Characterization and protective properties of attenuated mutants of Salmonella choleraesuis

Impact of the Resistance Responses to Stress Conditions Encountered in Food and Food Processing Environments on the Virulence and Growth Fitness of Non-Typhoidal Salmonellae

The success of Salmonella as a foodborne pathogen can probably be attributed to two major features: its remarkable genetic diversity and its extraordinary ability to adapt. Salmonella cells can survive in harsh environments, successfully compete for nutrients, and cause disease once inside the host. Furthermore, they are capable of rapidly reprogramming their metabolism, evolving in a short time from a stress-resistance mode to a growth or virulent mode, or even to express stress resistance and virulence factors at the same time if needed, thanks to a complex and fine-tuned regulatory network. It is nevertheless generally acknowledged that the development of stress resistance usually has a fitness cost for bacterial cells and that induction of stress resistance responses to certain agents can trigger changes in Salmonella virulence. In this review, we summarize and discuss current knowledge concerning the effects that the development of resistance responses to stress conditions encountered in food and food processing environments (including acid, osmotic and oxidative stress, starvation, modified atmospheres, detergents and disinfectants, chilling, heat, and non-thermal technologies) exerts on different aspects of the physiology of non-typhoidal Salmonellae, with special emphasis on virulence and growth fitness.

Effect of cAMP Receptor Protein Gene on Growth Characteristics and Stress Resistance of Haemophilus parasuis Serovar 5

Haemophilus parasuis (HPS), a member of the family Pasteurellaceae, is a common bacteria in the upper respiratory tract of pigs but under certain circumstances can cause serious systemic disease (Glasser's disease) characterized by severe infection of the upper respiratory tract, fibrinous polyserositis, polyarthritis, and meningitis. cAMP receptor protein (CRP) is among the most important global regulators, playing a vital role in adapting to environmental changes during the process of bacterial infection. In order to investigate the function of the crp gene in the growth characteristics of H. parasuis serovar 5 (HPS5) and its ability to overcome adverse environmental stresses, a crp mutant strain (Δcrp) was constructed and verified. In this study, we found that the crp gene was involved in growth rate, biofilm formation, stress tolerance, serum resistance, and iron utilization. Compared with the wild type, both the growth rate of the crp mutant and its resistance to osmotic pressure decreased significantly. Similar phenomena were also found in biofilm formation and iron utilization. However, the resistance to heat shock and serum complement of the crp mutant were enhanced. This study aimed to reveal the function in growth characteristics and stress resistance of the crp gene in HPS5. Whether it relates to virulence requires additional in-depth research.

Phenotype of Aeromonas salmonicida sp. salmonicida cyclic adenosine 3',5'-monophosphate receptor protein (Crp) mutants and its virulence in rainbow trout (Oncorhynchus mykiss)

Precise deletion of genes related to virulence can be used as a strategy to produce attenuated bacterial vaccines. Here, we study the deletion of the cyclic-3',5'-adenosine monophosphate (cAMP) receptor protein (Crp) in Aeromonas salmonicida, the aetiologic agent of furunculosis in marine and freshwater fish. The Crp protein is a conserved global regulator, controlling physiology processes, like sugar utilization. Deletion of the crp gene has been utilized in live attenuated vaccines for mammals, birds and warm water fish. Here, we characterized the crp gene and reported the effect of a crp deletion in A. salmonicida virulent and non-virulent isolates. We found that A. salmonicida Δcrp was not able to utilize maltose and other sugars, and its generation time was similar to the wild type. A. salmonicida ∆crp showed a higher ability of cell invasion compared to the wild type. Fish challenges showed that A. salmonicida ∆crp is ~6 times attenuated in Oncorhynchus mykiss and conferred protective immunity against the intraperitoneal challenge with A. salmonicida wild type. We concluded that deletion of A. salmonicida crp influences sugar utilization, cell invasion and virulence. Deletion of crp in A. salmonicida could be considered as part of an effective strategy to develop immersion live attenuated vaccines against furunculosis.

Edwardsiella piscicida lacking the cyclic AMP receptor protein (Crp) is avirulent and immunogenic in fish

Edwardsiella piscicida is a Gram-negative pathogen that generally causes lethal septicemia in marine and freshwater fish. We generated a E. piscicida CK216 Δcrp mutant to investigate various biological roles related to this organism, including pathogenesis. Lack of Crp in CK216 was demonstrated by immunoblotting using a Crp-specific antibody. Compared to the parental strain, the mutant exhibited changes in three biochemical phenotypes, including ornithine decarboxylation, citrate utilization, and H₂S production. Complementation of crp deletion in trans rescued the phenotype of the parental strain. This study proved that hemolytic activity in E. piscicida is controlled by Crp. In addition, significantly reduced motility of E. piscicida CK216 was observed, which resulted from a lack of flagella synthesis. To examine the virulence in fish, E. piscicida cells were injected into the goldfish (Carassius auratus) via intraperitoneal route. The LD₅₀ of CK216 was 9.25 × 10⁸ CFU, while that of the CK108 parental strain was 9.24 × 10⁵ CFU, attenuated 1000 fold in goldfish. Fish immunized with CK216 elicited IgM responses. Moreover, 80% of goldfish immunized with 1 × 10⁶ CFU survived after administration of a lethal dose (1 × 10⁷ CFU) of virulent E. piscicida CK41, suggesting the potential for E. piscicida CK216 to serve as a live attenuated vaccine in aquaculture.

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.

Identification of the crp gene in avian Pasteurella multocida and evaluation of the effects of crp deletion on its phenotype, virulence and immunogenicity

Background

Pasteurella multocida (P. multocida) is an important veterinary pathogen that can cause severe diseases in a wide range of mammals and birds. The global regulator crp gene has been found to regulate the virulence of some bacteria, and crp mutants have been demonstrated to be effective attenuated vaccines against Salmonella enterica and Yersinia enterocolitica. Here, we first characterized the crp gene in P. multocida, and we report the effects of a crp deletion.

