Shigella’ s ways of manipulating the host intestinal innate and adaptive immune system: a tool box for survival?

Systematic review of associations between gut microbiome composition and stunting in under-five children

Childhood stunting is associated with impaired cognitive development and increased risk of infections, morbidity, and mortality. The composition of the enteric microbiota may contribute to the pathogenesis of stunting. We systematically reviewed and synthesized data from studies using high-throughput genomic sequencing methods to characterize the gut microbiome in stunted versus non-stunted children under 5 years in LMICs. We included 14 studies from Asia, Africa, and South America. Most studies did not report any significant differences in the alpha diversity, while a significantly higher beta diversity was observed in stunted children in four out of seven studies that reported beta diversity. At the phylum level, inconsistent associations with stunting were observed for Bacillota, Pseudomonadota, and Bacteroidota phyla. No single genus was associated with stunted children across all 14 studies, and some associations were incongruent by specific genera. Nonetheless, stunting was associated with an abundance of pathobionts that could drive inflammation, such as Escherichia/Shigella and Campylobacter, and a reduction of butyrate producers, including Faecalibacterium, Megasphera, Blautia, and increased Ruminoccoccus. An abundance of taxa thought to originate in the oropharynx was also reported in duodenal and fecal samples of stunted children, while metabolic pathways, including purine and pyrimidine biosynthesis, vitamin B biosynthesis, and carbohydrate and amino acid degradation pathways, predicted linear growth. Current studies show that stunted children can have distinct microbial patterns compared to non-stunted children, which could contribute to the pathogenesis of stunting.

Proteomic analysis and protein structure prediction of Shigella phage Sfk20 based on a comparative study using structure prediction approaches

Bacteriophages are the natural predators of bacteria and are available abundantly everywhere in nature. Lytic phages can specifically infect their bacterial host (through attachment to the receptor) and use their host replication machinery to replicate rapidly, a feature that enables them to kill a disease-causing bacteria. Hence, phage attachment to the host bacteria is the first important step of the infection process. It is reported in this study that the receptor could be an LPS which is responsible for the attachment of the Sfk20 phage to its host (Shigella flexneri 2a). Phage Sfk20 bacteriolytic activity was examined for preliminary optimization of phage titer. The phage Sfk20 viability at different saline conditions was conducted. The LC-MS/MS technique used here for detecting and identifying 40 Sfk20 phage proteins helped us to get an initial understanding of the structural landscape of phage Sfk20. From the identified proteins, six structurally significant proteins were selected for structure prediction using two neural network systems: AlphaFold2 and ESMFold, and one homology modeling software: Phyre2. Later the performance of these modeling systems was compared using various metrics. We conclude from the available and generated information that AlphaFold2 and Phyre2 perform better than ESMFold for predicting Sfk20 phage protein structures.

Age-dependent acquisition of IgG antibodies to Shigella serotypes-a retrospective analysis of seroprevalence in Kenyan children with implications for infant vaccination

Background

Shigellosis mainly affects children under 5 years of age living in low- and middle-income countries, who are the target population for vaccination. There are, however, limited data available to define the appropriate timing for vaccine administration in this age group. Information on antibody responses following natural infection, proxy for exposure, could help guide vaccination strategies.

Methods

We undertook a retrospective analysis of antibodies to five of the most prevalent Shigella serotypes among children aged <5 years in Kenya. Serum samples from a cross-sectional serosurvey in three Kenyan sites (Nairobi, Siaya, and Kilifi) were analyzed by standardized ELISA to measure IgG against Shigella sonnei and Shigella flexneri 1b, 2a, 3a, and 6. We identified factors associated with seropositivity to each Shigella serotype, including seropositivity to other Shigella serotypes.

Results

A total of 474 samples, one for each participant, were analyzed: Nairobi (n = 169), Siaya (n = 185), and Kilifi (n = 120). The median age of the participants was 13.4 months (IQR 7.0-35.6), and the male:female ratio was 1:1. Geometric mean concentrations (GMCs) for each serotype increased with age, mostly in the second year of life. The overall seroprevalence of IgG antibodies increased with age except for S. flexneri 6 which was high across all age subgroups. In the second year of life, there was a statistically significant increase of antibody GMCs against all five serotypes (p = 0.01-0.0001) and a significant increase of seroprevalence for S. flexneri 2a (p = 0.006), S. flexneri 3a (p = 0.006), and S. sonnei (p = 0.05) compared with the second part of the first year of life. Among all possible pairwise comparisons of antibody seropositivity, there was a significant association between S. flexneri 1b and 2a (OR = 6.75, 95% CI 3-14, p < 0.001) and between S. flexneri 1b and 3a (OR = 23.85, 95% CI 11-54, p < 0.001).

Conclusion

Children living in low- and middle-income settings such as Kenya are exposed to Shigella infection starting from the first year of life and acquire serotype-specific antibodies against multiple serotypes. The data from this study suggest that Shigella vaccination should be targeted to infants, ideally at 6 or at least 9 months of age, to ensure children are protected in the second year of life when exposure significantly increases.

Molecular Signature of Monocytes Shaped by the Shigella sonnei 1790-Generalized Modules for Membrane Antigens Vaccine

Shigellosis, an acute gastroenteritis infection caused by Shigella species, remains a public health burden in developing countries. Recently, many outbreaks due to Shigella sonnei multidrug-resistant strains have been reported in high-income countries, and the lack of an effective vaccine represents a major hurdle to counteract this bacterial pathogen. Vaccine candidates against Shigella sonnei are under clinical development, including a Generalized Modules for Membrane Antigens (GMMA)-based vaccine. The mechanisms by which GMMA-based vaccines interact and activate human immune cells remain elusive. Our previous study provided the first evidence that both adaptive and innate immune cells are targeted and functionally shaped by the GMMA-based vaccine. Here, flow cytometry and confocal microscopy analysis allowed us to identify monocytes as the main target population interacting with the S. sonnei 1790-GMMA vaccine on human peripheral blood. In addition, transcriptomic analysis of this cell population revealed a molecular signature induced by 1790-GMMA mostly correlated with the inflammatory response and cytokine-induced processes. This also impacts the expression of genes associated with macrophages' differentiation and T cell regulation, suggesting a dual function for this vaccine platform both as an antigen carrier and as a regulator of immune cell activation and differentiation.

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.

Differential invasiveness & expression of antimicrobial peptides in Shigella serotypes

Background & objectives

The study of Shigella pathogenesis at present is severely hampered by the lack of a relevant animal model that replicates human bacillary dysentery. Different Shigella serogroups cause varying severity of clinical illness. Ex vivo colonization of Shigella flexneri, S. dysenteriae and S. sonnei were characterized in human paediatric colonic pinch biopsies in the in vitro organ culture (IVOC) model to study the invasiveness of Shigella by gentamicin protection assay (GPA). Furthermore, the expression of antimicrobial peptides (AMPs) in response to different serotypes of Shigella was also studied in IVOC model.

Methods

IVOC explants were inoculated with 109 colony forming units of different serotypes of Shigella and recovery of bacteria studied. Histopathological analysis was carried out to study inflammatory immune responses. GPA was done to elucidate the invasiveness of different serotypes of Shigella. Secretions of AMPs were measured by enzyme-linked immunosorbent assay (ELISA). Western blotting was performed to check the expression of AMPs and nuclear factor kappa B in IVOC explants.

Results

After 24 h post-infection, the colon biopsies showed intense inflammatory reaction. In both IVOC and GPA, S. dysenteriae 1 was the most invasive as compared to S. flexneri and S. sonnei. S. sonnei was the least invasive. ELISA demonstrated that S. sonnei dampened the HBD (human β-defensin)-2 responses whereas there was augmentation by S. dysenteriae and there was a modest but non-significant increase by S. flexneri. A modest increase in HBD-3 by S. sonnei and S. flexneri was observed but was not found to be significant. However, western blotting data showed upregulation of all AMPs by all serotypes. Western blotting is more sensitive than ELISA.

Interpretation & conclusions

In the present study, differences in invasiveness and AMP production induced by different serotypes of Shigella were found. Human intestinal IVOC represents a model system to investigate early interaction between pathogenic bacteria and the human gut.

