Salmonella typhimurium mutants that downregulate phagocyte nitric oxide production

RyhB Paralogs Downregulate the Expressions of Multiple Survival-Associated Genes and Attenuate the Survival of Salmonella Enteritidis in the Chicken Macrophage HD11

RyhB-1 and RyhB-2 are small non-coding RNAs in Salmonella that act as regulators of iron homeostasis by sensing the environmental iron concentration. Expressions of RyhB paralogs from Salmonella Typhimurium are increased within microphages. RyhB paralogs restrain the growth of S. Typhimurium in RAW264.7 macrophages by modulating the expression of Salmonella pathogenicity island 1 (SPI-1) genes sicA and rtsB. However, little is known about the regulatory role of RyhBs and their virulence-associated targets in Salmonella Enteritidis. We studied candidate targets of RyhB paralogs via RNA-Seq in conditions of iron limitation and hypoxia. RyhB paralogs were expressed when the S. Enteritidis strain CMCC(B)50336 (SE50336) interacted with the chicken macrophage line HD11. We analyzed gene expression associated with Salmonella survival and replication in macrophages in wild-type strain SE50336 and the RyhB deletion mutants after co-incubation with HD11 and screened out targets regulated by RyhBs. The expressions of both RyhB-1 and RyhB-2 were increased after co-incubation with HD11 for 8 h and several survival-associated genes within macrophages, such as ssaI, sseA, pagC, sodC, mgtC, yaeB, pocR, and hns, were upregulated in the ryhB-1 deletion mutant. Specifically, ssaI, the type-three secretion system 2 (T3SS-2) effector encoded by SPI-2, which promoted the survival of Salmonella in macrophages, was upregulated more than 3-fold in the ryhB-1 deletion mutant. We confirmed that both RyhB-1 and RyhB-2 downregulated the expression of ssaI to repress its mRNA translation by directly interacting with its coding sequence (CDS) region via an incomplete complementary base-pairing mechanism. The SPI-2 gene sseA was indirectly modulated by RyhB-1. The survival assays in macrophages showed that the ability of intracellular survival of ryhB-1 and/or ryhB-2 deletion mutants in HD11 was higher than that of the wild-type strain. These results indicate that RyhB paralogs downregulate survival-related virulence factors and attenuate the survival of S. Enteritidis inside chicken macrophage HD11.

Cytokines Induced by Edwardsiella tarda: Profile and Role in Antibacterial Immunity

Edwardsiella tarda is a Gram-negative bacterial pathogen with a broad range of hosts, including fish and mammals. In the present study, we used an advanced antibody array technology to identify the expression pattern of cytokines induced by E. tarda in a mouse infection model. In total, 31 and 24 differentially expressed cytokines (DECs) were identified in the plasma at 6 h and 24 h post-infection (hpi), respectively. The DECs were markedly enriched in the Gene Ontology (GO) terms associated with cell migration and response to chemokine and in the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways associated with immunity, diseases, and infection. Ten key DECs, including IL6 and TNF-α, were found to form extensive protein-protein interaction networks. IL6 was demonstrated to inhibit E. tarda infection and be required for E. tarda-induced inflammatory response. TNF-α also exerted an inhibitory effect on E. tarda infection, and knockdown of fish (Japanese flounder) TNF-α promoted E. tarda invasion in host cells. Together, the results of this study revealed a comprehensive profile of cytokines induced by E. tarda, thus adding new insights into the role of cytokine-associated immunity against bacterial infection and also providing the potential plasma biomarkers of E. tarda infection for future studies.

The Small RNA RyhB Homologs from Salmonella Typhimurium Restrain the Intracellular Growth and Modulate the SPI-1 Gene Expression within RAW264.7 Macrophages

Growing evidence indicates that small noncoding RNAs (sRNAs) play important regulatory roles during bacterial infection. In Salmonella Typhimurium, several sRNAs are strongly up-regulated within macrophages, but little is known about their role during the infection process. Among these sRNAs, the well-characterized paralogs RyhB-1 and RyhB-2 are two regulators of gene expression mainly related with the response to iron availability. To investigate the role of the sRNAs RyhB-1 and RyhB-2 from S. Typhimurium in the infection of RAW264.7 macrophages, we analyzed several phenotypic traits from intracellular mutant strains lacking one and both sRNAs. Deletion of RyhB-1 and/or RyhB-2 resulted in increased intracellular survival and faster replication within macrophages. The bacterial metabolic status inside macrophages was also analyzed, revealing that all the mutant strains exhibited higher intracellular levels of ATP and lower NAD⁺/NADH ratios than the wild type. Expression analyses from bacteria infecting macrophages showed that RyhB-1 and RyhB-2 affect the intra-macrophage expression of bacterial genes associated with the Salmonella pathogenicity island 1 (SPI-1) and the type III secretion system (T3SS). With a two-plasmid system and compensatory mutations, we confirmed that RyhB-1 and RyhB-2 directly interact with the mRNAs of the invasion chaperone SicA and the regulatory protein RtsB. Altogether, these results indicate that the RyhB homologs contribute to the S. Typhimurium virulence modulation inside macrophages by reducing the intracellular growth and down-regulating the SPI-1 gene expression.

Mechanisms involved in the adaptation of Escherichia coli O157:H7 to the host intestinal microenvironment

Host adaptation of pathogens may increase intra- and interspecies transmission. We showed previously that the passage of a clinically isolated enterohemorrhagic Escherichia coli (EHEC) O157 strain (125/99) through the gastrointestinal tract of mice increases its pathogenicity in the same host. In this work, we aimed to elucidate the underlying mechanism(s) involved in the patho-adaptation of the stool-recovered (125RR) strain. We assessed the global transcription profile by microarray and found almost 100 differentially expressed genes in 125RR strain compared with 125/99 strain. We detected an overexpression of Type Three Secretion System (TTSS) proteins at the mRNA and protein levels and demonstrated increased adhesion to epithelial cell lines for the 125RR strain. Additional key attributes of the 125RR strain were: increased motility on semisolid agar, which correlated with an increased fliC mRNA level; reduced Stx2 production at the mRNA and protein levels; increased survival at pH 2.5, as determined by acid resistance assays. We tested whether the overexpression of the LEE-encoded regulator (ler) in trans in the 125/99 strain could recreate the increased pathogenicity observed in the 125RR strain. As anticipated ler overexpression led to increased expression of TTSS proteins and bacterial adhesion to epithelial cells in vitro but also increased mortality and intestinal colonization in vivo. We conclude that this host-adaptation process required changes in several mechanisms that improved EHEC O157 fitness in the new host. The research highlights some of the bacterial mechanisms required for horizontal transmission of these zoonotic pathogens between their animal and human populations.

Replication of Salmonella enterica serovar Typhimurium in RAW264.7 Phagocytes Correlates With Hypoxia and Lack of iNOS Expression

Salmonella infection associates with tissue hypoxia, while inducible nitric oxide synthase (iNOS), relying for its activity on molecular oxygen, stands as a central host defence measure in murine salmonellosis. Here, we have detailed hypoxia and iNOS responses of murine macrophage-like RAW264.7 cells upon infection with Salmonella enterica serovar Typhimurium. We noted that only a proportion of the infected RAW264.7 cells became hypoxic or expressed iNOS. Heavily infected cells became hypoxic, while in parallel such cells tended not to express iNOS. While a proportion of the infected RAW264.7 cells revealed shutdown of protein synthesis, this was only detectable after 12 h post infection and after iNOS expression was induced in the cell culture. Our data implicate an intrinsic heterogeneity with regard to hypoxia and iNOS expression in a cell culture-based infection setting.

