Coordinated regulation of two independent cell-cell signaling systems and swarmer differentiation in Salmonella enterica serovar Typhimurium

Drug repositioning: doxazosin attenuates the virulence factors and biofilm formation in Gram-negative bacteria

The resistance development is an increasing global health risk that needs innovative solutions. Repurposing drugs to serve as anti-virulence agents is suggested as an advantageous strategy to diminish bacterial resistance development. Bacterial virulence is controlled by quorum sensing (QS) system that orchestrates the expression of biofilm formation, motility, and virulence factors production as enzymes and virulent pigments. Interfering with QS could lead to bacterial virulence mitigation without affecting bacterial growth that does not result in bacterial resistance development. This study investigated the probable anti-virulence and anti-QS activities of α-adrenoreceptor blocker doxazosin against Proteus mirabilis and Pseudomonas aeruginosa. Besides in silico study, in vitro and in vivo investigations were conducted to assess the doxazosin anti-virulence actions. Doxazosin significantly diminished the biofilm formation and release of QS-controlled Chromobacterium violaceum pigment and virulence factors in P. aeruginosa and P. mirabilis, and downregulated the QS encoding genes in P. aeruginosa. Virtually, doxazosin interfered with QS proteins, and in vivo protected mice against P. mirabilis and P. aeruginosa. The role of the membranal sensors as QseC and PmrA was recognized in enhancing the Gram-negative virulence. Doxazosin downregulated the membranal sensors PmR and QseC encoding genes and could in silico interfere with them. In conclusion, this study preliminary documents the probable anti-QS and anti-virulence activities of doxazosin, which indicate its possible application as an alternative or in addition to antibiotics. However, extended toxicological and pharmacological investigations are essential to approve the feasible clinical application of doxazosin as novel efficient anti-virulence agent. KEY POINTS: • Anti-hypertensive doxazosin acquires anti-quorum sensing activities • Doxazosin diminishes the virulence of Proteus mirabilis and Pseudomonas aeruginosa • Doxazosin could dimmish the bacterial espionage.

Hiring of the Anti-Quorum Sensing Activities of Hypoglycemic Agent Linagliptin to Alleviate the Pseudomonas aeruginosa Pathogenesis

Bacteria communicate with each other using quorum sensing (QS) which works in an inducer/receptor manner. QS plays the main role in orchestrating diverse bacterial virulence factors. Pseudomonas aeruginosa is one of the most clinically important bacterial pathogens that can cause infection in almost all body tissues. Besides its efficient capability to develop resistance to different antibiotics, P. aeruginosa acquires a huge arsenal of virulence factors that are controlled mainly by QS. Challenging QS with FDA-approved drugs and natural products was proposed as a promising approach to mitigate bacterial virulence enabling the host immunity to complete the eradication of bacterial infection. The present study aims to evaluate the dipeptidase inhibitor-4 inhibitor hypoglycemic linagliptin anti-QS and anti-virulence activities against P. aeruginosa in vitro, in vivo, and in silico. The current results revealed the significant ability to diminish the production of protease and pyocyanin, motility, and biofilm formation in P. aeruginosa. Furthermore, the histopathological examination of liver and kidney tissues of mice injected with linagliptin-treated bacteria showed an obvious reduction of pathogenesis. Linagliptin downregulation to QS-encoding genes, besides the virtual ability to interact with QS receptors, indicates its anti-QS activities. In conclusion, linagliptin is a promising anti-virulence and anti-QS candidate that can be used solely or in combination with traditional antimicrobial agents in the treatment of P. aeruginosa aggressive infections.

