsend KM, Frost AJ, Ghodussi M, Adler B. Role of capsule in the pathogenesis of fowl cholera caused by Pasteurella multocida serogroup A. Infect. Immun. 2001;69:2487-2492). In this study, we have assessed the ability of this acapsular strain (PBA930) to induce protection against wild-type challenge in mice and the natural host chickens. Intramuscular administration of PBA930 to mice stimulated significant protection against X-73 and the heterologous strain P-1059 (A:3), but not against challenge with P-1662 (A:4). No protection was observed when PBA930 was introduced by the intraperitoneal or subcutaneous routes in mice. Significantly, the acapsular strain PBA930 was able to induce protection against challenge with wild type X-73 in chickens.

Vaccines against Infections Caused by Salmonella, Shigella, and Pathogenic Escherichia coli

Infectious diseases represent one of the most common causes of death worldwide, with the enteropathogenic bacteria Salmonella and Shigella and pathogenic Escherichia coli being among the most detrimental. Currently, vaccination represents the preferred method of preventing such infections. For stimulating the adaptive immune response, immunizations are frequently based on formulations which include inactivated whole-cell vaccines, live attenuated vaccines, or subunit vaccines. These can be administered via a parenteral or mucosal route, the latter having the advantage that it most closely mimics the actual course of infection. In addition to the type of vaccine and method of application, important consideration needs to be paid to safety, efficacy, and cost, which are often major bottlenecks in the successful implementation of vaccines. In this chapter we take a limited look at the history surrounding vaccinations involving Salmonella, Shigella, and pathogenic E. coli. Salmonella infections, which can lead to typhoid fever, are becoming increasing difficult to treat with antibiotics due to multi-drug-resistant strains. At present, the parenteral Vi-based subunit vaccines and the live attenuated oral vaccine Ty21a have proven to be the vaccines of choice, with high levels of protective efficacy and limited side effects. Shigella infections are responsible for the diarrheal disease shigellosis. Various live and nonliving mucosal and parenteral vaccines have been tested, with the most promising candidates evolving around those that stimulate the production of O-antigen-specific antibodies. Pathogenic Escherichia coli infections can lead to severe diseases due to the bacterium's production of several specific toxins. Vaccines against this bacterium target its toxins, as well as surface-exposed antigens, all of which have been found to be effective as immunogens.

Growth, virulence, and immunogenicity of Listeria monocytogenes aro mutants

Mutants of Listeria monocytogenes with deletions in genes of the common branch of the biosynthesis pathway leading to aromatic compounds were constructed as possible virulence-attenuated carrier strains for protein antigens or vaccine DNA. aroA, aroB, and in particular aroE mutants showed strongly reduced growth rates in epithelial cells and even in rich culture media. The metabolism of the aro mutants under these conditions was predominantly anaerobic. Aerobic metabolism and a wild-type growth rate were, however, regained upon the addition of vitamin K2, suggesting that the aro mutants are deficient in oxidative respiration due to the lack of menaquinone. Replication of the aro mutants in the host cell's cytosol and cell-to-cell spread were drastically slowed down, and all aro mutants showed high virulence attenuation in mice, i.e., the 50% lethal dose in BALB/c mice was increased at least 10(4)-fold for the aroA, aroB, and aroA/B mutants and >10(5)-fold for the aroE mutant compared to the parent strain. Nevertheless, mice preimmunized with aro mutant bacteria elicited good T-cell response and full protection against a subsequent challenge with the virulent wild-type strain. A total of 5 x 10(6) aroA, aroB, and aroA/B mutant bacteria were sufficient to obtain a protective T-cell response, while 5 x 10(8) aroE or aroA/E mutants were necessary to achieve comparable numbers of antigen-specific T cells. These numbers were well tolerated without causing any signs of disease, indicating that Listeria strains with deletions in genes of the basic branch of the aromatic amino acid pathway could be useful vaccine carriers for inducing T-cell immunity.

Construction, characterization, and evaluation of the vaccine potential of three genetically defined mutants of avian pathogenic Escherichia coli