Integrative analysis of gut microbiome and metabolites revealed novel mechanisms of intestinal Salmonella carriage in chicken

Intestinal carriage of Salmonella Enteritidis (SE) in the chicken host serves as a reservoir for transmission of Salmonella to humans through the consumption of poultry products. The aim of the current study was to examine the three-way interaction that occurred between host metabolites, resident gut microbiota and Salmonella following inoculation of SE in two-week-old layer chicks. Our results revealed an overall alteration in gut microbiome and metabolites in association with SE infection. Enriched colonization by different microbial members throughout the course of experimental infection highlighted significant fluctuation in the intestinal microbial community in response to Salmonella infection. As changes in community membership occurred, there was also subsequent impact on differential regulation of interlinked predicted functional activities within the intestinal environment dictated by Salmonella-commensal interaction. Alteration in the overall microbial community following infection also has a ripple effect on the host regulation of cecum-associated metabolic networks. The findings showed that there was differential regulation in many of the metabolites in association with SE colonization in chickens. Perturbation in metabolic pathways related to arginine and proline metabolism as well as TCA cycle was most prominently detected. Taken together, the present findings provided a starting point in understanding the effect of intestinal Salmonella carriage on the microbiome and metabolome of developing young layer chicks.

The Transcription Factor ArcA Modulates Salmonella's Metabolism in Response to Neutrophil Hypochlorous Acid-Mediated Stress

Salmonella Typhimurium, a bacterial pathogen with high metabolic plasticity, can adapt to different environmental conditions; these traits enhance its virulence by enabling bacterial survival. Neutrophils play important roles in the innate immune response, including the production of microbicidal reactive oxygen species (ROS). In addition, the myeloperoxidase in neutrophils catalyzes the formation of hypochlorous acid (HOCl), a highly toxic molecule that reacts with essential biomolecules, causing oxidative damage including lipid peroxidation and protein carbonylation. The bacterial response regulator ArcA regulates adaptive responses to oxygen levels and influences the survival of Salmonella inside phagocytic cells. Here, we demonstrate by whole transcriptomic analyses that ArcA regulates genes related to various metabolic pathways, enabling bacterial survival during HOCl-stress in vitro. Also, inside neutrophils, ArcA controls the transcription of several metabolic pathways by downregulating the expression of genes related to fatty acid degradation, lysine degradation, and arginine, proline, pyruvate, and propanoate metabolism. ArcA also upregulates genes encoding components of the oxidative pathway. These results underscore the importance of ArcA in ATP generation inside the neutrophil phagosome and its participation in bacterial metabolic adaptations during HOCl stress.

A Comparative Analysis of Edwardsiella tarda-Induced Transcriptome Profiles in RAW264.7 Cells Reveals New Insights into the Strategy of Bacterial Immune Evasion

Edwardsiella tarda is a Gram-negative bacterial pathogen with a broad host range, including fish, reptiles, and mammals. One prominent virulence feature of E. tarda is its ability to survive and replicate in host phagocytes, but the relevant molecular mechanism is largely unknown. In this study, we examined the transcriptome profiles of RAW264.7 cells, a murine macrophage cell line, infected with live E. tarda or stimulated with dead E. tarda for 4 h and 8 h. Eighteen libraries were constructed, and an average of 69 million clean reads per library were obtained, with ~81.63% of the reads being successfully mapped to the reference genome. In total, 208 and 232 differentially expressed genes (DEGs) were identified between live and dead E. tarda-treated cells at 4 h and 8 h post-infection, respectively. The DEGs were markedly enriched in the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways associated with immunity. Live E. tarda differed strikingly from dead E. tarda in the regulation of immune related genes. Compared with dead E. tarda-treated cells, live E. tarda-treated cells exhibited marked and significant suppression in the induction of a large amount of immune genes, including RIG-I-like receptors, cytokines, and interferon-related genes. Furthermore, some of the immune genes highly regulated by live E. tarda formed complicated interaction networks with each other. Together, the results of this study revealed a transcriptome profile specifically induced by the active virulence elements of live E. tarda during the infection process, thus adding new insights into the intracellular infection mechanism of E. tarda. This study also provided a valuable set of target genes for further study of the immune evasion strategy of E. tarda.

Salmonella and Reactive Oxygen Species: A Love-Hate Relationship

Salmonella enterica represents an enterobacterial species including numerous serovars that cause infections at, or initiated at, the intestinal epithelium. Many serovars also act as facultative intracellular pathogens with a tropism for phagocytic cells. These bacteria not only survive in phagocytes but also undergo de facto replication therein. Phagocytes, through the activities of phagocyte NADPH-dependent oxidase and inducible nitric oxide synthase, are very proficient in converting molecular oxygen to reactive oxygen (ROS) and nitrogen species (RNS). These compounds represent highly efficient effectors of the innate immune defense. Salmonella is by no means resistant to these effectors, which may stand in contrast to the host niches chosen. To cope with this paradox, these bacteria rely on an array of detoxification and repair systems. Combination these systems allows for a high enough tolerance to ROS and RNS to enable establishment of infection. In addition, salmonella possesses protein factors that have the potential to dampen the infection-associated inflammation, which evidently results in a reduced exposure to ROS and RNS. This review attempts to summarize the activities and strategies by which salmonella tries to cope with ROS and RNS and how the bacterium can make use of these innate defense factors.

Zinc-dependent substrate-level phosphorylation powers Salmonella growth under nitrosative stress of the innate host response

The metabolic processes that enable the replication of intracellular Salmonella under nitrosative stress conditions engendered in the innate response of macrophages are poorly understood. A screen of Salmonella transposon mutants identified the ABC-type high-affinity zinc uptake system ZnuABC as a critical determinant of the adaptation of Salmonella to the nitrosative stress generated by the enzymatic activity of inducible nitric oxide (NO) synthase of mononuclear phagocytic cells. NO limits the virulence of a znuB mutant in an acute murine model of salmonellosis. The ZnuABC transporter is crucial for the glycolytic function of fructose bisphosphate aldolase, thereby fueling growth of Salmonella during nitrosative stress produced in the innate response of macrophages. Our investigations demonstrate that glycolysis mediates resistance of Salmonella to the antimicrobial activity of NO produced in an acute model of infection. The ATP synthesized by substrate-level phosphorylation at the payoff phase of glycolysis and acetate fermentation powers the replication of Salmonella experiencing high levels of nitrosative stress. In contrast, despite its high potential for ATP synthesis, oxidative phosphorylation is a major target of inhibition by NO and contributes little to the antinitrosative defenses of intracellular Salmonella. Our investigations have uncovered a previously unsuspected conjunction between zinc homeostasis, glucose metabolism and cellular energetics in the adaptation of intracellular Salmonella to the reactive nitrogen species synthesized in the innate host response.