Controlling of Bacterial Virulence: Evaluation of Anti-Virulence Activities of Prazosin against Salmonella enterica

Salmonella enterica is a Gram-negative orofecal transmitted pathogen that causes a wide diversity of local and systemic illnesses. Salmonella enterica utilizes several interplayed systems to regulate its invasion and pathogenesis: namely, quorum sensing (QS) and type three secretion system (T3SS). In addition, S. enterica could sense the adrenergic hormones in the surroundings that enhance its virulence. The current study aimed to evaluate the ability of α-adrenoreceptor antagonist prazosin to mitigate the virulence of S. enterica serovar Typhimurium. The prazosin effect on biofilm formation and the expression of sdiA, qseC, qseE, and T3SS-type II encoding genes was evaluated. Furthermore, the prazosin intracellular replication inside macrophage and anti-virulence activity was evaluated in vivo against S. typhimurium. The current finding showed a marked prazosin ability to compete on SdiA and QseC and downregulate their encoding genes. Prazosin significantly downregulated the virulence factors encoding genes and diminished the biofilm formation, intracellular replication inside macrophages, and in vivo protected mice. To sum up, prazosin showed significant inhibitory activities against QS, T3SS, and bacterial espionage, which documents its considered anti-virulence activities.

The Roles of Microbial Cell-Cell Chemical Communication Systems in the Modulation of Antimicrobial Resistance

Rapid emergence of antimicrobial resistance (AMR) has become a critical challenge worldwide. It is of great importance to understand how AMR is modulated genetically in order to explore new antimicrobial strategies. Recent studies have unveiled that microbial communication systems, which are known to play key roles in regulation of bacterial virulence, are also associated with the formation and regulation of AMR. These microbial cell-to-cell chemical communication systems, including quorum sensing (QS) and pathogen-host communication mechanisms, rely on detection and response of various chemical signal molecules, which are generated either by the microbe itself or host cells, to activate the expression of virulence and AMR genes. This article summarizes the generic signaling mechanisms of representative QS and pathogen-host communications systems, reviews the current knowledge regarding the roles of these chemical communication systems in regulation of AMR, and describes the strategies developed over the years for blocking bacterial chemical communication systems in disease control. The research progress in this field suggests that the bacterial cell-cell communication systems are a promising target not only for disease control but also for curbing the problem of microbial drug resistance.

Rck of Salmonella Typhimurium Delays the Host Cell Cycle to Facilitate Bacterial Invasion

Salmonella Typhimurium expresses on its outer membrane the protein Rck which interacts with the epidermal growth factor receptor (EGFR) of the plasma membrane of the targeted host cells. This interaction activates signaling pathways, leading to the internalization of Salmonella. Since EGFR plays a key role in cell proliferation, we sought to determine the influence of Rck mediated infection on the host cell cycle. By analyzing the DNA content of uninfected and infected cells using flow cytometry, we showed that the Rck-mediated infection induced a delay in the S-phase (DNA replication phase) of the host cell cycle, independently of bacterial internalization. We also established that this Rck-dependent delay in cell cycle progression was accompanied by an increased level of host DNA double strand breaks and activation of the DNA damage response. Finally, we demonstrated that the S-phase environment facilitated Rck-mediated bacterial internalization. Consequently, our results suggest that Rck can be considered as a cyclomodulin with a genotoxic activity.

Detection of bacterial contamination in food matrices by integration of quorum sensing in a paper-strip test

There are an estimated 48 million cases of foodborne illness in the United States every year. In general, these illnesses are the result of unintentional contamination and improper food handling. Because bacterial contamination plays a major role in food spoilage and, hence, in foodborne illnesses, it is important to design easy, portable methods to detect bacteria in food. Quorum sensing (QS) enables bacteria to communicate with one another and by doing so they can modulate their behavior in a cell-density dependent manner. In bacteria, quorum sensing molecules (QSMs) are known to control several factors such as virulence factor production, antibiotic production, biofilm formation, and gene regulation. Herein, we demonstrate the applicability of whole cell biosensing systems for the early identification of food contamination via detection of QSMs. Additionally, we have developed a portable system for detection of bacterial contamination using microdots of immobilized whole cell-based biosensors on paper that boast nanomolar level detection of QSMs in two different food matrices, namely beef and milk. Limits of detection ranged from 1 × 10-7 M to 1 × 10-9 M with relative standard deviations (RSDs) of 1-16%. This rapid, easy, and portable test could be a useful tool for use in the field and during all stages of food manipulation, i.e., from farms to distribution, storage, sales, and preparation prior to consumption, to ensure that food is free of bacterial contamination.