The metabolic impact of bacterial infection in the gut

Bacterial infections of the gut are one of the major causes of morbidity and mortality worldwide. The interplay between the pathogen and the host is finely balanced, with the bacteria evolving to proliferate and establish infection. In contrast, the host mounts a response to first restrict and then eliminate the infection. The intestine is a rapidly proliferating tissue, and metabolism is tuned to cater to the demands of proliferation and differentiation along the crypt-villus axis (CVA) in the gut. As bacterial pathogens encounter the intestinal epithelium, they elicit changes in the host cell, and core metabolic pathways such as the tricarboxylic acid (TCA) cycle, lipid metabolism and glycolysis are affected. This review highlights the mechanisms utilized by diverse gut bacterial pathogens to subvert host metabolism and describes host responses to the infection.

Fatty Acids Abolish Shigella Virulence by Inhibiting Its Master Regulator, VirF

The pathogenicity of Shigella, the intracellular pathogen responsible for human bacillary dysentery, depends on a coordinated and tightly regulated expression of its virulence determinants. This is the result of a cascade organization of its positive regulators, with VirF, a transcriptional activator belonging to the AraC-XylS family, in a pivotal position. VirF itself is submitted to several well-known regulations at the transcriptional level. In this work, we present evidence for a novel posttranslational regulatory mechanism of VirF mediated by the inhibitory interaction with specific fatty acids. By homology modeling and molecular docking analyses, we identify a jelly roll motif in the structure of ViF capable of interacting with medium-chain saturated and long-chain unsaturated fatty acids. In vitro and in vivo assays show that capric, lauric, myristoleic, palmitoleic, and sapienic acids interact effectively with the VirF protein, abolishing its transcription-promoting activity. This silences the virulence system of Shigella, leading to a drastic reduction in its ability to invade epithelial cells and proliferate in their cytoplasm. IMPORTANCE In the absence of a valid vaccine, the main therapeutic approach currently used to treat shigellosis is based on the use of antibiotics. The emergence of antibiotic resistance jeopardizes the future effectiveness of this approach. The importance of the present work resides both in the identification of a new level of posttranslational regulation of the Shigella virulence system and in the characterization of a mechanism offering new opportunities for the design of antivirulence compounds, which may change the treatment paradigm of Shigella infections by limiting the emergence of antibiotic-resistant bacteria.

Toward a Shigella Vaccine: Opportunities and Challenges to Fight an Antimicrobial-Resistant Pathogen

Shigellosis causes more than 200,000 deaths worldwide and most of this burden falls on Low- and Middle-Income Countries (LMICs), with a particular incidence in children under 5 years of age. In the last decades, Shigella has become even more worrisome because of the onset of antimicrobial-resistant strains (AMR). Indeed, the WHO has listed Shigella as one of the priority pathogens for the development of new interventions. To date, there are no broadly available vaccines against shigellosis, but several candidates are being evaluated in preclinical and clinical studies, bringing to light very important data and information. With the aim to facilitate the understanding of the state-of-the-art of Shigella vaccine development, here we report what is known about Shigella epidemiology and pathogenesis with a focus on virulence factors and potential antigens for vaccine development. We discuss immunity after natural infection and immunization. In addition, we highlight the main characteristics of the different technologies that have been applied for the development of a vaccine with broad protection against Shigella.

Bifidobacterium animalis subsp. lactis BL-99 ameliorates colitis-related lung injury in mice by modulating short-chain fatty acid production and inflammatory monocytes/macrophages

Pulmonary inflammation as one of the extraintestinal manifestations of ulcerative colitis (UC) has attracted extensive attention, and its pathogenesis is closely related to gut dysbiosis. Bifidobacterium animalis subsp. lactis BL-99 (BL-99) can alleviate osteoporosis caused by UC, but less research has been done on other extraintestinal manifestations (EIM) caused by UC. This study aimed to explore the role and potential mechanisms of BL-99 on DSS-induced pulmonary complications in colitis mice. The results showed that BL-99 decreased weight loss, disease activity index score, colonic pathology score, and the production of pro-inflammatory cytokines (e.g., TNF-α, IL-1β, and IL-6) in colitis mice. BL-99 also alleviated DSS-induced lung pathological damage by suppressing the infiltration of pro-inflammatory cytokines, inflammatory monocytes, and macrophages. Furthermore, 16S rRNA gene sequencing showed lower abundances of several potentially pathogenic bacteria (e.g., Burkholderia, Shigella, and Clostridium perfringens) and enrichment in specific beneficial bacteria (e.g., Adlercreutzia and Bifidobacterium animalis) in colitis mice with BL-99 treatment. Targeted metabolomics suggested that BL-99 intervention promoted the production of intestinal acetate and butyrate. Finally, we observed that the pulmonary expression of primary acetate and butyrate receptors, including FFAR2, FFAR3, and, GPR109a, was up-regulated in BL-99-treated mice, which negatively correlated with inflammatory monocytes and macrophages. Altogether, these results suggest that BL-99 might be utilized as a probiotic intervention to prevent the incidence of colitis-related lung injury owing to its ability to shape the intestinal microbiota and suppress inflammation.

Immune evasion and persistence in enteric bacterial pathogens

The major function of the mammalian immune system is to prevent and control infections caused by enteropathogens that collectively have altered human destiny. In fact, as the gastrointestinal tissues are the major interface of mammals with the environment, up to 70% of the human immune system is dedicated to patrolling them The defenses are multi-tiered and include the endogenous microflora that mediate colonization resistance as well as physical barriers intended to compartmentalize infections. The gastrointestinal tract and associated lymphoid tissue are also protected by sophisticated interleaved arrays of active innate and adaptive immune defenses. Remarkably, some bacterial enteropathogens have acquired an arsenal of virulence factors with which they neutralize all these formidable barriers to infection, causing disease ranging from mild self-limiting gastroenteritis to in some cases devastating human disease.

Mucosal Immune Profiles Associated with Diarrheal Disease Severity in Shigella- and Enteropathogenic Escherichia coli-Infected Children Enrolled in the Global Enteric Multicenter Study

Enteropathogenic Escherichia coli (EPEC) and Shigella are etiologic agents of diarrhea in children <5 years old living in resource-poor countries. Repeated bouts of infection lead to lifelong morbidity and even death. The goal of this study was to characterize local mucosal immune responses in Shigella- and EPEC-infected children <5 years of age with moderate to severe diarrhea (MSD) enrolled in the Global Enteric Multicenter Study (GEMS). We hypothesized that infection with each of these pathogens would induce distinct gut mucosal immune profiles indicative of disease etiology and severity. To test this hypothesis, innate and adaptive immune markers were measured in stools from children with diarrhea due to EPEC, Shigella, or other organisms and in children who had no diarrhea. Shigella-positive diarrhea evoked robust proinflammatory and T_H1/T_H2 cytokine responses compared to diarrhea caused by EPEC or other organisms, with the exception of interleukin 5 (IL-5), which was associated with EPEC infection. The presence of IL-1β, IL-4, IL-16, and tumor necrosis factor beta (TNF-β) was associated with the absence of dysentery. EPEC-positive diarrhea evoked high levels of IL-1β, vascular endothelial growth factor (VEGF), and IL-10. Granulocyte-macrophage colony-stimulating factor (GM-CSF) had opposing roles in disease severity, being associated with absence of diarrhea in EPEC-infected children and with dysenteric Shigella infection. High levels of antigen-specific antibodies were detected in the controls and children with Shigella without dysentery, which suggests a protective role against severe disease. In summary, this study identified distinct local immune responses associated with two clinically relevant diarrheagenic pathogens, Shigella and EPEC, in children and identified protective immune phenotypes that can inform the development of preventive measures.

Genomic and proteomic characterization of two strains of Shigella flexneri 2 isolated from infants' stool samples in Argentina

Background

Shigella specie is a globally important intestinal pathogen disseminated all over the world. In this study we analyzed the genome and the proteomic component of two Shigella flexneri 2a clinical isolates, collected from pediatric patients with gastroenteritis of the Northwest region of Argentina (NWA) in two periods of time, with four years of difference. Our goal was to determine putative changes at molecular levels occurred during these four years, that could explain the presence of this Shigella`s serovar as the prevalent pathogen in the population under study.

Results

As previously reported, our findings support the idea of Shigella has a conserved “core” genome, since comparative studies of CI133 and CI172 genomes performed against 80 genomes obtained from the NCBI database, showed that there is a large number of genes shared among all of them. However, we observed that CI133 and CI172 harbors a small number of strain-specific genes, several of them present in mobile genetic elements, supporting the hypothesis that these isolates were established in the population by horizontal acquisition of genes. These differences were also observed at proteomic level, where it was possible to detect the presence of certain secreted proteins in a culture medium that simulates the host environment.

