Pleiotropic phenotypes of a Yersinia enterocolitica flhD mutant include reduced lethality in a chicken embryo model

Evaluation of the Correlation between the mRNA Expression Levels of ystA and ymoA Genes in Y. enterocolitica Strains with Different Enterotoxic Properties

Yersinia enterocolitica is one of the main causative agents of human diarrhea. Pigs are a reservoir and the most common source of infection for humans. The aim of this study was to analyze the expression of ystA and ymoA genes in Y. enterocolitica strains with different enterotoxic properties, isolated from humans and pigs. The experiment involved two groups of Y. enterocolitica strains producing and not producing enterotoxin YstA, which were isolated from humans and pigs. All strains were ystA- and ymoA-positive. The expression of ystA and ymoA genes was analyzed by quantitative real-time PCR (qPCR). The relative expression level of the ystA gene was significantly higher than the expression level of the ymoA gene in Y. enterocolitica strains isolated from humans with clinical symptoms of yersiniosis. In other strains, a significant decrease in ystA gene transcription was observed, and the relative expression level of the ymoA gene was significantly higher than the expression level of the ystA gene. Statistically significant differences were not observed in either group of strains isolated from pigs. The results of our study revealed a correlation between mRNA expression levels of ystA and ymoA genes in Y. enterocolitica strains isolated from humans.

The use of embryonic chicken eggs as an alternative model to evaluate the virulence of Salmonella enterica serovar Gallinarum

Salmonella enterica serovar Gallinarum (S. Gallinarum) can cause fowl typhoid, a severe systemic disease responsible for considerable economic losses. Chicken pathogenicity test is the traditional method for assessing the virulence of S. Gallinarum. However, this method is limited by several factors, including ethical considerations, costs, and the need for specialized facilities. Hence, we established a chicken embryo lethality assay (ELA) model to determine the virulence of S. Gallinarum. Three virulent and three avirulent representative strains, which were confirmed by the chicken pathogenicity test, were used to perform the ELA. The most significant difference between the virulent and avirulent strains could be observed when 13-day-old embryos were inoculated via the AC route and incubated for 5 days. Based on a 50% embryo lethal dose (ELD₅₀), isolates considered to be virulent had a Log₁₀ELD₅₀ of ≤ 4.0, moderately virulent strains had a Log₁₀ELD₅₀ of 4.0−6.1, and avirulent isolates had a Log₁₀ELD₅₀ of ≥ 6.1. Different abilities to invade the liver of embryos were found between the virulent and avirulent strains by a growth curve experiment in vitro. The maximum colony-forming units (CFU) of the virulent strain was about 10,000 times higher than that of the avirulent strain in the liver at 5 days post infection. The ELA results of 42 field strains showed that thirty-two strains (76.2%) were virulent, nine were moderately virulent (21.4%), and one strain was avirulent (2.4%). In conclusion, these results suggest that the ELA can be used as an alternative method to assess the virulence of S. Gallinarum, which will contribute to the study of virulence genes, virulence evolution, pathogenic mechanisms and vaccine development.

Isolate Specific Cold Response of Yersinia enterocolitica in Transcriptional, Proteomic, and Membrane Physiological Changes

Yersinia enterocolitica, a zoonotic foodborne pathogen, is able to withstand low temperatures. This psychrotrophic ability allows it to multiply in food stored in refrigerators. However, little is known about the Y. enterocolitica cold response. In this study, isolate-specific behavior at 4°C was demonstrated and the cold response was investigated by examining changes in phenotype, gene expression, and the proteome. Altered expression of cold-responsive genes showed that the ability to survive at low temperature depends on the capacity to acclimate and adapt to cold stress. This cold acclimation at the transcriptional level involves the transient induction and effective repression of cold-shock protein (Csp) genes. Moreover, the resumption of expression of genes encoding other non-Csp is essential during prolonged adaptation. Based on proteomic analyses, the predominant functional categories of cold-responsive proteins are associated with protein synthesis, cell membrane structure, and cell motility. In addition, changes in membrane fluidity and motility were shown to be important in the cold response of Y. enterocolitica. Isolate-specific differences in the transcription of membrane fluidity- and motility-related genes provided evidence to classify strains within a spectrum of cold response. The combination of different approaches has permitted the systematic description of the Y. enterocolitica cold response and gives a better understanding of the physiological processes underlying this phenomenon.

