A diffusible signal factor of the intestine dictates Salmonella invasion through its direct control of the virulence activator HilD.

Successful intestinal infection by Salmonella requires optimized invasion of the gut epithelium, a function that is energetically costly. Salmonella have therefore evolved to intricately regulate the expression of their virulence determinants by utilizing specific environmental cues. Here we show that a powerful repressor of Salmonella invasion, a cis-2 unsaturated long chain fatty acid, is present in the murine large intestine. Originally identified in Xylella fastidiosa as a diffusible signal factor for quorum sensing, this fatty acid directly interacts with HilD, the master transcriptional regulator of Salmonella, and prevents hilA activation, thus inhibiting Salmonella invasion. We further identify the fatty acid binding region of HilD and show it to be selective and biased in favour of signal factors with a cis-2 unsaturation over other intestinal fatty acids. Single mutation of specific HilD amino acids to alanine prevented fatty acid binding, thereby alleviating their repressive effect on invasion. Together, these results highlight an exceedingly sensitive mechanism used by Salmonella to colonize its host by detecting and exploiting specific molecules present within the complex intestinal environment.

AraC-type regulators HilC and RtsA are directly controlled by an intestinal fatty acid to regulate Salmonella invasion.

Invasion of the intestinal epithelium is an essential but energetically expensive survival strategy and is, therefore, tightly regulated by using specific cues from the environment. The enteric pathogen Salmonella controls its invasion machinery through the elegant coordination of three AraC-type transcription activators, HilD, HilC, and RtsA. Most environmental signals target HilD to control invasion, whereas HilC and RtsA are known only to augment these effects on HilD. Here we show that a fatty acid found in the murine colon, cis-2-hexadecenoic acid (c2-HDA), represses Salmonella invasion by directly targeting HilC and RtsA, in addition to HilD. c2-HDA directly binds each of these regulators and inhibits their attachment to DNA targets, repressing invasion even in the absence of HilD. Fatty acid binding, however, does not affect HilC and RtsA protein stability, unlike HilD. Importantly, we show that HilC and RtsA are highly effective in restoring HilD production and invasion gene expression after elimination of the repressive fatty acid c2-HDA. Together, these results illuminate a precise mechanism by which HilC and RtsA may modulate invasion as Salmonella navigates through different regions of the intestine, contributing to our understanding of how this enteric pathogen senses and adapts to a diverse intestinal environment while maintaining its virulence.

Salmonella invasion is controlled through the secondary structure of the hilD transcript.

Virulence functions of bacterial pathogens are often energetically costly and thus are subjected to intricate regulatory mechanisms. In Salmonella, invasion of the intestinal epithelium, an essential early step in virulence, requires the production of a multi-protein type III secretion apparatus. The pathogen mitigates the overall cost of invasion by inducing it in only a fraction of its population. This constitutes a successful virulence strategy as invasion by a small number is sufficient to promote the proliferation of the non-invading majority. Such a system suggests the existence of a sensitive triggering mechanism that permits only a minority of Salmonella to reach a threshold of invasion-gene induction. We show here that the secondary structure of the invasion regulator hilD message provides such a trigger. The 5' end of the hilD mRNA is predicted to contain two mutually exclusive stem-loop structures, the first of which (SL1) overlaps the ribosome-binding site and the ORF start codon. Changes that reduce its stability enhance invasion gene expression, while those that increase stability reduce invasion. Conversely, disrupting the second stem-loop (SL2) represses invasion genes. Although SL2 is the energetically more favorable, repression through SL1 is enhanced by binding of the global regulator CsrA. This system thus alters the levels of hilD mRNA and is so sensitive that changing a single base pair within SL1, predicted to augment its stability, eliminates expression of invasion genes and significantly reduces Salmonella virulence in mice. This system thus provides a possible means to rapidly and finely tune an essential virulence function.

Diffusible Signal Factors Act through AraC-Type Transcriptional Regulators as Chemical Cues To Repress Virulence of Enteric Pathogens.