The delta galE, delta purA, and delta aroA derivatives of avian septicemic Escherichia coli EC99 strain (O78 serogroup) were constructed with a suicide vector containing the pir-dependent R6K replicon and the sacB gene of Bacillus subtilis. The resultant isogenic mutants were stable and lacked approximately 45%, 36%, and 52% of the genes for galE, purA, and aroA, respectively. The delta purA and delta aroA mutants did not grow on minimal medium, whereas the delta galE mutant grew on minimal medium but was sensitive to galactose-induced lysis. The reversion frequencies of all three mutants were <10(-12). The mutants were highly attenuated for virulence as determined by administration of approximately 10(7) colony-forming units of each mutant to 1-day-old chicks by the subcutaneous route. Chickens were vaccinated with the mutants by spray (droplet size approximately 20 microm) at 1 and 14 days of age to determine safety, immunogenicity, and efficacy. The mutants were found to be safe. Seven days after a second vaccination, immunoglobulin (Ig)Y antibodies to E. coli in serum and air sac washings were detected by enzyme-linked immunosorbent assay. In both serum and air sac washings, IgY antibodies were significantly higher in chickens vaccinated with the mutants as compared with phosphate-buffered saline-treated controls but were significantly lower compared with chickens that were vaccinated with the parent strain. In serum, but not in air sac washings, IgY antibodies were significantly lower in chickens vaccinated with the mutants compared with the parent strain. The vaccinated chickens were given infectious bronchitis virus intranasally at 17 days of age and were challenged with homologous (EC99 strain) or heterologous (O2 serogroup) E. coli 4 days later. Chickens that received wild-type EC99 strain or its mutant derivatives were protected from homologous but not from heterologous challenge. This study indicates that the delta galE, delta purA, and delta aroA mutants are safe and moderately immunogenic but the protection conferred by the mutants is serogroup specific.

Salmonella enterica serotype Choleraesuis: epidemiology, pathogenesis, clinical disease, and treatment

Nontyphoid Salmonella strains are important causes of reportable food-borne infection. Among more than 2,000 serotypes, Salmonella enterica serotype Choleraesuis shows the highest predilection to cause systemic infections in humans. The most feared complication of serotype Cholearesuis bacteremia in adults is the development of mycotic aneurysm, which previously was almost uniformally fatal. The advances in diagnostic techniques, surgical care, and antimicrobial therapy have greatly improved the survival of these patients. However, the recent emergence of serotype Choleraesuis that is resistant to ampicillin, chloramphenicol, trimethoprim-sulfamethoxazole, and, notably, fluoroquinolone antibiotics has aroused concern about the use of these agents for the empirical treatment of systemic infection caused by this organism. In view of the serious implications of the situation, the chain of transmission and mechanism of resistance should be carefully studied to reduce the spread of infection and threat to human health. To date, there are no vaccines available to prevent serotype Choleraesuis infections in humans. The availability, in the near future, of the genome sequence of serotype Cholearesuis will facilitate the development of effective vaccines as well as the discovery of new targets for novel antimicrobial agents.

Salmonella typhimurium thyA mutants fail to grow intracellularly in vitro and are attenuated in mice

Salmonella typhimurium ATCC14028 readily multiplies in professional phagocytes in vitro and is highly virulent in mice. Mutants lacking thymidylate synthase activity (thyA) were isolated and shown to be strictly dependent on thymidine monophosphate precursors in the growth medium. The thyA mutants were found to be virtually incapable of intracellular growth and survival in vitro, both in macrophage-like cell line P338D(1) and in the human epithelial cell line Hep-2, and their virulence was impaired in BALB/c mice. Intraperitoneal immunization of mice with two doses of live S. typhimurium thyA provided protection against a challenge with 10(3) times the 50% lethal dose of the virulent parent strain.

Attenuation and immunogenicity of Deltacya Deltacrp 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.

All accessible epitopes in the Salmonella lipopolysaccharide core are associated with branch residues

Antisera generated against each of the nine known chemotypes of Salmonella lipopolysaccharide (LPS) core were characterized in order to delineate cross-reactive epitopes and define the bases for their accessibility. Strongly cross-reactive epitopes were associated with three chemotypes: Ra and Rb4, which recognized alpha-GlcNAc-1-->2-alpha-Glc, and Rd1, which recognized L-alpha-D-heptose-1-->7-L-alpha-D-heptose. Both these disaccharides and the more weakly cross-reactive alpha-Gal-1-->6-alpha-Glc terminal in Rb3 LPS represent branch points along the core oligosaccharide. Therefore, branch points in endotoxin core oligosaccharides may generally be cross-reactive.

Attenuation and vaccine potential of aroQ mutants of Corynebacterium pseudotuberculosis

Corynebacterium pseudotuberculosis, a gram-positive intracellular bacterial pathogen, is the etiological agent of the disease caseous lymphadenitis (CLA) in both sheep and goats. Attenuated mutants of C. pseudotuberculosis have the potential to act as novel live veterinary vaccine vectors. We have cloned and sequenced the aroB and aroQ genes from C. pseudotuberculosis C231. By allelic exchange, aroQ mutants of both C231, designated CS100, and a pld mutant strain TB521, designated CS200, were constructed. Infection of BALB/c mice indicated that introduction of the aroQ mutation into C231 and TB521 attenuated both strains. In sublethally infected BALB/c mice, both CS100 and CS200 were cleared from spleens and livers by day 8 postinfection. The in vivo persistence of these strains was increased when the intact aroQ gene was supplied on a plasmid in trans. Mice infected with TB521 harbored bacteria in organs at least till day 8 postinfection without ill effect. When used as a vaccine, only the maximum tolerated dose of CS100 had the capacity to protect mice from homologous challenge. Vaccination with TB521 also elicited protective immunity, and this was associated with gamma interferon (IFN-gamma) production from splenocytes stimulated 7 days postvaccination. The role of IFN-gamma in controlling primary infections with C. pseudotuberculosis was examined in mice deficient for the IFN-gamma receptor (IFN-gammaR(-/-) mice). IFN-gammaR(-/-) mice cleared an infection with CS100 but were significantly more susceptible than control littermates to infection with C231 or TB521. These studies support an important role for IFN-gamma in control of primary C. pseudotuberculosis infections and indicate that aroQ mutants remain attenuated even in immunocompromised animals. This is the first report of an aroQ mutant of a bacterial pathogen, and the results may have implications for the construction of aromatic mutants of Mycobacterium tuberculosis for use as vaccines.