Use of a Chicken Embryo Lethality Assay to Assess the Efficacy of Phage Therapy

To combat infectious diseases induced by antibiotic-resistant bacteria in human and animals, phage therapy has regained attention by the scientific community. Before phages can be widely accepted as therapeutics in the same way as antibiotics, convincing detailed applied experimental evidence must be available. The embryonated chicken egg model has been used to study the virulence of many pathogens. We describe here a procedure to test the efficacy of phage therapy to treat colibacillosis using a chicken embryo lethality assay, this being potentially applied to others bacterial infection.

Differential impact of lipopolysaccharide defects caused by loss of RfaH in Yersinia pseudotuberculosis and Yersinia pestis

RfaH enhances transcription of a select group of operons controlling bacterial surface features such as lipopolysaccharide (LPS). Previous studies have suggested that rfaH may be required for Yersinia pseudotuberculosis resistance to antimicrobial chemokines and survival during mouse infections. In order to further investigate the role of RfaH in LPS synthesis, resistance to host defense peptides, and virulence of Yersinia, we constructed ΔrfaH mutants of Y. pseudotuberculosis IP32953 and Y. pestis KIM6+. Loss of rfaH affected LPS synthesis in both species, resulting in a shorter core oligosaccharide. Susceptibility to polymyxin and the antimicrobial chemokine CCL28 was increased by loss of rfaH in Y. pseudotuberculosis but not in Y. pestis. Transcription of genes in the ddhD-wzz O-antigen gene cluster, but not core oligosaccharide genes, was reduced in ΔrfaH mutants. In addition, mutants with disruptions in specific ddhD-wzz O-antigen cluster genes produced LPS that was indistinguishable from the ΔrfaH mutant. This suggests that both Y. pseudotuberculosis and Y. pestis produce an oligosaccharide core with a single O-antigen unit attached in an RfaH-dependent fashion. Despite enhanced sensitivity to host defense peptides, the Y. pseudotuberculosis ΔrfaH strain was not attenuated in mice, suggesting that rfaH is not required for acute infection.

Flagellum density regulates Proteus mirabilis swarmer cell motility in viscous environments

Proteus mirabilis is an opportunistic pathogen that is frequently associated with urinary tract infections. In the lab, P. mirabilis cells become long and multinucleate and increase their number of flagella as they colonize agar surfaces during swarming. Swarming has been implicated in pathogenesis; however, it is unclear how energetically costly changes in P. mirabilis cell morphology translate into an advantage for adapting to environmental changes. We investigated two morphological changes that occur during swarming--increases in cell length and flagellum density--and discovered that an increase in the surface density of flagella enabled cells to translate rapidly through fluids of increasing viscosity; in contrast, cell length had a small effect on motility. We found that swarm cells had a surface density of flagella that was ∼5 times larger than that of vegetative cells and were motile in fluids with a viscosity that inhibits vegetative cell motility. To test the relationship between flagellum density and velocity, we overexpressed FlhD(4)C(2), the master regulator of the flagellar operon, in vegetative cells of P. mirabilis and found that increased flagellum density produced an increase in cell velocity. Our results establish a relationship between P. mirabilis flagellum density and cell motility in viscous environments that may be relevant to its adaptation during the infection of mammalian urinary tracts and movement in contact with indwelling catheters.

Embryonated chicken eggs as alternative infection model for pathogenic fungi

Embryonated eggs have been used as infection models for decades in virology and bacteriology. However, they can also be used as an attractive alternative infection model for studying fungal pathogenesis. Here, we discuss some general aspects which need to be considered when working with embryonated eggs as infection models. Furthermore, we provide detailed protocols and technical tips for infection of embryonated eggs with Aspergillus fumigatus and Candida albicans via the chorioallantois membrane, as well as sampling methods for downstream analyses.