Microbial pathogens are exposed to multiple antimicrobial defenses during their associations with host cells. Nitric oxide generated in the innate response exerts widespread antimicrobial activity against a variety of pathogenic microorganisms. Nitric oxide has high affinity for metal groups of terminal cytochromes of the respiratory chain, and thus nitrosative stress exerts extreme deleterious actions against the cellular energetics that rely on oxidative phosphorylation. Intracellular Salmonella have resolved this dilemma by satisfying a significant portion of their energetic demands via substrate level phosphorylation in the payoff phase of glycolysis and acetate fermentation. A high affinity zinc uptake system promotes antinitrosative defense of intracellular Salmonella by in great part supporting the enzymatic activity of an essential enzyme in the preparatory phase of glycolysis. Our research provides novel insights into the metabolic and energetic adaptations that allow a bacterial pathogen to thrive in the midst of the innate host response of vertebrate cells.

The ArcAB two-component regulatory system promotes resistance to reactive oxygen species and systemic infection by Salmonella Typhimurium

Salmonella enterica Serovar Typhimurium (S. Typhimurium) is an intracellular bacterium that overcomes host immune system barriers for successful infection. The bacterium colonizes the proximal small intestine, penetrates the epithelial layer, and is engulfed by macrophages and neutrophils. Intracellularly, S. Typhimurium encounters highly toxic reactive oxygen species including hydrogen peroxide and hypochlorous acid. The molecular mechanisms of Salmonella resistance to intracellular oxidative stress is not completely understood. The ArcAB two-component system is a global regulatory system that responds to oxygen. In this work, we show that the ArcA response regulator participates in Salmonella adaptation to changing oxygen levels and is also involved in promoting intracellular survival in macrophages and neutrophils, enabling S. Typhimurium to successfully establish a systemic infection.

Salmonella small RNA fragment Sal-1 facilitates bacterial survival in infected cells via suppressing iNOS induction in a microRNA manner

Salmonella can hijack host atypical miRNA processing machinery to cleave its small non-coding RNA into a ~22-nt RNA fragment, Sal-1, which facilitates Salmonella survival in the infected host. The mechanism through which Sal-1 promotes Salmonella survival, however, remains unknown. In the present study, we reported that Sal-1 targets cellular inducible nitric oxide synthase (iNOS) in a miRNA manner, leading to attenuation of host cell iNOS/NO-mediated anti-microbial capacity. First, depletion of Sal-1 in Salmonella-infected epithelial cells significantly increased the iNOS level but not the levels of various inflammatory cytokines. Bioinformatics analysis and mutagenesis strategies were consistent with the identification of mRNA of iNOS as a target of Sal-1 in both human and mice. Second, western blot and immunohistochemical analysis confirmed that Sal-1 suppressed iNOS expression in vitro and in vivo, thus reducing the production of NO. Finally, Sal-1 facilitating Salmonella survival through suppressing iNOS induction was confirmed in mouse model by expressing mutated iNOS that is not targeted by Sal-1 in mice colon. In conclusion, our study provides new insight into the pathogenic mechanism of intracellular bacteria to modulate host innate immune response.

In silico clustering of Salmonella global gene expression data reveals novel genes co-regulated with the SPI-1 virulence genes through HilD

A wide variety of Salmonella enterica serovars cause intestinal and systemic infections to humans and animals. Salmonella Patogenicity Island 1 (SPI-1) is a chromosomal region containing 39 genes that have crucial virulence roles. The AraC-like transcriptional regulator HilD, encoded in SPI-1, positively controls the expression of the SPI-1 genes, as well as of several other virulence genes located outside SPI-1. In this study, we applied a clustering method to the global gene expression data of S. enterica serovar Typhimurium from the COLOMBOS database; thus genes that show an expression pattern similar to that of SPI-1 genes were selected. This analysis revealed nine novel genes that are co-expressed with SPI-1, which are located in different chromosomal regions. Expression analyses and protein-DNA interaction assays showed regulation by HilD for six of these genes: gtgE, phoH, sinR, SL1263 (lpxR) and SL4247 were regulated directly, whereas SL1896 was regulated indirectly. Interestingly, phoH is an ancestral gene conserved in most of bacteria, whereas the other genes show characteristics of genes acquired by Salmonella. A role in virulence has been previously demonstrated for gtgE, lpxR and sinR. Our results further expand the regulon of HilD and thus identify novel possible Salmonella virulence genes.

Nutritional and metabolic requirements for the infection of HeLa cells by Salmonella enterica serovar Typhimurium

Salmonella is the causative agent of a spectrum of human and animal diseases ranging from gastroenteritis to typhoid fever. It is a food - and water - borne pathogen and infects via ingestion followed by invasion of intestinal epithelial cells and phagocytic cells. In this study we employed a mutational approach to define the nutrients and metabolic pathways required by Salmonella enterica serovar Typhimurium during infection of a human epithelial cell line (HeLa). We deleted the key glycolytic genes, pfkA and pfkB to show that S. Typhimurium utilizes glycolysis for replication within HeLa cells; however, glycolysis was not absolutely essential for intracellular replication. Using S. Typhimurium strains deleted for genes encoding components of the phosphotransferase system and glucose transport, we show that glucose is a major substrate required for the intracellular replication of S. Typhimurium in HeLa cells. We also deleted genes encoding enzymes involved in the utilization of gluconeogenic substrates and the glyoxylate shunt and show that neither of these pathways were required for intracellular replication of S. Typhimurium within HeLa cells.

The EAL-like protein STM1697 regulates virulence phenotypes, motility and biofilm formation in Salmonella typhimurium

The ubiquitous second messenger c-di-GMP regulates the switching of bacterial lifestyles from motility to sessility and acute to chronic virulence to adjust bacterial fitness to altered environmental conditions. Conventionally, EAL proteins being c-di-GMP phosphodiesterases promote motility and acute virulence phenotypes such as invasion into epithelial cells and inhibit biofilm formation. We report here that in contradiction, the EAL-like protein STM1697 of Salmonella typhimurium suppresses motility, invasion into HT-29 epithelial cell line and secretion of the type three secretion system 1 effector protein SipA, whereas it promotes rdar biofilm formation and CsgD expression. STM1697 can, however, functionally replace the EAL-like protein STM1344 and vice versa, whereby both proteins neither degrade nor bind c-di-GMP. Like STM1344, STM1697 suppresses the transcription of class 2 and class 3 flagella regulon genes by binding to FlhD, a component of the master regulator of the flagella regulon FlhD4 C2 and act additively under numerous conditions. Interestingly, the interaction interface of STM1697 with FlhD2 is distinct from its paralogue STM1344. We predict that the stand alone EAL domain proteins STM1697 and STM1344 belong to a subclass of EAL domain proteins in S. typhimurium, which are all involved in motility, biofilm and virulence regulation through interaction with proteins that regulate flagella function.

The FUN of identifying gene function in bacterial pathogens; insights from Salmonella functional genomics

The availability of thousands of genome sequences of bacterial pathogens poses a particular challenge because each genome contains hundreds of genes of unknown function (FUN). How can we easily discover which FUN genes encode important virulence factors? One solution is to combine two different functional genomic approaches. First, transcriptomics identifies bacterial FUN genes that show differential expression during the process of mammalian infection. Second, global mutagenesis identifies individual FUN genes that the pathogen requires to cause disease. The intersection of these datasets can reveal a small set of candidate genes most likely to encode novel virulence attributes. We demonstrate this approach with the Salmonella infection model, and propose that a similar strategy could be used for other bacterial pathogens.

