Salmonella plasmid virulence gene spvB enhances bacterial virulence by inhibiting autophagy in a zebrafish infection model

Detection of Salmonella Pathogenicity Islands and Antimicrobial-Resistant Genes in Salmonella enterica Serovars Enteritidis and Typhimurium Isolated from Broiler Chickens

Rapid growth in commercial poultry production is one of the major sources of Salmonella infections that leads to human salmonellosis. The two main Salmonella enterica serovars associated with human salmonellosis are enteritidis and typhimurium. The aim of this study was to determine the prevalence of S. enterica serovars Enteritidis and S. Typhimurium as well as their Salmonella pathogenicity islands (SPI) and antibiotic resistance profiles in broiler chicken feces from slaughterhouses. A total of 480 fecal samples from broiler chickens that were grouped into 96 pooled samples were identified to have Salmonella spp. using the invA gene, whilst the Spy and sdfI genes were used to screen for the presence of S. Enteritidis and S. Typhimurium serovars, respectively, by polymerase chain reaction (PCR) assays. The isolates were also screened for the presence of Salmonella pathogenicity islands (SPIs) using PCR. The disc diffusion assay was performed to determine the antibiotic resistance profiles of the isolates. A total of 36 isolates were confirmed as Salmonella spp. through amplification of the invA gene. Out of 36 confirmed Salmonella spp. a total of 22 isolates were classified as S. Enteritidis (n = 8) and were S. Typhimurium (n = 14) serovars. All (n = 22) S. Enteritidis and S. Typhimurium isolates possessed the hilA (SPI-1), ssrB (SPI-2) and pagC (SPI-11) pathogenicity islands genes. Amongst these serovars, 50% of the isolates (n = 11/22) were resistant to tetracycline and nalidixic acid. Only 22% of the isolates, S. Typhimurium (13.6%) and S. Enteritidis (9.1%) demonstrated resistance against three or more antibiotic classes. The most detected antibiotic resistance genes were tet(K), mcr-1, sulI and strA with 13 (59.1%), 9 (40.9%), 9 (40.9%) and 7 (31.8%), respectively. The findings of this study revealed that S. Typhimurium is the most prevalent serotype detected in chicken feces. To reduce the risk to human health posed by salmonellosis, a stringent public health and food safety policy is required.

Effects and mechanisms of Salmonella plasmid virulence gene spv on host-regulated cell death

Salmonella is responsible for the majority of food poisoning outbreaks around the world. Pathogenic Salmonella mostly carries a virulence plasmid that contains the Salmonella plasmid virulence gene (spv), a highly conserved sequence encoding effector proteins that can manipulate host cells. Intestinal epithelial cells are crucial components of the innate immune system, acting as the first barrier of defense against infection. When the barrier is breached, Salmonella encounters the underlying macrophages in lamina propria, triggering inflammation and engaging in combat with immune cells recruited by inflammatory factors. Host regulated cell death (RCD) provides a variety of means to fight against or favour Salmonella infection. However, Salmonella releases effector proteins to regulate RCD, evading host immune killing and neutralizing host antimicrobial effects. This review provides an overview of pathogen-host interactions in terms of (1) pathogenicity of Salmonella spv on intestinal epithelial cells and macrophages, (2) mechanisms of host RCD to limit or promote pathogenic Salmonella expansion, and (3) effects and mechanisms of Salmonella spv gene on host RCD.

Manipulation of host immune defenses by effector proteins delivered from multiple secretion systems of Salmonella and its application in vaccine research

Salmonella is an important zoonotic bacterial species and hazardous for the health of human beings and livestock globally. Depending on the host, Salmonella can cause diseases ranging from gastroenteritis to life-threatening systemic infection. In this review, we discuss the effector proteins used by Salmonella to evade or manipulate four different levels of host immune defenses: commensal flora, intestinal epithelial-mucosal barrier, innate and adaptive immunity. At present, Salmonella has evolved a variety of strategies against host defense mechanisms, among which various effector proteins delivered by the secretory systems play a key role. During its passage through the digestive system, Salmonella has to face the intact intestinal epithelial barrier as well as competition with commensal flora. After invasion of host cells, Salmonella manipulates inflammatory pathways, ubiquitination and autophagy processes with the help of effector proteins. Finally, Salmonella evades the adaptive immune system by interfering the migration of dendritic cells and interacting with T and B lymphocytes. In conclusion, Salmonella can manipulate multiple aspects of host defense to promote its replication in the host.