Evaluation of an aroA mutant Salmonella typhimurium vaccine in chickens using modified semisolid Rappaport Vassiliadis medium to monitor faecal shedding

In groups of chickens vaccinated orally or intramuscularly with a live aroA mutant Salmonella typhimurium vaccine strain and then experimentally inoculated with 10(8) CFU of wild type S. typhimurium or 10(9) CFU of S. enteritidis, faecal shedding of the vaccine and wild type strains was monitored by the buffered peptone water-modified semisolid Rappaport Vassiliadis medium method, which detected less than 10(2) CFU per gram of faeces. The vaccine strain was shed in the faeces for up to 26 days. Vaccination failed to reduce the faecal shedding of wild type S. typhimurium or S. enteritidis. The variation in the shedding patterns of chickens within each group was greater than between treatment groups.

A riboflavin auxotroph of Actinobacillus pleuropneumoniae is attenuated in swine

Actinobacillus pleuropneumoniae is the etiological agent of a highly contagious and often fatal pleuropneumonia in swine. A riboflavin-requiring mutant of A. pleuropneumoniae serotype 1, designated AP233, was constructed by deleting a portion of the riboflavin biosynthetic operon (ribGBAH) and replacing it with a gene cassette encoding kanamycin resistance. The genes affected included both the alpha- and beta-subunits of riboflavin synthase as well as a bifunctional enzyme containing GTP cyclohydrase and 3,4-dihydroxy-2-butanone-4-phosphate synthase activities. AP233 was unable to grow in the absence of exogenous riboflavin but otherwise was phenotypically identical to the parent wild-type strain. Experimental infection studies with pigs demonstrated that the riboflavin-requiring mutant was unable to cause disease, on the basis of mortality, lung pathology, and clinical signs, at dosages as high as 500 times the normal 50% lethal dose for the wild-type parent. This is the first demonstration of the attenuation of A. pleuropneumoniae by introduction of a defined mutation in a metabolic gene and the first demonstration that mutations in the genes required for riboflavin biosynthesis can lead to attenuation in a bacterial pathogen.

Construction and vaccine potential of an aroA mutant of Pasteurella haemolytica

The aroA gene, encoding 5-enolpyruvylshikimate 3-phosphate synthase, from Pasteurella haemolytica biotype A, serotype 1 was cloned by complementation of the aroA mutation in Escherichia coli strain AB2829 after electroporation with a DNA library constructed in pUC18. The cloned P. haemolytica aroA gene was inactivated by insertion of a kanamycin resistance gene and reintroduced by allelic exchange into the chromosome of the parental P. haemolytica using PbluescriptII SK+. The P. haemolytica aroA mutant was highly attenuated in a mouse septicaemic model. Mice immunized intraperitoneally with two doses of live P. haemolytica aroA mutant were protected against a lethal parental strain challenge.

Development of mucosal protection against the heat-stable enterotoxin (ST) of Escherichia coli by oral immunization with a genetic fusion delivered by a bacterial vector

An LT-B-ST (LT-B/ST) fusion peptide was constructed by genetically joining the 5' terminus of a synthetic gene coding for the heat-stable enterotoxin (ST) of Escherichia coli to the 3' terminus of the gene coding for the binding subunit of the heat-labile enterotoxin (LT-B) of E. coli. An eight-amino-acid, proline-containing linker was included between the LT-B and ST moieties. An aroA mutant of Salmonella dublin transformed with a plasmid carrying this genetic construct was shown to express a fusion peptide with antigenic determinants of both LT-B and ST. Mice were immunized orally with this strain or with a control strain expressing just LT-B from the same plasmid. Sera and mucosal secretions were obtained and analyzed for the presence of serum immunoglobulin G and mucosal immunoglobulin A that were able to recognize LT-B and ST by enzyme-linked immunosorbent assay (ELISA) and, more importantly, were able to neutralize native ST in the suckling mouse assay. Sera and mucosal secretions from animals immunized with the strain expressing the LT-B/ST fusion exhibited detectable ELISA reactivity against LT-B but not against native ST. However, even in the absence of detectable ELISA reactivity, both sera and mucosal secretions from these animals were able to neutralize the biological activity of native ST in the suckling mouse assay. These findings are important because they demonstrate the development of mucosal protection against ST by oral immunization with a genetic fusion delivered by a bacterial vector.