Conclusion

Great similarities were observed between the CI133 and CI172 strains, confirming the high percentage of genes constituting the “core” genome of S. flexneri 2. However, numerous strain specific genes were also determined. The presence of the here identified molecular elements into other strain of our culture collation, is currently used to develop characteristic markers of local pathogens. In addition, the most outstanding result of this study was the first description of a S. flexneri 2 producing Colicin E, as one of the characteristics that allows S. flexneri 2 to persist in the microbial community. These findings could also contribute to clarify the mechanism and the evolution strategy used by this pathogen to specifically colonize, survive, and cause infection within the NWA population.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08711-5.

Molecular mechanisms of Shigella effector proteins: a common pathogen among diarrheic pediatric population

Different gastrointestinal pathogens cause diarrhea which is a very common problem in children aged under 5 years. Among bacterial pathogens, Shigella is one of the main causes of diarrhea among children, and it accounts for approximately 11% of all deaths among children aged under 5 years. The case-fatality rates for Shigella among the infants and children aged 1 to 4 years are 13.9% and 9.4%, respectively. Shigella uses unique effector proteins to modulate intracellular pathways. Shigella cannot invade epithelial cells on the apical site; therefore, it needs to pass epithelium through other cells rather than the epithelial cell. After passing epithelium, macrophage swallows Shigella, and the latter should prepare itself to exhibit at least two types of responses: (I) escaping phagocyte and (II) mediating invasion of and injury to the recurrent PMN. The presence of PMN and invitation to a greater degree resulted in gut membrane injuries and greater bacterial penetration. Infiltration of Shigella to the basolateral space mediates (A) cell attachment, (B) cell entry, (C) evasion of autophagy recognition, (D) vacuole formation and and vacuole rapture, (E) intracellular life, (F) Shiga toxin, and (G) immune response. In this review, an attempt is made to explain the role of each factor in Shigella infection.

The AraC/XylS Protein MxiE and Its Coregulator IpgC Control a Negative Feedback Loop in the Transcriptional Cascade That Regulates Type III Secretion in Shigella flexneri

Members of the AraC family of transcriptional regulators (AFTRs) control the expression of many genes important to cellular processes, including virulence. In Shigella species, the type III secretion system (T3SS), a key determinant for host cell invasion, is regulated by the three-tiered VirF/VirB/MxiE transcriptional cascade. Both VirF and MxiE belong to the AFTRs and are characterized as positive transcriptional regulators. Here, we identify a novel regulatory activity for MxiE and its coregulator IpgC, which manifests as a negative feedback loop in the VirF/VirB/MxiE transcriptional cascade. Our findings show that MxiE and IpgC downregulate the virB promoter and, hence, VirB protein production, thus decreasing VirB-dependent promoter activity at ospD1, one of the nearly 50 VirB-dependent genes. At the virB promoter, regions required for negative MxiE- and IpgC-dependent regulation were mapped and found to be coincident with regions required for positive VirF-dependent regulation. In tandem, negative MxiE- and IpgC-dependent regulation of the virB promoter only occurred in the presence of VirF, suggesting that MxiE and IpgC can function to counter VirF activation of the virB promoter. Lastly, MxiE and IpgC do not downregulate another VirF-activated promoter, icsA, demonstrating that this negative feedback loop targets the virB promoter. Our study provides insight into a mechanism that may reprogram Shigella virulence gene expression following type III secretion and provides the impetus to examine if MxiE and IpgC homologs in other important bacterial pathogens, such as Burkholderia pseudomallei and Salmonella enterica serovars Typhimurium and Typhi, coordinate similar negative feedback loops. IMPORTANCE The large AraC family of transcriptional regulators (AFTRs) control virulence gene expression in many bacterial pathogens. In Shigella species, the AraC/XylS protein MxiE and its coregulator IpgC positively regulate the expression of type III secretion system genes within the three-tiered VirF/VirB/MxiE transcriptional cascade. Our findings suggest a negative feedback loop in the VirF/VirB/MxiE cascade, in which MxiE and IpgC counter VirF-dependent activation of the virB promoter, thus making this the first characterization of negative MxiE- and IpgC-dependent regulation. Our study provides insight into a mechanism that likely reprograms Shigella virulence gene expression following type III secretion, which has implications for other important bacterial pathogens with functional homologs of MxiE and IpgC.

The tale of the traveling cheese: Shigella in a lung transplant patient

Shigellae are Gram-negative, nonmotile, facultatively anaerobic, non-spore-forming rods. Shigella is a common cause of gastroenteritis in areas of overcrowding and poor sanitation, but is seen less frequently in the developed world. Infection is mainly acquired through the fecal-oral route, but consumption of unpasteurized dairy remains a high risk for transmission. In the developing world, Shigella is a childhood illness and with adequate hydration is fairly self-limiting. The use of antibiotics depends on the severity of illness, the age of the patient and immune status. In immunocompromised patients, chronic symptomatic or relapsing infection has been described. In this report, we describe a case of a lung transplant patient, one year out of his transplant, on triple immunosuppressive therapy, who presented with septic shock secondary to Shigella gastroenteritis after ingesting unpasteurized cheese brought back from Peru. This case highlights the importance of educating transplant patients on how to reduce certain harmful exposures that may be fatal in immunosuppressed individuals.

A Comprehensive Computational Investigation into the Conserved Virulent Proteins of Shigella species Unveils Potential Small-Interfering RNA Candidates as a New Therapeutic Strategy against Shigellosis

Shigella species account for the second-leading cause of deaths due to diarrheal diseases among children of less than 5 years of age. The emergence of multi-drug-resistant Shigella isolates and the lack of availability of Shigella vaccines have led to the pertinence in the efforts made for the development of new therapeutic strategies against shigellosis. Consequently, designing small-interfering RNA (siRNA) candidates against such infectious agents represents a novel approach to propose new therapeutic candidates to curb the rampant rise of anti-microbial resistance in such pathogens. In this study, we analyzed 264 conserved sequences from 15 different conserved virulence genes of Shigella sp., through extensive rational validation using a plethora of first-generation and second-generation computational algorithms for siRNA designing. Fifty-eight siRNA candidates were obtained by using the first-generation algorithms, out of which only 38 siRNA candidates complied with the second-generation rules of siRNA designing. Further computational validation showed that 16 siRNA candidates were found to have a substantial functional efficiency, out of which 11 siRNA candidates were found to be non-immunogenic. Finally, three siRNA candidates exhibited a sterically feasible three-dimensional structure as exhibited by parameters of nucleic acid geometry such as: the probability of wrong sugar puckers, bad backbone confirmations, bad bonds, and bad angles being within the accepted threshold for stable tertiary structure. Although the findings of our study require further wet-lab validation and optimization for therapeutic use in the treatment of shigellosis, the computationally validated siRNA candidates are expected to suppress the expression of the virulence genes, namely: IpgD (siRNA 9) and OspB (siRNA 15 and siRNA 17) and thus act as a prospective tool in the RNA interference (RNAi) pathway. However, the findings of our study require further wet-lab validation and optimization for regular therapeutic use for treatment of shigellosis.

The effect of Lacticaseibacillus casei on inflammatory cytokine (IL-8) gene expression induced by exposure to Shigella sonnei in Zebrafish (Danio rerio)

ABSTRACT This study aimed to evaluate the protective function of probiotics against Shigella sonnei pathogenicity. For this purpose, 400 zebrafish were divided into four groups with two replications: (T1): receiving Lacticaseibacillus casei for 27 days, (T2): receiving L. casei for 27 days followed by 72 hr exposure to S. sonnei, (T3): receiving basal diet for 27 days followed by 72 hr exposure to S. sonnei, and control group (C): receiving basal diet without exposure to the pathogen. According to the results, feeding with L. casei for 27 days reduced the interleukin-8 (IL-8) expression significantly (P<0.05). The results showed a decrease in IL-8 expression in the group exposed to the pathogen and fed with the probiotic compared to the group only fed with the basal diet (P<0.05). Considering the role of IL-8 as a pro-inflammatory cytokine, our results indicated that feeding with L. casei could modulate inflammatory responses.