Antigen Presentation and Autophagy in Teleost Adaptive Immunity

Infectious diseases are a burden for aquaculture. Antigen processing and presentation (APP) to the immune effector cells that fight pathogens is key in the adaptive immune response. At the core of the adaptive immunity that appeared in lower vertebrates during evolution are the variable genes encoding the major histocompatibility complex (MHC). MHC class I molecules mainly present peptides processed in the cytosol by the proteasome and transported to the cell surface of all cells through secretory compartments. Professional antigen-presenting cells (pAPC) also express MHC class II molecules, which normally present peptides processed from exogenous antigens through lysosomal pathways. Autophagy is an intracellular self-degradation process that is conserved in all eukaryotes and is induced by starvation to contribute to cellular homeostasis. Self-digestion during autophagy mainly occurs by the fusion of autophagosomes, which engulf portions of cytosol and fuse with lysosomes (macroautophagy) or assisted by chaperones (chaperone-mediated autophagy, CMA) that deliver proteins to lysosomes. Thus, during self-degradation, antigens can be processed to be presented by the MHC to immune effector cells, thus, linking autophagy to APP. This review is focused on the essential components of the APP that are conserved in teleost fish and the increasing evidence related to the modulation of APP and autophagy during pathogen infection.

Selective autophagy: the rise of the zebrafish model

Selective autophagy is a specific elimination of certain intracellular substrates by autophagic pathways. The most studied macroautophagy pathway involves tagging and recognition of a specific cargo by the autophagic membrane (phagophore) followed by the complete sequestration of targeted cargo from the cytosol by the double-membrane vesicle, autophagosome. Until recently, the knowledge about selective macroautophagy was minimal, but now there is a panoply of links elucidating how phagophores engulf their substrates selectively. The studies of selective autophagy processes have further stressed the importance of using the in vivo models to validate new in vitro findings and discover the physiologically relevant mechanisms. However, dissecting how the selective autophagy occurs yet remains difficult in living organisms, because most of the organelles are relatively inaccessible to observation and experimental manipulation in mammals. In recent years, zebrafish (Danio rerio) is widely recognized as an excellent model for studying autophagic processes in vivo because of its optical accessibility, genetic manipulability and translational potential. Several selective autophagy pathways, such as mitophagy, xenophagy, lipophagy and aggrephagy, have been investigated using zebrafish and still need to be studied further, while other selective autophagy pathways, such as pexophagy or reticulophagy, could also benefit from the use of the zebrafish model. In this review, we shed light on how zebrafish contributed to our understanding of these selective autophagy processes by providing the in vivo platform to study them at the organismal level and highlighted the versatility of zebrafish model in the selective autophagy field.Abbreviations: AD: Alzheimer disease; ALS: amyotrophic lateral sclerosis; Atg: autophagy-related; CMA: chaperone-mediated autophagy; CQ: chloroquine; HsAMBRA1: human AMBRA1; KD: knockdown; KO: knockout; LD: lipid droplet; MMA: methylmalonic acidemia; PD: Parkinson disease; Tg: transgenic.

A preliminary investigation on the mechanism of action of 4-(8-(2-ethylimidazole)octyloxy)-arctigenin against IHNV

Arctigenin derivatives form an elite class of naturally occurring compounds that possess promising antiviral therapeutic perspectives. In a previous study, we design and synthesize a arctigenin derivative, 4-(8-(2-ethylimidazole)octyloxy)-arctigenin (EOA), to evaluate its antiviral activity on infectious hematopoietic necrosis virus (IHNV). In this study, we find that the half maximal inhibitory concentrations (IC₅₀) of EOA on IHNV nucleoprotein (N), phosphoprotein (P), matrix protein (M), nonvirion protein (NV) and polymerase (L) mRNA expression is 0.92, 0.80, 0.98, 0.89 and 0.87 μM, respectively. Mechanistically, our results show that EOA do not damage the viral particles directly, indicating EOA does not possess antiviral activity by destroying virions. Viral binding assays reveal that EOA do not interfere with IHNV adsorption. Because rapamycin has been shown to exhibit anti-IHNV activity by inducing autophagy of epithelioma papulosum cyprini (EPC) cells, we further investigate the relationship between EOA and autophagy in EPC cells. Autophagy fluorescence detection shows that EPC cells have a strong autophagy body after being treated with derivative EOA. The electron microscopy results show that EOA could induce typical autophagosomes which are representative structures of autophagy activation. Moreover, the punctate accumulation of green fluorescence-tagged microtubule-associate protein 1 light chain 3 (LC3) and the protein conversion from LC3-I to LC3-II are respectively confirmed by confocal fluorescence microscopy and western blotting. Overall, these findings demonstrate that EOA plays an anti-IHNV role via inducing autophagy in EPC cells.