Results

The P. multocida crp mutant exhibited a similar lipopolysaccharide and outer membrane protein profile but displayed defective growth and serum complement resistance in vitro compared with the parent strain. Furthermore, crp deletion decreased virulence but did not result in full attenuation. The 50 % lethal dose (LD₅₀) of the Δcrp mutant was 85-fold higher than that of the parent strain for intranasal infection. Transcriptome sequencing analysis showed that 92 genes were up-regulated and 94 genes were down-regulated in the absence of the crp gene. Finally, we found that intranasal immunization with the Δcrp mutant triggered both systematic and mucosal antibody responses and conferred 60 % protection against virulent P. multocida challenge in ducks.

Conclusion

The deletion of the crp gene has an inhibitory effect on bacterial growth and bacterial resistance to serum complement in vitro. The P. multocida crp mutant was attenuated and conferred moderate protection in ducks. This work affords a platform for analyzing the function of crp and aiding the formulation of a novel vaccine against P. multocida.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0739-y) contains supplementary material, which is available to authorized users.

Use of Attenuated but Metabolically Competent Salmonella as a Probiotic To Prevent or Treat Salmonella Infection

Salmonella enterica is among the most burdensome of foodborne disease agents. There are over 2,600 serovars that cause a range of disease manifestations ranging from enterocolitis to typhoid fever. While there are two vaccines in use in humans to protect against typhoid fever, there are none that prevent enterocolitis. If vaccines preventing enterocolitis were to be developed, they would likely protect against only one or a few serovars. In this report, we tested the hypothesis that probiotic organisms could compete for the preferred nutrient sources of Salmonella and thus prevent or treat infection. To this end, we added the fra locus, which encodes a utilization pathway for the Salmonella-specific nutrient source fructose-asparagine (F-Asn), to the probiotic bacterium Escherichia coli Nissle 1917 (Nissle) to increase its ability to compete with Salmonella in mouse models. We also tested a metabolically competent, but avirulent, Salmonella enterica serovar Typhimurium mutant for its ability to compete with wild-type Salmonella The modified Nissle strain became more virulent and less able to protect against Salmonella in some instances. On the other hand, the modified Salmonella strain was safe and effective in preventing infection with wild-type Salmonella While we tested for efficacy only against Salmonella Typhimurium, the modified Salmonella strain may be able to compete metabolically with most, if not all, Salmonella serovars, representing a novel approach to control of this pathogen.

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.

Complete Genome Sequence of Salmonella enterica Serovar Choleraesuis Vaccine Strain C500 Attenuated by Chemical Mutation

Salmonella enterica serovar Choleraesuis strain C500 is a live vaccine attenuated by chemical methods. Here, we report the complete genome sequence of the strain, which may be helpful for elucidating the attenuation mechanism of the vaccine strain.

Safety and protective efficacy of live attenuated Salmonella Gallinarum mutants in Rhode Island Red chickens

Salmonella enterica serovar Gallinarum is the causative agent of fowl typhoid, an important systemic disease of poultry with economic consequences in developing nations. A live attenuated orally applied S. Gallinarum vaccine could provide a low cost method for controlling this disease. We constructed S. Gallinarum strains in which the expression of the crp, rfc and rfaH genes, important for virulence of Salmonella Typhimurium in mice, were under the control of an arabinose-regulated promoter. We evaluated the virulence of these strains compared to wild-type S. Gallinarum and to mutants carrying deletions in these genes. We found that rfc mutants were fully virulent, indicating that, unlike the S. Typhimurium mouse model, the rfc gene is dispensable in S. Gallinarum for virulence in birds. In the case of rfaH, the deletion mutant was attenuated and protective, while the strain with arabinose-regulated rfaH expression retained full virulence. The strain exhibiting arabinose-regulated crp expression was attenuated. Its virulence was not affected by the inclusion of 0.2% arabinose in the drinking water. Birds immunized with this strain were protected against a lethal S. Gallinarum challenge and against colonization with the human pathogen Salmonella Enteritidis. This work shows that an arabinose-regulated crp strain provides a basis for further development of a fowl typhoid vaccine.

Natural lysogenization and transduction in Salmonella enterica serovar Choleraesuis by bacteriophage P1

It has been reported that bacteriophage P1 injects DNA into serovar Choleraesuis without evidence of productive infection. However, we found that P1 generates progeny and is capable of transduction in serovar Choleraesuis. This is not the case with other serovars of Salmonella enterica we tested. Therefore, P1 could play a role in serovar Choleraesuis evolution and contribute to its genetic manipulation and analysis.

Evolution of Salmonella enterica virulence via point mutations in the fimbrial adhesin

Whereas the majority of pathogenic Salmonella serovars are capable of infecting many different animal species, typically producing a self-limited gastroenteritis, serovars with narrow host-specificity exhibit increased virulence and their infections frequently result in fatal systemic diseases. In our study, a genetic and functional analysis of the mannose-specific type 1 fimbrial adhesin FimH from a variety of serovars of Salmonella enterica revealed that specific mutant variants of FimH are common in host-adapted (systemically invasive) serovars. We have found that while the low-binding shear-dependent phenotype of the adhesin is preserved in broad host-range (usually systemically non-invasive) Salmonella, the majority of host-adapted serovars express FimH variants with one of two alternative phenotypes: a significantly increased binding to mannose (as in S. Typhi, S. Paratyphi C, S. Dublin and some isolates of S. Choleraesuis), or complete loss of the mannose-binding activity (as in S. Paratyphi B, S. Choleraesuis and S. Gallinarum). The functional diversification of FimH in host-adapted Salmonella results from recently acquired structural mutations. Many of the mutations are of a convergent nature indicative of strong positive selection. The high-binding phenotype of FimH that leads to increased bacterial adhesiveness to and invasiveness of epithelial cells and macrophages usually precedes acquisition of the non-binding phenotype. Collectively these observations suggest that activation or inactivation of mannose-specific adhesive properties in different systemically invasive serovars of Salmonella reflects their dynamic trajectories of adaptation to a life style in specific hosts. In conclusion, our study demonstrates that point mutations are the target of positive selection and, in addition to horizontal gene transfer and genome degradation events, can contribute to the differential pathoadaptive evolution of Salmonella.