Regulation of cell division, biofilm formation, and virulence by FlhC in Escherichia coli O157:H7 grown on meat

To understand the continuous problems that Escherichia coli O157:H7 causes as food pathogen, this study assessed global gene regulation in bacteria growing on meat. Since FlhD/FlhC of E. coli K-12 laboratory strains was previously established as a major control point in transducing signals from the environment to several cellular processes, this study compared the expression pattern of an E. coli O157:H7 parent strain to that of its isogenic flhC mutant. This was done with bacteria that had been grown on meat. Microarray experiments revealed 287 putative targets of FlhC. Real-time PCR was performed as an alternative estimate of transcription and confirmed microarray data for 13 out of 15 genes tested (87%). The confirmed genes are representative of cellular functions, such as central metabolism, cell division, biofilm formation, and pathogenicity. An additional 13 genes from the same cellular functions that had not been hypothesized as being regulated by FlhC by the microarray experiment were tested with real-time PCR and also exhibited higher expression levels in the flhC mutant than in the parent strain. Physiological experiments were performed and confirmed that FlhC reduced the cell division rate, the amount of biofilm biomass, and pathogenicity in a chicken embryo lethality model. Altogether, this study provides valuable insight into the complex regulatory network of the pathogen that enables its survival under various environmental conditions. This information may be used to develop strategies that could be used to reduce the number of cells or pathogenicity of E. coli O157:H7 on meat by interfering with the signal transduction pathways.

OmpR controls Yersinia enterocolitica motility by positive regulation of flhDC expression

Flagella and invasin play important roles during the early stages of infection by the enteric pathogen Yersinia enterocolitica. Our previous study demonstrated that OmpR negatively regulates invasin gene expression at the transcriptional level. The present study focused on the role of OmpR in the regulation of flagella expression. Motility assays and microscopic observations revealed that an ompR mutant strain exhibits a non-motile phenotype due to the lack of flagella. An analysis of flhDC::lacZYA chromosomal fusions demonstrated a decrease in flhDC expression in ompR mutant cells, suggesting a role for OmpR in the positive control of flagellar master operon flhDC, which is in contrast to the negative role it plays in Escherichia coli. Moreover, high temperature or osmolarity and low pH decreased flhDC expression and OmpR was not required for the response to these factors. Evidence from an examination of the DNA binding properties of OmpR in vitro indicated that the mechanism by which OmpR regulates flhDC is direct. Electrophoretic mobility shift assays confirmed that OmpR binds specifically to the flhDC promoter region and suggested the presence of more than one OmpR-binding site. In addition, phosphorylation of OmpR by acetyl-P appeared to stimulate the binding abilities of OmpR. Together with the results of our previous studies revealing the negative role of OmpR in the regulation of invasin expression, these findings support a model in which invasion and motility might be reciprocally regulated by OmpR.

Embryonated eggs as an alternative infection model to investigate Aspergillus fumigatus virulence

Infection models are essential tools for studying microbial pathogenesis. Murine models are considered the "gold standard" for studying in vivo infections caused by Aspergillus species, such as A. fumigatus. Recently developed molecular protocols allow rapid construction of high numbers of fungal deletion mutants, and alternative infection models based on cell culture or invertebrates are widely used for screening such mutants to reduce the number of rodents in animal experiments. To bridge the gap between invertebrate models and mice, we have developed an alternative, low-cost, and easy-to-use infection model for Aspergillus species based on embryonated eggs. The outcome of infections in the egg model is dose and age dependent and highly reproducible. We show that the age of the embryos affects the susceptibility to A. fumigatus and that increased resistance coincides with altered chemokine production after infection. The progress of disease in the model can be monitored by using egg survival and histology. Based on pathological analyses, we hypothesize that invasion of embryonic membranes and blood vessels leads to embryonic death. Defined deletion mutant strains previously shown to be fully virulent or partially or strongly attenuated in a mouse model of bronchopulmonary aspergillosis showed comparable degrees of attenuation in the egg model. Addition of nutrients restored the reduced virulence of a mutant lacking a biosynthetic gene, and variations of the infectious route can be used to further analyze the role of distinct genes in our model. Our results suggest that embryonated eggs can be a very useful alternative infection model to study A. fumigatus virulence and pathogenicity.

