Construction and characterization of a cigR deletion mutant of Salmonella enterica serovar Pullorum

The influence of cigR gene on the pathogenicity of Salmonella paratyphi A in vitro and in vivo

Salmonella Paratyphi A is the causative agent of paratyphoid fever A which is a serious threat to human health in many countries. The cigR gene located in Salmonella pathogenicity island 3 is a type III secretion system 2 effector gene. However, the influence of cigR gene on the pathogenicity of Salmonella Paratyphi A remains unclear. Here, a cigR gene deletion mutant of Salmonella Paratyphi A was constructed and its pathogenic changes were also evaluated. It was found that both the growth and biochemical features have not changed after the loss of cigR, but the absence of cigR significantly enhanced the replication and/or survival ability in phorbol-12-myristate-13-acetate (PMA)-differentiated human macrophage THP-1 cells and in mouse; the proliferative activity and apoptosis of PMA-differentiated THP-1 cell were significantly decreased and increased, respectively, after the lack of cigR gene; and the mutant showed increased virulence to a mouse infection model by decreased half-lethal dose (LD50) value and enhanced the proliferation ratio of bacteria in vivo. These results demonstrated that CigR is an anti-virulence factor and plays an important role in the pathogenicity of Salmonella Paratyphi A.

Coupling CRISPR/Cas9 and Lambda Red Recombineering System for Genome Editing of Salmonella Gallinarum and the Effect of ssaU Knock-Out Mutant on the Virulence of Bacteria

The poultry industry in developing countries still faces a significant threat from fowl typhoid, a disease caused by Salmonella Gallinarum that has been well contained in more economically developed countries. In addition to the virulence exhibited by large virulence plasmid (85 kb), Salmonella Pathogenicity Island 2 in S. Gallinarum plays a key role in mediating disease through its type III secretion systems (TTSS). The TTSS secrete effector protein across the Salmonella containing vacuoles and mediate the internalization of bacteria by modulating vesicular passage. In this study, candidate virulent ssaU gene (~1 kb) encoding type III secretion system was successfully deleted from indigenously isolated S. Gallinarum genome through homology-directed repair using CRISPR/Cas9 and lambda recombination systems. CRISPR/Cas9-based genome editing of poultry-derived Salmonella Gallinarum has not been previously reported, which might be linked to a lack of efficiency in its genetic tools. This is the first study which demonstrates a complete CRISPR/Cas9-based gene deletion from this bacterial genome. More importantly, a poultry experimental model was employed to assess the virulence potential of this mutant strain (ΔssaU_SG18) which was unable to produce any mortality in the experimentally challenged birds as compared to the wild type strain. No effect on weight gain was observed whereas bacteria were unable to colonize the intestine and liver in our challenge model. This in vivo loss of virulence in mutant strain provides an excellent functionality of this system to be useful in live vaccine development against this resistant and patho genic bacteria.

Contribution of prgH gene for Salmonella Pullorum to virulence and the expression of NLRP3, Caspase-1 and IL-1β in chickens

Type III secretion system 1 (T3SS1) encoded by Salmonella pathogenicity island 1 (SPI1) is associated with invasion of host cells by Salmonella, PrgH encoded by prgH gene is an important component of T3SS1. This study aimed to explore the contribution of prgH gene for Salmonella Pullorum to virulence and the expression of NLRP3, Caspase-1 and IL-1β in chickens. A prgH gene deletion mutant (C79-13ΔprgH) was firstly generated, and the result of LD₅₀ showed that deletion of prgH significantly decreased the virulence of Salmonella Pullorum in one-day-old HY-line white chickens, and the colonization also decreased in chickens after loss of prgH. Next, the expressions of NLRP3, Caspase-1, and IL-1β were detected in acute infection model of chickens by qRT-PCR and/or ELISA, respectively, and the results showed that the mutant strain C79-13ΔprgH reduced the expression levels of NLRP3, Caspase-1, and IL-1β in chickens compared to the group infected with the wild type strain C79-13. Taken together, all of these findings indicated that prgH promotes the virulence and the expression of NLRP3, Caspase-1, and IL-1β for Salmonella Pullorum in chickens.