Identification of genes responsible for biofilm formation or virulence in Salmonella enterica serovar pullorum

Salmonella living in biofilms are more resistant to chemical and physical stresses. However, information regarding the regulation of genes involved in biofilm formation for Salmonella enterica serovar Pullorum remains limited. In this study, eight mutants with knockout of genes ompR, rpoS, rfaG, rfbH, rhlE, metE, spiA, or steB from the Salmonella enterica serovar Pullorum strain S6702 were constructed. Phenotypic analysis revealed that all mutants were similar to the wild-type strain in growth rate. Only the ompR mutant showed a complete loss of production ofcurli and biofilm formation. The other mutants showed a modified production of curli and cellulose with less effect related to biofilm formation. The results of animal experiments indicated that the deletion of genes ompR, spiA, rfaG, or metE in wild-type strains contributed to attenuation of virulence in 1-day-old chickens. This study may bring new insights into novel vaccines or therapeutic interventions against Salmonella enterica serovar Pullorum infections.

Phenotype, virulence and immunogenicity of Edwardsiella ictaluri cyclic adenosine 3',5'-monophosphate receptor protein (Crp) mutants in catfish host

Edwardsiella ictaluri is an Enterobacteriaceae that causes lethal enteric septicemia in catfish. Being a mucosal facultative intracellular pathogen, this bacterium is an excellent candidate to develop immersion-oral live attenuated vaccines for the catfish aquaculture industry. Deletion of the cyclic 3',5'-adenosine monophosphate (cAMP) receptor protein (crp) gene in several Enterobacteriaceae has been utilized in live attenuated vaccines for mammals and birds. Here we characterize the crp gene and report the effect of a crp deletion in E. ictaluri. The E. ictaluri crp gene and encoded protein are similar to other Enterobacteriaceae family members, complementing Salmonella enterica Δcrp mutants in a cAMP-dependent fashion. The E. ictaluri Δcrp-10 in-frame deletion mutant demonstrated growth defects, loss of maltose utilization, and lack of flagella synthesis. We found that the E. ictaluri Δcrp-10 mutant was attenuated, colonized lymphoid tissues, and conferred immune protection against E. ictaluri infection to zebrafish (Danio rerio) and catfish (Ictalurus punctatus). Evaluation of the IgM titers indicated that bath immunization with the E. ictaluri Δcrp-10 mutant triggered systemic and skin immune responses in catfish. We propose that deletion of the crp gene in E. ictaluri is an effective strategy to develop immersion live attenuated antibiotic-sensitive vaccines for the catfish aquaculture industry.

A clinical field trial to evaluate the efficacy of vaccination in controlling Salmonella infection and the association of Salmonella-shedding and weight gain in pigs

A clinical field trial was performed to determine the effectiveness of an autogenous Salmonella Typhimurium bacterin compared with a commercial live S. Choleraesuis vaccine in pigs. The association between Salmonella shedding and weight gain was also investigated. Nine cohorts of weaned pigs, (330 to 350 pigs per cohort), were randomly assigned to 1 of 3 treatment groups (injection with S. Typhimurium bacterin, vaccination via water with S. Choleraesuis vaccine, or a control group receiving no vaccine). In each cohort, the average daily gain was calculated for a selected pen throughout the production stage. Pen (pooled) fecal samples were collected bi-weekly and cultured. The odds of Salmonella shedding in both vaccinated groups was higher than in the control group (P < 0.05). The prevalence of Salmonella shedding declined overall as pigs aged (P = 0.04). However, the control pigs showed the smallest decrease in Salmonella shedding over the entire production stage, while prevalence of Salmonella shedding in the vaccinated groups decreased twice as much as the control group over the entire production stage. Salmonella Typhimurium var. Copenhagen DT104, S. Cerro, and S. Agona, which had been isolated on the study farm previously, were recovered from pigs in this study. Shedding of S. Typhimurium var. Copenhagen decreased over time in both vaccine treatment groups. On the other hand, S. Cerro shedding rate was lower in the control pigs compared with vaccinated pigs and S. Agona could be recovered only from the samples collected from S. Choleraesuis vaccinated pigs. The pigs from pens with a higher Salmonella recovery rate experienced slower growth compared with pigs from pens where Salmonella was not isolated. This latter finding indicates that there might be an economic incentive for producers to try to control endemic salmonellosis if effective programs could be developed.

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.

Coordinated regulation of virulence during systemic infection of Salmonella enterica serovar Typhimurium

To cause a systemic infection, Salmonella must respond to many environmental cues during mouse infection and express specific subsets of genes in a temporal and spatial manner, but the regulatory pathways are poorly established. To unravel how micro-environmental signals are processed and integrated into coordinated action, we constructed in-frame non-polar deletions of 83 regulators inferred to play a role in Salmonella enteriditis Typhimurium (STM) virulence and tested them in three virulence assays (intraperitoneal [i.p.], and intragastric [i.g.] infection in BALB/c mice, and persistence in 129X1/SvJ mice). Overall, 35 regulators were identified whose absence attenuated virulence in at least one assay, and of those, 14 regulators were required for systemic mouse infection, the most stringent virulence assay. As a first step towards understanding the interplay between a pathogen and its host from a systems biology standpoint, we focused on these 14 genes. Transcriptional profiles were obtained for deletions of each of these 14 regulators grown under four different environmental conditions. These results, as well as publicly available transcriptional profiles, were analyzed using both network inference and cluster analysis algorithms. The analysis predicts a regulatory network in which all 14 regulators control the same set of genes necessary for Salmonella to cause systemic infection. We tested the regulatory model by expressing a subset of the regulators in trans and monitoring transcription of 7 known virulence factors located within Salmonella pathogenicity island 2 (SPI-2). These experiments validated the regulatory model and showed that the response regulator SsrB and the MarR type regulator, SlyA, are the terminal regulators in a cascade that integrates multiple signals. Furthermore, experiments to demonstrate epistatic relationships showed that SsrB can replace SlyA and, in some cases, SlyA can replace SsrB for expression of SPI-2 encoded virulence factors.