Successful colonization by enteric pathogens is contingent upon effective interactions with the host and the resident microbiota. These pathogens thus respond to and integrate myriad signals to control virulence. Long-chain fatty acids repress the virulence of the important enteric pathogens Salmonella enterica and Vibrio cholerae by repressing AraC-type transcriptional regulators in pathogenicity islands. While several fatty acids are known to be repressive, we show here that cis-2-unsaturated fatty acids, a rare chemical class used as diffusible signal factors (DSFs), are highly potent inhibitors of virulence functions. We found that DSFs repressed virulence gene expression of enteric pathogens by interacting with transcriptional regulators of the AraC family. In Salmonella enterica serovar Typhimurium, DSFs repress the activity of HilD, an AraC-type activator essential to the induction of epithelial cell invasion, by both preventing its interaction with target DNA and inducing its rapid degradation by Lon protease. cis-2-Hexadecenoic acid (c2-HDA), a DSF produced by Xylella fastidiosa, was the most potent among those tested, repressing the HilD-dependent transcriptional regulator hilA and the type III secretion effector sopB >200- and 68-fold, respectively. Further, c2-HDA attenuated the transcription of the ToxT-dependent cholera toxin synthesis genes of V. cholerae c2-HDA significantly repressed invasion gene expression by Salmonella in the murine colitis model, indicating that the HilD-dependent signaling pathway functions within the complex milieu of the animal intestine. These data argue that enteric pathogens respond to DSFs as interspecies signals to identify appropriate niches in the gut for virulence activation, which could be exploited to control the virulence of enteric pathogens.

Salmonella Pathogenicity Island 1 Is Expressed in the Chicken Intestine and Promotes Bacterial Proliferation.

Salmonella enterica serovar Enteritidis is a common cause of foodborne illness in the United States. The bacterium can be transmitted to humans via contaminated chicken meat and eggs, and virulence in humans requires type III secretion system 1 (TTSS-1), encoded on Salmonella pathogenicity island 1 (SPI-1). Chickens often carry S Enteritidis subclinically, obscuring the role of SPI-1 in facilitating bacterial colonization. To evaluate the role of SPI-1 in the infection of chicks by Salmonella, we created and utilized strains harboring a stable fluorescent reporter fusion designed to quantify SPI-1 expression within the intestinal tracts of animals. Using mutants unable to express TTSS-1, we demonstrated the important role of the secretion system in facilitating bacterial colonization. We further showed that coinoculation of an SPI-1 mutant with the wild-type strain increased the number of mutant organisms in intestinal tissue and contents, suggesting that the wild type rescues the mutant. Our results support the hypothesis that SPI-1 facilitates S Enteritidis colonization of the chicken and make SPI-1 an attractive target in preventing Salmonella carriage and colonization in chickens to reduce contamination of poultry meat and eggs by this foodborne pathogen.

The protein acyltransferase Pat post-transcriptionally controls HilD to repress Salmonella invasion.

N-Lysine acylation is a post-translational modification important for both prokaryotic and eukaryotic cells to control a wide array of cellular functions. Here we demonstrate that the protein acyltransferase Pat regulates genes on Salmonella Pathogenicity Island 1 (SPI1) that are required for the invasion of the intestinal epithelium. Mutation of pat slightly increased spleen colonization by Salmonella in streptomycin-treated mice, with more of the pat mutant reaching the spleen than the wild type strain. Growth of Salmonella under specific conditions selectively induced expression of Pat, and deletion of pat increased SPI1 gene expression under the same growth conditions. In addition, over-expression of Pat repressed SPI1 expression and bacterial entry into epithelial cells. These results demonstrate that Salmonella invasion is negatively controlled by Pat. Regulation of the SPI1 central regulator HilD was essential for Pat to exert its effects. The control of HilD by Pat was through post-transcriptional mechanisms, moderately repressing hilD translation while significantly reducing HilD stability. Additionally, growth of Salmonella in the presence of histone deacetylases inhibitors reduced expression of SPI1 by affecting HilD stability, supporting the concept that altering the stability of this regulator is required for Pat to control Salmonella invasion.