Phosphomimetic Tyrosine Mutations in Spa47 Inhibit Type Three Secretion ATPase Activity and Shigella Virulence Phenotype

Shigella is a highly infectious human pathogen responsible for 269 million infections and 200,000 deaths per year. Shigella virulence is absolutely reliant on the injection of effector proteins into the host cell cytoplasm via its type three secretion system (T3SS). The protein Spa47 is a T3SS ATPase whose activity is essential for the proper function of the Shigella T3SS needle-like apparatus through which effectors are secreted. A phosphoproteomics study recently found several Shigella T3SS proteins, including Spa47, to be tyrosine phosphorylated, suggesting a means of regulating Spa47 enzymatic activity, T3SS function, and overall Shigella virulence. The work presented here employs phosphomimetic mutations in Spa47 to probe the effects of phosphorylation at these targeted tyrosines through in vitro radiometric ATPase assays and circular dichroism as well as in vivo characterization of T3SS secretion activity, erythrocyte hemolysis, and cellular invasion. Results presented here demonstrate a direct correlation between Spa47 tyrosine phosphorylation state, Spa47 ATPase activity, T3SS function, and Shigella virulence. Together, these findings provide a strong foundation that leads the way to uncovering the specific pathway(s) that Shigella employ to mitigate wasteful ATP hydrolysis and effector protein secretion when not required as well as T3SS activation in preparation for host infection and immune evasion.

Isolation, phenotypic characterization and comparative genomic analysis of 2019SD1, a polyvalent enterobacteria phage

Shigella has the remarkable capability to acquire antibiotic resistance rapidly thereby posing a significant public health challenge for the effective treatment of dysentery (Shigellosis). The phage therapy has been proven as an effective alternative strategy for controlling Shigella infections. In this study, we illustrate the isolation and detailed characterization of a polyvalent phage 2019SD1, which demonstrates lytic activity against Shigella dysenteriae, Escherichia coli, Vibrio cholerae, Enterococcus saccharolyticus and Enterococcus faecium. The newly isolated phage 2019SD1 shows adsorption time < 6 min, a latent period of 20 min and burst size of 151 PFU per bacterial cell. 2019SD1 exhibits considerable stability in a wide pH range and survives an hour at 50 °C. Under transmission electron microscope, 2019SD1 shows an icosahedral capsid (60 nm dia) and a 140 nm long tail. Further, detailed bioinformatic analyses of whole genome sequence data obtained through Oxford Nanopore platform revealed that 2019SD1 belongs to genus Hanrivervirus of subfamily Tempevirinae under the family Drexlerviridae. The concatenated protein phylogeny of 2019SD1 with the members of Drexlerviridae taking four genes (DNA Primase, ATP Dependent DNA Helicase, Large Terminase Protein, and Portal Protein) using the maximum parsimony method also suggested that 2019SD1 formed a distinct clade with the closest match of the taxa belonging to the genus Hanrivervirus. The genome analysis data indicate the occurrence of putative tail fiber proteins and DNA methylation mechanism. In addition, 2019SD1 has a well-established anti-host defence system as suggested through identification of putative anti-CRISPR and anti-restriction endonuclease systems thereby also indicating its biocontrol potential.

YhjC is a novel transcriptional regulator required for Shigella flexneri virulence

Shigella is an intracellular pathogen that primarily infects the human colon and causes shigellosis. Shigella virulence relies largely on the type III secretion system (T3SS) and secreted effectors. VirF, the master Shigella virulence regulator, is essential for the expression of T3SS-related genes. In this study, we found that YhjC, a LysR-type transcriptional regulator, is required for Shigella virulence through activating the transcription of virF. Pathogenicity of the yhjC mutant, including colonization in the colons of guinea pigs as well as its ability for host cell adhesion and invasion, was significantly lowered. Expression levels of virF and nearly all VirF-dependent genes were downregulated by yhjC deletion, indicating that YhjC can activate virF transcription. Electrophoretic mobility shift assay analysis demonstrated that YhjC could bind directly to the virF promoter region. Therefore, YhjC is a novel virulence regulator that positively regulates the virF expression and promotes Shigella virulence. Additionally, genome-wide expression analysis identified the presence of other genes in the large virulence plasmid and a genome exhibiting differential expression in response to yhjC deletion, with 169 downregulated and 99 upregulated genes, indicating that YhjC also functioned as a global regulatory factor.

Bacterial Manipulation of the Integrated Stress Response: A New Perspective on Infection

Host immune activation forms a vital line of defence against bacterial pathogenicity. However, just as hosts have evolved immune responses, bacteria have developed means to escape, hijack and subvert these responses to promote survival. In recent years, a highly conserved group of signalling cascades within the host, collectively termed the integrated stress response (ISR), have become increasingly implicated in immune activation during bacterial infection. Activation of the ISR leads to a complex web of cellular reprogramming, which ultimately results in the paradoxical outcomes of either cellular homeostasis or cell death. Therefore, any pathogen with means to manipulate this pathway could induce a range of cellular outcomes and benefit from favourable conditions for long-term survival and replication. This review aims to outline what is currently known about bacterial manipulation of the ISR and present key hypotheses highlighting areas for future research.

An antibody targeting type III secretion system induces broad protection against Salmonella and Shigella infections

Salmonella and Shigella bacteria are food- and waterborne pathogens that are responsible for enteric infections in humans and are still the major cause of morbidity and mortality in the emerging countries. The existence of multiple Salmonella and Shigella serotypes as well as the emergence of strains resistant to antibiotics requires the development of broadly protective therapies. Recently, the needle tip proteins of the type III secretion system of these bacteria were successfully utilized (SipD for Salmonella and IpaD for Shigella) as vaccine immunogens to provide good prophylactic cross-protection in murine models of infections. From these experiments, we have isolated a cross-protective monoclonal antibody directed against a conserved region of both proteins. Its conformational epitope determined by Deep Mutational Scanning is conserved among needle tip proteins of all pathogenic Shigella species and Salmonella serovars, and are well recognized by this antibody. Our study provides the first in vivo experimental evidence of the importance of this common region in the mechanism of virulence of Salmonella and Shigella and opens the way to the development of cross-protective therapeutic agents.

Salmonella and Shigella are responsible for gastrointestinal diseases and continue to remain a serious health hazard in South and South-East Asia and African countries, even more with the new emergence of multi drug resistances. Developed vaccines are either not commercialized (for Shigella) or cover only a limited number of serotypes (for Salmonella). There is thus a crucial need to develop cross-protective therapies. By targeting proteins SipD and IpaD belonging respectively to the injectisome of Salmonella and Shigella and necessary to their virulence, we have shown that a monoclonal antibody (mAb) directed against a conserved common region of their apical part provides good cross-protection prophylactic efficacy. We have determined the region targeted by this mAb which could explain why it is conserved among Salmonella and Shigella bacteria.

Loss of the virulence plasmid by Shigella sonnei promotes its interactions with CD207 and CD209 receptors

Introduction. Shigella sonnei, the cause of bacillary dysentery, belongs to Gram-negative enteropathogenic bacteria. S. sonnei contains a 210 kb virulence plasmid that encodes an O-antigen gene cluster of LPSs. However, this virulence plasmid is frequently lost during replication. It is well-documented that after losing the O-antigen and becoming rough strains, the Gram-negative bacteria may express an LPS core on its surface. Previous studies have suggested that by using the LPS core, Gram-negative bacteria can interact with several C-type lectin receptors that are expressed on antigen-presenting cells (APCs).Hypothesis/Gap Statement. S. sonnei by losing the virulence plasmid may hijack APCs via the interactions of LPS-CD209/CD207.Aim. This study aimed to investigate if the S. sonnei rough strain, by losing the virulence plasmid, interacted with APCs that express C-type lectins of human CD207, human CD209a and mouse CD209b.Methodology. SDS-PAGE silver staining was used to examine the O-antigen expression of S. sonnei WT and its rough strain. Invasion assays and inhibition assays were used to examine the ability of S. sonnei WT and its rough strain to invade APCs and investigate whether CD209 and CD207 are receptors for phagocytosis of rough S. sonnei. Animal assays were used to observe the dissemination of S. sonnei.Results. S. sonnei did not express O-antigens after losing the virulence plasmid. The S. sonnei rough strain invades with APCs, including human dendritic cells (DCs) and mouse macrophages. CD209 and CD207 are receptors for phagocytosis of rough S. sonnei. Expression of the O-antigen reduces the ability of the S. sonnei rough strain to be disseminated to mesenteric lymph nodes and spleens.Conclusion. This work demonstrated that S. sonnei rough strains - by losing the virulence plasmid - invaded APCs through interactions with CD209 and CD207 receptors.

