Adaptation to the chicken intestine in Salmonella Enteritidis PT4 studied by transcriptional analysis

Global transcriptional profiling of tyramine and d-glucuronic acid catabolism in Salmonella

Salmonella has evolved various metabolic pathways to scavenge energy from the metabolic byproducts of the host gut microbiota, however, the precise metabolic byproducts and pathways utilized by Salmonella remain elusive. Previously we reported that Salmonella can proliferate by deriving energy from two metabolites that naturally occur in the host as gut microbial metabolic byproducts, namely, tyramine (TYR, an aromatic amine) and d-glucuronic acid (DGA, a hexuronic acid). Salmonella Pathogenicity Island 13 (SPI-13) plays a critical role in the ability of Salmonella to derive energy from TYR and DGA, however the catabolic pathways of these two micronutrients in Salmonella are poorly defined. The objective of this study was to identify the specific genetic components and construct the regulatory circuits for the TYR and DGA catabolic pathways in Salmonella. To accomplish this, we employed TYR and DGA-induced global transcriptional profiling and gene functional network analysis approaches. We report that TYR induced differential expression of 319 genes (172 up-regulated and 157 down-regulated) when Salmonella was grown in the presence of TYR as a sole energy source. These included the genes originally predicted to be involved in the classical TYR catabolic pathway. TYR also induced expression of majority of genes involved in the acetaldehyde degradation pathway and aided identification of a few new genes that are likely involved in alternative pathway for TYR catabolism. In contrast, DGA induced differential expression of 71 genes (58 up-regulated and 13 down-regulated) when Salmonella was grown in the presence of DGA as a sole energy source. These included the genes originally predicted to be involved in the classical pathway and a few new genes likely involved in the alternative pathway for DGA catabolism. Interestingly, DGA also induced expression of SPI-2 T3SS, suggesting that DGA may also influence nutritional virulence of Salmonella. In summary, this is the first report describing the global transcriptional profiling of TYR and DGA catabolic pathways of Salmonella. This study will contribute to the better understanding of the role of TYR and DGA in metabolic adaptation and virulence of Salmonella.

Comparative genomics of Salmonella enterica serovar Montevideo reveals lineage-specific gene differences that may influence ecological niche association

Salmonella enterica serovar Montevideo has been linked to recent foodborne illness outbreaks resulting from contamination of products such as fruits, vegetables, seeds and spices. Studies have shown that Montevideo also is frequently associated with healthy cattle and can be isolated from ground beef, yet human salmonellosis outbreaks of Montevideo associated with ground beef contamination are rare. This disparity fuelled our interest in characterizing the genomic differences between Montevideo strains isolated from healthy cattle and beef products, and those isolated from human patients and outbreak sources. To that end, we sequenced 13 Montevideo strains to completion, producing high-quality genome assemblies of isolates from human patients (n=8) or from healthy cattle at slaughter (n=5). Comparative analysis of sequence data from this study and publicly available sequences (n=72) shows that Montevideo falls into four previously established clades, differentially occupied by cattle and human strains. The results of these analyses reveal differences in metabolic islands, environmental adhesion determinants and virulence factors within each clade, and suggest explanations for the infrequent association between bovine isolates and human illnesses.

Gene Expression Response of Salmonella enterica Serotype Enteritidis Phage Type 8 to Subinhibitory Concentrations of the Plant-Derived Compounds Trans-Cinnamaldehyde and Eugenol

Background:Salmonella Enteritidis phage type 8 (PT8) is a major poultry-associated Salmonella strain implicated in foodborne outbreaks in the United States. We previously reported that two plant-derived compounds generally recognized as safe (GRAS), trans-cinnamaldehyde (TC), and eugenol (EG), significantly reduced S. Enteritidis colonization in broiler and layer chickens. To elucidate potential PT8 genes affected by TC and EG during colonization, a whole-genome microarray analysis of the bacterium treated with TC and EG was conducted. Results:S. Enteritidis PT8 was grown in Luria-Bertani broth at 37°C to an OD₆₀₀ of ~0.5. Subinhibitory concentrations (SICs; concentration that does not inhibit bacterial growth) of TC (0.01%; 0.75 mM) or EG (0.04%; 2.46 mM) were then added to the culture. S. Enteritidis PT8 RNA was extracted before and 30 min after TC or EG addition. Labeled cDNA from three replicate experiments was subsequently hybridized to a microarray of over 99% of S. Enteritidis PT4 genes, and the hybridization signals were quantified. The plant-derived compounds down-regulated (P < 0.005) expression of S. Enteritidis PT8 genes involved in flagellar motility, regulation of the Salmonella Pathogenicity Island 1, and invasion of intestinal epithelial cells. TC and EG also suppressed transcription of genes encoding multiple transport systems and outer membrane proteins. Moreover, several metabolic and biosynthetic pathways in the pathogen were down-regulated during exposure to the plant-derived compounds. Both TC and EG stimulated the transcription of heat shock genes, such as dnaK, dnaJ, ibpB, and ibpA in S. Enteritidis PT8 (P < 0.005). The results obtained from microarray were validated using a quantitative real-time PCR. Conclusion: The plant-derived compounds TC and EG exert antimicrobial effects on S. Enteritidis PT8 by affecting multiple genes, including those associated with virulence, colonization, cell membrane composition, and transport systems.