Bile Acids Function Synergistically To Repress Invasion Gene Expression in Salmonella by Destabilizing the Invasion Regulator HilD.

Salmonella spp. are carried by and can acutely infect agricultural animals and humans. After ingestion, salmonellae traverse the upper digestive tract and initiate tissue invasion of the distal ileum, a virulence process carried out by the type III secretion system encoded within Salmonella pathogenicity island 1 (SPI-1). Salmonellae coordinate SPI-1 expression with anatomical location via environmental cues, one of which is bile, a complex digestive fluid that causes potent repression of SPI-1 genes. The individual components of bile responsible for SPI-1 repression have not been previously characterized, nor have the bacterial signaling processes that modulate their effects been determined. Here, we characterize the mechanism by which bile represses SPI-1 expression. Individual bile acids exhibit repressive activity on SPI-1-regulated genes that requires neither passive diffusion nor OmpF-mediated entry. By using genetic methods, the effects of bile and bile acids were shown to require the invasion gene transcriptional activator hilD and to function independently of known upstream signaling pathways. Protein analysis techniques showed that SPI-1 repression by bile acids is mediated by posttranslational destabilization of HilD. Finally, we found that bile acids function synergistically to achieve the overall repressive activity of bile. These studies demonstrate a common mechanism by which diverse environmental cues (e.g., certain short-chain fatty acids and bile acids) inhibit SPI-1 expression. These data provide information relevant to Salmonella pathogenesis during acute infection in the intestine and during chronic infection of the gallbladder and inform the basis for development of therapeutics to inhibit invasion as a means of repressing Salmonella pathogenicity.

Acquisition of Iron Is Required for Growth of Salmonella spp. in Tomato Fruit.

Salmonella remains a leading cause of bacterial food-borne disease, sickening millions each year. Although outbreaks of salmonellosis have traditionally been associated with contaminated meat products, recent years have seen numerous disease cases caused by the consumption of produce. Tomatoes have been specifically implicated, due to the ability of Salmonella spp. to enter the tomato fruit and proliferate within, making the decontamination of the raw product impossible. To investigate the genetic means by which Salmonella is able to survive and proliferate within tomatoes, we conducted a screen for bacterial genes of Salmonella enterica serovar Montevideo specifically induced after inoculation into ripe tomato fruit. Among these genes, we found 17 members of the previously described anaerobic Fur (ferric uptake regulator) regulon. Fur is a transcriptional and posttranscriptional regulator known to sense iron, suggesting the importance of this mineral to Salmonella within tomatoes. To test whether iron acquisition is essential for Salmonella growth in tomatoes, we tested a ΔfepDGC mutant, which lacks the ability to import iron-associated siderophores. This mutant grew significantly more poorly within tomatoes than did the wild type, but the growth defect of the mutant was fully reversed by the addition of exogenous iron, demonstrating the need for bacterial iron scavenging. Further, dependence upon iron was not apparent for Salmonella growing in filtered tomato juice, implicating the cellular fraction of the fruit as an important mediator of iron acquisition by the bacteria.

Antimicrobial resistance trends among canine Escherichia coli isolates obtained from clinical samples in the northeastern USA, 2004-2011.

Our objectives were to describe the antimicrobial susceptibility of Escherichia coli isolates from dogs in the northeastern USA and to identify temporal trends in resistance to selected antimicrobial agents. Data were collected retrospectively for all canine E. coli isolates from clinical samples submitted to Cornell University's Animal Health Diagnostic Center between January 1, 2004 and December 31, 2011. Antimicrobial susceptibility testing was performed on 3519 canine E. coli isolates; frequency of resistance to each agent ranged from 0.4% (amikacin) to 34.3% (ampicillin). No trends were evident among urinary isolates, but cephalosporin resistance remained consistently high. Among non-urinary isolates, there was evidence of a significantly increasing trend in prevalence of resistance to several agents, including cephalosporins, enrofloxacin, and tetracycline. These data suggest that some of the most commonly used antimicrobial agents in companion animal practice are becoming less effective against canine E. coli infections outside the urinary tract.