An Updated View on the Rck Invasin of Salmonella: Still Much to Discover

Salmonella is a facultative intracellular Gram-negative bacterium, responsible for a wide range of food- and water-borne diseases ranging from gastroenteritis to typhoid fever depending on hosts and serotypes. Salmonella thus represents a major threat to public health. A key step in Salmonella pathogenesis is the invasion of phagocytic and non-phagocytic host cells. To trigger its own internalization into non-phagocytic cells, Salmonella has developed different mechanisms, involving several invasion factors. For decades, it was accepted that Salmonella could only enter cells through a type three secretion system, called T3SS-1. Recent research has shown that this bacterium expresses outer membrane proteins, such as the Rck protein, which is able to induce Salmonella entry mechanism. Rck mimics natural host cell ligands and triggers engulfment of the bacterium by interacting with the epidermal growth factor receptor. Salmonella is thus able to use multiple entry pathways during the Salmonella infection process. However, it is unclear how and when Salmonella exploits the T3SS-1 and Rck entry mechanisms. As a series of reviews have focused on the T3SS-1, this review aims to describe the current knowledge and the limitations of our understanding of the Rck outer membrane protein. The regulatory cascade which controls Rck expression and the molecular mechanisms underlying Rck-mediated invasion into cells are summarized. The potential role of Rck-mediated invasion in Salmonella pathogenesis and the intracellular behavior of the bacteria following a Salmonella Rck-dependent entry are discussed.

A Modular, Tn7-Based System for Making Bioluminescent or Fluorescent Salmonella and Escherichia coli Strains

Our goal was to develop a robust tagging method that can be used to track bacterial strains in vivo To address this challenge, we adapted two existing systems: a modular plasmid-based reporter system (pCS26) that has been used for high-throughput gene expression studies in Salmonella and Escherichia coli and Tn7 transposition. We generated kanamycin- and chloramphenicol-resistant versions of pCS26 with bacterial luciferase, green fluorescent protein (GFP), and mCherry reporters under the control of σ(70)-dependent promoters to provide three different levels of constitutive expression. We improved upon the existing Tn7 system by modifying the delivery vector to accept pCS26 constructs and moving the transposase genes from a nonreplicating helper plasmid into a temperature-sensitive plasmid that can be conditionally maintained. This resulted in a 10- to 30-fold boost in transposase gene expression and transposition efficiencies of 10(-8) to 10(-10) in Salmonella enterica serovar Typhimurium and E. coli APEC O1, whereas the existing Tn7 system yielded no successful transposition events. The new reporter strains displayed reproducible signaling in microwell plate assays, confocal microscopy, and in vivo animal infections. We have combined two flexible and complementary tools that can be used for a multitude of molecular biology applications within the Enterobacteriaceae This system can accommodate new promoter-reporter combinations as they become available and can help to bridge the gap between modern, high-throughput technologies and classical molecular genetics.

IMPORTANCE

This article describes a flexible and efficient system for tagging bacterial strains. Using our modular plasmid system, a researcher can easily change the reporter type or the promoter driving expression and test the parameters of these new constructs in vitro Selected constructs can then be stably integrated into the chromosomes of desired strains in two simple steps. We demonstrate the use of this system in Salmonella and E. coli, and we predict that it will be widely applicable to other bacterial strains within the Enterobacteriaceae This technology will allow for improved in vivo analysis of bacterial pathogens.

A GFP promoter fusion library for the study of Salmonella biofilm formation and the mode of action of biofilm inhibitors

Salmonella, an important foodborne pathogen, forms biofilms in many different environments. The composition of these biofilms differs depending on the growth conditions, and their development is highly coordinated in time. To develop efficient treatments, it is therefore essential that biofilm formation and its inhibition be understood in different environments and in a time-dependent manner. Many currently used techniques, such as transcriptomics or proteomics, are still expensive and thus limited in their application. Therefore, a GFP-promoter fusion library with 79 important Salmonella biofilm genes was developed (covering among other things matrix production, fimbriae and flagella synthesis, and c-di-GMP regulation). This library is a fast, inexpensive, and easy-to-use tool, and can therefore be conducted in different experimental setups in a time-dependent manner. In this paper, four possible applications are highlighted to illustrate and validate the use of this reporter fusion library.