Dietary purslane (Portulaca oleracea L.) promotes the growth performance of broilers by modulation of gut microbiota

Purslane is a widespread wild vegetable with both medicinal and edible properties. It is highly appreciated for its high nutritional value and is also considered as a high-quality feed resource for livestock and poultry. In this study, Sanhuang broilers were used to investigate the effect of feeding purslane diets on the growth performance in broilers and their gut microbiota. A total of 48 birds with good growth and uniform weight were selected and randomly allocated to four treatment groups A (control), B, C and D. Dietary treatments were fed with basal diet without purslane and diets containing 1%, 2% and 3% purslane. The 16S rDNA was amplified by PCR and sequenced using the Illumina HiSeq platform to analyze the composition and diversity of gut microbiota in the four sets of samples. The results showed that dietary inclusion of 2% and 3% purslane could significantly improve the growth performance and reduce the feed conversion ratio. Microbial diversity analysis indicated that the composition of gut microbiota of Sanhuang broilers mainly included Gallibacterium, Bacteroides and Escherichia-Shigella, etc. As the content of purslane was increased, the abundance of Lactobacillus increased significantly, and Escherichia-Shigella decreased. LEfSe analysis revealed that Bacteroides_caecigallinarum, Lachnospiraceae, Lactobacillales and Firmicutes had significant differences compared with the control group. PICRUSt analysis revealed bacteria mainly enriched in carbohydrate metabolism pathway due to the additon of purslane in the diet. These results suggest that the addition of purslane to feed could increase the abundance of Lactobacillus in intestine, modulate the environment of gut microbiota and promote the metabolism of carbohydrates to improve its growth performance. This study indicates that the effect of purslane on the growth-promoting performance of broilers might depend on its modulation on gut microbiota, so as to provide a certain scientific basis for the application of purslane in the feed industry.

Shigella sonnei: virulence and antibiotic resistance

Shigella sonnei is the emerging pathogen globally, as it is the second common infectious species of shigellosis (bloody diarrhoea) in low- and middle-income countries (LMICs) and the leading one in developed world. The multifactorial processes and novel mechanisms have been identified in S. sonnei, that are collectively playing apart a substantial role in increasing its prevalence, while replacing the S. flexneri and other Gram-negative gut pathogens niche occupancy. Recently, studies suggest that due to improvement in sanitation S. sonnei has reduced cross-immunization from Plesiomonas shigelliodes (having same O-antigen as S. sonnei) and also found to outcompete the two major species of Enterobacteriaceae family (Shigella flexneri and Escherichia coli), due to encoding of type VI secretion system (T6SS). This review aimed to highlight S. sonnei as an emerging pathogen in the light of recent research with pondering aspects on its epidemiology, transmission, and pathogenic mechanisms. Additionally, this paper aimed to review S. sonnei disease pattern and related complications, symptoms, and laboratory diagnostic techniques. Furthermore, the available treatment reigns and antibiotic-resistance patterns of S. sonnei are also discussed, as the ciprofloxacin and fluoroquinolone-resistant S. sonnei has already intensified the global spread and burden of antimicrobial resistance. In last, prevention and controlling strategies are briefed to limit and tackle S. sonnei and possible future areas are also explored that needed more research to unravel the hidden mysteries surrounding S. sonnei.

Human small intestinal organotypic culture model for drug permeation, inflammation, and toxicity assays

The gastrointestinal tract (GIT), in particular, the small intestine, plays a significant role in food digestion, fluid and electrolyte transport, drug absorption and metabolism, and nutrient uptake. As the longest portion of the GIT, the small intestine also plays a vital role in protecting the host against pathogenic or opportunistic microbial invasion. However, establishing polarized intestinal tissue models in vitro that reflect the architecture and physiology of the gut has been a challenge for decades and the lack of translational models that predict human responses has impeded research in the drug absorption, metabolism, and drug-induced gastrointestinal toxicity space. Often, animals fail to recapitulate human physiology and do not predict human outcomes. Also, certain human pathogens are species specific and do not infect other hosts. Concerns such as variability of results, a low throughput format, and ethical considerations further complicate the use of animals for predicting the safety and efficacy xenobiotics in humans. These limitations necessitate the development of in vitro 3D human intestinal tissue models that recapitulate in vivo–like microenvironment and provide more physiologically relevant cellular responses so that they can better predict the safety and efficacy of pharmaceuticals and toxicants. Over the past decade, much progress has been made in the development of in vitro intestinal models (organoids and 3D-organotypic tissues) using either inducible pluripotent or adult stem cells. Among the models, the MatTek’s intestinal tissue model (EpiIntestinal™ Ashland, MA) has been used extensively by the pharmaceutical industry to study drug permeation, metabolism, drug-induced GI toxicity, pathogen infections, inflammation, wound healing, and as a predictive model for a clinical adverse outcome (diarrhea) to pharmaceutical drugs. In this paper, our review will focus on the potential of in vitro small intestinal tissues as preclinical research tool and as alternative to the use of animals.

Characterization of ShigETEC, a Novel Live Attenuated Combined Vaccine against Shigellae and ETEC

Background: Shigella spp. and enterotoxigenic Escherichia coli (ETEC) remain the two leading bacterial causes of diarrheal diseases worldwide. Attempts to develop preventive vaccines against Shigella and ETEC have not yet been successful. The major challenge for a broad Shigella vaccine is the serotype-specific immune response to the otherwise protective LPS O-antigen. ETEC vaccines mainly rely on the heat-labile enterotoxin (LT), while heat-stable toxin (ST) has also been shown to be an important virulence factor. Methods: We constructed a combined Shigella and ETEC vaccine (ShigETEC) based on a live attenuated Shigella strain rendered rough and non-invasive with heterologous expression of two ETEC antigens, LTB and a detoxified version of ST (ST_N12S). This new vaccine strain was characterized and tested for immunogenicity in relevant animal models. Results: Immunization with ShigETEC resulted in serotype independent protection in the mouse lung shigellosis model and induced high titer IgG and IgA antibodies against bacterial lysates, and anti-ETEC toxin antibodies with neutralizing capacity. Conclusions: ShigETEC is a promising oral vaccine candidate against Shigella and ETEC infections and currently in Phase 1 testing.

T Cell Immunity to Bacterial Pathogens: Mechanisms of Immune Control and Bacterial Evasion

The human body frequently encounters harmful bacterial pathogens and employs immune defense mechanisms designed to counteract such pathogenic assault. In the adaptive immune system, major histocompatibility complex (MHC)-restricted αβ T cells, along with unconventional αβ or γδ T cells, respond to bacterial antigens to orchestrate persisting protective immune responses and generate immunological memory. Research in the past ten years accelerated our knowledge of how T cells recognize bacterial antigens and how many bacterial species have evolved mechanisms to evade host antimicrobial immune responses. Such escape mechanisms act to corrupt the crosstalk between innate and adaptive immunity, potentially tipping the balance of host immune responses toward pathological rather than protective. This review examines the latest developments in our knowledge of how T cell immunity responds to bacterial pathogens and evaluates some of the mechanisms that pathogenic bacteria use to evade such T cell immunosurveillance, to promote virulence and survival in the host.

Novel Noncompetitive Type Three Secretion System ATPase Inhibitors Shut Down Shigella Effector Secretion

Shigella is the causative agent of bacillary dysentery and is responsible for an estimated 165 million infections and 600,000 deaths annually. Like many Gram-negative pathogens, Shigella relies on a type three secretion system (T3SS) to initiate and sustain infection by directly injecting effector proteins into host cells. Protein secretion through the needle-like injectisome and overall Shigella virulence rely on the T3SS ATPase Spa47, making it a likely means for T3SS regulation and an attractive target for therapeutic small molecule inhibitors. Here, we utilize a recently solved 2.15 Å crystal structure of Spa47 to computationally screen 7.6 million drug-like compounds for candidates which avoid the highly conserved active site by targeting a distal, but critical, interface between adjacent protomers of the Spa47 homohexamer. Ten of the top inhibitor candidates were characterized, identifying novel Spa47 inhibitors that reduce in vitro ATPase activity by as much as 87.9 ± 10.5% with IC₅₀'s as low as 25 ± 20 μM and reduce in vivo Shigella T3SS protein secretion by as much as 94.7 ± 3.0%. Kinetic analyses show that the inhibitors operate through a noncompetitive mechanism that likely supports the inhibitors' low cytotoxicity, as they avoid off-target ATPases involved in either Shigella or mammalian cell metabolism. Interestingly, the inhibitors display nearly identical inhibition profiles for Spa47 and the T3SS ATPases EscN from E. coli and FliI from Salmonella. Together, the results of this study provide much-needed insight into T3SS ATPase inhibition mechanisms and a strong platform for developing broadly effective cross-pathogen T3SS ATPase inhibitors.