Tendances de la résistance antimicrobienne parmi les isolats canins d'Escherichia coliobtenus dans des échantillons cliniques dans le nord-est des États-Unis de 2004 à 2011. Nos objectifs consistaient à décrire la susceptibilité des isolats d'Escherichia coli chez des chiens dans le nord-est des États-Unis et à identifier les tendances de résistance temporelles aux agents antimicrobiens sélectionnés. Des données ont été recueillies rétrospectivement pour tous les isolats canins d'E. coli provenant d'échantillons cliniques soumis à l'Animal Health Diagnostic Center de l'Université Cornell entre le 1er janvier 2004 et le 31 décembre 2011. Des épreuves de sensibilité antimicrobienne ont été réalisées sur 3519 isolats canins E. coli; la fréquence de résistance à chaque agent allait de 0,4 % (amikacine) à 34,3 % (ampicilline). Aucunes tendances n'étaient évidentes parmi les isolats urinaires, mais la résistance à la céphalosporine demeurait constamment élevée. Parmi les isolats non urinaires, il y avait des preuves d'une tendance significative à la hausse de la prévalence de la résistance à plusieurs agents, y compris les céphalosporines, l'enrofloxacine et la tétracycline. Ces données suggèrent que certains des agents antimicrobiens les plus communément utilisés en pratique des animaux de compagnie deviennent de moins en moins efficaces contre les infections canines par E. coli à l'extérieur des voies urinaires.(Traduit par Isabelle Vallières).

The intestinal fatty acid propionate inhibits Salmonella invasion through the post-translational control of HilD.

To cause disease, Salmonella must invade the intestinal epithelium employing genes encoded within Salmonella Pathogenicity Island 1 (SPI1). We show here that propionate, a fatty acid abundant in the intestine of animals, repressed SPI1 at physiologically relevant concentration and pH, reducing expression of SPI1 transcriptional regulators and consequently decreasing expression and secretion of effector proteins, leading to reduced bacterial penetration of cultured epithelial cells. Essential to repression was hilD, which occupies the apex of the regulatory cascade within SPI1, as loss of only this gene among those of the regulon prevented repression of SPI1 transcription by propionate. Regulation through hilD, however, was achieved through the control of neither transcription nor translation. Instead, growth of Salmonella in propionate significantly reduced the stability of HilD. Extending protein half-life using a Lon protease mutant demonstrated that protein stability itself did not dictate the effects of propionate and suggested modification of HilD with subsequent degradation as the means of action. Furthermore, repression was significantly lessened in a mutant unable to produce propionyl-CoA, while further metabolism of propionyl-CoA appeared not to be required. These results suggest a mechanism of control of Salmonella virulence in which HilD is post-translationally modified using the high-energy intermediate propionyl-CoA.

Antimicrobial resistance trends among Escherichia coli isolates obtained from dairy cattle in the northeastern United States, 2004-2011.

Monitoring antimicrobial resistance trends among bacteria isolated from food animals and people is necessary to inform risk analyses and guide public policy regarding antimicrobial use. Our objectives were to describe the antimicrobial resistance status of Escherichia coli isolates from dairy cattle in the northeastern United States and to identify trends in resistance to selected antimicrobial agents over time. We collected data retrospectively for all bovine E. coli isolates that were obtained from samples submitted to Cornell University's Animal Health Diagnostic Center between January 1, 2004 and December 31, 2011. We investigated temporal trends in the prevalence of resistant E. coli for each antimicrobial agent using the Cochran-Armitage trend test. Antimicrobial susceptibility testing was performed on 3373 bovine E. coli isolates from clinical samples submitted during the study period. Overall resistance to each antimicrobial agent ranged from 2.7% (enrofloxacin) to 91.3% (oxytetracycline). There was evidence of a significantly decreasing trend in prevalence of resistance to several agents: chlortetracycline, florfenicol, neomycin, oxytetracycline, spectinomycin, and trimethoprim/sulfamethoxazole. However, a significantly increasing trend in prevalence of resistance to enrofloxacin was also evident. These results do not support the idea that current antimicrobial use practices on dairy operations are driving a general increase in the emergence and dissemination of drug-resistant E. coli in the region served by the laboratory. However, resistance to some drugs remained consistently high during the study period, and increasing resistance to enrofloxacin is a key area of concern.

