Bile-induced biofilm formation in Bacteroides thetaiotaomicron requires magnesium efflux by an RND pump

Bacteroides thetaiotaomicron is a prominent member of the human gut microbiota contributing to nutrient exchange, gut function, and maturation of the host's immune system. This obligate anaerobe symbiont can adopt a biofilm lifestyle, and it was recently shown that B. thetaiotaomicron biofilm formation is promoted by the presence of bile. This process also requires a B. thetaiotaomicron extracellular DNase, which is not, however, regulated by bile. Here, we showed that bile induces the expression of several Resistance-Nodulation-Division (RND) efflux pumps and that inhibiting their activity with a global competitive efflux inhibitor impaired bile-dependent biofilm formation. We then showed that, among the bile-induced RND-efflux pumps, only the tripartite BT3337-BT3338-BT3339 pump, re-named BipABC [for Bile Induced Pump A (BT3337), B (BT3338), and C (BT3339)], is required for biofilm formation. We demonstrated that BipABC is involved in the efflux of magnesium to the biofilm extracellular matrix, which leads to a decrease of extracellular DNA concentration. The release of magnesium in the biofilm matrix also impacts biofilm structure, potentially by modifying the electrostatic repulsion forces within the matrix, reducing interbacterial distance and allowing bacteria to interact more closely and form denser biofilms. Our study therefore, identified a new molecular determinant of B. thetaiotaomicron biofilm formation in response to bile salts and provides a better understanding on how an intestinal chemical cue regulates biofilm formation in a major gut symbiont.IMPORTANCEBacteroides thetaiotaomicron is a prominent member of the human gut microbiota able to degrade dietary and host polysaccharides, altogether contributing to nutrient exchange, gut function, and maturation of the host's immune system. This obligate anaerobe symbiont can adopt a biofilm community lifestyle, providing protection against environmental factors that might, in turn, protect the host from dysbiosis and dysbiosis-related diseases. It was recently shown that B. thetaiotaomicron exposure to intestinal bile promotes biofilm formation. Here, we reveal that a specific B. thetaiotaomicron membrane efflux pump is induced in response to bile, leading to the release of magnesium ions, potentially reducing electrostatic repulsion forces between components of the biofilm matrix. This leads to a reduction of interbacterial distance and strengthens the biofilm structure. Our study, therefore, provides a better understanding of how bile promotes biofilm formation in a major gut symbiont, potentially promoting microbiota resilience to stress and dysbiosis events.

Concomitant Morphological Modifications of the Avian Eggshell, Eggshell Membranes and the Chorioallantoic Membrane During Embryonic Development

The chicken eggshell (ES) consists of 95% calcium carbonate and 3.5% organic matter, and represents the first physical barrier to protect the developing embryo, while preventing water loss. During the second half of development, calcium ions from the inner ES are progressively solubilized to support mineralization of the embryonic skeleton. This process is mediated by the chorioallantoic membrane (CAM), which is an extraembryonic structure that adheres to the eggshell membranes (ESM) lining the inner ES. The CAM surrounds the embryo and all egg contents by day 11 of incubation (Embryonic Incubation Day 11, EID11) and is fully differentiated and functionally active by day 15 of incubation (Embryonic Incubation Day 15, EID15). In this study, we explored the simultaneous morphological modifications in the ES, ESM and the CAM at EID11 and EID15 by scanning electron microscopy. We observed that the tips of the mammillary knobs of the ES remain tightly attached to the ESM fibers, while their bases become progressively eroded and then detached from the bulk ES. Concomitantly, the CAM undergoes major structural changes that include the progressive differentiation of villous cells whose villi extend to reach the ESM and the ES. These structural data are discussed with respect to the importance of ES decalcification in providing the calcium necessary for mineralization of embryo's skeleton. In parallel, eggshell decalcification and weakening during incubation is likely to impair the ability of the ES to protect the embryo. It is assumed that the CAM could counteract this apparent weakening as an additional layer of physical, cellular and molecular barriers against environmental pressures, including pathogens, dehydration and shocks. However, such hypothesis needs to be further investigated.