SipD and IpaD induce a cross-protection against Shigella and Salmonella infections

Salmonella and Shigella species are food- and water-borne pathogens that are responsible for enteric infections in both humans and animals and are still the major cause of morbidity and mortality in the emerging countries. The existence of multiple Salmonella and Shigella serotypes as well as the emergence of strains resistant to antibiotics require the development of broadly protective therapies. Those bacteria utilize a Type III Secretion System (T3SS), necessary for their pathogenicity. The structural proteins composing the T3SS are common to all virulent Salmonella and Shigella spp., particularly the needle-tip proteins SipD (Salmonella) and IpaD (Shigella). We investigated the immunogenicity and protective efficacy of SipD and IpaD administered by intranasal and intragastric routes, in a mouse model of Salmonella enterica serotype Typhimurium (S. Typhimurium) intestinal challenge. Robust IgG (in all immunization routes) and IgA (in intranasal and oral immunization routes) antibody responses were induced against both proteins. Mice immunized with SipD or IpaD were protected against lethal intestinal challenge with S. Typhimurium or Shigella flexneri (100 Lethal Dose 50%). We have shown that SipD and IpaD are able to induce a cross-protection in a murine model of infection by Salmonella and Shigella. We provide the first demonstration that Salmonella and Shigella T3SS SipD and IpaD are promising antigens for the development of a cross-protective Salmonella-Shigella vaccine. These results open the way to the development of cross-protective therapeutic molecules.

Transcytosis subversion by M cell-to-enterocyte spread promotes Shigella flexneri and Listeria monocytogenes intracellular bacterial dissemination

Microfold (M) cell host-pathogen interaction studies would benefit from the visual analysis of dynamic cellular and microbial interplays. We adapted a human in vitro M cell model to physiological bacterial infections, expression of fluorescent localization reporters and long-term three-dimensional time-lapse microscopy. This approach allows following key steps of M cell infection dynamics at subcellular resolution, from the apical onset to basolateral epithelial dissemination. We focused on the intracellular pathogen Shigella flexneri, classically reported to transcytose through M cells to initiate bacillary dysentery in humans, while eliciting poorly protective immune responses. Our workflow was critical to reveal that S. flexneri develops a bimodal lifestyle within M cells leading to rapid transcytosis or delayed vacuolar rupture, followed by direct actin motility-based propagation to neighboring enterocytes. Moreover, we show that Listeria monocytogenes, another intracellular pathogen sharing a tropism for M cells, disseminates in a similar manner and evades M cell transcytosis completely. We established that actin-based M cell-to-enterocyte spread is the major dissemination pathway for both pathogens and avoids their exposure to basolateral compartments in our system. Our results challenge the notion that intracellular pathogens are readily transcytosed by M cells to inductive immune compartments in vivo, providing a potential mechanism for their ability to evade adaptive immunity.

Microfold (M) epithelial cells are important for the onset of infections and induction of immune responses in many mucosal diseases. We extended a human in vitro M cell model to apical infections, expression of fluorescent host and microbial reporters and real-time fluorescence microscopy. Focusing on the human intracellular pathogen S. flexneri, responsible for bacillary dysentery, this workflow allowed us to uncover that the bacterium can subvert the immunological sampling function of M cells by promoting a cytosolic lifestyle and spreading directly to neighboring enterocytes. This mechanism was shared with the etiologic agent of listeriosis, the intracellular pathogen L. monocytogenes and allowed both pathogens to avoid exposure to underlying immune compartments. These results may provide a mechanism for the ability of intracellular pathogens to evade adaptive immunity in vivo, emphasizing the importance of advanced studies of M cell host-pathogen interactions to understand early steps of mucosal invasion and their consequences on immunity.

High Rates of Antimicrobial Resistance and Virulence Gene Distribution Among Shigella spp. Isolated from Pediatric Patients in Tehran, Iran

Background

Shigella continues to be important causes of acute pediatric diarrhea worldwide. Shigella produces numerous virulence factors involved in colonization and invasion into epithelial cells which eventually result in the disease. The present study was conducted to evaluate the prevalence of virulence genes and to investigate antibiotic resistance profiles among Shigella isolates obtained from pediatric patients in Iran.

Methods

A total of 141 Shigella isolates were collected between March 2017 and September 2018 from stool of children under 14 who were suspected to have shigellosis. Shigella isolates were identified using standard microbiological and serological tests and antimicrobial susceptibility testing was carried out via Kirby–Bauer disk diffusion method. In addition, the presence of seven virulence determinants including ipaH, ipaB, ipaC, ipaD, ipgD, sen, and virA were evaluated using PCR.

Results

S. sonnei (78.7%) was the most prevalent shigella spp. among children with shigellosis followed by S. flexneri (19.9%) and S. boydii (1.4%). Antimicrobial susceptibility testing revealed that most of the isolates were considered as multidrug-resistant (MDR) strains. Our findings also showed a high resistance rate against trimethoprim-sulfamethoxazole in Shigella isolates. The prevalence of ipaH, ipaC, sen, ipaD, virA, ipaB, and ipgD were 100%, 95.7%, 95.7%, 94.3%, 93.6%, 92.9%, and 80.8%, respectively.

Conclusion

The current study revealed that S. sonnei was the predominant species isolated from children with shigellosis in Iran. Our results also indicated a high distribution of type III secretion system effector protein-encoding genes and high multidrug-resistance among shigella spp. in Iran. Therefore, it is suggested that antimicrobial susceptibility testing be performed prior to antibiotic prescription.

A human pathogenic bacterium Shigella proliferates in plants through adoption of type III effectors for shigellosis

Shigella, which infects primates, can be transmitted via fresh vegetables; however, its molecular interactions with plants have not been elucidated. Here, we show that four Shigella strains, Shigella boydii, Shigella sonnei, Shigella flexneri 2a, and S. flexneri 5a, proliferate at different levels in Arabidopsis thaliana. Microscopic studies revealed that these bacteria were present inside leaves and damaged plant cells. Green fluorescent protein (GFP)-tagged S. boydii and S. flexneri 5a colonized leaves only, whereas S. flexneri 2a colonized both leaves and roots. Using Shigella mutants lacking type III secretion systems (T3SSs), we found that T3SSs that regulate the pathogenesis of shigellosis in humans also play a central role in bacterial proliferation in Arabidopsis. Strikingly, the immunosuppressive activity of two T3S effectors, OspF and OspG, was required for proliferation of Shigella in Arabidopsis. Of note, delivery of OspF or OspG effectors inside plant cells upon Shigella inoculation was confirmed using a split GFP system. These findings demonstrate that the human pathogen Shigella can proliferate in plants by adapting immunosuppressive machinery used in the original host human.

Deciphering the Role Played by Autophagy in Leishmania Infection

In recent decades, studies have shown that, depending on parasite species and host background, autophagy can either favor infection or promote parasite clearance. To date, relatively few studies have attempted to assess the role played by autophagy in Leishmania infection. While it has been consistently shown that Leishmania spp. induce autophagy in a variety of cell types, published results regarding the effects of autophagic modulation on Leishmania survival are contradictory. The present review, after a short overview of the general aspects of autophagy, aims to summarize the current body of knowledge surrounding how Leishmania spp. adaptively interact with macrophages, the host cells mainly involved in controlling leishmaniasis. We then explore the scarce studies that have investigated interactions between these parasite species and the autophagic pathway, and finally present a critical perspective on how autophagy influences infection outcome.

Recent insights into Shigella

PURPOSE OF REVIEW

Diarrhoea is a major global health problem, and recent studies have confirmed Shigella as a major contributor to this burden. Here, we review recent advances in Shigella research; focusing on their epidemiology, pathogenesis, antimicrobial resistance, and the role of the gut microbiome during infection.