Functionalization of cotton nonwoven with cyclodextrin/lawsone inclusion complex nanofibrous coating for antibacterial wound dressing.

Functionalizing cotton to induce biological activity is a viable approach for developing wound dressing. This study explores the development of cotton-based wound dressing through coating with biologically active nanofibers. Bioactive compounds like lawsone offer dual benefits of wound healing and infection prevention, however, their limited solubility and viability hinder their applications. To address this, Hydroxypropyl-beta-cyclodextrin (HP-ß-CD) and Hydroxypropyl-gamma-cyclodextrin (HP-γ-CD) were employed. Inclusion complexations of CD/lawsone were achieved at 2:1 and 4:1 M ratios, followed by the fabrication of CD/lawsone nanofibrous systems via electrospinning. Phase solubility studies indicated a twofold increase in lawsone water-solubility with HP-ß-CD. Electrospinning yielded smooth and uniform nanofibers with an average diameter of ∼300-700 nm. The results showed that while specific crystalline peaks of lawsone are apparent in the samples with a 2:1 M ratio, they disappeared in 4:1, indicating complete complexation. The nanofibers exhibited ∼100 % loading efficiency of lawsone and its rapid release upon dissolution. Notably, antibacterial assays demonstrated the complete elimination of Escherichia coli and Staphylococcus aureus colonies. The CD/lawsone nanofibers also showed suitable antioxidant activity ranging from 50 % to 70 %. This integrated approach effectively enhances lawsone's solubility through CD complexation and offers promise for bilayer cotton-based wound dressings.

Serotype and anti-microbial resistance trends among bovine Salmonella isolates from samples submitted to a veterinary diagnostic laboratory in central New York, 2007-2021.

AIMS: Salmonella enterica is a leading cause of acute enteritis in people, and dairy cattle are an important reservoir of this pathogen. The objective of this study was to analyse serotype and anti-microbial resistance trends of Salmonella isolated from dairy cattle in the United States between 2007 and 2021. METHODS AND RESULTS: We collected data for bovine Salmonella isolates obtained from samples submitted to Cornell University's Animal Health Diagnostic Center (AHDC). We analysed 5114 isolates for serotype trends, and a subset of 2521 isolates tested for anti-microbial susceptibility were analysed for resistance trends. The most frequently identified serotypes were Salmonella Cerro, Dublin, Typhimurium, Montevideo, 4,[5],12:i:-, and Newport. Among these serotypes, a Cochran-Armitage trend test determined there was a significant increase in the proportion of isolates serotyped as Salmonella Dublin (p < 0.0001) and Montevideo (p < 0.0001) over time. There was a significant decrease in the proportion of isolates serotyped as Salmonella Cerro (p < 0.0001), Typhimurium (p < 0.0001), and Newport (p < 0.0001). For the anti-microbial resistance (AMR) analysis, we found an overall increase in the proportion of multi-drug-resistant isolates over time (p = 0.009). There was a significant increase in the proportion of isolates resistant to ampicillin (p = 0.007), florfenicol (p = 0.0002), and ceftiofur (p < 0.0001) and a marginal increase in resistance to enrofloxacin (p = 0.05). There was a significant decrease in the proportion of isolates resistant to spectinomycin (p = 0.0002), trimethoprim/sulphamethoxazole (p = 0.01), sulphadimethoxine (p = 0.003), neomycin (p < 0.0001), and gentamicin (p = 0.0002). CONCLUSIONS: Our results provide evidence of an increase in resistance to key anti-microbial agents, although the observed trends were driven by the sharp increase in the proportion of Salmonella Dublin isolates over time.

