Carboxyl-Terminal Residues N478 and V479 Required for the Cytolytic Activity of Listeriolysin O Play a Critical Role in Listeria monocytogenes Pathogenicity

Destroying glutathione peroxidase improves the oxidative stress resistance and pathogenicity of Listeria monocytogenes

Introduction

Glutathione peroxidase is abundant in eukaryotes as an important antioxidant enzyme. However, prokaryotic glutathione peroxidase has not been thoroughly studied. Listeria monocytogenes is a facultative intracellular pathogen that is capable of causing listeriosis in animals as well as humans. Despite the fact that L. monocytogenes encodes a putative glutathione peroxidase, GSH-Px (encoded by lmo0983)), the functions of the enzyme are still unknown. Here we revealed the unusual roles of L. monocytogenes GSH-Px in bacterial antioxidants and pathogenicity.

Methods

L. monocytogenes Lm850658 was taken as the parental strain to construct the gsh-px deletion strain and related complement strain. The effect of the gsh-px gene on the resistance of L. monocytogenes to oxidative stress was determined by measuring the concentrations of glutathione and assaying the stress survival rates under different oxidative conditions. In addition, the pathogenicity of L. monocytogenes was determined by cellular adhesion and invasion assays and mice virulence tests, and the expression of virulence factors was determined by Western blot.

Results

Deficiency of GSH-Px not only increased glutathione concentrations in L. monocytogenes but also enhanced its resistance to oxidative stress when exposed to copper and iron ions. In addition, the absence of gsh-px significantly improved the adhesion and invasion efficiency of L. monocytogenes to Caco-2 cells. More importantly, L. monocytogenes lacking GSH-Px could colonize and proliferate more efficiently in mice livers and spleens, enhancing the pathogenicity of L. monocytogenes by increasing the expression of virulence factors like InlA, InlB, and LLO.

Discussion

Taken together, we confirmed that GSH-Px of L. monocytogenes has a counter-intuitive effect on the antioxidant capacity and pathogenicity.

YjbH mediates the oxidative stress response and infection by regulating SpxA1 and the phosphoenolpyruvate-carbohydrate phosphotransferase system (PTS) in Listeria monocytogenes

The foodborne pathogen Listeria monocytogenes relies on its ability to fine-tune the expression of virulence factors and stress regulators in response to rapidly changing environments. Here, we reveal that YjbH, a putative thioredoxin family oxidoreductase, plays a pivotal role in bacterial adaption to oxidative stress and host infection. YjbH directly interacts with SpxA1, an ArsC family oxidative stress response regulator, and the deletion of YjbH compromised the oxidative stress tolerance of L. monocytogenes. Also, YjbH is required for the bacterial spread in host cells and proliferation in mouse organs, thereby contributing to virulence. Transcriptomic analysis of strains treated with Cd²⁺ revealed that most virulence genes and phosphoenolpyruvate-carbohydrate phosphotransferase system (PTS) genes were significantly downregulated in the absence of YjbH. However, YjbH inhibits PrfA expression when bacteria were grown in the media, suggesting that YjbH participates in regulating the virulence genes via a complicated regulatory network involving PrfA and PTS. Collectively, these findings provide a valuable model for clarifying the roles of thioredoxins from foodborne pathogens regarding improving survival in the external environment and, more importantly, successfully establishing infection within the host.

Development of a Genomics-Based Approach To Identify Putative Hypervirulent Nontyphoidal Salmonella Isolates: Salmonella enterica Serovar Saintpaul as a Model

While differences in human virulence have been reported across nontyphoidal Salmonella (NTS) serovars and associated subtypes, a rational and scalable approach to identify Salmonella subtypes with differential ability to cause human diseases is not available. Here, we used NTS serovar Saintpaul (S. Saintpaul) as a model to determine if metadata and associated whole-genome sequence (WGS) data in the NCBI Pathogen Detection (PD) database can be used to identify (i) subtypes with differential likelihoods of causing human diseases and (ii) genes and single nucleotide polymorphisms (SNPs) potentially responsible for such differences. S. Saintpaul SNP clusters (n = 211) were assigned different epidemiology types (epi-types) based on statistically significant over- or underrepresentation of human clinical isolates, including human associated (HA; n = 29), non-human associated (NHA; n = 23), and other (n = 159). Comparative genomic analyses identified 384 and 619 genes overrepresented among isolates in 5 HA and 4 NHA SNP clusters most significantly associated with the respective isolation source. These genes included 5 HA-associated virulence genes previously reported to be present on Gifsy-1/Gifsy-2 prophages. Additionally, premature stop codons in 3 and 7 genes were overrepresented among the selected HA and NHA SNP clusters, respectively. Tissue culture experiments with strains representing 4 HA and 3 NHA SNP clusters did not reveal evidence for enhanced invasion or intracellular survival for HA strains. However, the presence of sodCI (encoding a superoxide dismutase), found in 4 HA and 1 NHA SNP clusters, was positively correlated with intracellular survival in macrophage-like cells. Post hoc analyses also suggested a possible difference in intracellular survival among S. Saintpaul lineages. IMPORTANCE Not all Salmonella isolates are equally likely to cause human disease, and Salmonella control strategies may unintentionally focus on serovars and subtypes with high prevalence in source populations but are rarely associated with human clinical illness. We describe a framework leveraging WGS data in the NCBI PD database to identify Salmonella subtypes over- and underrepresented among human clinical cases. While we identified genomic signatures associated with HA/NHA SNP clusters, tissue culture experiments failed to identify consistent phenotypic characteristics indicative of enhanced human virulence of HA strains. Our findings illustrate the challenges of defining hypo- and hypervirulent S. Saintpaul and potential limitations of phenotypic assays when evaluating human virulence, for which in vivo experiments are essential. Identification of sodCI, an HA-associated virulence gene associated with enhanced intracellular survival, however, illustrates the potential of the framework and is consistent with prior work identifying specific genomic features responsible for enhanced or reduced virulence of nontyphoidal Salmonella.