RECENT FINDINGS

Enhanced epidemiological data, combined with new generation diagnostics, has highlighted a greater burden of Shigella disease than was previously estimated, which is not restricted to vulnerable populations in low-middle income countries. As we gain an ever more detailed insight into the orchestrated mechanisms that Shigella exploit to trigger infection, we can also begin to appreciate the complex role of the gut microbiome in preventing and inducing such infections. The use of genomics, in combination with epidemiological data and laboratory investigations, has unravelled the evolution and spread of various species. Such measures have identified resistance to antimicrobials as a key contributor to the success of specific clones.

SUMMARY

We need to apply novel findings towards sustainable approaches for treating and preventing Shigella infections. Vaccines and alternative treatments are under development and may offer an opportunity to reduce the burden of Shigella disease and restrict the mobility of antimicrobial resistant clones.

Shigella promotes major alteration of gut epithelial physiology and tissue invasion by shutting off host intracellular transport

Intracellular trafficking pathways in eukaryotic cells are essential to maintain organelle identity and structure, and to regulate cell communication with its environment. Shigella flexneri invades and subverts the human colonic epithelium by the injection of virulence factors through a type 3 secretion system (T3SS). In this work, we report the multiple effects of two S. flexneri effectors, IpaJ and VirA, which target small GTPases of the Arf and Rab families, consequently inhibiting several intracellular trafficking pathways. IpaJ and VirA induce large-scale impairment of host protein secretion and block the recycling of surface receptors. Moreover, these two effectors decrease clathrin-dependent and -independent endocytosis. Therefore, S. flexneri infection induces a global blockage of host cell intracellular transport, affecting the exchange between cells and their external environment. The combined action of these effectors disorganizes the epithelial cell polarity, disturbs epithelial barrier integrity, promotes multiple invasion events, and enhances the pathogen capacity to penetrate into the colonic tissue in vivo.

Regulated Delayed Shigella flexneri 2a O-antigen Synthesis in Live Recombinant Salmonella enterica Serovar Typhimurium Induces Comparable Levels of Protective Immune Responses with Constitutive Antigen Synthesis System

Shigella flexneri (S. flexneri), a leading cause of bacillary dysentery, is a major public health concern particularly affecting children in developing nations. We have constructed a novel attenuated Salmonella vaccine system based on the regulated delayed antigen synthesis (RDAS) and regulated delayed expression of attenuating phenotype (RDEAP) systems for delivering the S. flexneri 2a (Sf2a) O-antigen.

Methods: The new Salmonella vaccine platform was constructed through chromosomal integration of the araC P_BAD lacI and araC P_BAD wbaP cassettes, resulting in a gradual depletion of WbaP enzyme. An expression vector, encoding Sf2a O-antigen biosynthesis under the control of the LacI-repressible P_trc promoter, was maintained in the Salmonella vaccine strain through antibiotic-independent selection. Mice immunized with the vaccine candidates were evaluated for cell-mediate and humoral immune responses.

Results: In the presence of exogenous arabinose, the Salmonella vaccine strain synthesized native Salmonella LPS as a consequence of WbaP expression. Moreover, arabinose supported LacI expression, thereby repressing Sf2a O-antigen production. In the absence of arabinose in vivo, native Salmonella LPS synthesis is repressed whilst the synthesis of the Sf2a O-antigen is induced. Murine immunization with the Salmonella vaccine strain elicited robust Sf2a-specific protective immune responses together with long term immunity.

Conclusion: These findings demonstrate the protective efficacy of recombinant Sf2a O-antigen delivered by a Salmonella vaccine platform.

Zinc ameliorates intestinal barrier dysfunctions in shigellosis by reinstating claudin-2 and -4 on the membranes

Whether zinc (Zn²⁺) regulates barrier functions by modulating tight-junction (TJ) proteins when pathogens such as Shigella alter epithelial permeability is still unresolved. We investigated the potential benefits of Zn²⁺ in restoring impaired barrier function in vivo in Shigella-infected mouse tissue and in vitro in T84 cell monolayers. Basolateral Shigella infection triggered a time-dependent decrease in transepithelial resistance followed by an increase in paracellular permeability of FITC-labeled dextran and altered ion selectivity. This led to ion and water loss into the intestinal lumen. Immunofluorescence studies revealed redistribution of claudin-2 and -4 to an intracellular location and accumulation of these proteins in the cytoplasm following infection. Zn²⁺ ameliorated this perturbed barrier by redistribution of claudin-2 and -4 back to the plasma membrane and by modulating the phosphorylation state of TJ proteins t hough extracellular signal-regulated kinase (ERK)1/2 dependency. Zn²⁺ prevents elevation of IL-6 and IL-8. Mice challenged with Shigella showed that oral Zn²⁺supplementation diminished diverse pathophysiological symptoms of shigellosis. Claudin-2 and -4 were susceptible to Shigella infection, resulting in altered barrier function and increased levels of IL-6 and IL-8. Zn²⁺ supplementation ameliorated this barrier dysfunction, and the inflammatory response involving ERK-mediated change of phosphorylation status for claudin-2 and -4. Thus, Zn²⁺ may have potential therapeutic value in inflammatory diarrhea and shigellosis. NEW & NOTEWORTHY Our study addresses whether Zn²⁺ could be an alternative strategy to reduce Shigella-induced inflammatory response and epithelial barrier dysfunction. We have defined a mechanism in terms of intracellular signaling pathways and tight-junction protein expression by Zn²⁺. Claudin-2 and -4 are susceptible to Shigella infection, whereas in the presence of Zn²⁺ they are resistant to infection-related barrier dysfunction involving ERK-mediated change of phosphorylation status of claudins.

Aspherical and Spherical InvA497-Functionalized Nanocarriers for Intracellular Delivery of Anti-Infective Agents

Purpose

The objective of this work was to evaluate the potential of polymeric spherical and aspherical invasive nanocarriers, loaded with antibiotic, to access and treat intracellular bacterial infections.

Methods

Aspherical nanocarriers were prepared by stretching of spherical precursors, and both aspherical and spherical nanocarriers were surface-functionalized with the invasive protein InvA497. The relative uptake of nanocarriers into HEp-2 epithelial cells was then assessed. Nanocarriers were subsequently loaded with a preparation of the non-permeable antibiotic gentamicin, and tested for their ability to treat HEp-2 cells infected with the enteroinvasive bacterium Shigella flexneri.

Results

InvA497-functionalized nanocarriers of both spherical and aspherical shape showed a significantly improved rate and extent of uptake into HEp-2 cells in comparison to non-functionalized nanocarriers. Functionalized and antibiotic-loaded nanocarriers demonstrated a dose dependent killing of intracellular S. flexneri. A slight but significant enhancement of intracellular bacterial killing was also observed with aspherical as compared to spherical functionalized nanocarriers at the highest tested concentration.

Conclusions

InvA497-functionalized, polymer-based nanocarriers were able to efficiently deliver a non-permeable antibiotic across host cell membranes to affect killing of intracellular bacteria. Functionalized nanocarriers with an aspherical shape showed an interesting future potential for intracellular infection therapy.

Electronic supplementary material

The online version of this article (10.1007/s11095-018-2521-3) contains supplementary material, which is available to authorized users.

Human Enteric α-Defensin 5 Promotes Shigella Infection by Enhancing Bacterial Adhesion and Invasion

Shigella is a Gram-negative bacterium that causes bacillary dysentery worldwide. It invades the intestinal epithelium to elicit intense inflammation and tissue damage, yet the underlying mechanisms of its host selectivity and low infectious inoculum remain perplexing. Here, we report that Shigella co-opts human α-defensin 5 (HD5), a host defense peptide important for intestinal homeostasis and innate immunity, to enhance its adhesion to and invasion of mucosal tissues. HD5 promoted Shigella infection in vitro in a structure-dependent manner. Shigella, commonly devoid of an effective host-adhesion apparatus, preferentially targeted HD5 to augment its ability to colonize the intestinal epithelium through interactions with multiple bacterial membrane proteins. HD5 exacerbated infectivity and Shigella-induced pathology in a culture of human colorectal tissues and three animal models. Our findings illuminate how Shigella exploits innate immunity by turning HD5 into a virulence factor for infection, unveiling a mechanism of action for this highly proficient human pathogen.