The influence of CsgD on the expression of genes of folate metabolism and hmp in Escherichia coli K-12

The csgD gene codes for the regulatory protein CsgD. CsgD upregulates the synthesis of the adhesive fimbriae, curli, that are important for biofilm formation and downregulates flagellar synthesis. We compared the expression of genes involved in folate metabolism and a gene (hmp) in strains with an intact csgD gene and with a deletion in csgD. The hmp gene codes a flavohemoglobin that inactivates nitric oxide. Expression was monitored by measuring light production from single copy lux operon fusions. At late times of growth, expression of genes responsible for methylene tetrahydrofolate synthesis (glyA and gcvTHP) and formyltetrahydrofolate recycling (purU) was higher in cells with CsgD than those without. In contrast, expression of hmp was lower in the presence of CsgD throughout the period monitored. We used a novel defined medium which should assist in defining nutritional factors that contribute to curli formation.

Transcriptional regulation of sdiA by cAMP-receptor protein, LeuO, and environmental signals in Salmonella enterica serovar Typhimurium

The sdiA gene encodes for a LuxR-type transcription factor, which is active when bound to N-acyl homoserine lactones (AHLs). Because Salmonella enterica serovar Typhimurium does not produce AHLs, SdiA senses signals produced by other organisms. SdiA is not expressed constitutively, and response is limited to conditions in which elevated expression occurs, but little is known about the regulation of sdiA expression. Here we map the sdiA promoter and define several regulators that directly or indirectly act on the promoter. The major activator of sdiA expression is cAMP-receptor protein (CRP), and we define the CRP operator in the sdiA promoter using promoter and crp mutants. LeuO activates sdiA expression to a lesser extent than does CRP. We demonstrate that LeuO directly binds the sdiA promoter and the Rcs phosphorelay represses sdiA expression. In this study, NhaR, IlvY, and Fur affected sdiA expression indirectly and weakly. Expression in late-stationary phase depended on RpoS. AHL-dependent expression of the SdiA-regulated gene rck correlated to the observed sdiA transcriptional changes in regulator mutants. The data demonstrate that regulation of sdiA involves integration of multiple environmental and metabolic signals.

Light producing reporter plasmids for Escherichia coli K-12 that can be integrated into the chromosome

Plasmid vectors using the Photorhabdus luminescenslux operon can be used for real time measurements of promoter activity. We have generated a series of lux vectors that have a conditional origin of replication, different selectable markers and the attP sequence from λ. Single copies of these plasmids can be integrated into the λ attachment site in the Escherichia coli chromosome. We constructed reporter derivatives and compared light production when the plasmids were present in multiple copies and in single copies. We also demonstrated that expression could be induced under the appropriate conditions.

Heterogeneity of type III secretion system (T3SS)-1-independent entry mechanisms used by Salmonella Enteritidis to invade different cell types

Salmonella causes a wide range of diseases from acute gastroenteritis to systemic typhoid fever, depending on the host. To invade non-phagocytic cells, Salmonella has developed different mechanisms. The main invasion system requires a type III secretion system (T3SS) known as T3SS-1, which promotes a Trigger entry mechanism. However, other invasion factors have recently been described in Salmonella, including Rck and PagN, which were not expressed under our bacterial culture conditions. Based on these observations, we used adhesion and invasion assays to analyse the respective roles of Salmonella Enteritidis T3SS-1-dependent and -independent invasion processes at different times of infection. Diverse cell lines and cell types were tested, including endothelial, epithelial and fibroblast cells. We demonstrated that cell susceptibility to the T3SS-1-independent entry differs by a factor of nine between the most and the least permissive cell lines tested. In addition, using scanning electron and confocal microscopy, we showed that T3SS-1-independent entry into cells was characterized by a Trigger-like alteration, as for the T3SS-1-dependent entry, and also by Zipper-like cellular alteration. Our results demonstrate for what is believed to be the first time that Salmonella can induce Trigger-like entry independently of T3SS-1 and can induce Zipper-like entry independently of Rck. Overall, these data open new avenues for discovering new invasion mechanisms in Salmonella.