Antibacterial, Anti-Inflammatory, and Antioxidant Cotton-Based Wound Dressing Coated with Chitosan/Cyclodextrin-Quercetin Inclusion Complex Nanofibers.

Quercetin, recognized for its antioxidant, anti-inflammatory, and antibacterial properties, faces limited biomedical application due to its low solubility. Cotton, a preferred wound dressing material over synthetic ones, lacks inherent antibacterial and wound-healing attributes and can benefit from quercetin features. This study explores the potential of overcoming these challenges through the inclusion complexation of quercetin with cyclodextrins (CDs) and the development of a nanofibrous coating on a cotton nonwoven textile. Hydroxypropyl-beta-cyclodextrin (HP-ß-CD) and hydroxypropyl-gamma-cyclodextrin (HP-γ-CD) formed inclusion complexes of quercetin, with chitosan added to enhance antibacterial properties. Phase solubility results showed that inclusion complexation can enhance quercetin solubility up to 20 times, with HP-γ-CD forming a more stable inclusion complexation compared with HP-ß-CD. Electrospinning of the nanofibers from HP-ß-CD/Quercetin and HP-γ-CD/Quercetin aqueous solutions without the use of a polymeric matrix yielded a uniform, smooth fiber morphology. The structural and thermal analyses of the HP-ß-CD/Quercetin and HP-γ-CD/Quercetin nanofibers confirmed the presence of inclusion complexes between quercetin and each of the CDs (HP-ß-CD and HP-γ-CD). Moreover, HP-ß-CD/Quercetin and HP-γ-CD/Quercetin nanofibers showed a near-complete loading efficiency of quercetin and followed a fast-releasing profile of quercetin. Both HP-ß-CD/Quercetin and HP-γ-CD/Quercetin nanofibers showed significantly higher antioxidant activity compared to pristine quercetin. The HP-ß-CD/Quercetin and HP-γ-CD/Quercetin nanofibers also showed antibacterial activity, and with the addition of chitosan in the HP-γ-CD/Quercetin system, the Chitosan/HP-γ-CD/Quercetin nanofibers completely eliminated the investigated bacteria species. The nanofibers were nontoxic and well-tolerated by cells, and exploiting the quercetin and chitosan anti-inflammatory activities resulted in the downregulation of IL-6 and NO secretion in both immune as well as regenerative cells. Overall, CD inclusion complexation markedly enhances quercetin solubility, resulting in a biofunctional antioxidant, antibacterial, and anti-inflammatory wound dressing through a nanofibrous coating on cotton textiles.

Genome-wide screen of Mycobacterium tuberculosis- infected macrophages identified the GID/CTLH complex as a determinant of intracellular bacterial growth.

The eukaryotic GID/CTLH complex is a highly conserved E3 ubiquitin ligase involved in a broad range of biological processes. However, a role of this complex in host antimicrobial defenses has not been described. We exploited Mycobacterium tuberculosis ( Mtb ) induced cytotoxicity in macrophages in a FACS based CRISPR genetic screen to identify host determinants of intracellular Mtb growth restriction. Our screen identified 5 ( GID8 , YPEL5 , WDR26 , UBE2H , MAEA ) of the 10 predicted members of the GID/CTLH complex as determinants of intracellular growth of both Mtb and Salmonella serovar Typhimurium. We show that the antimicrobial properties of the GID/CTLH complex knockdown macrophages are mediated by enhanced GABAergic signaling, activated AMPK, increased autophagic flux and resistance to cell death. Meanwhile, Mtb isolated from GID/CTLH knockdown macrophages are nutritionally starved and oxidatively stressed. Our study identifies the GID/CTLH complex activity as broadly suppressive of host antimicrobial responses against intracellular bacterial infections.

Genomic characterization provides new insight into Salmonella phage diversity.