Francisella tularensis DeltapyrF mutants show that replication in nonmacrophages is sufficient for pathogenesis in vivo

The pathogenesis of Francisella tularensis has been associated with this bacterium's ability to replicate within macrophages. F. tularensis can also invade and replicate in a variety of nonphagocytic host cells, including lung and kidney epithelial cells and hepatocytes. As uracil biosynthesis is a central metabolic pathway usually necessary for pathogens, we characterized DeltapyrF mutants of both F. tularensis LVS and Schu S4 to investigate the role of these mutants in intracellular growth. As expected, these mutant strains were deficient in de novo pyrimidine biosynthesis and were resistant to 5-fluoroorotic acid, which is converted to a toxic product by functional PyrF. The F. tularensis DeltapyrF mutants could not replicate in primary human macrophages. The inability to replicate in macrophages suggested that the F. tularensis DeltapyrF strains would be attenuated in animal infection models. Surprisingly, these mutants retained virulence during infection of chicken embryos and in the murine model of pneumonic tularemia. We hypothesized that the F. tularensis DeltapyrF strains may replicate in cells other than macrophages to account for their virulence. In support of this, F. tularensis DeltapyrF mutants replicated in HEK-293 cells and normal human fibroblasts in vitro. Moreover, immunofluorescence microscopy showed abundant staining of wild-type and mutant bacteria in nonmacrophage cells in the lungs of infected mice. These findings indicate that replication in nonmacrophages contributes to the pathogenesis of F. tularensis.

Polyphasic study of plant- and clinic-associated Pantoea agglomerans strains reveals indistinguishable virulence potential

Pantoea species are ubiquitous in nature and occasionally associated with infections caused by contaminated clinical material. Hence, Pantoea agglomerans is considered as an opportunistic pathogen of humans. Since species of the genus Pantoea and closely related species of other Enterobacteriaceae genera are phenotypically very similar, many clinical isolates are misassigned into P. agglomerans based on the use of quick commercial-offered biochemical tests. Our objective was to find markers enabling discrimination between clinical and plant isolates and to assess their virulence potential. We characterized 27 Pantoea strains, including 8 P. agglomerans isolates of clinical, and 11 of plant origin by biochemical tests and genotyping, including analysis of 16S rDNA and gapA gene sequences, and pattern polymorphisms of ITS- and ERIC/REP-DNA. All data showed that no discrete evolution occurred between plant-associated and clinical P. agglomerans isolates. Based on the typing results, five clinical- and five plant-associated P. agglomerans strains representing the majority of clades were tested on a model plant and in embryonated eggs. On soybean plants P. agglomerans strains independent of their origin could develop stable epiphytic populations. Surprisingly, in the embryonated egg model there was no difference of virulence between clinical and vegetable P. agglomerans isolates. However, these strains were significantly less virulent than a phytopathogenic P. ananatis isolate. We suggest that, independent of their origin, all P. agglomerans strains might possess indistinguishable virulence potential.

Global transcriptional response to mammalian temperature provides new insight into Francisella tularensis pathogenesis

Background

After infecting a mammalian host, the facultative intracellular bacterium, Francisella tularensis, encounters an elevated environmental temperature. We hypothesized that this temperature change may regulate genes essential for infection.

Results

Microarray analysis of F. tularensis LVS shifted from 26°C (environmental) to 37°C (mammalian) showed ~11% of this bacterium's genes were differentially-regulated. Importantly, 40% of the protein-coding genes that were induced at 37°C have been previously implicated in virulence or intracellular growth of Francisella in other studies, associating the bacterial response to this temperature shift with pathogenesis. Forty-four percent of the genes induced at 37°C encode proteins of unknown function, suggesting novel Francisella virulence traits are regulated by mammalian temperature. To explore this possibility, we generated two mutants of loci induced at 37°C [FTL_1581 and FTL_1664 (deoB)]. The FTL_1581 mutant was attenuated in a chicken embryo infection model, which was likely attributable to a defect in survival within macrophages. FTL_1581 encodes a novel hypothetical protein that we suggest naming temperature-induced, virulence-associated locus A, tivA. Interestingly, the deoB mutant showed diminished entry into mammalian cells compared to wild-type LVS, including primary human macrophages and dendritic cells, the macrophage-like RAW 264.7 line, and non-phagocytic HEK-293 cells. This is the first study identifying a Francisella gene that contributes to uptake into both phagocytic and non-phagocytic host cells.

Conclusion

Our results provide new insight into mechanisms of Francisella virulence regulation and pathogenesis. F. tularensis LVS undergoes considerable gene expression changes in response to mammalian body temperature. This temperature shift is important for the regulation of genes that are critical for the pathogenesis of Francisella. Importantly, the compilation of temperature-regulated genes also defines a rich collection of novel candidate virulence determinants, including tivA (FTL_1581). An analysis of tivA and deoB (FTL_1664) revealed that these genes contribute to intracellular survival and entry into mammalian cells, respectively.