Eavesdropping by bacteria: the role of SdiA in Escherichia coli and Salmonella enterica serovar Typhimurium quorum sensing

Many gram-negative bacteria utilize N-acyl-L-homoserine lactones (AHLs) to bind to transcriptional regulators leading to activation or repression of target genes. Escherichia coli and Salmonella enterica do not synthesize AHLs but do contain the AHL receptor, SdiA. Studies reveal that SdiA can bind AHLs produced by other bacterial species and thereby allow E. coli and S. enterica to regulate gene transcription. The Salmonella sdiA gene regulates the rck gene, which mediates Salmonella adhesion and invasion of epithelial cells and the resistance of the organism to complement. In E. coli, there is some evidence that SdiA may regulate genes associated with acid resistance, virulence, motility, biofilm formation, and autoinducer-2 transport and processing. However, there is a lack of information concerning the role of SdiA in regulating growth and survival of E. coli and Salmonella in food environments, and therefore studies in this area are needed.

Salmonella SdiA recognizes N-acyl homoserine lactone signals from Pectobacterium carotovorum in vitro, but not in a bacterial soft rot

Genomes of Salmonella enterica isolates, including those linked to outbreaks of produce-associated gastroenteritis, contain sdiA, which encodes a receptor of N-acyl homoserine lactones (AHL). AHL are the quorum-sensing signals used by bacteria to coordinately regulate gene expression within -their populations. Because S. enterica does not produce its own AHL, SdiA is hypothesized to function in the interspecies cross-talk with AHL-producing bacteria. Under laboratory conditions, S. enterica responded to AHL from phytobacteria by upregulating expression of srgE. AHL-dependent expression of srgE required a functional sdiA. Essentially, no sdiA-dependent resolution of the srgE recombinase-based (RIVET) reporter was observed inside a soft rot formed on a tomato by an AHL-producing strain of Pectobacterium carotovorum. The results of the control experiments suggest that sdiA is not expressed inside tomato, pepper, green onion, or carrot affected by the soft rot, and the lack of sdiA expression in planta prevents Salmonella spp. from responding to AHL. Despite its inability to detect and respond to AHL during colonization of soft rots, S. enterica reached higher final cell numbers inside a tomato soft rot compared with its growth in intact tomato fruit. The synergistic effect was the strongest under the conditions that are typical for the Florida fall/winter production season.

Cell density and mobility protect swarming bacteria against antibiotics

Swarming bacteria move in multicellular groups and exhibit adaptive resistance to multiple antibiotics. Analysis of this phenomenon has revealed the protective power of high cell densities to withstand exposure to otherwise lethal antibiotic concentrations. We find that high densities promote bacterial survival, even in a nonswarming state, but that the ability to move, as well as the speed of movement, confers an added advantage, making swarming an effective strategy for prevailing against antimicrobials. We find no evidence of induced resistance pathways or quorum-sensing mechanisms controlling this group resistance, which occurs at a cost to cells directly exposed to the antibiotic. This work has relevance to the adaptive antibiotic resistance of bacterial biofilms.

E. coli K-12 and EHEC genes regulated by SdiA

Background

Escherichia and Salmonella encode SdiA, a transcription factor of the LuxR family that regulates genes in response to N-acyl homoserine lactones (AHLs) produced by other species of bacteria. E. coli genes that change expression in the presence of plasmid-encoded sdiA have been identified by several labs. However, many of these genes were identified by overexpressing sdiA on a plasmid and have not been tested for a response to sdiA produced from its natural position in the chromosome or for a response to AHL.