Carbon catabolite repression in bacteria: many ways to make the most out of nutrients

Most bacteria can selectively use substrates from a mixture of different carbon sources. The presence of preferred carbon sources prevents the expression, and often also the activity, of catabolic systems that enable the use of secondary substrates. This regulation, called carbon catabolite repression (CCR), can be achieved by different regulatory mechanisms, including transcription activation and repression and control of translation by an RNA-binding protein, in different bacteria. Moreover, CCR regulates the expression of virulence factors in many pathogenic bacteria. In this Review, we discuss the most recent findings on the different mechanisms that have evolved to allow bacteria to use carbon sources in a hierarchical manner.

Feeding a diet containing a fructooligosaccharide mix can enhance Salmonella vaccine efficacy in mice

Fructooligosaccharides (FOS) are considered prebiotics because of their ability to promote growth of specific beneficial gut bacteria, such as bifidobacteria. Some studies reported potential immune-modulating properties. The aim of this study was to investigate the effect of FOS:inulin mix on murine response to Salmonella vaccine and evaluate the relevance toward protection against Salmonella infection. Balb/c mice were fed a diet containing 5% FOS:inulin mix or a control diet 1 wk before oral immunization with a suboptimal dose of live attenuated Salmonella typhimurium vaccine. Four weeks after vaccination, mice were infected with LD100 of virulent S. typhimurium. Specific blood Salmonella immunoglobulin G and fecal immunoglobulin A significantly increased in mice fed the diet containing prebiotics compared with control mice 4 wk postimmunization. Peritoneal macrophage phagocytic activity also significantly increased in FOS:inulin-fed mice at 1 wk postimmunization compared with control mice. No detectable effects were observed on the percentage of lymphoid cell subsets in the spleen. However, production of cytokines, interferon-gamma, interleukin-12, and tumor necrosis factor alpha, was numerically increased in spleen cell cultures stimulated with mitogens from FOS:inulin-fed mice 1 and 4 wk postimmunization. Salmonella translocation to lymphoid organs was not affected by feeding FOS:inulin. However, the improved response to Salmonella vaccine was concomitant with an increase in the survival rate of FOS:inulin-fed mice upon challenge with virulent Salmonella. No detectable effects were observed on the composition or the metabolic activity of the microbiota. Overall, the data suggest that a diet supplemented with FOS:inulin mix stimulates mucosal immunity and seems to improve efficacy of an oral vaccine.

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.

Effects of crp deletion in Salmonella enterica serotype Gallinarum

Background

Salmonella enterica serotype Gallinarum (S. Gallinarum) remains an important pathogen of poultry, especially in developing countries. There is a need to develop effective and safe vaccines. In the current study, the effect of crp deletion was investigated with respect to virulence and biochemical properties and the possible use of a deletion mutant as vaccine candidate was preliminarily tested.

Methods

Mutants were constructed in S. Gallinarum by P22 transduction from Salmonella Typhimurium (S. Typhimurium) with deletion of the crp gene. The effect was characterized by measuring biochemical properties and by testing of invasion in a chicken loop model and by challenge of six-day-old chickens. Further, birds were immunized with the deleted strain and challenged with the wild type isolate.

Results

The crp deletions caused complete attenuation of S. Gallinarum. This was shown by ileal loop experiments not to be due to significantly reduced invasion. Strains with such deletions may have vaccine potential, since oral inoculatoin with S. Gallinarum Δcrp completely protected against challenge with the same dose of wild type S. Gallinarum ten days post immunization. Interestingly, the mutations did not cause the same biochemical and growth changes to the two biotypes of S. Gallinarum. All biochemical effects but not virulence could be complemented by providing an intact crp-gene from S. Typhimurium on the plasmid pSD110.

Conclusion

Transduction of a Tn10 disrupted crp gene from S. Typhimurium caused attenuation in S. Gallinarum and mutated strains are possible candidates for live vaccines against fowl typhoid.

Cancer immunotherapy based on the killing of Salmonella typhimurium-infected tumour cells

The use of bacteria for anticancer purposes was first attempted over 100 years ago. Recently, several strains of Salmonella have been shown to possess anticancer activity and, interestingly, Salmonella possesses a unique propensity to track to tumour tissue in vivo. This review presents the use of Salmonella for cancer immunotherapy, including the distinguishing characteristics of this organism and preclinical as well as clinical data.

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.

Salmonella enterica serovar Choleraesuis infection of the porcine jejunal Peyer's patch rapidly induces IL-1beta and IL-8 expression

Salmonella enterica serovar Choleraesuis is an enteric pathogen of swine, producing septicemia, enterocolitis, pneumonia, and hepatitis. The initial molecular events at the site of Salmonella infection are hypothesized to be critical in the initiation of innate and adaptive immune responses; however, the acute immune response elicited by porcine intestinal tissues is not well understood. To address this need, we employed explants of jejunal Peyer's patch (JPP) mucosa from pigs to examine Salmonella-induced immune responses under controlled conditions as well as to overcome limitations of whole animal approaches. JPP explants mounted in Ussing chambers maintained normal histological structure for 2 h and stable short-circuit current and electrical conductance for 2.5 h. After ex vivo luminal exposure to Salmonella serovar Choleraesuis, JPP responded with an increase in mRNA expression of IL-1beta and IL-8, but not TNFalpha. Increased IL-1beta and IL-8 expression were dependent on efficient Salmonella adhesion and internalization, whereas mutant Salmonella did not induce inflammatory cytokine expression. Commensal enteric bacteria, present in some experiments, also did not induce inflammatory cytokine expression. These findings indicate that Salmonella uptake by Peyer's patch is important in the induction of an innate response involving expression of IL-1beta and IL-8, and that ex vivo intestinal immune tissue explants provide an intact tissue model that will facilitate investigation of mucosal immunity in swine.