The putative thiosulfate sulfurtransferases PspE and GlpE contribute to virulence of Salmonella Typhimurium in the mouse model of systemic disease

The phage-shock protein PspE and GlpE of the glycerol 3-phosphate regulon of Salmonella enterica serovar Typhimurium are predicted to belong to the class of thiosulfate sulfurtransferases, enzymes that traffic sulfur between molecules. In the present study we demonstrated that the two genes contribute to S. Typhimurium virulence, as a glpE and pspE double deletion strain showed significantly decreased virulence in a mouse model of systemic infection. However, challenge of cultured epithelial cells and macrophages did not reveal any virulence-associated phenotypes. We hypothesized that their contribution to virulence could be in sulfur metabolism or by contributing to resistance to nitric oxide, oxidative stress, or cyanide detoxification. In vitro studies demonstrated that glpE but not pspE was important for resistance to H₂O₂. Since the double mutant, which was the one affected in virulence, was not affected in this assay, we concluded that resistance to oxidative stress and the virulence phenotype was most likely not linked. The two genes did not contribute to nitric oxid stress, to synthesis of essential sulfur containing amino acids, nor to detoxification of cyanide. Currently, the precise mechanism by which they contribute to virulence remains elusive.

Oxidoreductases that act as conditional virulence suppressors in Salmonella enterica serovar Typhimurium

In Salmonella enterica serovar Typhimurium, oxidoreductases of the thioredoxin superfamily contribute to bacterial invasiveness, intracellular replication and to the virulence in BALB/c mice as well as in the soil nematode Caenorhabditis elegans. The scsABCD gene cluster, present in many but not all enteric bacteria, codes for four putative oxidoreductases of the thioredoxin superfamily. Here we have analyzed the potential role of the scs genes in oxidative stress tolerance and virulence in S. Typhimurium. An scsABCD deletion mutant showed moderate sensitization to the redox-active transition metal ion copper and increased protein carbonylation upon exposure to hydrogen peroxide. Still, the scsABCD mutant was not significantly affected for invasiveness or intracellular replication in respectively cultured epithelial or macrophage-like cells. However, we noted a significant copper chloride sensitivity of SPI1 T3SS mediated invasiveness that strongly depended on the presence of the scs genes. The scsABCD deletion mutant was not attenuated in animal infection models. In contrast, the mutant showed a moderate increase in its competitive index upon intraperitoneal challenge and enhanced invasiveness in small intestinal ileal loops of BALB/c mice. Moreover, deletion of the scsABCD genes restored the invasiveness of a trxA mutant in epithelial cells and its virulence in C. elegans. Our findings thus demonstrate that the scs gene cluster conditionally affects virulence and underscore the complex interactions between oxidoreductases of the thioredoxin superfamily in maintaining host adaptation of S. Typhimurium.

Inhibited Production of iNOS by Murine J774 Macrophages Occurs via a phoP-Regulated Differential Expression of NFκB and AP-1

Background. There are no reported data to explain how Salmonella suppress nitrite ion production in macrophages or whether this phenomenon is unique to typhoidal or non-typhoidal serovars. The aims of this study were, therefore, to investigate these phenomena. Methods. We measured survival of S. typhimurium 14028 and its phoP mutant in murine J774 macrophages, cultured with or without interferon gamma. We compared expression of inducible nitric oxide synthase (iNOS) mRNA and protein, and nitrite ion production and also examined binding of nuclear factor κB (NFκB) and activator protein 1 (AP-1) to macrophage DNA. Results. S. typhimurium 14028 inhibited binding of NFκB and AP-1 to DNA in murine J774. A macrophages via an intact phoP regulon. This correlated with increased survival and reduced iNOS expression. Suppression of NFκB activity was ameliorated in macrophages cultured with IFN-γ and this correlated with increased expression of iNOS mRNA and nitrite ion production, although IFN-γ had no effect on AP-1/DNA interaction. We show, that with one exception, suppression of iNOS is unique to typhoidal serovars. Conclusion. S. typhimurium inhibit NFκB and AP-1 interaction with macrophage DNA via the PhoP regulon, this reduces nitrite ion production and is principally associated with typhoidal serovars.

Nitric oxide and salmonella pathogenesis

Nitric oxide (NO) and its congeners contribute to the innate immune response to Salmonella. This enteric pathogen is exposed to reactive nitrogen species (RNS) in the environment and at different anatomical locations during its infectious cycle in vertebrate hosts. Chemical generation of RNS enhances the gastric barrier to enteropathogenic bacteria, while products of the Salmonella pathogenicity island 1 type III secretion system and Salmonella-associated molecular patterns stimulate transcription of inducible NO synthase (iNOS) by cells of the mononuclear phagocytic cell lineage. The resulting NO, or products that arise from its interactions with oxygen (O₂) or iron and low-molecular weight thiols, are preferentially bacteriostatic against Salmonella, while reaction of NO and superoxide (O2−) generates the bactericidal compound peroxynitrite (ONOO−). The anti-Salmonella activity of RNS emanates from the modification of redox active thiols and metal prosthetic groups of key molecular targets of the electron transport chain, central metabolic enzymes, transcription factors, and DNA and DNA-associated proteins. In turn, Salmonella display a plethora of defenses that modulate the delivery of iNOS-containing vesicles to phagosomes, scavenge and detoxify RNS, and repair biomolecules damaged by these toxic species. Traditionally, RNS have been recognized as important mediators of host defense against Salmonella. However, exciting new findings indicate that Salmonella can exploit the RNS produced during the infection to foster virulence. More knowledge of the primary RNS produced in response to Salmonella infection, the bacterial processes affected by these toxic species, and the adaptive bacterial responses that protect Salmonella from nitrosative and oxidative stress associated with NO will increase our understanding of Salmonella pathogenesis. This information may assist in the development of novel therapeutics against this common enteropathogen.

An incomplete TCA cycle increases survival of Salmonella Typhimurium during infection of resting and activated murine macrophages

BACKGROUND

In comparison to the comprehensive analyses performed on virulence gene expression, regulation and action, the intracellular metabolism of Salmonella during infection is a relatively under-studied area. We investigated the role of the tricarboxylic acid (TCA) cycle in the intracellular replication of Salmonella Typhimurium in resting and activated macrophages, epithelial cells, and during infection of mice.

METHODOLOGY/PRINCIPAL FINDINGS

We constructed deletion mutations of 5 TCA cycle genes in S. Typhimurium including gltA, mdh, sdhCDAB, sucAB, and sucCD. We found that the mutants exhibited increased net intracellular replication in resting and activated murine macrophages compared to the wild-type. In contrast, an epithelial cell infection model showed that the S. Typhimurium ΔsucCD and ΔgltA strains had reduced net intracellular replication compared to the wild-type. The glyoxylate shunt was not responsible for the net increased replication of the TCA cycle mutants within resting macrophages. We also confirmed that, in a murine infection model, the S. Typhimurium ΔsucAB and ΔsucCD strains are attenuated for virulence.

CONCLUSIONS/SIGNIFICANCE

Our results suggest that disruption of the TCA cycle increases the ability of S. Typhimurium to survive within resting and activated murine macrophages. In contrast, epithelial cells are non-phagocytic cells and unlike macrophages cannot mount an oxidative and nitrosative defence response against pathogens; our results show that in HeLa cells the S. Typhimurium TCA cycle mutant strains show reduced or no change in intracellular levels compared to the wild-type. The attenuation of the S. Typhimurium ΔsucAB and ΔsucCD mutants in mice, compared to their increased net intracellular replication in resting and activated macrophages suggest that Salmonella may encounter environments within the host where a complete TCA cycle is advantageous.