BACKGROUND: Salmonella is a widely distributed foodborne pathogen that causes tens of millions of salmonellosis cases globally every year. While the genomic diversity of Salmonella is increasingly well studied, our knowledge of Salmonella phage genomic diversity is still rather limited, despite the contributions of both lysogenic and lytic phages to Salmonella virulence, diversity and ecology (e.g., through horizontal gene transfer and Salmonella lysis). To gain a better understanding of phage diversity in a specific ecological niche, we sequenced 22 Salmonella phages isolated from a number of dairy farms from New York State (United States) and analyzed them using a comparative genomics approach. RESULTS: Classification of the 22 phages according to the presence/absence of orthologous genes allowed for classification into 8 well supported clusters. In addition to two phage clusters that represent novel virulent Salmonella phages, we also identified four phage clusters that each contained previously characterized phages from multiple continents. Our analyses also identified two clusters of phages that carry putative virulence (e.g., adhesins) and antimicrobial resistance (tellurite and bicyclomycin) genes as well as virulent and temperate transducing phages. Insights into phage evolution from our analyses include (i) identification of DNA metabolism genes that may facilitate nucleotide synthesis in phages with a G+C % distinct from Salmonella, and (ii) evidence of Salmonella phage tailspike and fiber diversity due to both single nucleotide polymorphisms and major re-arrangements, which may affect the host specificity of Salmonella phages. CONCLUSIONS: Genomics-based characterization of 22 Salmonella phages isolated from dairy farms allowed for identification of a number of novel Salmonella phages. While the comparative genomics analyses of these phages provide a number of new insights in the evolution and diversity of Salmonella phages, they only represent a first glimpse into the diversity of Salmonella phages that is likely to be discovered when phages from different environments are characterized.

Salmonella colonization activates the plant immune system and benefits from association with plant pathogenic bacteria.

Despite increasing incidences of human salmonellosis caused by consumption of contaminated vegetables, relatively little is known about how the plant immune system responds to and may inhibit Salmonella colonization. Here we show that Salmonella Typhimurium activates the plant immune system primarily due to its recognition of the flg22 region in Salmonella flagellin. Several previously identified plant genes that play a role in immunity were found to affect the host response to Salmonella. The Salmonella flg22 (Seflg22) peptide induced the immune response in leaves which effectively restricted the growth of Salmonella as well as the plant pathogenic bacterium, Pseudomonas syringae pv. tomato. Induction of immune responses by Seflg22 was dependent on the plant FLS2 receptor. Salmonella multiplied poorly on plant tissues similar to other bacteria which are non-pathogenic to plants. However, Salmonella populations increased significantly when co-inoculated with P. syringae pv. tomato but not when co-inoculated with a type III secretion system mutant of this pathogen. Our results suggest that Salmonella benefits from the immune-suppressing effects of plant pathogenic bacteria, and this growth enhancement may increase the risk of salmonellosis.

Antimicrobial resistance trends among Salmonella isolates obtained from dairy cattle in the northeastern United States, 2004-2011.