Enterobacteria and host resistance to infection

Enterobacteriaceae are a large family of Gram-negative, non-spore-forming bacteria. Although many species exist as part of the natural flora of animals including humans, some members are associated with both intestinal and extraintestinal diseases. In this review, we focus on members of this family that have important roles in human disease: Salmonella, Escherichia, Shigella, and Yersinia, providing a brief overview of the disease caused by these bacteria, highlighting the contribution of animal models to our understanding of their pathogenesis and of host genetic determinants involved in susceptibility or resistance to infection.

MxiN Differentially Regulates Monomeric and Oligomeric Species of the Shigella Type Three Secretion System ATPase Spa47

Shigella rely entirely on the action of a single type three secretion system (T3SS) to support cellular invasion of colonic epithelial cells and to circumvent host immune responses. The ATPase Spa47 resides at the base of the Shigella needle-like type three secretion apparatus (T3SA), supporting protein secretion through the apparatus and providing a likely means for native virulence regulation by Shigella and a much needed target for non-antibiotic therapeutics to treat Shigella infections. Here, we show that MxiN is a differential regulator of Spa47 and that its regulatory impact is determined by the oligomeric state of the Spa47 ATPase, with which it interacts. In vitro and in vivo characterization shows that interaction of MxiN with Spa47 requires the six N-terminal residues of Spa47 that are also necessary for stable Spa47 oligomer formation and activation. This interaction with MxiN negatively influences the activity of Spa47 oligomers while upregulating the ATPase activity of monomeric Spa47. Detailed kinetic analyses of monomeric and oligomeric Spa47 in the presence and absence of MxiN uncover additional mechanistic insights into the regulation of Spa47 by MxiN, suggesting that the MxiN/Spa47 species resulting from interaction with monomeric and oligomeric Spa47 are functionally distinct and that both could be involved in Shigella T3SS regulation. Uncovering regulation of Spa47 by MxiN addresses an important gap in the current understanding of how Shigella controls T3SA activity and provides the first description of differential T3SS ATPase regulation by a native T3SS protein.

T cell mediated immunity induced by the live-attenuated Shigella flexneri 2a vaccine candidate CVD 1208S in humans

Background

Shigellosis persists as a public health problem worldwide causing ~ 165,000 deaths every year, of which ~ 55,000 are in children less than 5 years of age. No vaccine against shigellosis is currently licensed. The live-attenuated Shigella flexneri 2a vaccine candidate CVD 1208S (S. flexneri 2a; ΔguaBA, Δset, Δsen) demonstrated to be safe and immunogenic in phase 1 and 2 clinical trials. Earlier reports focused on humoral immunity. However, Shigella is an intracellular pathogen and therefore, T cell mediated immunity (T-CMI) is also expected to play an important role. T-CMI responses after CVD 1208S immunization are the focus of the current study.

Methods

Consenting volunteers were immunized orally (3 doses, 10⁸ CFU/dose, 28 days apart) with CVD 1208S. T-CMI to IpaB was assessed using autologous EBV-transformed B-Lymphocytic cell lines as stimulator cells. T-CMI was assessed by the production of 4 cytokines (IFN-γ, IL-2, IL-17A and TNF-α) and/or expression of the degranulation marker CD107a in 14 volunteers (11 vaccine and 3 placebo recipients).

Results

Following the first immunization, T-CMI was detected in CD8 and CD4 T cells obtained from CVD 1208S recipients. Among CD8 T cells, the T effector memory (T_EM) and central memory (T_CM) subsets were the main cytokine/CD107a producers/expressors. Multifunctional (MF) cells were also detected in CD8 T_EM cells. Cells with 2 and 3 functions were the most abundant. Interestingly, TNF-α appeared to be dominant in CD8 T_EM MF cells. In CD4 T cells, T_EM responses predominated. Following subsequent immunizations, no booster effect was detected. However, production of cytokines/expression of CD107a was detected in individuals who had previously not responded. After three doses, production of at least one cytokine/CD107a was detected in 8 vaccinees (73%) in CD8 T_EM cells and in 10 vaccinees (90%) in CD4 T_EM cells.

Conclusions

CVD 1208S induces diverse T-CMI responses, which likely complement the humoral responses in protection from disease.

Trial registration This study was approved by the Institutional Review Board and registered on ClinicalTrials.gov (identifier NCT01531530)

Electronic supplementary material

The online version of this article (10.1186/s12967-018-1439-1) contains supplementary material, which is available to authorized users.

Use of zebrafish to study Shigella infection

Shigella is a leading cause of dysentery worldwide, responsible for up to 165 million cases of shigellosis each year. Shigella is also recognised as an exceptional model pathogen to study key issues in cell biology and innate immunity. Several infection models have been useful to explore Shigella biology; however, we still lack information regarding the events taking place during the Shigella infection process in vivo Here, we discuss a selection of mechanistic insights recently gained from studying Shigella infection of zebrafish (Danio rerio), with a focus on cytoskeleton rearrangements and cellular immunity. We also discuss how infection of zebrafish can be used to investigate new concepts underlying infection control, including emergency granulopoiesis and the use of predatory bacteria to combat antimicrobial resistance. Collectively, these insights illustrate how Shigella infection of zebrafish can provide fundamental advances in our understanding of bacterial pathogenesis and vertebrate host defence. This information should also provide vital clues for the discovery of new therapeutic strategies against infectious disease in humans.

Innate Recognition of Intracellular Bacterial Growth Is Driven by the TIFA-Dependent Cytosolic Surveillance Pathway

Intestinal epithelial cells (IECs) act as sentinels for incoming pathogens. Cytosol-invasive bacteria, such as Shigella flexneri, trigger a robust pro-inflammatory nuclear factor κB (NF-κB) response from IECs that is believed to depend entirely on the peptidoglycan sensor NOD1. We found that, during Shigella infection, the TRAF-interacting forkhead-associated protein A (TIFA)-dependent cytosolic surveillance pathway, which senses the bacterial metabolite heptose-1,7-bisphosphate (HBP), functions after NOD1 to detect bacteria replicating free in the host cytosol. Whereas NOD1 mediated a transient burst of NF-κB activation during bacterial entry, TIFA sensed HBP released during bacterial replication, assembling into large signaling complexes to drive a dynamic inflammatory response that reflected the rate of intracellular bacterial proliferation. Strikingly, IECs lacking TIFA were unable to discriminate between proliferating and stagnant intracellular bacteria, despite the NOD1/2 pathways being intact. Our results define TIFA as a rheostat for intracellular bacterial replication, escalating the immune response to invasive Gram-negative bacteria that exploit the host cytosol for growth.

Antibiotic resistance and molecular characterization of shigella isolates recovered from children aged less than 5 years in Manhiça, Southern Mozambique

The objective of this study was to assess antibiotic resistance and the molecular epidemiology of shigella isolates from a case-control study of diarrhoea, conducted from 2007 to 2012 in children aged less than 5 years in Manhiça district, southern Mozambique. All isolates were tested for antimicrobial susceptibility using the disc diffusion method. Polymerase chain reaction was used to detect different molecular mechanisms of antibiotic resistance. Serotyping was performed using specific antisera. The clonal relationship of Shigella flexneri and Shigella sonnei was assessed by pulsed-field gel electrophoresis (PFGE). Of the 67 shigella isolates analysed, 59 were diarrhoeal cases and eight were controls. S. flexneri (70.1%; 47/67) was the most common species, followed by S. sonnei (23.9%; 16/67). The most prevalent S. flexneri serotypes were 2a (38.3%; 18/47), 6 (19.2%; 9/47) and 1b (14.9%; 7/47). High rates of antimicrobial resistance were observed for trimethoprim-sulfametoxazole (92.5%; 62/67), tetracycline (68.7%; 46/67), chloramphenicol (53.7%; 36/67) and ampicillin (50.7%; 34/67). Multi-drug resistance (MDR) was present in 55.2% (37/67) of the isolates and was associated with a case fatality rate of 8.1% (3/37). PFGE revealed 22 clones (16 S. flexneri and 6 S. sonnei), among which P1 (31.9%; 15/47), P9 (17%; 8/47) and P2 (10.6%; 5/47) were the most prevalent clones of S. flexneri. In conclusion, S. flexneri was the most prevalent species, with MDR isolates mainly belonging to three specific clones (P1, P9 and P2). The case fatality rate observed among MDR isolates is a matter of concern, indicating the need for appropriate treatment.