SteE Enhances the Virulence of Salmonella Pullorum in Chickens by Regulating the Inflammation Response

Salmonella enterica serovar Pullorum (S. Pullorum) is a host-specific pathogen, which causes acute gastroenteritis with high mortality in poultry. However, the association between steE, encoded by type III secretion system 2, and Salmonella virulence is not well-understood. To elucidate the functions of steE in S. Pullorum, ΔsteE strain was constructed using the λ-Red recombination technology. Compared to that in the wild-type, the deletion of steE in S. Pullorum reduced bacterial invasion, proliferation, and late apoptosis in the infected HD-11 cells. In addition, we analyzed the mRNA expression levels of effector genes and cytokines by qRT-PCR. SteE was associated with the regulation of various effector genes and inflammatory cytokines in HD-11 cells during S. Pullorum infection. The wild-type effector steE promoted the expression of anti-inflammatory cytokines (IL-4 and IL-10) and reduced that of pro-inflammatory cytokines (IL-1β, IL-6, and IL-12) compared to that in the ΔsteE-infected HD-11 cells and chicken spleens. Results from the chicken infection model showed that the deletion of steE resulted in significantly decreased colonization and long-term survival of the bacteria and alleviated pathological lesions compared to those in the wild-type. Further, steE increased the virulence of S. Pullorum in chickens by regulating the expression of inflammatory cytokines. Our findings provide insights into the persistent infection and autoimmunity associated with steE in S. Pullorum.

Genetic Determinants in Salmonella enterica Serotype Typhimurium Required for Overcoming In Vitro Stressors in the Mimicking Host Environment

Salmonella enterica serotype Typhimurium, a nontyphoidal Salmonella (NTS), results in a range of enteric diseases, representing a major disease burden worldwide. There is still a significant portion of Salmonella genes whose mechanistic basis to overcome host innate defense mechanisms largely remains unknown. Here, we have applied transposon insertion sequencing (Tn-seq) method to unveil the genetic factors required for the growth or survival of S. Typhimurium under various host stressors simulated in vitro. A highly saturating Tn5 library of S. Typhimurium 14028s was subjected to selection during growth in the presence of short-chain fatty acid (100 mM propionate), osmotic stress (3% NaCl), or oxidative stress (1 mM H2O2) or survival in extreme acidic pH (30 min in pH 3) or starvation (12 days in 1× phosphate-buffered saline [PBS]). We have identified a total of 339 conditionally essential genes (CEGs) required to overcome at least one of these conditions mimicking host insults. Interestingly, all eight genes encoding FoF1-ATP synthase subunit proteins were required for fitness in all five stresses. Intriguingly, a total of 88 genes in Salmonella pathogenicity islands (SPI), including SPI-1, SPI-2, SPI-3, SPI-5, SPI-6, and SPI-11, are also required for fitness under the in vitro conditions. Additionally, by comparative analysis of the genes identified in this study and the genes previously shown to be required for in vivo fitness, we identified novel genes (marBCT, envF, barA, hscA, rfaQ, rfbI, and the genes encoding putative proteins STM14_1138, STM14_3334, STM14_4825, and STM_5184) that have compelling potential for the development of vaccines and antibacterial drugs to curb Salmonella infection. IMPORTANCE Salmonella enterica serotype Typhimurium is a major human bacterial pathogen that enters the food chain through meat animals asymptomatically carrying this pathogen. Despite the rich genome sequence data, a significant portion of Salmonella genes remain to be characterized for their potential contributions to virulence. In this study, we used transposon insertion sequencing (Tn-seq) to elucidate the genetic factors required for growth or survival under various host stressors, including short-chain fatty acids, osmotic stress, oxidative stress, extreme acid, and starvation. Among the total of 339 conditionally essential genes (CEGs) that are required under at least one of these five stress conditions were 221 previously known virulence genes required for in vivo fitness during infection in at least one of four animal species, including mice, chickens, pigs, and cattle. This comprehensive map of virulence phenotype-genotype in S. Typhimurium provides a roadmap for further interrogation of the biological functions encoded by the genome of this important human pathogen to survive in hostile host environments.