Microarray analysis of response of Salmonella during infection of HLA-B27- transfected human macrophage-like U937 cells

BACKGROUND

Human leukocyte antigen (HLA)-B27 is strongly associated with the development of reactive arthritis (ReA) in humans after salmonellosis. Human monocytic U937 cells transfected with HLA-B27 are less able to eliminate intracellular Salmonella enterica serovar Enteritidis than those transfected with control HLA antigens (e.g. HLA-A2). To investigate further the mechanisms by which HLA-B27-transfected cells allow increased replication of these bacteria, a DNA-based microarray was used for comparative genomic analysis of S. Enteritidis grown in HLA-B27- or HLA-A2-transfected cells. The microarray consisted of 5080 oligonucleotides from different serovars of Salmonella including S. Enteritidis PT4-specific genes. Bacterial RNA was isolated from the infected HLA-B27- or HLA-A2-transfected cells, reverse-transcribed to cDNA, and hybridized with the oligonucleotides on the microarrays. Some microarray results were confirmed by RT-PCR.

RESULTS

When gene expression was compared between Salmonella grown in HLA-B27 cells and in HLA-A2 cells, 118 of the 4610 S. Enteritidis-related genes differed in expression at 8 h after infection, but no significant difference was detectable at 2 h after infection. These differentially expressed genes are mainly involved in Salmonella virulence, DNA replication, energy conversion and metabolism, and uptake and metabolism of nutrient substances, etc. The difference suggests HLA-B27-dependent modulation of Salmonella gene expression, resulting in increased Salmonella replication in HLA-B27-positive cells. Among the up-regulated genes were those located in Salmonella pathogenicity island (SPI)-2, which play a central role in intracellular survival and replication of Salmonella.

CONCLUSIONS

This is the first report to show the regulation of Salmonella gene expression by HLA-B27 during infection of host cells. This regulation probably leads to increased Salmonella survival and replication in HLA-B27-positive cells. SPI-2 genes seem to contribute significantly to the increased replication.

Bacterial manipulation of innate immunity to promote infection

The mammalian innate immune response provides a barrier against invading pathogens. Innate immune mechanisms are used by the host to respond to a range of bacterial pathogens in an acute and conserved fashion. Host cells express pattern recognition receptors that sense pathogen-associated molecular patterns. After detection, an arsenal of antimicrobial mechanisms is deployed to kill bacteria in infected cells. Innate immunity also stimulates antigen-specific responses mediated by the adaptive immune system. In response, pathogens manipulate host defence mechanisms to survive and eventually replicate. This Review focuses on the control of host innate immune responses by pathogenic intracellular bacteria.

Salmonella enterica serovar Typhimurium lipopolysaccharide deacylation enhances its intracellular growth within macrophages

Modification of lipid A is essential for bacterial adaptation to its host. Salmonella Typhimurium LpxR potentially detoxifies lipid A by 3'-O-deacylation; however, the involvement of deacylation in its adaptation remains unclear. LpxR-dependent 3'-O-deacylation was observed in the stationary phase. When macrophages were infected with stationary phase bacteria, the intracellular growth of the lpxR-null strain was lower than that of the wild-type strain. Furthermore, the expression level of inducible nitric oxide synthase was higher in the cells infected with the lpxR-null strain than in the cells infected with the wild-type strain. These results indicate that lipid A 3'-O-deacylation is beneficial for intracellular growth.

Molecular insights into farm animal and zoonotic Salmonella infections

Salmonella enterica is a facultative intracellular pathogen of worldwide importance. Infections may present in a variety of ways, from asymptomatic colonization to inflammatory diarrhoea or typhoid fever depending on serovar- and host-specific factors. Human diarrhoeal infections are frequently acquired via the food chain and farm environment by virtue of the ability of selected non-typhoidal serovars to colonize the intestines of food-producing animals and contaminate the avian reproductive tract and egg. Colonization of reservoir hosts often occurs in the absence of clinical symptoms; however, some S. enterica serovars threaten animal health owing to their ability to cause acute enteritis or translocate from the intestines to other organs causing fever, septicaemia and abortion. Despite the availability of complete genome sequences of isolates representing several serovars, the molecular mechanisms underlying Salmonella colonization, pathogenesis and transmission in reservoir hosts remain ill-defined. Here we review current knowledge of the bacterial factors influencing colonization of food-producing animals by Salmonella and the basis of host range, differential virulence and zoonotic potential.

Glucose and glycolysis are required for the successful infection of macrophages and mice by Salmonella enterica serovar typhimurium

Salmonella is a widespread zoonotic enteropathogen that causes gastroenteritis and fatal typhoidal disease in mammals. During systemic infection of mice, Salmonella enterica serovar Typhimurium resides and replicates in macrophages within the "Salmonella-containing vacuole" (SCV). It is surprising that the substrates and metabolic pathways necessary for growth of S. Typhimurium within the SCV of macrophages have not been identified yet. To determine whether S. Typhimurium utilized sugars within the SCV, we constructed a series of S. Typhimurium mutants that lacked genes involved in sugar transport and catabolism and tested them for replication in mice and macrophages. These mutants included a mutant with a mutation in the pfkAB-encoded phosphofructokinase, which catalyzes a key committing step in glycolysis. We discovered that a pfkAB mutant is severely attenuated for replication and survival within RAW 264.7 macrophages. We also show that disruption of the phosphoenolpyruvate:carbohydrate phosphotransferase system by deletion of the ptsHI and crr genes reduces S. Typhimurium replication within RAW 264.7 macrophages. We discovered that mutants unable to catabolize glucose due to deletion of ptsHI, crr, and glk or deletion of ptsG, manXYZ, and glk showed reduced replication within RAW 264.7 macrophages. This study proves that S. Typhimurium requires glycolysis for infection of mice and macrophages and that transport of glucose is required for replication within macrophages.

Omeprazole antagonizes virulence and inflammation in Salmonella enterica-infected RAW264.7 cells

The proton pump inhibitor omeprazole reduced the intracellular replication of Salmonella enterica serovar Typhimurium in RAW264.7 cells without affecting bacterial growth in vitro or the viability of the host cells. The mechanism was bacteriostatic and interfered with replication mediated by the virulence-associated SPI2 type III secretion system. The proton pump inhibitor bafilomycin A(1), in contrast, mediated killing of intracellular bacteria and imposed a marked cytotoxicity on RAW264.7 cells. The two compounds also differentially affected the proinflammatory responses of the infected cells. Bafilomycin A(1) enhanced nitric oxide production, whereas omeprazole delayed IkappaB degradation and blocked nitric oxide production and the secretion of proinflammatory cytokines. These results imply that omeprazole can be used to block the virulence factor-mediated intracellular replication of S. Typhimurium, and that its mechanism of growth inhibition is different from that mediated by bafilomycin A(1).

A dynamic view of the spread and intracellular distribution of Salmonella enterica

The events that determine the dynamics of proliferation, spread and distribution of microbial pathogens within their hosts are surprisingly heterogeneous and poorly understood. We contend that understanding these phenomena at a sophisticated level with the help of mathematical models is a prerequisite for the development of truly novel, targeted preventative measures and drug regimes. We describe here recent studies of Salmonella enterica infections in mice which suggest that bacteria resist the antimicrobial environment inside host cells and spread to new sites, where infection foci develop, and thus avoid local escalation of the adaptive immune response. We further describe implications for our understanding of the pathogenic mechanism inside the host.