Salmonella Effector SpvB Disrupts Intestinal Epithelial Barrier Integrity for Bacterial Translocation

Salmonella are common enteric bacterial pathogens that infect both humans and animals. Intestinal epithelial barrier, formed by a single layer of epithelial cells and apical junctional complex (AJC), plays a crucial role in host defense against enteric pathogens to prevent bacterial translocation. However, the underlying mechanisms of intestinal epithelial barrier dysfunction caused by Salmonella are poorly understood. It is found that a locus termed Salmonella plasmid virulence (spv) gene exists extensively in clinically important Salmonella serovars. SpvB is a key effector encoded within this locus, and closely related to Salmonella pathogenicity such as interfering with autophagy and iron homeostasis. To investigate the interaction between SpvB and intestinal epithelial barrier and elucidate the underlying molecular mechanism, we used the typical foodborne disease agent Salmonella enterica serovar Typhimurium (Salmonella typhimurium) carrying spvB or not to construct infection models in vivo and in vitro. C57BL/6 mice were orally challenged with S. typhimurium wild-type strain SL1344 or spvB-deficient mutant strain SL1344-ΔspvB. Caco-2 cell monolayer model, as a widely used model to mimic the human intestinal epithelium in vitro, was infected with SL1344, SL1344-ΔspvB, or spvB complementary strain SL1344-c-ΔspvB, respectively. The results showed that SpvB enhanced bacterial pathogenicity during S. typhimurium infection in vivo, and contributed to intestinal epithelial barrier dysfunction in both infection systems. This SpvB-mediated barrier dysfunction was attributed to the cellular redistribution of Claudin-1, Occludin, and E-cadherin junctional proteins. Moreover, by using pharmacological inhibitors, we found that F-actin rearrangement and suppression of protein kinase C (PKC) signaling pathway were involved in SpvB-mediated barrier dysfunction. In conclusion, the study reveals the contribution of Salmonella effector SpvB to the dysfunction of intestinal epithelial barrier integrity, which facilitates bacterial translocation via the paracellular route to promote Salmonella systemic dissemination. Our findings broaden the understanding of host–pathogen interactions in salmonellosis, and provide new strategies for the therapy in limiting bacterial dissemination during infection.

Evidence and speculation: the response of Salmonella confronted by autophagy in macrophages

Bacteria of the Salmonella genus cause diseases ranging from self-limited gastroenteritis to typhoid fever. Macrophages are immune cells that engulf and restrict Salmonella. These cells will carry Salmonella into the circulatory system and provoke a systemic infection. Therefore, the interaction between macrophages and intracellular Salmonella is vital for its pathogenicity. As one of the immune responses of macrophages, autophagy, along with the fusion of autophagosomes with lysosomes, occupies an important position in eliminating Salmonella. However, Salmonella that can overcome cellular defensive responses and infect neighboring cells must derive strategies to escape autophagy. This review introduces novel findings on Salmonella and macrophage autophagy as a mechanism against infection and explores the strategies used by Salmonella to escape autophagy.