Intracytoplasmic growth and virulence of Listeria monocytogenes auxotrophic mutants

The intracellular growth of several auxotrophic mutants of Listeria monocytogenes was examined in cell culture, and virulence was evaluated in mice by intravenous injection of log-phase bacteria. L. monocytogenes transposon insertion mutants requiring either uracil, phenylalanine, glycine, proline, or nicotinic acid for growth were fully virulent and grew similarly to the parental strain as shown by their growth rates in cell culture. Those requiring all three aromatic amino acids (phenylalanine, tryptophan, and tyrosine) or adenine were 1.5 log10 less virulent than the wild type. A threonine auxotroph, which showed enhanced growth in the presence of threonine-containing peptides as compared with that in the presence of free threonine, was approximately 1 log10 less virulent than the wild type. When host cells were deprived of specific amino acids required by both the host cell and L. monocytogenes, the bacteria continued to grow intracellularly. These studies suggest that the cytoplasm of eucaryotic cells behaves like rich medium, facilitating the growth of an intracellular bacterial pathogen with complex growth requirements. In addition, results related to amino acid deprivation during intracellular growth and specific extracellular growth requirements of a threonine auxotroph suggest that L. monocytogenes may utilize intracellular peptides as a source of amino acids.

An aromatic-dependent mutant of the fish pathogen Aeromonas salmonicida is attenuated in fish and is effective as a live vaccine against the salmonid disease furunculosis

Aeromonas salmonicida is the etiological agent of furunculosis in salmonid fish. The disease is responsible for severe economic losses in intensively cultured salmon and trout. Bacterin vaccines provide inadequate protection against infection. We have constructed an aromatic-dependent mutant of A. salmonicida in order to investigate the possibility of an effective live-attenuated vaccine. The aroA gene of A. salmonicida was cloned in Escherichia coli, and the nucleotide sequence was determined. The codon usage pattern of aroA was found to be quite distinct from that of the vapA gene coding for the surface array protein layer (A layer). The aroA gene was inactivated by inserting a fragment expressing kanamycin resistance within the coding sequence. The aroA::Kar mutation was introduced into the chromosome of virulent A. salmonicida 644Rb and 640V2 by allele replacement by using a suicide plasmid delivery system. The aroA mutation did not revert at a detectable frequency (< 10(-11). The mutation resulted in attenuation when bacteria were injected intramuscularly into Atlantic salmon (Salmo salar L.). Introduction of the wild-type aroA gene into the A. salmonicida mutants on a broad-host-range plasmid restored virulence. A. salmonicida mutant 644Rb aroA::Kar persisted in the kidney of brown trout (Salmo trutta L.) for 12 days at 10 degrees C. Vaccination of brown trout with 10(7) CFU of A. salmonicida 644Rb aroA by intraperitoneal injection resulted in a 253-fold increase in the 50% lethal dose (LD50) compared with unvaccinated controls challenged with a virulent clinical isolate 9 weeks later. A second vaccination after 6 weeks increased the LD50 by a further 16-fold.

Oral immunization of mice and swine with an attenuated Salmonella choleraesuis [delta cya-12 delta(crp-cdt)19] mutant containing a recombinant plasmid

Salmonella choleraesuis chi 3781, an attenuated [delta cya-12 delta(crp-cdt)19] mutant, was electroporated with the plasmid pBA31-R7, which codes for the expression of a 31-kDa protein from Brucella abortus (BCSP31). Recombinant S. choleraesuis chi 3781 stably maintained the pBA31-R7 plasmid and continued to express the cloned protein following recovery of the organism from orally inoculated animals. Unlike previous studies using S. typhimurium chi 4064(pBA31-R7), S. choleraesuis chi 3781(pBA31-R7) was able to colonize both the gut mucosa and deep tissues of both BALB/cByJ mice and crossbred swine. Orally inoculated mice developed serum antibodies to both the cloned 31-kDa protein (rBCSP31) and to S. choleraesuis chi 3781 endotoxin. These mice also developed a local intestinal antibody response to Salmonella endotoxin but not to rBCSP31. Similarly, mice inoculated with recombinant S. choleraesuis chi 3781 did not develop a delayed-type hypersensitivity (DTH) footpad response following injection with rBCSP31; however, these mice did respond to S. choleraesuis chi 3781 soluble antigen. Conversely, orally inoculated swine did not develop significant serum or intestinal antibody responses to cloned protein or Salmonella endotoxin, but DTH responses to both cloned protein and S. choleraesuis chi 3781 soluble antigen were strongly positive. The cell-mediated nature of these DTH responses was confirmed by histological examination. Results suggest that S. choleraesuis chi 3781 may be a suitable choice for further studies of vaccine efficacy in swine, especially for diseases which require cell-mediated immunity for resolution.