Genetic Basis of Salmonella Enteritidis Pathogenesis in Chickens

Salmonella Enteritidis (SE) is the predominant cause of the food-borne salmonellosis in humans, in part because this serotype has the unique ability to contaminate chicken eggs without causing discernible illness in the infected birds. Attempts to develop effective vaccines and eradicate SE from chickens are undermined by significant limitations in our current understanding of the genetic basis of pathogenesis of SE in this reservoir host. In this chapter, we summarize the infection kinetics and provide an overview of the current understanding of genetic factors underlying SE infection in the chicken host. We also discuss the important knowledge gaps that, if addressed, will improve our understanding of the complex biology of SE in young chickens and in egg laying hens.

Distinct Salmonella Enteritidis lineages associated with enterocolitis in high-income settings and invasive disease in low-income settings

An epidemiological paradox surrounds Salmonella enterica serovar Enteritidis. In high-income settings, it has been responsible for an epidemic of poultry-associated, self-limiting enterocolitis, whereas in sub-Saharan Africa it is a major cause of invasive nontyphoidal Salmonella disease, associated with high case fatality. By whole-genome sequence analysis of 675 isolates of S. Enteritidis from 45 countries, we show the existence of a global epidemic clade and two new clades of S. Enteritidis that are geographically restricted to distinct regions of Africa. The African isolates display genomic degradation, a novel prophage repertoire, and an expanded multidrug resistance plasmid. S. Enteritidis is a further example of a Salmonella serotype that displays niche plasticity, with distinct clades that enable it to become a prominent cause of gastroenteritis in association with the industrial production of eggs and of multidrug-resistant, bloodstream-invasive infection in Africa.

Patterns of genome evolution that have accompanied host adaptation in Salmonella

Many bacterial pathogens are specialized, infecting one or few hosts, and this is often associated with more acute disease presentation. Specific genomes show markers of this specialization, which often reflect a balance between gene acquisition and functional gene loss. Within Salmonella enterica subspecies enterica, a single lineage exists that includes human and animal pathogens adapted to cause infection in different hosts, including S. enterica serovar Enteritidis (multiple hosts), S. Gallinarum (birds), and S. Dublin (cattle). This provides an excellent evolutionary context in which differences between these pathogen genomes can be related to host range. Genome sequences were obtained from ∼ 60 isolates selected to represent the known diversity of this lineage. Examination and comparison of the clades within the phylogeny of this lineage revealed signs of host restriction as well as evolutionary events that mark a path to host generalism. We have identified the nature and order of events for both evolutionary trajectories. The impact of functional gene loss was predicted based upon position within metabolic pathways and confirmed with phenotyping assays. The structure of S. Enteritidis is more complex than previously known, as a second clade of S. Enteritidis was revealed that is distinct from those commonly seen to cause disease in humans or animals, and that is more closely related to S. Gallinarum. Isolates from this second clade were tested in a chick model of infection and exhibited a reduced colonization phenotype, which we postulate represents an intermediate stage in pathogen-host adaptation.

RNA sequencing reveals differences between the global transcriptomes of Salmonella enterica serovar enteritidis strains with high and low pathogenicities

Salmonella enterica serovar Enteritidis is one of the important causes of bacterial food-borne gastroenteritis worldwide. Field strains of S. Enteritidis are relatively genetically homogeneous; however, they show extensive phenotypic diversity and differences in virulence potential. RNA sequencing (RNA-Seq) was used to characterize differences in the global transcriptome between several genetically similar but phenotypically diverse poultry-associated field strains of S. Enteritidis grown in laboratory medium at avian body temperature (42°C). These S. Enteritidis strains were previously characterized as high-pathogenicity (HP; n = 3) and low-pathogenicity (LP; n = 3) strains based on both in vitro and in vivo virulence assays. Using the negative binomial distribution-based statistical tools edgeR and DESeq, 252 genes were identified as differentially expressed in LP strains compared with their expression in the HP strains (P < 0.05). A majority of genes (235, or 93.2%) showed significantly reduced expression, whereas a few genes (17, or 6.8%) showed increased expression in all LP strains compared with HP strains. LP strains showed a unique transcriptional profile that is characterized by significantly reduced expression of several transcriptional regulators and reduced expression of genes involved in virulence (e.g., Salmonella pathogenicity island 1 [SPI-1], SPI-5, and fimbrial and motility genes) and protection against osmotic, oxidative, and other stresses, such as iron-limiting conditions commonly encountered within the host. Several functionally uncharacterized genes also showed reduced expression. This study provides a first concise view of the global transcriptional differences between field strains of S. Enteritidis with various levels of pathogenicity, providing the basis for future functional characterization of several genes with potential roles in virulence or stress regulation of S. Enteritidis.