Salmonella Interacts With Autophagy to Offense or Defense

Autophagy is an important component of the innate immune system in mammals. Low levels of basic autophagy are sustained in normal cells, to help with the clearance of aging organelles and misfolded proteins, thus maintaining their structural and functional stability. However, when cells are faced with challenges, such as starvation or pathogenic infection, their level of autophagy increases significantly. Salmonella is a facultative intracellular pathogen, which imposes an economic burden on the poultry farming industry and human public health. Previous studies have shown that Salmonella can induce the autophagy of cells following invasion, which to a certain extent helps to protect the cells from bacterial colonization. This review summarizes the latest research in the field of Salmonella-induced autophagy, including: (i) the autophagy induction and escape mechanisms employed by Salmonella during the infection of host cells; (ii) the effect of autophagy on intracellular Salmonella; (iii) the important autophagy adaptors that recognize intracellular Salmonella in host cells; and (iv) the effect of autophagy-modulating drugs on Salmonella infection.

Optimal translational fidelity is critical for Salmonella virulence and host interactions

Translational fidelity is required for accurate flow of genetic information, but is frequently altered by genetic changes and environmental stresses. To date, little is known about how translational fidelity affects the virulence and host interactions of bacterial pathogens. Here we show that surprisingly, either decreasing or increasing translational fidelity impairs the interactions of the enteric pathogen Salmonella Typhimurium with host cells and its fitness in zebrafish. Host interactions are mediated by Salmonella pathogenicity island 1 (SPI-1). Our RNA sequencing and quantitative RT-PCR results demonstrate that SPI-1 genes are among the most down-regulated when translational fidelity is either increased or decreased. Further, this down-regulation of SPI-1 genes depends on the master regulator HilD, and altering translational fidelity destabilizes HilD protein via enhanced degradation by Lon protease. Our work thus reveals that optimal translational fidelity is pivotal for adaptation of Salmonella to the host environment, and provides important mechanistic insights into this process.

Neutrophil plays critical role during Edwardsiella piscicida immersion infection in zebrafish larvae

Edwardsiella piscicida is a facultative intracellular pathogen that causes hemorrhagic septicemia and haemolytic ascites disease in aquaculture fish. During bacterial infection, macrophages and neutrophils are the first line of host innate immune system. However, the role of neutrophils in response to E. piscicida infection in vivo remains poorly understood. Here, through developing an immersion infection model in the 5 day-post fertilization (dpf) zebrafish larvae, we found that E. piscicida was mainly colonized in intestine, and resulted into significant pathological changes in paraffin sections. Moreover, a dynamic up-regulation of inflammatory cytokines (TNF-α, IL-1β, GCSFb, CXCL8 and MMP9) was detected in zebrafish larvae during E. piscicida infection. Furthermore, a significant recruitment of neutrophils was observed during the E. piscicida infection in Tg(mpx:eGFP) zebrafish larvae. Thus, we utilized the CRISPR/Cas9 system to generate the neutrophil-knockdown (gcsfr^-/- crispants) larvae, and found a comparative higher mortality and bacterial colonization in gcsfr^-/- crispants, which reveals the critical role of fish neutrophils in bacterial clearance. Taken together, our results developed an effective E. piscicida immersion challenge model in zebrafish larvae to clarify the dynamic of bacterial infection in vivo, which would provide a better understanding of the action about innate immune cells during infection.

Resveratrol-Induced Xenophagy Promotes Intracellular Bacteria Clearance in Intestinal Epithelial Cells and Macrophages

Autophagy is a lysosomal degradation process that contributes to host immunity by eliminating invasive pathogens and the modulating inflammatory response. Several infectious and immune disorders are associated with autophagy defects, suggesting that stimulation of autophagy in these diseases should be beneficial. Here, we show that resveratrol is able to boost xenophagy, a selective form of autophagy that target invasive bacteria. We demonstrated that resveratrol promotes in vitro autophagy-dependent clearance of intracellular bacteria in intestinal epithelial cells and macrophages. These results were validated in vivo using infection in a transgenic GFP-LC3 zebrafish model. We also compared the ability of resveratrol derivatives, designed to improve the bioavailability of the parent molecule, to stimulate autophagy and to induce intracellular bacteria clearance. Together, our data demonstrate the ability of resveratrol to stimulate xenophagy, and thereby enhance the clearance of two invasive bacteria involved life-threatening diseases, Salmonella Typhimurium and Crohn's disease-associated Adherent-Invasive Escherichia coli. These findings encourage the further development of pro-autophagic nutrients to strengthen intestinal homeostasis in basal and infectious states.