Characterization and protective properties of attenuated mutants of Salmonella choleraesuis

We have constructed crp::Tn10 and cya::Tn10 Salmonella choleraesuis mutants and their fusaric acid-resistant derivatives with deletions (delta) of the Tn10 and adjacent DNA sequences and found them to be avirulent and able to induce protection against a wild-type challenge in 8-week-old BALB/c mice. Mice survived infection with the crp and cya mutants at doses of more than 7 x 10(3) times the oral (p.o.) 50% lethal dose (LD50) and more than 8 x 10(2) times the intraperitoneal LD50 of the wild-type S. choleraesuis parent. Mice vaccinated with attenuated strains were protected against challenge with more than 1.6 x 10(4) times the p.o. LD50 and more than 80 times the intraperitoneal LD50 of the wild-type virulent S. choleraesuis parent. One deletion mutation isolated in the crp region extends to an adjacent gene(s) that was shown to be associated with avirulence. This gene or operon has been designated cdt (colonization of deep tissues). A delta (crp-cdt)19 strain, when complemented with the wild-type crp gene and promoter on a pBR-derived plasmid, had p.o. LD50 values 10(3) times higher than those for the wild type. A delta cya delta (crp-cdt)19 double mutant was less virulent than and afforded more complete protection against a challenge with the wild-type strain than a delta crp-11 delta cya double mutant or the individual cya, crp, or crp+/cdt mutants. The deletion derivatives exhibited reduced invasion of CHO cells in vitro, and the numbers of the mutants recovered from mouse tissues were less than that of the parent strain. These studies suggest that one or more of the genes involved in cell attachment to and/or invasion of S. choleraesuis may be under catabolite repression. In addition, we describe a new deletion of a gene(s) located in the crp region between cysG and argD that is associated with virulence in S. choleraesuis.

Oral immunization using live attenuated Salmonella spp. as carriers of foreign antigens

A variety of techniques, including the use of live oral vaccines, have been used to deliver antigens to the gut-associated lymphoid tissues in an attempt to initiate production of specific secretory immunoglobulin A for protection against pathogens that colonize or cross mucosal surfaces to initiate infection. A number of attenuated Salmonella mutants are able to interact with the lymphoid tissues in the Peyer's patches but are not able to cause systemic disease. Some of these mutants are effective as live vaccines (i.e., able to protect against infection with the virulent Salmonella parent) and are candidates for use as carriers for virulence determinants of other mucosal pathogens. This has been shown to be an effective means of stimulating significant levels of specific mucosal secretory immunoglobulin A directed against the carrier strains and against a variety of heterologous antigens and has been shown to stimulate production of serum antibodies and cell-mediated responses as well. This review examines the history of this mechanism of vaccine delivery and summarizes the most recent applications of this evolving technology. This is a technique for vaccine delivery with significant potential for influencing the management of infectious diseases on a large scale. It can be used not only for vaccines against enteric bacterial pathogens but also for vaccines against a variety of other bacteria, viruses, and parasites. The results obtained to date are encouraging, and there is great potential for development of safe, effective, affordable vaccines.

Salmonella choleraesuis and Salmonella typhimurium associated with liver cells after intravenous inoculation of rats are localized mainly in Kupffer cells and multiply intracellularly

Male Sprague-Dawley rats were inoculated intravenously with Salmonella choleraesuis or Salmonella typhimurium and used over 3 consecutive days to produce highly enriched (greater than 95% homogenous) preparations of Kupffer and mononuclear cells (KC), liver endothelial cells (LEC), and hepatocytes. The methods involved collagenase perfusion of the liver in situ, differential centrifugation of liver cells over a Percoll gradient, and selective attachment of the cells to plastic or to culture dishes coated with collagen. The different cell preparations were then assayed for the number and location, intracellular or extracellular, of associated viable bacteria. Most of the viable bacteria recovered were associated with KC and were mainly intracellular. The intracellular bacteria in KC from rats infected with either bacterial strain increased about 20- to 50-fold over 2 days. Some of the bacteria associated with LEC and in some experiments with hepatocytes also survived treatment with gentamicin and increased in number with time. Intracellular bacteria were readily visualized in KC by light microscopy and transmission electron microscopy. On rare occasions, bacteria were seen within LEC from rats infected with S. choleraesuis but not from those infected with S. typhimurium. Microcolonies of S. typhimurium but not of S. choleraesuis were occasionally found on the surface of some LEC. Bacteria were not seen within or on the surface of hepatocytes by transmission or scanning electron microscopy. The integration of microscopic and viability data suggested that most intracellular S. choleraesuis organisms in KC had been killed whereas most intracellular S. typhimurium organisms were viable.