Enhancement of humoral and cellular immune responses by an oral Salmonella choleraesuis vaccine expressing porcine prothymosin alpha

Previously, we showed that murine prothymosin alpha (ProT) enhances the efficacy of a pseudorabies DNA vaccine delivered by bacterial vectors. In this study, we cloned and sequenced the cDNA for porcine ProT. The deduced amino acid sequence of porcine ProT exhibited high homology to ProT from other mammals. Oral Salmonella choleraesuis vaccine carrying the ProT eukaryotic expression plasmid protected mice against virulent S. choleraesuis challenge. The adjuvant effect of ProT on humoral and cellular immune responses enhanced protective efficacy of the vaccine. Furthermore, both humoral and cellular immune responses played roles in the protective immune responses induced by the vaccine. Collectively, our results show that delivery of the ProT gene carried by attenuated S. choleraesuis augmented the immunogenicity of oral S. choleraesuis vaccine.

Virulence attenuation and live vaccine potential of aroA, crp cdt cya, and plasmid-cured mutants of Salmonella enterica serovar Abortusovis in mice and sheep

Three live vaccine candidates of Salmonella enterica subspecies I serotype Abortusovis (aroA, cya crp cdt, and plasmid-cured strains) have been developed, and their efficacies in inducing humoral antibodies and protecting against abortion after challenge with wild-type strain SS44 were evaluated in sheep. Following estrus synchronization, animals were immunized 3 weeks after fertilization and boosted once 3 weeks later. Following challenge with wild-type SS44, pregnancy failure of vaccinated ewes was reduced compared to that of nonimmunized controls. Attenuation of each vaccine was also assessed in challenge experiments with nonimmunized pregnant ewes and in BALB/c mice. All three vaccine candidates appear to be safe for use in sheep and provide a model for the development of live vaccine candidates against naturally occurring ovine salmonellosis.

Novel targets for antibiotics - an update

The emergence of widespread antibiotic resistance as an impediment in the treatment of bacterial diseases is of growing concern. In some instances, clinicians are left with few or no antibiotics for treatment of infections and this problem will more than likely grow in magnitude. One approach to get around the problem of antibiotic resistance is to develop new drugs with novel targets and mechanisms of action. Due to the 'newness' of these novel targets as therapeutic targets, the likelihood that resistance will initially be widespread is low. Three approaches are discussed in this overview: discovery of new essential genes that are expressed exclusively in vivo development of compounds that act on global bacterial gene regulators; and interference with virulence determinants. By exploiting virulence related attributes or genes expressed exclusively in vivo, the risk of resistance is reduced since inhibiting these products will probably alter the ecology (habitats) of these organisms rather than causing direct cell death. This might also lead to a selective targeting of pathogens with the beneficial consequence of ignoring organisms growing in their normal habitat, such as in the gastrointestinal tract or skin.

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.

Antimicrobial use and resistance in animals

Food animals in the United States are often exposed to antimicrobials to treat and prevent infectious disease or to promote growth. Many of these antimicrobials are identical to or closely resemble drugs used in humans. Precise figures for the quantity of antimicrobials used in animals are not publicly available in the United States, and estimates vary widely. Antimicrobial resistance has emerged in zoonotic enteropathogens (e.g., Salmonella spp., Campylobacter spp.), commensal bacteria (e.g., Escherichia coli, enterococci), and bacterial pathogens of animals (e.g., Pasteurella, Actinobacillus spp.), but the prevalence of resistance varies. Antimicrobial resistance emerges from the use of antimicrobials in animals and the subsequent transfer of resistance genes and bacteria among animals and animal products and the environment. To slow the development of resistance, some countries have restricted antimicrobial use in feed, and some groups advocate similar measures in the United States. Alternatives to growth-promoting and prophylactic uses of antimicrobials in agriculture include improved management practices, wider use of vaccines, and introduction of probiotics. Monitoring programs, prudent use guidelines, and educational campaigns provide approaches to minimize the further development of antimicrobial resistance.

Bacteremia associated with live attenuated chi8110 Salmonella enterica serovar Typhi ISP1820 in healthy adult volunteers

Live attenuated chi8110 Salmonella enterica serovar Typhi ISP1820 vaccine was given in a dose-escalation trial to healthy, adult volunteers. Positive stool and blood cultures were noted, but limited, as were immune responses measured by ELISA and ELISPOT. Only volunteers with bacteremia developed immune responses; however, no symptoms were associated with bacteremia. The vaccine was insufficiently immunogenic for use as a vaccine. It is possible that reduced survival in the gut and reduced immunogenicity may have been due to the thawing of frozen inocula immediately prior to use.

Intranasal immunogenicity of a Deltacya Deltacrp-pabA mutant of Salmonella enterica serotype Typhimurium for the horse

The aim of this study was to investigate the intranasal immunogenicity for the horse of a Deltacya Deltacrp-pabA mutant (MGN-707) of Salmonella enterica serotype Typhimurium (S. typhimurium). MGN-707 caused no sign of disease, was not detected in feces and a single administration induced strong Salmonella-specific serum and nasal mucosal antibody responses. All ponies had made strong salmonella specific serum IgGa, IgGb, IgA and IgM antibody responses by day 25 after the first immunization. IgM responses to salmonella lipopolysaccharide (LPS) were short lived whereas salmonella specific serum IgGa and IgGb persisted at high levels in all ponies until 83 and 140 days, respectively. Specific nasal mucosal antibody responses dominated by IgA and IgM were evident by day 25 in all ponies except one in which only specific IgGa and IgGb were evident. Specific nasal mucosal IgA persisted in most ponies until day 69. A second immunization on day 140 boosted antibody responses, and stimulated a strong nasal mucosal IgA response in the pony that failed to make an IgA response after primary immunization. At the termination of the experiment, IgA and IgGb dominated jejunal antibody responses whereas vaginal responses were mainly IgA. The latter response unequivocally confirms the existence of a common mucosal immune system in equids. The results indicate that a S. typhimurium Deltacya Deltacrp-pabA mutant has potential as an intranasal vaccine against salmonellosis in the horse.