Pathogenicity differences of Salmonella enterica serovars Typhimurium, Enteritidis, and Choleraesuis-specific virulence plasmids and clinical S. Choleraesuis strains with large plasmids to the human THP-1 cell death

Salmonella is a common foodborne and zoonotic pathogen. Only a few serovars carry a virulence plasmid (pSV), which enhances the pathogenicity of the host. Here, we investigated the pathogenicity roles of the pSVs among wild-type, plasmid-less, and complemented S. Typhimurium, S. Enteritidis S. Choleraesuis in invasion, phagocytosis, and intracellular bacterial survival in human THP-1 cells and cell death patterns by flow cytometry and difference in cell death patterns between pig and human S. Choleraesuis isolates with large pSCVs. Virulence plasmid (pSTV) led to slightly increasing cellular apoptosis for S. Typhimurium; virulence plasmid (pSEV) enhanced apoptosis and necrosis significantly for S. Enteritidis; and pSCV reduced apoptosis significantly for S. Choleraesuis. After complementation, pSTV increased the intracellular survival of pSCV-less Choleraesuis and the cytotoxicity against human THP-1 cells. Using the Cytochalasin D to differentiate the invasion of S. Choleraaesuis and phagocytosis of THP-1 cells determined that pSCV were responsible for invasion and phagocytosis at 0 h and inhibited intracellular replication in THP-1 cells, and pSTV were responsible for invasion and increased intracellular survival for S. Choleraesuis in THP-1 cells. The human isolates with large pSCV induced more cellular apoptosis and necrosis than the pig isolates. In conclusion, human S. Choleraesuis isolates carrying large pSCVs were more adapted to human THP-1 cells for more cell death than pig isolates with large pSCV. The role of pSVs in invasion, phagocytosis, intracellular survival and apoptosis differed among hosted serovars.

Autophagy induced by infectious hematopoietic necrosis virus inhibits intracellular viral replication and extracellular viral yields in epithelioma papulosum cyprini cell line

Infectious hematopoietic necrosis virus (IHNV) is a common pathogen that causes severe disease in the salmonid aquaculture industry. Recent work demonstrated that autophagy plays an important role in pathogen invasion by activating innate and adaptive immunity. This study investigated the relationship between IHNV and autophagy in epithelioma papulosum cyprini cells. The electron microscopy results show that IHNV infection can induce typical autophagosomes which are representative structures of autophagy activation. The punctate accumulation of green fluorescence-tagged microtubule-associate protein 1 light chain 3 (LC3) and the protein conversion from LC3-I to LC3-II were respectively confirmed by confocal fluorescence microscopy and western blotting. Furthermore, the effects of autophagy on IHNV replication were also clarified by altering the autophagy pathway. The results showed that rapamycin induced autophagy can inhibit both intracellular viral replication and extracellular viral yields, while autophagy inhibitor produced the opposite results. These findings demonstrated that autophagy plays an antiviral role during IHNV infection.

spv locus aggravates Salmonella infection of zebrafish adult by inducing Th1/Th2 shift to Th2 polarization

Salmonella enterica serovar typhimurium (S. typhimurium) are facultative intracellular enteric pathogens causing disease with a broad range of hosts. It was known that Th1-type cytokines such as IFN-γ, IL-12, and TNF-α etc. could induce protective immunity against intracellular pathogens, while Th2-type cytokines such as IL-4, IL-10, and IL-13 etc. are proved to help pathogens survive inside hosts and cause severe infection. One of the critical virulence factor attributes to the pathogenesis of S. typhimurium is Salmonella plasmid virulence genes (spv). Until now, the interaction between spv locus and the predictable generation of Th1 or Th2 immune responses to Salmonella has not been identified. In this study, zebrafish adults were employed to explore the effect of spv locus on Salmonella pathogenesis as well as host adaptive immune responses especially shift of Th1/Th2 balance. The pathological changes of intestines and livers in zebrafish were observed by hematoxylin-eosin (HE) staining and electron microscopy. Levels of the transcription factors of Th1 (Tbx21) and Th2 (GATA3) were measured by real-time quantitative PCR (RT-qPCR). Expression of cytokines were determined by using RT-qPCR and ELISA, respectively. Results showed that spv operon aggravates damage of zebrafish. Furthermore, it demonstrated that spv locus could inhibit the transcription of tbx21 gene and suppress the expression of cytokines IFN-γ, IL-12 and TNF-α. On the contrary, the transcription of gata3 gene could be promoted and the expression of cytokines IL-4, IL-10 and IL-13 were enhanced by spv locus. Taken together, our data revealed that spv locus could aggravate Salmonella infection of zebrafish adult by inducing an imbalance of Th1/Th2 immune response and resulting in a detrimental Th2 bias of host.