The mechanism of ciprofloxacin resistance in dihydrogen peroxide-induced mutants of Salmonella enterica subsp. enterica serovar typhimurium consists mainly in mutations in gyrA gene and less in mutations affecting ciprofloxacin uptake

The effect of H(2)O(2) on the induction of ciprofloxacin (CFL) resistant mutants of Salmonella enterica subsp. enterica serovar Typhimurium was evaluated and determinants of CFL resistance in the mutants were analyzed. Factors associated with CFL resistance in H(2)O(2)-induced mutants included (i) mutations in gyrA gene, predominantly (63 %) Asp(87)-->Asn and less (37 %) Ser(83)-->Phe substitutions, (ii) mutations in the regulatory genes of MarRAB or SoxRS or in the individual structural genes of these operons. Such mutations are induced by H(2)O(2) in a much lower extent. Reduced OmpF expression simultaneously with enhanced efflux was detected only in one mutant strain and 20 % of mutant strains had increased CFL efflux from the cells.

Arginase modulates Salmonella induced nitric oxide production in RAW264.7 macrophages and is required for Salmonella pathogenesis in mice model of infection

Arginine is a common substrate for both inducible nitric oxide synthase (iNOS) and arginase. The competition between iNOS and arginase for arginine contributes to the outcome of several parasitic and bacterial infections. Salmonella infection in macrophage cell line RAW264.7 induces iNOS. Because the availability of l-arginine is a major determinant for nitric oxide (NO) synthesis, we hypothesize that in the Salmonella infected macrophages NO production may be regulated by arginase. Here we report for the first time that Salmonella up-regulates arginase II but not arginase I isoform in RAW264.7 macrophages. Blocking arginase increases the substrate l-arginine availability to iNOS for production of more nitric oxide and perhaps peroxynitrite molecules in the infected cells allowing better killing of virulent Salmonella in a NO dependent manner. RAW264.7 macrophages treated with iNOS inhibitor Aminoguanidine reverts the attenuation in arginase-blocked condition. Further, the NO block created by Salmonella was removed by increasing concentration of l-arginine. The whole-mice system arginase I, although constitutive, is much more abundant than the inducible arginase II isoform. Inhibition of arginase activity in mice during the course of Salmonella infection reduces the bacterial burden and delays the disease outcome in a NO dependent manner.

Reactive nitrogen species contribute to innate host defense against Campylobacter jejuni

Campylobacter jejuni, a gram-negative, invasive organism, is a common cause of food-borne bacterial diarrheal disease. However, the relationship between C. jejuni and the innate immune system is not well described. To better characterize host defense against C. jejuni, we investigated the ability of nitric oxide/reactive nitrogen species to kill two strains of C. jejuni. C. jejuni viability was measured after exposure to reactive nitrogen species produced biochemically as acidified nitrite and by bone marrow-derived macrophages. We report that acidified nitrite caused a 3-log-increased kill of C. jejuni (P < 0.05) at doses that did not affect the viability of Salmonella enterica serovar Typhimurium. Expression of NOS2, the gene responsible for the production of inducible nitric oxide, was increased >100-fold in murine macrophages after incubation with C. jejuni (P < 0.001). These macrophages effected a 2-log-increased kill of C. jejuni over 24 h compared to that by NOS2-/- macrophages unable to produce nitric oxide (P < 0.05). These findings suggest that the mammalian host upregulates the production of nitric oxide in response to exposure to C. jejuni and that nitric oxide and reactive nitrogen species comprise part of the innate defense mechanisms that contribute to the resolution of C. jejuni infection.

During infection of epithelial cells Salmonella enterica serovar Typhimurium undergoes a time-dependent transcriptional adaptation that results in simultaneous expression of three type 3 secretion systems

The biogenesis of the Salmonella-containing vacuole within mammalian cells has been intensively studied over recent years. However, the ability of Salmonella to sense and adapt to the intracellular environment of different types of host cells has received much less attention. To address this issue, we report the transcriptome of Salmonella enterica serovar Typhimurium SL1344 within epithelial cells and show comparisons with Salmonella gene expression inside macrophages. We report that S. Typhimurium expresses a characteristic intracellular transcriptomic signature in response to the environments it encounters within different cell types. The signature involves the upregulation of the mgtBC, pstACS and iro genes for magnesium, phosphate and iron uptake, and Salmonella pathogenicity island 2 (SPI2). Surprisingly, in addition to SPI2, the invasion-associated SPI1 pathogenicity island and the genes involved in flagellar biosynthesis were expressed inside epithelial cells at later stages of the infection, while they were constantly downregulated in macrophage-like cells. To our knowledge, this is the first report of the simultaneous transcription of all three Type Three Secretion Systems (T3SS) within an intracellular Salmonella population. We discovered that S. Typhimurium strain SL1344 was strongly cytotoxic to epithelial cells after 6 h of infection and hypothesize that the time-dependent changes in Salmonella gene expression within epithelial cells reflects the bacterial response to host cells that have been injured by the infection process.

Net replication of Salmonella enterica serovars Typhimurium and Choleraesuis in porcine intestinal mucosa and nodes is associated with their differential virulence

Salmonella enterica is a facultative intracellular pathogen of worldwide importance and causes a spectrum of diseases depending on serovar- and host-specific factors. Oral infection of pigs with S. enterica serovar Typhimurium strain 4/74 produces acute enteritis but is rarely fatal, whereas serovar Choleraesuis strain A50 causes systemic disease with a high mortality rate. With a porcine ligated ileal loop model, we observed that systemic virulence of serovar Choleraesuis A50 is not associated with enhanced intestinal invasion, secretory responses, or neutrophil recruitment compared to serovar Typhimurium 4/74. The net growth in vivo of serovar Choleraesuis A50 and serovar Typhimurium 4/74 was monitored following oral inoculation of pigs with strains harboring pHSG422, which exhibits temperature-sensitive replication. Analysis of plasmid partitioning revealed that the enteric virulence of serovar Typhimurium 4/74 relative to that of serovar Choleraesuis A50 is associated with rapid replication in the intestinal wall, whereas systemic virulence of serovar Choleraesuis A50 is associated with enhanced persistence in intestinal mesenteric lymph nodes. Faster replication of serovar Typhimurium, compared to that of serovar Choleraesuis, in the intestinal mucosa was associated with greater induction of the proinflammatory cytokines tumor necrosis factor alpha, interleukin-8 (IL-8), and IL-18 as detected by reverse transcriptase PCR analysis of transcripts from infected mucosa. During replication in batch culture and porcine alveolar macrophages, transcription of genes encoding components of type III secretion systems 1 (sipC) and 2 (sseC) was observed to be significantly higher in serovar Typhimurium 4/74 than in serovar Choleraesuis A50, and this may contribute to the differences in epithelial invasion and intracellular proliferation. The rapid induction of proinflammatory responses by strain 4/74 may explain why pigs confine serovar Typhimurium infection to the intestines, whereas slow replication of serovar Choleraesuis may enable it to evade host innate immunity and thus disseminate by stealth.