Survival and infectivity of Salmonella choleraesuis in swine feces

Many serotypes of Salmonella survive well in the environment. Conversely, it is believed that Salmonella Choleraesuis, the host-adapted serotype of swine, does not survive well outside the host. We examined the survival capability of Salmonella Choleraesuis in swine feces. Six pigs were infected with Salmonella Choleraesuis and feces were collected and pooled on days 2, 4, 7, and 10 postinoculation (PI). Feces were stored in a wet and a dry form, and survival was measured over 13 months. Salmonella Choleraesuis was recovered from wet feces through 3 months of storage. In a desiccated (dry) form, Salmonella Choleraesuis was recovered from at least 13 months. Salmonella Choleraesuis shed from swine prior to 4 days PI did not survive as well as that shed 4 days PI or later. We also examined the infectivity of Salmonella Choleraesuis resident in dry feces. Six- or 13-week-old pigs were inoculated with dry feces that had been stored either 2 months or 4 months, respectively. Pigs were inoculated either intranasally or by mixing dry feces with the swine ration. Although clinical signs were mild, Salmonella Choleraesuis was widely disseminated among the tissues of all the pigs inoculated. This study demonstrates that Salmonella Choleraesuis remains viable and infective in the environment. Therefore, contaminated fecal matter can serve as a reservoir for Salmonella Choleraesuis as well as other Salmonella spp. Control measures must consider this environmental reservoir as a source of new infections.

Intranasal immunogenicity of a Delta cya Delta crp-pabA mutant of Salmonella enterica serotype Typhimurium for the horse

The aim of this study was to investigate the intranasal immunogenicity for the horse of a Deltacya Deltacrp-pabA mutant (MGN-707) of Salmonella enterica serotype Typhimurium (S. typhimurium). MGN-707 caused no sign of disease, was not detected in feces and a single administration induced strong Salmonella-specific serum and nasal mucosal antibody responses. All ponies had made strong salmonella specific serum IgGa, IgGb, IgA and IgM antibody responses by day 25 after the first immunization. IgM responses to salmonella lipopolysaccharide (LPS) were short lived whereas salmonella specific serum IgGa and IgGb persisted at high levels in all ponies until 83 and 140 days, respectively. Specific nasal mucosal antibody responses dominated by IgA and IgM were evident by day 25 in all ponies except one in which only specific IgGa and IgGb were evident. Specific nasal mucosal IgA persisted in most ponies until day 69. A second immunization on day 140 boosted antibody responses, and stimulated a strong nasal mucosal IgA response in the pony that failed to make an IgA response after primary immunization. At the termination of the experiment, IgA and IgGb dominated jejunal antibody responses whereas vaginal responses were mainly IgA. The latter response unequivocally confirms the existence of a common mucosal immune system in equids. The results indicate that a S. typhimurium Deltacya Deltacrp-pabA mutant has potential as an intranasal vaccine against salmonellosis in the horse.

Experimental Salmonella typhi infection in the domestic pig, Sus scrofa domestica

The domestic pig, Sus scrofa domestica, was examined as a model for typhoid fever, a severe and systemic disease of humans caused by Salmonella typhi. Six pigs were inoculated 1 week post-weaning with approximately 10(10)colony forming units (cfu) of wild type Salmonella typhi strain ISP1820 intranasally and observed for 3 weeks. S. typhi was cultured from the tonsils of 50% of the pigs at necropsy. Cultures from all other organs analysed (ileum, colon, spleen and liver) were negative. No clinical or histopathological signs of disease were observed. Pigs inoculated in parallel with swine-virulent S. choleraesuis all exhibited signs of systemic salmonellosis indicating that the parameters of the experimental infection with S. typhi (e.g. route) were appropriate. Whereas the pig has a gastrointestinal tract that is very similar to humans, our results indicated that the unique features of host and microbe interaction needed to produce typhoid fever were not mimicked in swine. Nevertheless, our observation of tonsillar involvement was consistent with former observations of S. choleraesuis and S. typhimurium infections in swine and supports a role for the tonsil in all porcine salmonella infections.

A genetically-defined riboflavin auxotroph of Actinobacillus pleuropneumoniae as a live attenuated vaccine

Actinobacillus pleuropneumoniae is a gram negative pleiomorphic rod that is the causative agent of a severe, highly infectious and often fatal pleuropneumonia in swine. We have previously reported the construction of genetically-defined stable riboflavin auxotrophs by replacement of a portion of the APP riboflavin biosynthetic operon (ribGBAH) with an antibiotic cassette encoding resistance to kanamycin, and have demonstrated that such riboflavin auxotrophs are avirulent. In this study, we evaluated riboflavin auxotrophs of A. pleuropneumoniae for their ability to stimulate protective immunity against pleuropneumonia. An initial challenge experiment demonstrated that intramuscular vaccination with a live attenuated serotype 1A rib mutant, in a vaccine formulation that included a limiting amount of exogenous riboflavin, provided better protection against challenge with virulent A. pleuropneumoniae than either intratracheal immunization or intramuscular immunization with live bacteria in the absence of exogenous riboflavin. Subsequent studies in which the vaccine inoculating dose, concentration of exogenous riboflavin, and serotype of the vaccine strain were varied demonstrated that immunization with live avirulent riboflavin auxotrophs could elicit significant protection against experimental challenge with both homologous and heterologous virulent serotypes of A. pleuropneumoniae.

Mechanisms of vaccine adjuvanticity at mucosal surfaces

The vast majority of pathogens invade via mucosal surfaces, including those of the intestine. Vaccination directly on these surfaces may induce local protective immunity and prevent infection and disease. Although vaccine delivery to the gut mucosa is fraught with obstacles, immunization can be enhanced using adjuvants with properties specific to intestinal immunity. In this review, we present three general mechanisms of vaccine adjuvant function as originally described by Freund, and we discuss these principles with respect to intestinal adjuvants in general and to the prototypical mucosal adjuvant, cholera toxin. The key property of intestinal adjuvants is to induce an immunogenic context for the presentation of the vaccine antigen. The success of oral vaccine adjuvants is determined by their ability to induce a controlled inflammatory response in the gut-associated lymphoid tissues, characterized by the expression of various costimulatory molecules and cytokines. An understanding of the specific molecular mechanisms of adjuvanticity in the gut will allow the rational development of safe and effective oral vaccines.