Evaluation of Safety and Protective Efficacy of a waaJ and spiC Double Deletion Korean Epidemic Strain of Salmonella enterica Serovar Gallinarum

With an aim to develop a highly attenuated and strongly immunogenic distinguishable vaccine candidate, a waaJ (a gene involved in the synthesis of lipopolysaccharide) and spiC (a virulence gene) double deletion Korean epidemic strain of S. enterica ser. Gallinarum (SG005) was constructed. Our results showed that the growth and biochemical characteristics were not altered by this double deletion. The double deletion strain contained dual markers. One was a bacteriological marker (rough phenotype) and the other was a serological marker helping distinguish infected chickens from vaccinated chickens. The double deletion strain showed good genetic stability and reduced resistance to environmental stresses in vitro; furthermore, it was extremely safe and highly avirulent in broilers. Single intramuscular or oral immunization of 7-day-old broilers with the double deletion strain could stimulate the body to produce antibody levels similar to the conventional vaccine strain SG9R. In addition, against a lethal wild-type challenge, it conferred effective protection that was comparable to that seen in the group vaccinated with SG9R. In conclusion, this double deletion strain may be an effective vaccine candidate for controlling S. enterica ser. Gallinarum infection in broilers.

Salmonella Pullorum lacking srfA is attenuated, immunogenic and protective in chickens

Sallmonella Pullorum is a host-restricted pathogen for poultry and causes severe economic importance in many developing countries. The development of novel vaccines for Salmonella Pullorum is necessary to eradicate the prevalence of the pathogen. In our study, a srfA deletion mutant (C79-13ΔsrfA) of Salmonella Pullorum was constructed, and then the biological characteristics and protective efficacy of the mutant were evaluated. The mutant C79-13ΔsrfA was much less virulent than its parental strain C79-13 in one-day-old HY-line white chickens, immunization with C79-13ΔsrfA (4 × 10⁷ CFU) through oral pathway induced highly specific humoral and cellular immune responses, the growth performance of vaccinated chickens was consistent with that of unvaccinated chickens. The survival percentages of vaccinated chickens reached 90% and 80%, after challenge with Salmonella Pullorum strain C79-13 and Salmonella Gallinarum strain SG9 at 10 days post-immunization (dpi), respectively. Collectively, our results indicate that C79-13ΔsrfA is a live attenuated vaccine candidate.

Mechanisms adopted by Salmonella to colonize plant hosts

Fruits and vegetables consumed fresh or as minimally-processed produce, have multiple benefits for our diet. Unfortunately, they bring a risk of food-borne diseases, for example salmonellosis. Interactions between Salmonella and crop plants are indeed a raising concern for the global health. Salmonella uses multiple strategies to manipulate the host defense system, including plant's defense responses. The main focus of this review are strategies used by this bacterium during the interaction with crop plants. Emphasis was put on how Salmonella avoids the plant defense responses and successfully colonizes plants. In addition, several factors were reviewed assessing their impact on Salmonella persistence and physiological adaptation to plants and plant-related environment. The understanding of those mechanisms, their regulation and use by the pathogen, while in contact with plants, has significant implication on the growth, harvest and processing steps in plant production system. Consequently, it requires both the authorities and science to advance and definite methods aiming at prevention of crop plants contamination. Thus, minimizing and/or eliminating the potential of human diseases.

SspH2 as anti-inflammatory candidate effector and its contribution in Salmonella Enteritidis virulence

Salmonella enterica serovar Enteritidis (S. Enteritidis) is a facultative intracellular pathogen deploying the type III secretion system (T3SS) encoded by Salmonella Pathogenicity Island 2 (SPI2) to transfer effector proteins into host cells to modify its functions and accomplish intracellular replication. To study the effect of SspH2 on immune response induced by S. Enteritidis, we generated a deletion mutant of the effector gene sspH2 and a plasmid mediated complementary strain in S. Enteritidis C50336. The results of LD50 showed that SspH2 has no obvious effect on the virulence of S. Enteritidis. However, deletion of sspH2 decreased the invasion and intercellular colonization of the bacteria in Caco2 BBE cells. Using bacteriological counts from tissue homogenates the result of colonization in internal organs showed that in spleen and liver tissues, at 3rd and 4th day p.i. there is a significance decreased number of C50336-ΔsspH2 compared to the C50336-WT and C50336-ΔsspH2-psspH2, respectively. The qRT-PCR analysis results of both in vivo and in vitro experiments clearly showed that the mutant strain C50336ΔsspH2 significantly promoted expression of IL-1β, INF-γ, IL-12, and iNOS cytokines compared to the groups infected with the wild type or complementary strains, while the IL-8 synthesis was decreased in the mutant strain infected group. All of these findings revealed that SspH2 promotes the colonization of S. Enteritidis in host cells, and it is an important anti-inflammatory biased effector in Salmonella.