Relevance of inoculation route to virulence of three Salmonella spp. strains in mice

The virulence of three Salmonella species strains was compared by the i.p. and i.v. routes in BALB/c mice. Salmonella choleraesuis, SL2824 (serogroup C1, O-6,7), was more virulent by the i.v. than i.p. route. A strain of S. typhimurium, SL1260 (serogroup B; O-1,4,12) was more virulent i.p. than i.v. while another strain, SL3201 (O-4,5,12) was equally virulent i.p. or i.v. The LD50 of each strain by either route correlated with the number of bacteria in the liver and spleen on day one after inoculation and thus seems determined mainly by initial bactericidal mechanisms. The rate of bacterial growth in the liver and spleen was independent of inoculation route but differed between the three strains. Salmonella choleraesuis multiplied faster than either strain of S. typhimurium. Non-virulent aromatic-dependent (aro) derivatives of these strains were tested, instead of their virulent ancestors, for survival within peritoneal macrophages in vitro. Salmonella choleraesuis SL 2824 aro and S. typhimurium SL1260 aro were much more readily killed intracellularly than S. typhimurium SL3201 aro. The data indicate that the survival and multiplication of different Salmonella serotypes or strains in vivo may depend on different critical properties or mechanisms to overcome host defenses.

Immunity conferred by Aro- Salmonella live vaccines

The specificity of protection conferred by Aro- salmonellae was studied in BALB/c mice challenged 3 months after intravenous (i.v.) vaccination, more than 1 month after the vaccine had been cleared. Oral challenge showed better protection than i.v. challenge. Salmonella typhimurium aroA SL3261 conferred very good protection against wild-type S. typhimurium C5 (over 10,000 x LD50). Cross protection experiments were performed using S. typhimurium, S. enteritidis and S. dublin for vaccination and challenge, including variants of S. typhimurium and S. enteritidis of similar virulence differing in the main LPS antigen (O-4 or O-9). Salmonella typhimurium aroA conferred solid protection against S. typhimurium (O-4), but no protection against wild-type S. enteritidis (O-9). However challenge with LPS variant strains showed that although protection was generally better to strains of the homologous LPS type, specificity of protection was determined more by the parent strain background (S. typhimurium or S. enteritidis) of the challenge than by O-factors 4 or 9, suggesting that other antigens are involved. The nature of the protective antigen(s) in this model is unclear, but it does not appear to be the main O-specific antigen. A S. enteritidis Se795 aroA vaccine gave good protection against wild-type S. enteritidis Se795 2 weeks after vaccination, but much less at 3 months (approximately 10-200 x LD50), although the persistence of the S. enteritidis aroA vaccine in the liver and spleen was similar to that of the S. typhimurium vaccine, and the wild-type Se795 challenge strain was of similar virulence to S. typhimurium C5. A S. dublin aroA vaccine conferred similar protection against wild-type S. dublin (approximately 300 x LD50).

Immunity induced by live attenuated Salmonella vaccines

Studies on the degree and specificity of protection conferred by immunization with aroA salmonella live vaccines in BALB/c mice are described. Animals were immunized i.v. and challenged orally 3 months later to ensure that the vaccine had been cleared from the tissues. Vaccination with Salmonella typhimurium aroA SL3261 conferred very good protection against virulent S. typhimurium C5 (over 10,000 x LD50). The specificity of cross protection was studied using S. typhimurium, Salmonella enteritidis and Salmonella dublin for vaccination and challenge, including challenge with variants of S. typhimurium and S. enteritidis of similar virulence which differed in the main LPS (lipopolysaccharide) antigen (0-4 or 0-9). S. typhimurium SL3261 gave very good protection against S. typhimurium C5 (0-4), but no protection against S. enteritidis Se795 (0-9). However, challenge with strains differing in the main 0 antigens showed that, although protection was generally better to strains expressing the same LPS type as the vaccine, specificity of protection was determined more by the background (S. typhimurium or S. enteritidis) of the parent strain used for the challenge than by 0 factors 4 or 9, suggesting that other factors could be involved. The nature of the antigen(s) responsible for protection in this model is unclear, but it would not appear to be the main 0-specific antigen. An S. enteritidis Se795 aroA vaccine was far less effective than S. typhimurium SL3261; it conferred good protection against the homologous wild type at 2 weeks post-vaccination, but far less at three months (approx 10-200 x LD50). This was unexpected, as the persistence of the S. enteritidis vaccine in the liver and spleen was similar to that of S. typhimurium SL3261, and the S. enteritidis and S. typhimurium challenge strains were of similar virulence. An S. dublin aroA vaccine conferred similar protection against wild type S. dublin (approx 300 x LD50).