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 microbiome of the chicken gastrointestinal tract

The modern molecular biology movement was developed in the 1960s with the conglomeration of biology, chemistry, and physics. Today, molecular biology is an integral part of studies aimed at understanding the evolution and ecology of gastrointestinal microbial communities. Molecular techniques have led to significant gains in our understanding of the chicken gastrointestinal microbiome. New advances, primarily in DNA sequencing technologies, have equipped researchers with the ability to explore these communities at an unprecedented level. A reinvigorated movement in systems biology offers a renewed promise in obtaining a more complete understanding of chicken gastrointestinal microbiome dynamics and their contributions to increasing productivity, food value, security, and safety as well as reducing the public health impact of raising production animals. Here, we contextualize the contributions molecular biology has already made to our understanding of the chicken gastrointestinal microbiome and propose targeted research directions that could further exploit molecular technologies to improve the economy of the poultry industry.

The salmonella transcriptome in lettuce and cilantro soft rot reveals a niche overlap with the animal host intestine

Fresh vegetables have been recurrently associated with salmonellosis outbreaks, and Salmonella contamination of retail produce has been correlated positively with the presence of soft rot disease. We observed that population sizes of Salmonella enterica serovar Typhimurium SL1344 increased 56-fold when inoculated alone onto cilantro leaves, versus 2,884-fold when coinoculated with Dickeya dadantii, a prevalent pathogen that macerates plant tissue. A similar trend in S. enterica populations was observed for soft-rotted lettuce leaves. Transcriptome analysis of S. enterica cells that colonized D. dadantii-infected lettuce and cilantro leaves revealed a clear shift toward anaerobic metabolism and catabolism of substrates that are available due to the degradation of plant cells by the pectinolytic pathogen. Twenty-nine percent of the genes that were upregulated in cilantro macerates were also previously observed to have increased expression levels in the chicken intestine. Furthermore, multiple genes induced in soft rot lesions are also involved in the colonization of mouse, pig, and bovine models of host infection. Among those genes, the operons for ethanolamine and propanediol utilization as well as for the synthesis of cobalamin, a cofactor in these pathways, were the most highly upregulated genes in lettuce and cilantro lesions. In S. Typhimurium strain LT2, population sizes of mutants deficient in propanediol utilization or cobalamin synthesis were 10- and 3-fold lower, respectively, than those of the wild-type strain in macerated cilantro (P < 0.0002); in strain SL1344, such mutants behaved similarly to the parental strain. Anaerobic conditions and the utilization of nutrients in macerated plant tissue that are also present in the animal intestine indicate a niche overlap that may explain the high level of adaptation of S. enterica to soft rot lesions, a common postharvest plant disease.

Transposon mutagenesis of Salmonella enterica serovar Enteritidis identifies genes that contribute to invasiveness in human and chicken cells and survival in egg albumen

Salmonella enterica serovar Enteritidis is an important food-borne pathogen, and chickens are a primary reservoir of human infection. While most knowledge about Salmonella pathogenesis is based on research conducted on Salmonella enterica serovar Typhimurium, S. Enteritidis is known to have pathobiology specific to chickens that impacts epidemiology in humans. Therefore, more information is needed about S. Enteritidis pathobiology in comparison to that of S. Typhimurium. We used transposon mutagenesis to identify S. Enteritidis virulence genes by assay of invasiveness in human intestinal epithelial (Caco-2) cells and chicken liver (LMH) cells and survival within chicken (HD-11) macrophages as a surrogate marker for virulence. A total of 4,330 transposon insertion mutants of an invasive G1 Nal(r) strain were screened using Caco-2 cells. This led to the identification of attenuating mutations in a total of 33 different loci, many of which include genes previously known to contribute to enteric infection (e.g., Salmonella pathogenicity island 1 [SPI-1], SPI-4, SPI-5, CS54, fliH, fljB, csgB, spvR, and rfbMN) in S. Enteritidis and other Salmonella serovars. Several genes or genomic islands that have not been reported previously (e.g., SPI-14, ksgA, SEN0034, SEN2278, and SEN3503) or that are absent in S. Typhimurium or in most other Salmonella serovars (e.g., pegD, SEN1152, SEN1393, and SEN1966) were also identified. Most mutants with reduced Caco-2 cell invasiveness also showed significantly reduced invasiveness in chicken liver cells and impaired survival in chicken macrophages and in egg albumen. Consequently, these genes may play an important role during infection of the chicken host and also contribute to successful egg contamination by S. Enteritidis.