Methodology/Principal Findings

We determined that two important loci reported to respond to plasmid-based sdiA, ftsQAZ and acrAB, do not respond to sdiA expressed from its natural position in the chromosome or to AHLs. To identify genes that are regulated by chromosomal sdiA and/or AHLs, we screened 10,000 random transposon-based luciferase fusions in E. coli K-12 and a further 10,000 in E. coli O157:H7 for a response to AHL and then tested these genes for sdiA-dependence. We found that genes encoding the glutamate-dependent acid resistance system are up-regulated, and fliE is down-regulated, by sdiA. Gene regulation by sdiA of E. coli is only partially dependent upon AHL.

Conclusions/Significance

The genes of E. coli that respond to plasmid-based expression of sdiA are largely different than those that respond to chromosomal sdiA and/or AHL. This has significant implications for determining the true function of AHL detection by E. coli.

Salmonella enterica serovar Typhimurium can detect acyl homoserine lactone production by Yersinia enterocolitica in mice

LuxR-type transcription factors detect acyl homoserine lactones (AHLs) and are typically used by bacteria to determine the population density of their own species. Escherichia coli and Salmonella enterica serovar Typhimurium cannot synthesize AHLs but can detect the AHLs produced by other bacterial species using the LuxR homolog, SdiA. Previously we determined that S. Typhimurium did not detect AHLs during transit through the gastrointestinal tract of a guinea pig, a rabbit, a cow, 5 mice, 6 pigs, or 12 chickens. However, SdiA was activated during transit through turtles colonized by Aeromonas hydrophila, leading to the hypothesis that SdiA is used for detecting the AHL production of other pathogens. In this report, we determined that SdiA is activated during the transit of S. Typhimurium through mice infected with the AHL-producing pathogen Yersinia enterocolitica. SdiA is not activated during transit through mice infected with a yenI mutant of Y. enterocolitica that cannot synthesize AHLs. However, activation of SdiA did not confer a fitness advantage in Yersinia-infected mice. We hypothesized that this is due to infrequent or short interactions between S. Typhimurium and Y. enterocolitica or that the SdiA regulon members do not function in mice. To test these hypotheses, we constructed an S. Typhimurium strain that synthesizes AHLs to mimic a constant interaction with Y. enterocolitica. In this background, sdiA(+) S. Typhimurium rapidly outcompetes the sdiA mutant in mice. All known members of the sdiA regulon are required for this phenotype. Thus, all members of the sdiA regulon are functional in mice.

Previously uncharacterized Salmonella enterica genes required for swarming play a role in seedling colonization

Incidences of bacterial foodborne illness caused by ingestion of fresh produce are rising. Instead of this being due to incidental contamination, the animal pathogen Salmonella enterica utilizes specific molecular mechanisms to attach to and colonize plants. This work characterizes two S. enterica genes of unknown function: a putative periplasmic protein, STM0278, and a putative protein with a hydrolase in the C-terminus, STM0650. STM0278 and STM0650 are important for seedling colonization but appear to have different roles during the process of colonization. Mutants of either STM0278 or STM0650 showed reduced colonization of alfalfa seedlings at 24 h, and the STM0278 mutant also showed reduced colonization at 48 h. Both genes were expressed in planta at 4 h following inoculation of 3-day-old seedlings and at 72 h after seed inoculation. This suggests that the role of STM0650 in seedling colonization is less important later in the process or is duplicated by other mechanisms. Mutants of STM0278 and STM0650 were defective in swarming. The STM0278 mutant failed to swarm in 24 h, while swarming of the STM0650 mutant was delayed. Addition of surfactant restored swarming of the STM0278 mutant, suggesting that STM0278 is involved in surfactant or osmotic agent production or deployment. Alfalfa seed exudates as the sole nutrient source were capable of perpetuating S. enterica swarming. Sequence analysis revealed sequences homologous to STM0278 and STM0650 in plant-associated bacteria, but none in Escherichia coli. Phylogenetic analysis of STM0650 showed similar sequences from diverse classes of plant-associated bacteria. Bacteria that preferentially colonize roots, including S. enterica, may use a similar hydrolase for swarming or biofilm production on plants. Multicellular behaviours by S. enterica appear central to plant colonization. S. enterica genes involved in plant colonization and survival outside of a host are most likely among the 'function unknown' genes of this bacterium.