Studying Autophagy in Zebrafish

Autophagy is an evolutionarily conserved catabolic process which allows lysosomal degradation of complex cytoplasmic components into basic biomolecules that are recycled for further cellular use. Autophagy is critical for cellular homeostasis and for degradation of misfolded proteins and damaged organelles as well as intracellular pathogens. The role of autophagy in protection against age-related diseases and a plethora of other diseases is now coming to light; assisted by several divergent eukaryotic model systems ranging from yeast to mice. We here give an overview of different methods used to analyse autophagy in zebrafish-a relatively new model for studying autophagy-and briefly discuss what has been done so far and possible future directions.

Lipopolysaccharide-induced autophagy participates in the control of pro-inflammatory cytokine release in grass carp head kidney leukocytes

Microtubule-associated protein 1 light chain 3B (LC3B) is a known marker of autophagy in mammals. In the present study, we isolated and identified grass carp LC3B (gcLC3B) cDNA and found its inductive expression in response to bacterial infection in vivo. To assess the occurrence of autophagy in immune response, the role of gcLC3B as an autophagy marker in grass carp was characterized. Accordingly, grass carp kidney cells (CIKs) with stable expression of GFP-gcLC3B were established and GFP-gcLC3B puncta were counted under a confocal fluorescence microscope. Results showed that starvation, a conventional inducer of autophagy, significantly enhanced GFP-gcLC3B puncta number, indicating the existence of gcLC3B-linked autophagy in fish cells. Moreover, a commercial antibody recognizing gcLC3B and 3-methyladenine (3-MA) were validated in grass carp CIKs, and used to evaluate autophagy in grass carp head kidney leukocytes (HKLs) in response to LPS. Western blotting assay showed that LPS significantly induced the conversion of gcLC3B protein, providing the evidence for autophagy induced by LPS in fish immune cells. Importantly, autophagy inhibition by 3-MA enhanced grass carp IL-1β and TNF-α secretion, indicating the involvement of autophagy in pro-inflammatory cytokine production. Besides, 3-MA could amplify LPS-induced IL-1β and TNF-α release, implying that autophagic induction may drive a mechanism for controlling inflammatory response in fish. Thus, our data highlight the role of autophagy in fish immunity and provide new insight into the mechanism for the regulation of inflammation in fish.

Salmonella spv locus suppresses host innate immune responses to bacterial infection

Salmonella enterica serovar typhimurium (S. typhimurium) is globally distributed and causes massive morbidity and mortality in humans and animals. S. typhimurium carries Salmonella plasmid virulence (spv) locus, which is highly conserved and closely related to bacterial pathogenicity, while its exact role in host immune responses during infection remains to be elucidated. To counteract the invaders, the host has evolved numerous strategies, among which the innate immunity and autophagy act as the first defense. Recently, zebrafish has been universally accepted as a valuable and powerful vertebrate model in analyzing bacteria-host interactions. To investigate whether spv locus enhances the virulence of Salmonella by exerting an effect on the host early defense, zebrafish larvae were employed in this study. LD₅₀ of S. typhimurium to zebrafish larvae and bacterial dissemination were analyzed. Sudan black B and neutral red staining were performed to detect the responses of neutrophils and macrophages to Salmonella infection. Autophagy agonist Torin1 and inhibitor Chloroquine were used to interfere in autophagic flux, and the protein level of Lc3 and p62 were measured by western blotting. Results indicated that spv locus could decrease the LD₅₀ of S. typhimurium to zebrafish larvae, accelerate the reproduction and dissemination of bacteria by inhibiting the function of neutrophils and macrophages. Moreover, spv locus restrained the formation of autophagosomes in the earlier stage of autophagy. These findings suggested the virulence of spv locus involving in suppressing host innate immune responses for the first time, which shed new light on the role of spv operon in Salmonella pathogenicity.