Salmonella choleraesuis strains deficient in O antigen remain fully virulent for mice by parenteral inoculation but are avirulent by oral administration

O-antigen-deficient derivatives of two mouse-virulent strains of Salmonella choleraesuis (serogroup C1; O-6,7) were constructed by transduction of a long deletion of the rfb operon. Strains SN36 and SN57 were derived from the smooth ancestor SL2824, while SN37 was derived from the smooth ancestor SL2840. These rfb deletion derivatives (rfb strains) had typical bacteriophage sensitivity patterns of "rough" Salmonella strains and were at least 200,000 times more sensitive to serum than their smooth ancestors. Lipopolysaccharides (LPS) extracted from them consisted only of two low-molecular-weight bands and lacked the ladderlike pattern of bands seen in the LPS of their smooth ancestors. The LPS from the rfb strains did not react in an enzyme immunoassay with any of three monoclonal antibodies directed against different epitopes of the O-6,7 antigen but reacted well with at least one of three monoclonal antibodies specific for core epitopes. The data were consistent with inability of these strains to synthesize O-specific chains and showed that the LPS extracted from SN57 was of chemotype Ra and that from SN36 was of chemotype Rb1, while that of SN37 consisted of a mixture of the two chemotypes. The virulence of these strains was tested by various routes in BALB/c mice. All three O-antigen-deficient derivatives were about as virulent as their "smooth" ancestors by the intraperitoneal and intravenous routes (50% lethal dose, 20 to 700 bacteria) but, unlike their ancestors, were avirulent by the oral route (50% lethal dose, greater than or equal to 5 x 10(9) bacteria). This suggests that the major role of smooth LPS in the mouse virulence of S. choleraesuis is to facilitate survival in the gastrointestinal tract and eventual entry into deeper tissues.

Lysogenization of Salmonella choleraesuis by phage 14 increases average length of O-antigen chains, serum resistance and intraperitoneal mouse virulence

Three clones from a strain of Salmonella choleraesuis (serogroup C1) were lysogenized with phage 14 (P14) which converts the O-antigen of serogroup C1 salmonellae from O-6,7 to O-6,7,14. The lysogens were compared with their parental non-lysogenic clones with respect to the following properties: average length of O-antigen polysaccharide chains, sensitivity to normal human serum, and mouse-virulence. SDS-polyacrylamide gel electrophoresis of lipopolysaccharides extracted from these bacteria showed that samples from lysogens consisted mainly of long-chained molecules whereas those from non-lysogens contained mainly short-chained molecules. The O-antigen polysaccharide from a lysogen was estimated by chemical analysis to be six times as long as that from a non-lysogen. Lysogens were serum-resistant whereas non-lysogens were serum-sensitive. About 10 times more colony forming units of a lysogen than of a non-lysogen were recovered from the livers and spleens of mice on day 1 and 3 after intraperitoneal inoculation of equal doses. By comparison with S. choleraesuis, lysogenization of S. typhimurium with phage P22 or phage A4 did not affect the chain-length distribution of O-antigen polysaccharide. Our data suggest that phage 14-coded determinants increase efficiency of O-antigen biosynthesis in S. choleraesuis leading to increase in average length of O-polysaccharide chains. Increased serum resistance and mouse virulence are logical consequences of increase in average length of O-polysaccharide chains and represent phage-conferred selective advantage not previously described in Salmonella.

Efficient production of mouse and rat monoclonal antibodies against the O antigens of Salmonella serogroup C1, using LPS-coated bacteria as immunogen

A variety of different immunogens and immunisation schemes were investigated for the production of monoclonal antibodies directed against the O antigenic polysaccharide of the lipopolysaccharide (LPS) of Salmonella serogroup C1 (O:6,7). Of 12 fusions performed, higher yields of stable, LPS-reactive hybridomas producing monoclonal antibodies were achieved in both the mouse and rat when using O:6,7 LPS-coated S. thompson bacteria as immunogens than with live and heat-killed bacteria, or O:6,7-BSA glycoconjugate as immunogens. All of the 17 hybrid clones obtained were shown to bind the O antigens of Salmonella serogroup C1 when tested in ELISA against a set of chemically defined LPS from Salmonella smooth and rough strains. The results are discussed with a view to bettering the immunisation strategy for production of monoclonal antibodies against the LPS antigens of bacteria.

Salmonella-based vaccines

There continues to be considerable interest in the development of a safe, effective, live, oral vaccine to combat typhoid fever of humans. Such a vaccine may be a derivative of the causative agent of the disease, Salmonella typhi. The prototype of such a vaccine, Ty21a, is not ideal, but no replacement for Ty21a is yet obvious. The construction and trial of bivalent vaccines, in which an attenuated Salmonella strain expresses determinants from another pathogen, awaits the development of a suitably attenuated derivative. In parallel with vaccine development programmes, a variety of techniques have been designed to effect stable association between Salmonella carrier and introduced cloned DNA.