Recombinant live Salmonella spp. for human vaccination against heterologous pathogens

Live attenuated Salmonella spp. are promising candidates as oral vaccine delivery systems for heterologous antigens. Clinical trials have demonstrated that this approach is feasible for human vaccinations but further optimisation is necessary to obtain a better efficacy. Here, we discuss how existing clinical and pre-clinical data can be used to guide such optimisation efforts.

Synergism between porcine reproductive and respiratory syndrome virus (PRRSV) and Salmonella choleraesuis in swine

Porcine reproductive and respiratory syndrome virus (PRRSV) and Salmonella choleraesuis are two leading causes of economic loss in the swine industry. While respiratory disease is common in both S. choleraesuis and PRRSV infections, the factors that contribute to its development remain largely undefined. We investigated the interaction of PRRSV, S. choleraesuis, and stress in 5-week-old swine. All combinations of three factors (inoculation with S. choleraesuis on Day 0, PRRSV on Day 3, and treatment with dexamethasone on Days 3-7) were used to produce eight treatment groups in two independent trials. Fecal samples, tonsil and nasal swabs, serum samples and postmortem tissues were collected for bacteriologic and virologic examinations. No clinical signs were observed in pigs inoculated with only PRRSV or only S. choleraesuis. In contrast, pigs which were dually infected with S. choleraesuis and PRRSV exhibited unthriftiness, rough hair coats, dyspnea, and diarrhea. The pigs which received all three treatment factors were the most severely affected and 43% (three of seven) of the animals in this group died. Individuals in this group shed significantly higher quantities of S. choleraesuis in feces and had significantly higher serum PRRSV titers compared to other treatments (p < or = 0.05). In addition, S. choleraesuis and PRRSV were shed longer and by more pigs in this group than other groups and S. choleraesuis was recovered from more tissues in this group on Day 21 post inoculation. These results suggested that PRRSV, S. choleraesuis, and dexamethasone acted synergistically to produce a syndrome similar to that observed in the field.

Live attenuated Salmonella: a paradigm of mucosal vaccines

Two key steps control immune responses in mucosal tissues: the sampling and transepithelial transport of antigens, and their targeting into professional antigen-presenting cells in mucosa-associated lymphoid tissue. Live Salmonella bacteria use strategies that allow them to cross the epithelial barrier of the gut, to survive in antigen-presenting cells where bacterial antigens are processed and presented to the immune cells, and to express adjuvant activity that prevents induction of oral tolerance. Two Salmonella serovars have been used as vaccines or vectors, S. typhimurium in mice and S. typhi in humans. S. typhimurium causes gastroenteritis in a broad host range, including humans, while S. typhi infection is restricted to humans. Attenuated S. typhimurium has been used successfully in mice to induce systemic and mucosal responses against more than 60 heterologous antigens. This review aims to revisit S. typhimurium-based vaccination, as an alternative to S. typhi, with special emphasis on the molecular pathogenesis of S. typhimurium and the host response. We then discuss how such knowledge constitutes the basis for the rational design of novel live mucosal vaccines.

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.

The nature of the attenuation of Salmonella typhimurium strains expressing human papillomavirus type 16 virus-like particles determines the systemic and mucosal antibody responses in nasally immunized mice

We have recently shown by using a recombinant Salmonella typhimurium PhoPc strain in mice the feasibility of using a Salmonella-based vaccine to prevent infection by the genital human papillomavirus type 16 (HPV16). Here, we compare the HPV16-specific antibody responses elicited by nasal immunization with recombinant S. typhimurium strains harboring attenuations that, in contrast to PhoPc, are suitable for human use. For this purpose, chi4989 (Deltacya Deltacrp) and chi4990 [Deltacya Delta(crp-cdt)] were constructed in the ATCC 14028 genetic background, and comparison was made with the isogenic PhoPc and PhoP- strains. Although the levels of expression of HPV16 virus-like particle (VLP) were similar in all strains, only PhoPc HPV16 induced sustained specific antibody responses after nasal immunization, while all strains induced high antibody responses with a single nasal immunization when an unrelated viral hepatitis B core antigen was expressed. The level of the specific antibody responses induced did not correlate with the number of recombinant bacteria surviving in various organs 2 weeks after immunization. Our data suggest that the immunogenicity of attenuated Salmonella vaccine strains does not correlate with either the number of persisting bacteria after immunization or the levels of in vitro expression of the antigen carried. Rather, the PhoPc phenotype appears to provide the unique ability in Salmonella to induce immune responses against HPV16 VLPs.

Molecular and functional characterization of Salmonella enterica serovar typhimurium poxA gene: effect on attenuation of virulence and protection

Salmonella enterica poxA mutants exhibit a pleiotropic phenotype, including reduced pyruvate oxidase activity; reduced growth rate; and hypersensitivity to the herbicide sulfometuron methyl, alpha-ketobutyrate, and amino acid analogs. These mutants also failed to grow in the presence of the host antimicrobial peptide, protamine. In this study, PoxA- mutants of S. enterica serovar Typhimurium (S. typhimurium) were found to be 10,000-fold attenuated in orally inoculated BALB/c mice and 1,000-fold attenuated in intraperitoneally inoculated BALB/c mice, compared to wild-type S. typhimurium UK-1. In addition, poxA mutants were found to be capable of colonizing the spleen, mesenteric lymph nodes, and Peyer's patches; to induce strong humoral immune responses; and to protect mice against a lethal wild-type Salmonella challenge. A 2-kb DNA fragment was isolated from wild-type S. typhimurium UK-1 based on its ability to complement an isogenic poxA mutant. The nucleotide sequence of this DNA fragment revealed an open reading frame of 325 amino acids capable of encoding a polypeptide of 36.8 kDa that was confirmed in the bacteriophage T7 expression system. Comparison of the translated sequence to the available databases indicated high homology to a family of lysyl-tRNA synthetases. Our results indicate that a mutation of poxA has an attenuating effect on Salmonella virulence. Further, poxA mutants are immunogenic and could be useful in designing live vaccines with a variety of bacterial species. To our knowledge, this is the first report on the effect of poxA mutation on bacterial virulence.