Storage-Induced Micro-Erythrocytes Can Be Quantified and Sorted by Flow Cytometry

Refrigerated storage of red cell concentrates before transfusion is associated with progressive alterations of red blood cells (RBC). Small RBC (type III echinocytes, sphero-echinocytes, and spherocytes) defined as storage-induced micro-erythrocytes (SME) appear during pretransfusion storage. SME accumulate with variable intensity from donor to donor, are cleared rapidly after transfusion, and their proportion correlates with transfusion recovery. They can be rapidly and objectively quantified using imaging flow cytometry (IFC). Quantifying SME using flow cytometry would further facilitate a physiologically relevant quality control of red cell concentrates. RBC stored in blood bank conditions were stained with a carboxyfluorescein succinimidyl ester (CFSE) dye and incubated at 37°C. CFSE intensity was assessed by flow cytometry and RBC morphology evaluated by IFC. We observed the accumulation of a CFSE ^high RBC subpopulation by flow cytometry that accounted for 3.3 and 47.2% at day 3 and 42 of storage, respectively. IFC brightfield images showed that this CFSE ^high subpopulation mostly contains SME while the CFSE ^low subpopulation mostly contains type I and II echinocytes and discocytes. Similar numbers of SME were quantified by IFC (based on projected surface area) and by flow cytometry (based on CFSE intensity). IFC and scanning electron microscopy showed that ≥95% pure subpopulations of CFSE ^high and CFSE ^low RBC were obtained by flow cytometry-based sorting. SME can now be quantified using a common fluorescent dye and a standard flow cytometer. The staining protocol enables specific sorting of SME, a useful tool to further characterize this RBC subpopulation targeted for premature clearance after transfusion.

Target Identification of an Antimalarial Oxaborole Identifies AN13762 as an Alternative Chemotype for Targeting CPSF3 in Apicomplexan Parasites

Boron-containing compounds represent a promising class of molecules with proven efficacy against a wide range of pathogens, including apicomplexan parasites. Following lead optimization, the benzoxaborole AN13762 was identified as a preclinical candidate against the human malaria parasite, yet the molecular target remained uncertain. Here, we uncovered the parasiticidal mechanisms of AN13762, by combining forward genetics with transcriptome sequencing and computational mutation discovery and using Toxoplasma gondii as a relevant model for Apicomplexa. AN13762 was shown to target TgCPSF3, the catalytic subunit of the pre-mRNA cleavage and polyadenylation complex, as the anti-pan-apicomplexan benzoxaborole compound, AN3661. However, unique mutations within the TgCPSF3 catalytic site conferring resistance to AN13762 do not confer cross-protection against AN3661, suggesting a divergent resistance mechanism. Finally, in agreement with the high sequence conservation of CPSF3 between Toxoplasma and Cryptosporidium, AN13762 shows oral efficacy in cryptosporidiosis mouse model, a disease for which new drug development is of high priority.

Metal-captured inhibition of pre-mRNA processing activity by CPSF3 controls Cryptosporidium infection

Cryptosporidium is an intestinal pathogen that causes severe but self-limiting diarrhea in healthy humans, yet it can turn into a life-threatening, unrelenting infection in immunocompromised patients and young children. Severe diarrhea is recognized as the leading cause of mortality for children below 5 years of age in developing countries. The only approved treatment against cryptosporidiosis, nitazoxanide, has limited efficacy in the most vulnerable patient populations, including malnourished children, and is ineffective in immunocompromised individuals. Here, we investigate inhibition of the parasitic cleavage and polyadenylation specificity factor 3 (CPSF3) as a strategy to control Cryptosporidium infection. We show that the oxaborole AN3661 selectively blocked Cryptosporidium growth in human HCT-8 cells, and oral treatment with AN3661 reduced intestinal parasite burden in both immunocompromised and neonatal mouse models of infection with greater efficacy than nitazoxanide. Furthermore, we present crystal structures of recombinantly produced Cryptosporidium CPSF3, revealing a mechanism of action whereby the mRNA processing activity of this enzyme is efficiently blocked by the binding of the oxaborole group at the metal-dependent catalytic center. Our data provide insights that may help accelerate the development of next-generation anti-Cryptosporidium therapeutics.

Single molecule localisation microscopy reveals how HIV-1 Gag proteins sense membrane virus assembly sites in living host CD4 T cells

Monitoring virus assembly at the nanoscale in host cells remains a major challenge. Human immunodeficiency virus type 1 (HIV-1) components are addressed to the plasma membrane where they assemble to form spherical particles of 100 nm in diameter. Interestingly, HIV-1 Gag protein expression alone is sufficient to produce virus-like particles (VLPs) that resemble the immature virus. Here, we monitored VLP formation at the plasma membrane of host CD4+ T cells using a newly developed workflow allowing the analysis of long duration recordings of single-molecule Gag protein localisation and movement. Comparison of Gag assembling platforms in CD4+ T cells expressing wild type or assembly-defective Gag mutant proteins showed that VLP formation lasts roughly 15 minutes with an assembly time of 5 minutes. Trapping energy maps, built from membrane associated Gag protein movements, showed that one third of the assembling energy is due to direct Gag capsid-capsid interaction while the remaining two thirds require the nucleocapsid-RNA interactions. Finally, we show that the viral RNA genome does not increase the attraction of Gag at the membrane towards the assembling site but rather acts as a spatiotemporal coordinator of the membrane assembly process.