Monitoring antimicrobial resistance trends among bacteria isolated from food animals and people is necessary to inform public policy regarding appropriate antimicrobial use. Our objectives were to describe the antimicrobial resistance status of Salmonella isolates from dairy cattle in the northeastern United States and to identify trends in resistance to various antimicrobial agents over time. Data were collected retrospectively for all bovine Salmonella isolates that were obtained from samples submitted to Cornell University's Animal Health Diagnostic Center between January 1, 2004 and December 31, 2011. Temporal trends in the prevalence of resistant Salmonella were investigated for each antimicrobial agent using the Cochran-Armitage trend test. Antimicrobial susceptibility testing was performed on 2745 bovine Salmonella isolates from clinical samples submitted during the study period. Overall resistance to each antimicrobial agent ranged from 0% (amikacin, ciprofloxacin, and nalidixic acid) to 72.0% (sulfadimethoxine). There was evidence of a significantly decreasing trend in prevalence of resistance to most agents: amoxicillin/clavulanic acid (AUG), ampicillin (AMP), cefoxitin (FOX), ceftiofur (TIO), ceftriaxone (AXO), chloramphenicol (CHL), chlortetracycline (CTET), florfenicol (FFN), kanamycin (KAN), neomycin (NEO), oxytetracycline (OXY), spectinomycin (SPE), streptomycin (STR), sulfadimethoxine (SDM), sulfisoxazole (FIS), and tetracycline (TET). Among the 265 isolates that were tested using the National Antimicrobial Resistance Monitoring System (NARMS) panel, the most common resistance patterns were pansusceptible (54.0%), AUG-AMP-FOX-TIO-AXO-CHL-KAN-STR-FIS-TET (18.1%), and AUG-AMP-FOX-TIO-AXO-CHL-STR-FIS-TET (12.1%). Increasing prevalence of S. enterica serovar Cerro over the course of the study period presumably had an impact on the observed resistance trends. Nevertheless, these results do not support the notion that the current level of antimicrobial use in dairy cattle is driving an increase in the emergence and dissemination of drug-resistant Salmonella in the region served by the laboratory.

Shigella flexneri utilizes intestinal signals to control its virulence.

The enteric pathogens have evolved to utilize elements from their surroundings to optimize their infection strategies. A common mechanism to achieve this is to employ intestinal compounds as signals to control the activity of a master regulator of virulence. Shigella flexneri (S. flexneri) is a highly infectious entero-invasive pathogen which requires very few organisms to cause invasion of the colonic mucosa. The invasion program is controlled by the virulence master regulator VirF. Here, we show that the fatty acids commonly found in the colon can be exploited by S. flexneri to repress its virulence, allowing it to energetically finance its proliferation, thus increasing its pathogenicity. Colonic fatty acids such as oleic, palmitoleic and cis-2-hexadecenoic acid were shown to directly bind to VirF and mediate its prompt degradation. These fatty acids also disrupted the ability of VirF to bind to its target DNA, suppressing the transcription of the downstream virulence genes and significantly reducing the invasion of S. flexneri to colonic epithelial cells. Treatment with colonic fatty acids significantly increased the growth rate of the pathogen only under invasion-inducing conditions, showing that the reduction in the burden of virulence promotes a growth advantage. These results demonstrate the process by which S. flexneri can employ intestinal compounds as signals to increase its numbers at its preferred site of invasion, highlighting the mechanism by which the full spectrum of shigellosis is achieved despite a miniscule infectious dose. This highlights an elegant model of environmental adaption by S. flexneri to maximize the pathogenic benefit.

Salmonella Invasion Is Controlled by Competition among Intestinal Chemical Signals.

The intestine is a complex, ever-changing environment replete with an array of signaling molecules. To colonize such a complex organ, pathogens have adapted to utilize specific cues from the local environment to intricately regulate the expression of their virulence determinants. Salmonella preferentially colonizes the distal ileum, a niche enriched in the metabolite formic acid. Here, we show that the relatively higher concentration of this metabolite in the distal ileum prevents other signals from repressing Salmonella invasion in that region. We show that imported and unmetabolized formic acid functions as a cytoplasmic signal that competitively binds to HilD, the master transcriptional regulator of Salmonella invasion, thus preventing repressive fatty acids from binding to the protein. This results in an increased lifetime of HilD and subsequent derepression of invasion genes. This study demonstrates an important mechanism by which Salmonella utilizes competition among signals in the gut to its advantage as a pathogen. IMPORTANCE Enteric pathogens acutely sense their environment for signals to regulate their virulence functions. We demonstrate here that the enteric pathogen Salmonella utilizes the competition among certain regional intestinal constituents to modulate its virulence determinants in that region. We show that the high concentration of formic acid in the ileum outcompetes other signals and triggers the activation of virulence genes in the ileum. This study shows a delicate spatial and temporal mechanism by which enteric pathogens may utilize the competition among environmental cues to optimize their pathogenicity.