Salmonella adapts to plants and their environment during colonization of tomatoes

Humans and animals are considered typical hosts for Salmonella, however, also plants can be colonized. Tomatoes were linked to salmonellosis outbreaks already on several occasions. The aim of this study was, therefore, to establish a comprehensive view on the interaction between Salmonella enterica and tomatoes, and to test the hypothesis that colonization of plants is an interactive process. We assessed the persistence of Salmonella in agricultural soil, the colonization pattern in and on tomatoes, as well as the reciprocal responses of tomatoes to different Salmonella strains and Salmonella to root exudates and tomato-related media. This study revealed that Salmonella can persist in the soil and inside the tomato plant. Additionally, we show that Salmonella strains have particular colonization pattern, although the persistence inside the plant differs between the tested strains. Furthermore, the transcriptome response of tomato showed an up-regulation of several defense-related genes. Salmonella transcriptome analysis in response to the plant-based media showed differentially regulated genes related to amino acid and fatty acid synthesis and stress response, while the response to root exudates revealed regulation of the glyoxylate cycle. Our results indicate that both organisms actively engage in the interaction and that Salmonella adapts to the plant environment.

Salmonella-containing vacuole development in avian cells and characteristic of cigR in Salmonella enterica serovar Pullorum replication within macrophages

Salmonella enterica serovar Pullorum (S. Pullorum) is one of the host-restricted serotypes causing systemic infection in poultry. Survival in macrophages is one of the mechanisms underlying the persistent infection of S. Pullorum in hosts. Formation of Salmonella-containing vacuole (SCV) is essential for bacteria to be concealed in macrophages. In this study, confocal microscopy was applied to detect the SCV development by S. Pullorum in the chicken hepatocellular carcinoma cell line LMH and the macrophage cell line HD-11, respectively. The results showed that macrophages were more appropriate for the SCV maturation during S. Pullorum infection compared to epithelial cells. We evaluated the role of the CigR effector protein in the formation of SCVs. CigR is a membrane-binding protein, which is one of the type III secretion system 2 (T3SS2) effectors encoded within Salmonella pathogenicity island 3 (SPI3). The deletion of cigR in S. Typhimurium and S. Pullorum enhanced bacterial virulence to both mice and chickens. To analyze the influence of CigR on the SCV development during S. Pullorum infection, this study compared the formation of SCVs by using S. Pullorum C79-13 and the cigR deleted strain C79-13ΔcigR. Compared to the wild type strain, the loss of cigR gene in strain C79-13ΔcigR caused a four-fold increase in the recruitment ability of the SCV marker protein Rab7 in infected macrophages after 30 min, 5 h, and 16 h post-infection. In addition, infection and proliferation of strain C79-13ΔcigR in the avian macrophage cell line HD-11 was higher than that of the wild type strain. Our findings suggest that CigR is an anti-virulence effector inhibiting replication of S. Pullorum and SCV development in macrophages.

A protein that controls the onset of a Salmonella virulence program

The mechanism of action and contribution to pathogenesis of many virulence genes are understood. By contrast, little is known about anti-virulence genes, which contribute to the start, progression, and outcome of an infection. We now report how an anti-virulence factor in Salmonella enterica serovar Typhimurium dictates the onset of a genetic program that governs metabolic adaptations and pathogen survival in host tissues. Specifically, we establish that the anti-virulence protein CigR directly restrains the virulence protein MgtC, thereby hindering intramacrophage survival, inhibition of ATP synthesis, stabilization of cytoplasmic pH, and gene transcription by the master virulence regulator PhoP. We determine that, like MgtC, CigR localizes to the bacterial inner membrane and that its C-terminal domain is critical for inhibition of MgtC. As in many toxin/anti-toxin genes implicated in antibiotic tolerance, the mgtC and cigR genes are part of the same mRNA. However, cigR is also transcribed from a constitutive promoter, thereby creating a threshold of CigR protein that the inducible MgtC protein must overcome to initiate a virulence program critical for pathogen persistence in host tissues.