Vaccination route, infectivity and thioglycollate broth administration: effects on live vaccine efficacy of auxotrophic derivatives of Salmonella choleraesuis

An aromatic-dependent, therefore non-virulent, derivative of a mouse-virulent strain of Salmonella choleraesuis previously shown not to be effective as a live vaccine when given intraperitoneally (i.p.) to Itys mice, was administered to BALB/c mice. Two doses given i.p. or by feeding did not protect against i.p. or oral challenge with 50 to 5000 LD50 of the virulent ancestor strain. By contrast two doses given intravenously (i.v.) gave almost complete protection against i.p. or oral challenge with 500 LD50 and some protection against larger doses. The number of live bacteria (cfu) in the liver and spleen 24 h after administration of the live vaccine was less than 1% of the number inoculated i.p., but c. 25% of the number injected i.v. The number of cfu in the gut 24 h after oral vaccine administration was only c 10(-5) of the number fed. Administration of thioglycollate broth i.p. 5 days before i.p. vaccination increased recovery of live vaccine cfu in the liver and spleen and its protective efficacy. In each case the live vaccine did not multiply extensively in vivo. We have previously shown that a purine- and a thymine-requiring derivative of S. choleraesuis were each considerably attenuated but unlike the aro derivative were effective as i.p. live vaccines in mice. Doses of these strains (c. 10(4) cfu) found protective were administered i.p. to BALB/c mice. Each strain multiplied extensively in the liver and spleen to c. 10(7) cfu by day 6. All these results are in agreement with a correlation of protective efficacy of a live vaccine with the persistence of a large number of the vaccine bacteria in the liver and spleen for several days.

Virulence and vaccine potential of phoP mutants of Salmonella typhimurium

We have constructed Salmonella typhimurium phoP mutants and found them to be avirulent and able to induce a protective immune response. BALB/c mice survived challenge with phoP derivatives of the highly virulent S. typhimurium strains SR-11 and SL1344 when inoculated intraperitoneally and per oral with doses equivalent to 10(4) 50% lethal doses (LD50) of the parent virulent strains. The avirulent mutants were able to establish an infection of the Peyer's patches of orally infected animals for up to 10 days after inoculation but were very inefficient at reaching the spleens. Despite the low level of infectivity of these mutants, immunized animals developed a delayed-type hypersensitivity (DTH) response to Salmonella antigens and resisted challenge with up to 10(4) LD50 of the virulent parent strain 30 days after immunization.

Characterization of aromatic- and purine-dependent Salmonella typhimurium: attention, persistence, and ability to induce protective immunity in BALB/c mice

Stable transposon-generated auxotrophic mutations in aroA, purA, and purE or aroA and purA together were introduced into Salmonella typhimurium strains which were virulent in mice. Strains harboring any of these mutations were attenuated when tested in BALB/c mice. purE strains were less attenuated than aroA or purA strains. Both aroA and purA mutants persisted for several weeks in the livers and spleens of the mice after intravenous infection, although the numbers of viable cells detected at various times after infection differed. aroA strains persisted at a higher level than purA strains and were effective live vaccines given intravenously or orally. purA strains were ineffective as oral vaccines and were poor intravenous vaccines. Strains harboring both aroA and purA mutations together were ineffective vaccines when administered orally or intravenously even though they persisted in the livers and spleens of the mice for long periods after intravenous infection.

The effects of O-antigen character and enterobacterial common antigen content on the in vivo persistence of aromatic-dependent Salmonella sp. live-vaccine strains

Aromatic-dependent (aro) derivatives of Salmonella choleraesuis like aro S. typhimurium are non-virulent but, unlike them, are ineffective as live vaccines in mice, given i.p. An aro derivative of S. choleraesuis did not persist in the liver and spleen (RES) of mice after i.p. inoculation whereas a similar derivative of S. typhimurium persisted. S. choleraesuis (O group C1; O-6,7) and S. typhimurium [O group B; O-(1),4(5),12] differ in O antigen of LPS, determined by chromosomal locus, rfb. Three pairs of nearly-isogenic aro derivatives, one member O-6,7 and the other O-(1),4,(5),12, were constructed in two lines of S. typhimurium by replacement of their B-rfb genes with the C1-rfb genes of S. choleraesuis. In tests for persistence after mixed or separate i.p. inoculation of equal doses into BALB/c mice the O-(1),4,(5),12 member of each pair was recovered as CFU in the RES at ca. 100-fold greater number than the O-6,7 member at 24 hours post-inoculation and subsequently. O-6,7 derivatives of S. typhimurium constructed as described above by a simple replacement of group B with group C-rfb locus synthesise only trace (tr) amounts of enterobacterial common antigen (ECA). An ECA+ (able to make normal levels of ECA) derivative of one aro, O-6,7 S. typhimurium strain was constructed by replacement of its B-rfe locus with the C-rfe locus of S. choleraesuis. Tested by mixed inoculation i.p. this strain persisted in the RES in numbers 10-fold greater than its O-6,7 ECAtr but 5-10-fold lesser than its O-(1),4,(5),12 cousins. Thus both O-specificity and ECA contribute to the survival of salmonella species in mice as determined by in vivo persistence of non-multiplying aro derivatives.