Deletion of glutaredoxin promotes oxidative tolerance and intracellular infection in Listeria monocytogenes

Thiol-disulfide glutaredoxin systems of bacterial cytoplasm favor reducing conditions for the correct disulfide bonding of functional proteins, and therefore were employed by bacteria to defend against oxidative stress. Listeria monocytogenes has been shown to encode a putative glutaredoxin, Grx (encoded by lmo2344), while the underlying roles remain unknown. Here we suggest an unexpected role of L. monocytogenes Grx in oxidative tolerance and intracellular infection. The recombinant Grx was able to efficiently catalyze the thiol-disulfide oxidoreduction of insulin in the presence of DTT as an election donor. Unexpectedly, the deletion of grx resulted in a remarkably increased tolerance and survival ability of this bacteria when exposed to various oxidizing agents, including diamide, and copper and cadmium ions. Furthermore, loss of grx significantly promoted bacterial invasion and proliferation in human epithelial Caco-2 cells and murine macrophages, as well as a notably increasing invasion but not cell-to-cell spread in the murine fibroblasts L929 cells. More importantly, L. monocytogenes lacking the glutaredoxin exhibited more efficient proliferation and recovery in the spleens and livers of the infected mice, and hence became more virulent by upregulating the virulence factors, InlA and InlB. In summary, we here for the first time demonstrated that L. monocytogenes glutaredoxin plays a counterintuitive role in bacterial oxidative resistance and intracellular infection, which is the first report to provide valuable evidence for the role of glutaredoxins in bacterial infection, and more importantly suggests a favorable model to illustrate the functional diversity of bacterial Grx systems during environmental adaption and host infection.

Listeriolysin O Pore-Forming Activity Is Required for ERK1/2 Phosphorylation During Listeria monocytogenes Infection

Listeriolysin O (LLO) is a cholesterol-dependent cytolysin that mediates escape of L. monocytogenes from phagosomes and enables the bacteria to grow within the host. LLO is a versatile tool allowing Listeria to trigger several cellular responses. In this study, rapid phosphorylation of ERK1/2 on Caco-2 cells caused by Listeria infection was demonstrated to be highly dependent on LLO activity. The effect could be strongly induced by adding purified recombinant LLO alone and could be inhibited by exogenous cholesterol. Lack of the PEST sequence, known to tightly control cytotoxicity of LLO, did not affect ERK1/2 activation. However, the recombinant non-cytolytic LLO_T515AL516A, with mutations in the cholesterol-binding motif, was unable to trigger this response. Recombinant LLO_N478AV479A, which lacks most of the cytolytic activity, also failed to activate ERK1/2 phosphorylation, and this effect could be rescued when the protein concentration reached a cytolytic level. Infection with an LLO-deficient mutant (Δhly) or the mutant complementing LLO_T515AL516A abrogated the capacity of the bacteria to activate ERK1/2. However, infection with the Δhly mutant complementing LLO_N478AV479A, which retained partial pore-forming ability and could grow intracellularly, was capable of triggering ERK1/2 phosphorylation. Collectively, these data suggest that ERK1/2 activation by L. monocytogenes depends on the permeabilization activity of LLO and more importantly correlates with the cholesterol-binding motif of LLO.

Betulin efficiently suppresses the process of an experimental Listeria monocytogenes infection as an antagonist against listeriolysin O

Listeria monocytogenes (Lm) is a widespread foodborne intracellular pathogen that invades a variety of cells, causing abortions and severe human diseases. After internalization into host cells, pore-forming cytolysin listeriolysin O (LLO) disrupts the phagosome, which allows the bacterium to survive and colonize the cytoplasm, providing the bacterium the chance to infect neighboring cells. Betulin is an extracted natural compound from birch bark with diverse pharmacological activities. Here, we showed that LLO-induced rabbit red blood cell lysis in vitro was inhibited by preincubation with betulin, which suppressed the oligomerization process. Infectious assays performed with human monocyte macrophages indicated that betulin significantly protected cells against Lm-induced cell injury. In addition, Balb/c mice were used to perform a general infection, and betulin administration obviously inhibited organ damage and bacterial burden in livers and spleens of infected mice. In conclusion, betulin obviously inhibited Lm-induced cell injury in vitro and protected against infection in vivo through an antivirulence effect. Our results showed betulin as a new candidate against listeriosis by targeting LLO and highlight the potential of natural product-based medicine to be applied in the treatment of pathogenic infections.