A novel secretion pathway of Salmonella enterica acts as an antivirulence modulator during salmonellosis

Salmonella spp. are Gram-negative enteropathogenic bacteria that infect a variety of vertebrate hosts. Like any other living organism, protein secretion is a fundamental process essential for various aspects of Salmonella biology. Herein we report the identification and characterization of a horizontally acquired, autonomous and previously unreported secretion pathway. In Salmonella enterica serovar Typhimurium, this novel secretion pathway is encoded by STM1669 and STM1668, designated zirT and zirS, respectively. We show that ZirT is localized to the bacterial outer membrane, expected to adopt a compact β-barrel conformation, and functions as a translocator for ZirS. ZirS is an exoprotein, which is secreted into the extracellular environment in a ZirT-dependent manner. The ZirTS secretion pathway was found to share several important features with two-partner secretion (TPS) systems and members of the intimin/invasin family of adhesions. We show that zirTS expression is affected by zinc; and that in vivo, induction of zirT occurs distinctively in Salmonella colonizing the small intestine, but not in systemic sites. Additionally, strong expression of zirT takes place in Salmonella shed in fecal pellets during acute and persistent infections of mice. Inactivation of ZirTS results in a hypervirulence phenotype of Salmonella during oral infection of mice. Cumulatively, these results indicate that the ZirTS pathway plays a unique role as an antivirulence modulator during systemic disease and is involved in fine-tuning a host–pathogen balance during salmonellosis.

Bacteria of the Salmonella genus are important human pathogens and a leading cause of food-borne illness. Like for all other living organisms, protein secretion is a fundamental process, which is required for many different aspects of Salmonella biology including biogenesis of organelles, nutrient acquisition, and virulence. In this work we describe a new secretion pathway in Salmonella termed ZirTS. This pathway consists of an exported protein (ZirS) and a designated membrane translocator (ZirT), which mediates the secretion of ZirS to the extracellular milieu. Using a mouse model of Salmonella infection, we found that the ZirTS system is induced in Salmonella colonizing the small intestine and in Salmonella shed in fecal pellets during acute and persistent infections. Interestingly, inactivation of ZirTS results in a hypervirulence phenotype of Salmonella during oral infection of mice. These observations indicate that the ZirTS pathway plays a unique role as an antivirulence modulator and is involved in fine-tuning host–pathogen interactions during disease. Our study elucidates an emerging theme in pathogenesis emphasizing the importance of pathogens to limit their effects upon the cells they infect in order to achieve a balance with their host.

Aggregation via the red, dry, and rough morphotype is not a virulence adaptation in Salmonella enterica serovar Typhimurium

The Salmonella rdar (red, dry, and rough) morphotype is an aggregative and resistant physiology that has been linked to survival in nutrient-limited environments. Growth of Salmonella enterica serovar Typhimurium was analyzed in a variety of nutrient-limiting conditions to determine whether aggregation would occur at low cell densities and whether the rdar morphotype was involved in this process. The resulting cultures consisted of two populations of cells, aggregated and nonaggregated, with the aggregated cells preferentially displaying rdar morphotype gene expression. The two groups of cells could be separated based on the principle that aggregated cells were producing greater amounts of thin aggregative fimbriae (Tafi or curli). In addition, the aggregated cells retained some physiological characteristics of the rdar morphotype, such as increased resistance to sodium hypochlorite. Competitive infection experiments in mice showed that nonaggregative DeltaagfA cells outcompeted rdar-positive wild-type cells in all tissues analyzed, indicating that aggregation via the rdar morphotype was not a virulence adaptation in Salmonella enterica serovar Typhimurium. Furthermore, in vivo imaging experiments showed that Tafi genes were not expressed during infection but were expressed once Salmonella was passed out of the mice into the feces. We hypothesize that the primary role of the rdar morphotype is to enhance Salmonella survival outside the host, thereby aiding in transmission.