Salicylidene acylhydrazides that affect type III protein secretion in Salmonella enterica serovar typhimurium

A collection of nine salicylidene acylhydrazide compounds were tested for their ability to inhibit the activity of virulence-associated type III secretion systems (T3SSs) in Salmonella enterica serovar Typhimurium. The compounds strongly affected Salmonella pathogenicity island 1 (SPI1) T3SS-mediated invasion of epithelial cells and in vitro secretion of SPI1 invasion-associated effector proteins. The use of a SPI1 effector beta-lactamase fusion protein implicated intracellular entrapment of the protein construct upon application of a salicylidene acylhydrazide, whereas the use of chromosomal transcriptional gene fusions revealed a compound-mediated transcriptional silencing of SPI1. Salicylidene acylhydrazides also affected intracellular bacterial replication in murine macrophage-like cells and blocked the transport of an epitope-tagged SPI2 effector protein. Two of the compounds significantly inhibited bacterial motility and expression of extracellular flagellin. We conclude that salicylidene acylhydrazides affect bacterial T3SS activity in S. enterica and hence could be used as lead substances when designing specific inhibitors of bacterial T3SSs in order to pharmaceutically intervene with bacterial virulence.

Thioredoxin 1 promotes intracellular replication and virulence of Salmonella enterica serovar Typhimurium

The effect of the cytoplasmic reductase and protein chaperone thioredoxin 1 on the virulence of Salmonella enterica serovar Typhimurium was evaluated by deleting the trxA, trxB, or trxC gene of the cellular thioredoxin system, the grxA or gshA gene of the glutathione/glutaredoxin system, or the dsbC gene coding for a thioredoxin-dependent periplasmic disulfide bond isomerase. Mutants were tested for tolerance to oxidative and nitric oxide donor substances in vitro, for invasion and intracellular replication in cultured epithelial and macrophage-like cells, and for virulence in BALB/c mice. In these experiments only the gshA mutant, which was defective in glutathione synthesis, exhibited sensitization to oxidative stress in vitro and a small decrease in virulence. In contrast, the trxA mutant did not exhibit any growth defects or decreased tolerance to oxidative or nitric oxide stress in vitro, yet there were pronounced decreases in intracellular replication and mouse virulence. Complementation analyses using defined catalytic variants of thioredoxin 1 showed that there is a direct correlation between the redox potential of thioredoxin 1 and restoration of intracellular replication of the trxA mutant. Attenuation of mouse virulence that was caused by a deficiency in thioredoxin 1 was restored by expression of wild-type thioredoxin 1 in trans but not by expression of a catalytically inactive variant. These results clearly imply that in S. enterica serovar Typhimurium, the redox-active protein thioredoxin 1 promotes virulence, whereas in vitro tolerance to oxidative stress depends on production of glutathione.

The O-antigen affects replication of Salmonella enterica serovar Typhimurium in murine macrophage-like J774-A.1 cells through modulation of host cell nitric oxide production

O-antigen-proficient and defined O-antigen-deficient mutants of Salmonella enterica serovar Typhimurium were compared for intracellular replication and induction of nitric oxide (NO) expression in the murine macrophage-like cell line J774-A.1. While O-antigen-proficient bacteria replicated and provoked induction of host cell NO synthesis to expected levels, DeltawaaK, DeltawaaL and DeltawaaKL mutants displayed increased growth yields and induction of significantly lower levels of macrophage NO production. The downregulation of NO production did not involve suppression of inducible nitric oxide synthase (iNOS) expression, yet it depended on bacterial protein synthesis during infection of J774-A.1 cells. In contrast, when inhibitor substances were used to block iNOS activity, the growth yield of the wild type significantly exceeded that of the DeltawaaL mutant bacteria. Inactivation of the Salmonella pathogenicity island 1 (SPI1)-associated bacterial type III secretion system did not affect intracellular replication in the wild type or the DeltawaaL background. However, inactivation of the SPI2-associated type III secretion strongly abrogated bacterial intracellular replication, and the DeltawaaLDeltassaV double mutant lost the ability to suppress NO expression. The results imply that a lack of O-antigen may increase bacterial fitness in J774-A.1 cells through suppression of iNOS activity, and that the O-antigen may protect against NO-independent restriction of bacterial intracellular replication.

Differential gene expression of pathogens inside infected hosts

DNA microarray is a useful technology for studying differential gene expression in the context of microbe-host interactions. This review concentrates on recent findings of the survival strategies of three intracellular pathogens: Shigella flexneri, Salmonella enterica serovar Typhimurium and Mycobacterium tuberculosis.

Nitric Oxide in Salmonella and Escherichia coli Infections

This review discusses the role that nitric oxide (NO) and its congeners play on various stages in the pathophysiology of Escherichia coli and Salmonella infections, with special emphasis on the regulatory pathways that lead to high NO synthesis, the role of reactive nitrogen species (RNS) in host resistance, and the bacterial molecular targets and defense mechanisms that protect enteric bacteria against the nitrosative stress encountered in diverse host anatomical sites. In general, NO can react directly with prosthetic groups containing transition metal centers, with other radicals, or with sulfhydryl groups in the presence of an electron acceptor. Binding to iron complexes is probably the best characterized direct reaction of NO in biological systems. The targets of RNS are numerous. RNS can facilitate oxidative modifications including lipid peroxidation, hydroxylation, and DNA base and protein oxidation. In addition, RNS can inflict nitrosative stress through the nitrosation of amines and sulfhydryls. Numerous vital bacterial molecules can be targeted by NO. It is therefore not surprising that enteropathogenic bacteria are armed with a number of sensors to coordinate the protective response to nitrosative stress, along with an assortment of antinitrosative defenses that detoxify, repair, or avoid the deleterious effects of RNS encountered within the host. NO and NO-derived RNS play important roles in innate immunity to Salmonella and E. coli. Enzymatic NO production by NO synthases can be enhanced by microbial and other inflammatory stimuli and it exerts direct antimicrobial actions as well as immunomodulatory and vasoregulatory effects.

New concepts in Salmonella virulence: the importance of reducing the intracellular growth rate in the host

The literature refers to Salmonella enterica as an intracellular bacterial pathogen that proliferates within vacuoles of mammalian cells. However, recent in vivo studies have revealed that the vast majority of infected cells contain very few intracellular bacteria (three to four organisms). Salmonella intracellular growth is also limited in cultured dendritic cells and fibroblasts, two cell types abundant in tissues located underneath the intestinal epithelium. Recently, a Salmonella factor previously known for its role as a negative regulator of intracellular growth has been shown to tightly repress certain pathogen functions upon host colonization and to be critical for virulence. The connection between virulence and the negative control of intracellular growth is further sustained by the fact that some attenuated mutants overgrow in non-phagocytic cells located in the intestinal lamina propria. These findings are changing our classical view of Salmonella as a fast growing intracellular pathogen and suggest that this pathogen may trigger responses directed to reduce the growth rate within the infected cell. These responses could play a critical role in modulating the delicate balance between disease and persistence.