Characterization and immunogenicity of Salmonella typhimurium SL1344 and UK-1 delta crp and delta cdt deletion mutants

S. typhimurium SL1344 and UK-1 mutants with deletions of the crp (cyclic AMP receptor protein) and cdt (colonization of deep tissues) genes have been constructed and characterized, and their levels of virulence and immunogenicity have been determined for BALB/c mice. All Crp- Cdt- and Crp+ Cdt- mutants were avirulent, as mice survived oral doses of 10(9) cells without illness. All the mutants colonized the gut-associated lymphoid tissue efficiently, but capacities to colonize deeper tissues, such as those of the spleen and liver, and blood were significantly reduced for the Crp- Cdt- and Crp+ Cdt- mutants compared with the Crp- Cdt+ mutant and the wild-type parent strain. The Crp- Cdt- and Crp+ Cdt- SL1344 strains induced complete protection, as all mice immunized with the mutants survived challenge with approximately 10(4) times the 50% lethal dose of the wild-type SL1344 strain. The Crp- UK-1 strain was least attenuated yet induced the highest level of protective immunity against challenge with the wild-type UK-1 strain. The Crp+ Cdt- and Crp- Cdt- strains, although totally attenuated, differed in immunogenicity to challenge with the highly virulent UK-1 parent, with the apparently hyperattenuated Crp- Cdt- strain inducing a lower level of protective immunity than the Crp+ Cdt- strain. Nevertheless, these UK-1 Crp- Cdt- and Crp+ Cdt- strains induced complete protective immunity to challenge with the less-virulent SL1344 wild-type strain. Taken collectively, the results indicate that the attenuation of a highly virulent S. typhimurium strain can yield a vaccine that induces excellent protective immunity to challenge with less-virulent S. typhimurium strains.

How Salmonella became a pathogen

In many pathogens, virulence can be conferred by a single region of the genome. In contrast, the facultative intracellular lifestyle of Salmonella demands a large number of genes distributed around the chromosome. The evolution of Salmonella has been marked by the acquisition of several 'pathogenicity islands', each contributing to the unique virulence properties of this microorganism.

Safety and immunogenicity in humans of an attenuated Salmonella typhi vaccine vector strain expressing plasmid-encoded hepatitis B antigens stabilized by the Asd-balanced lethal vector system

Attenuated Salmonella typhi organisms which express genes encoding protective antigens of other pathogens have been developed for use as experimental oral vaccines. A delta asd S. typhi strain attenuated by deletions in cya, crp, and cdt which contains hepatitis B core (HBc) and pre-S genes encoded on an Asd+ pBR-based plasmid vector was constructed. Healthy adult volunteers ingested a single dose of 5 x 10(5) to 5 x 10(8) CFU of strain chi4073 (delta cya delta crp delta cdt S. typhi Ty2), 6 x 10(7) or 1 x 10(9) CFU of strain chi4632(pYA3149), a further derivative of chi4073 deleted in asd and containing the Asd+ vector without the HBc-pre-S fusion, or 3 x 10(7) or 7 x 10(8) CFU of strain X4632(pYA3167), a derivative containing the vector with the HBc-pre-S fusion. Chi4073 was generally well tolerated by 22 volunteers. No volunteer had fever or positive blood cultures; 4 of 22 volunteers shed vaccine organisms in the stool in the first 48 h only. Two of 18 volunteers who received one of the plasmid-containing derivatives of chi4073 developed low-grade fevers on day 10 or 12 after ingestion. One of these volunteers had positive blood cultures on days 7 and 8. Seven of these 18 volunteers had vaccine organisms detected in their stools in the first 48 h only. Most volunteers developed S. typhi-specific serum responses and developed S. typhi-specific antibody-secreting cells. However, no volunteer developed serum antibody to hepatitis pre-S or pre-S-specific antibody-secreting cells. Although the parent strain chi4073 was well tolerated, induced immunoglobulin G seroconversion to S. typhi lipopolysaccharide in 80 to 100% of vaccinees and stimulated specific IgA-secreting lymphocytes in 80 to 100% of vaccinees given a single oral dose of 2 x 10(7) and 5 x 10(8) CFU, chi4073 derivatives containing the Asd+ vector with and without sequences encoding the HBc-pre-S fusion caused occasional febrile reactions at high doses and did not stimulate detectable immune responses to hepatitis B antigens.

Oral and rectal immunization of adult female volunteers with a recombinant attenuated Salmonella typhi vaccine strain

An attenuated strain of Salmonella typhi delta(cya) delta(crp-cdt) delta(asd) expressing a gene encoding a hepatitis B virus core-pre-S protein was tested in female adult volunteers for its ability to elicit a systemic and a mucosal immune response. Specifically, our purpose was to evaluate the potential of such a vaccine strain to induce specific secretory immunoglobulin A (sIgA) at genital and rectal surfaces. Oral and rectal routes of immunization were compared: oral immunization induced seroconversion against the bacterial lipopolysaccharide (LPS) in six out of seven volunteers, while after rectal immunization only one out of six volunteers seroconverted against LPS. To our disappointment, the latter volunteer was also the only one who seroconverted against the carried antigen (pre-S1), demonstrating the poor ability of this live vaccine to induce an immune response against the carried antigen. Anti-LPS sIgA was found in both the vaginal and cervical secretions of a volunteer who presented a strong seroconversion after oral immunization (16-fold increase in anti-LPS IgG). Smaller amounts of anti-LPS sIgA were found in the rectal secretions of one orally and one rectally immunized volunteer and in the saliva of three orally and one rectally immunized woman. Our data show for the first time that it is possible to induce specific sIgA in the genital and rectal tracts of women by using an S. typhi vaccine strain.

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.