Components of the peptidoglycan-recycling pathway modulate invasion and intracellular survival of Salmonella enterica serovar Typhimurium

beta-Lactam resistance in enteric bacteria is frequently caused by mutations in ampD encoding a cytosolic N-acetylmuramyl- l-alanine amidase. Such mutants are blocked in murein (peptidoglycan) recycling and accumulate cytoplasmic muropeptides that interact with the transcriptional activator ampR, which de-represses beta-lactamase expression. Salmonella enterica serovar Typhimurium, an extensively studied enteric pathogen, was used to show that mutations in ampD decreased the ability of S. typhimurium to enter a macrophage derived cell line and made the bacteria more potent as inducers of inducible nitric oxide synthase (iNOS), as compared with the wild-type. ampG mutants, defective in the transport of recycled muropeptides across the cytoplasmic membrane, behaved essentially as the wild-type in invasion assays and in activation of iNOS. As ampD mutants also have reduced in vivo fitness in a murine model, we suggest that the cytoplasmic accumulation of muropeptides affects the virulence of the ampD mutants.

Transcriptional adaptation of Shigella flexneri during infection of macrophages and epithelial cells: insights into the strategies of a cytosolic bacterial pathogen

Shigella flexneri, the etiologic agent of bacillary dysentery, invades epithelial cells as well as macrophages and dendritic cells and escapes into the cytosol soon after invasion. Dissection of the global gene expression profile of the bacterium in its intracellular niche is essential to fully understand the biology of Shigella infection. We have determined the complete gene expression profiles for S. flexneri infecting human epithelial HeLa cells and human macrophage-like U937 cells. Approximately one quarter of the S. flexneri genes showed significant transcriptional adaptation during infection; 929 and 1,060 genes were up- or down-regulated within HeLa cells and U937 cells, respectively. The key S. flexneri virulence genes, ipa-mxi-spa and icsA, were drastically down-regulated during intracellular growth. This theme seems to be common in bacterial infection, because the Ipa-Mxi-Spa-like type III secretion systems were also down-regulated during mammalian cell infection by Salmonella enterica serovar Typhimurium and Escherichia coli O157. The bacteria experienced restricted levels of iron, magnesium, and phosphate in both host cell types, as shown by up-regulation of the sitABCD system, the mgtA gene, and genes of the phoBR regulon. Interestingly, ydeO and other acid-induced genes were up-regulated only in U937 cells and not in HeLa cells, suggesting that the cytosol of U937 cells is acidic. Comparison with the gene expression of intracellular Salmonella serovar Typhimurium, which resides within the Salmonella-containing vacuole, indicated that S. flexneri is exposed to oxidative stress in U937 cells. This work will facilitate functional studies of hundreds of novel intracellularly regulated genes that may be important for the survival and growth strategies of Shigella in the human host.

Experimental adaptation of Salmonella typhimurium to mice

Experimental evolution is a powerful approach to study the dynamics and mechanisms of bacterial niche specialization. By serial passage in mice, we evolved 18 independent lineages of Salmonella typhimurium LT2 and examined the rate and extent of adaptation to a mainly reticuloendothelial host environment. Bacterial mutation rates and population sizes were varied by using wild-type and DNA repair-defective mutator (mutS) strains with normal and high mutation rates, respectively, and by varying the number of bacteria intraperitoneally injected into mice. After <200 generations of adaptation all lineages showed an increased fitness as measured by a faster growth rate in mice (selection coefficients 0.11-0.58). Using a generally applicable mathematical model we calculated the adaptive mutation rate for the wild-type bacterium to be >10(-6)/cell/generation, suggesting that the majority of adaptive mutations are not simple point mutations. For the mutator lineages, adaptation to mice was associated with a loss of fitness in secondary environments as seen by a reduced metabolic capability. During adaptation there was no indication that a high mutation rate was counterselected. These data show that S. typhimurium can rapidly and extensively increase its fitness in mice but this niche specialization is, at least in mutators, associated with a cost.

Uptake and replication of Salmonella enterica in Acanthamoeba rhysodes

The ability of salmonellae to become internalized and to survive and replicate in amoebae was evaluated by using three separate serovars of Salmonella enterica and five different isolates of axenic Acanthamoeba spp. In gentamicin protection assays, Salmonella enterica serovar Dublin was internalized more efficiently than Salmonella enterica serovar Enteritidis or Salmonella enterica serovar Typhimurium in all of the amoeba isolates tested. The bacteria appeared to be most efficiently internalized by Acanthamoeba rhysodes. Variations in bacterial growth conditions affected internalization efficiency, but this effect was not altered by inactivation of hilA, a key regulator in the expression of the invasion-associated Salmonella pathogenicity island 1. Microscopy of infected A. rhysodes revealed that S. enterica resided within vacuoles. Prolonged incubation resulted in a loss of intracellular bacteria associated with morphological changes and loss of amoebae. In part, these alterations were associated with hilA and the Salmonella virulence plasmid. The data show that Acanthamoeba spp. can differentiate between different serovars of salmonellae and that internalization is associated with cytotoxic effects mediated by defined Salmonella virulence loci.

Antigen selection based on expression levels during infection facilitates vaccine development for an intracellular pathogen

Vaccines effective against intracellular pathogens could save the lives of millions of people every year, but vaccine development has been hampered by the slow largely empirical search for protective antigens. In vivo highly expressed antigens might represent a small attractive antigen subset that could be rapidly evaluated, but experimental evidence supporting this rationale, as well as practical strategies for its application, is largely lacking because of technical difficulties. Here, we used Salmonella strains expressing differential amounts of a fluorescent model antigen during infection to show that, in a mouse typhoid fever model, CD4 T cells preferentially recognize abundant Salmonella antigens. To identify a large number of natural Salmonella antigens with high expression levels during infection, we used a quantitative in vivo screening strategy. Immunization studies with five particularly attractive candidates revealed two highly protective antigens that might permit the development of an improved typhoid fever vaccine. In conclusion, we have established a rationale and an experimental strategy that will substantially facilitate vaccine development for Salmonella and possibly other intracellular pathogens.

Single-copy green fluorescent protein gene fusions allow accurate measurement of Salmonella gene expression in vitro and during infection of mammalian cells

We developed a reliable and flexible green fluorescent protein (GFP)-based system for measuring gene expression in individual bacterial cells. Until now, most systems have relied upon plasmid-borne gfp gene fusions, risking problems associated with plasmid instability. We show that a recently developed GFP variant, GFP+, is suitable for assessing bacterial gene expression. Various gfp+ transcriptional fusions were constructed and integrated as single copies into the chromosome of Salmonella enterica serovar Typhimurium. A comparison of the expression levels of proU-lacZ and proU-gfp+ fusions showed that GFP+ reported proU activity in individual Salmonella cells as accurately as beta-galactosidase reported activity for entire populations. The single-copy gfp+ fusions were ideal for monitoring up- and downregulation of Salmonella virulence genes. We discovered that in vitro induction of the SPI1gene prgH occurs only in a portion of the population and that the proportion varies with the growth phase. We determined the level of expression of the SPI2 gene ssaG in bacteria released from murine macrophages. Our results demonstrate for the first time that single-copy GFP+ fusions reliably report gene expression in simple and complex environments. This approach promises to allow accurate measurement of gene expression in individual bacteria during animal infection.

Salmonella infections in the mouse model: host resistance factors and in vivo dynamics of bacterial spread and distribution in the tissues

The mouse model is widely used to study the mechanisms of the pathogenesis of, and immunity to, systemic salmonellosis. During infection, Salmonella grows in phagocytic cells that reside in well-defined pathological lesions, are activated by cytokines and control the growth of intracellular bacteria using oxygen and nitrogen derivatives. Salmonella growth in the tissues results in the spatial segregation of bacterial populations and in their continuous distribution to new phagocytes. High bacterial numbers within infected phagocytes are uncommon in vivo.