Salmonella Typhimurium Invalidated for the Three Currently Known Invasion Factors Keeps Its Ability to Invade Several Cell Models

To establish an infection, Salmonella has to interact with eukaryotic cells. Invasion of non-phagocytic cells (i.e., epithelial, fibroblast and endothelial cells) involves either a trigger or a zipper mechanism mediated by the T3SS-1 or the invasin Rck, respectively. Another outer membrane protein, PagN, was also implicated in the invasion. However, other unknown invasion factors have been previously suggested. Our goal was to evaluate the invasion capability of a Salmonella Typhimurium strain invalidated for the three known invasion factors. Non-phagocytic cell lines of several animal origins were tested in a gentamicin protection assay. In most cells, we observed a drastic decrease in the invasion rate between the wild-type and the triple mutant. However, in five cell lines, the triple mutant invaded cells at a similarly high level to the wild-type, suggesting the existence of unidentified invasion factors. For the wild-type and the triple mutant, scanning-electron microscopy, confocal imaging and use of biochemical inhibitors confirmed their cellular uptake and showed a zipper-like mechanism of internalization involving both clathrin- and non-clathrin-dependent pathways. Despite a functional T3SS-1, the wild-type bacteria seemed to use the same entry route as the mutant in our cell model. All together, these results demonstrate the existence of unknown Salmonella invasion factors, which require further characterization.

Transcriptomic profiling of a chicken lung epithelial cell line (CLEC213) reveals a mitochondrial respiratory chain activity boost during influenza virus infection

Avian Influenza virus (AIV) is a major concern for the global poultry industry. Since 2012, several countries have reported AIV outbreaks among domestic poultry. These outbreaks had tremendous impact on poultry production and socio-economic repercussion on farmers. In addition, the constant emergence of highly pathogenic AIV also poses a significant risk to human health. In this study, we used a chicken lung epithelial cell line (CLEC213) to gain a better understanding of the molecular consequences of low pathogenic AIV infection in their natural host. Using a transcriptome profiling approach based on microarrays, we identified a cluster of mitochondrial genes highly induced during the infection. Interestingly, most of the regulated genes are encoded by the mitochondrial genome and are involved in the oxidative phosphorylation metabolic pathway. The biological consequences of this transcriptomic induction result in a 2.5- to 4-fold increase of the ATP concentration within the infected cells. PB1-F2, a viral protein that targets the mitochondria was not found associated to the boost of activity of the respiratory chain. We next explored the possibility that ATP may act as a host-derived danger signal (through production of extracellular ATP) or as a boost to increase AIV replication. We observed that, despite the activation of the P2X7 purinergic receptor pathway, a 1mM ATP addition in the cell culture medium had no effect on the virus replication in our epithelial cell model. Finally, we found that oligomycin, a drug that inhibits the oxidative phosphorylation process, drastically reduced the AIV replication in CLEC213 cells, without apparent cellular toxicity. Collectively, our results suggest that AIV is able to boost the metabolic capacities of its avian host in order to provide the important energy needs required to produce progeny virus.

Marek's disease virus undergoes complete morphogenesis after reactivation in a T-lymphoblastoid cell line transformed by recombinant fluorescent marker virus

T-lymphocytes are central targets of Marek's disease, a major chicken disease induced by the oncogenic alphaherpesvirus Marek's disease virus (MDV). T-lymphocyte infection is also associated with immunosuppression and virus latency. To decipher viral morphogenesis in T-lymphocytes, we used the recombinant vRB-1B 47EGFP marker virus to generate a new lymphoblastoid cell line, 3867K, that exhibited typical properties of other MDV-transformed chicken cell lines in term of cell markers, reactivation rate and infectivity. Examination of reactivating EGFP-positive 3867K cells by transmission electron microscopy revealed the presence of most types of herpesvirus particles inside the cells but no extracellular ones. Quantification of virion types indicated only 5% cytoplasmic particles, with 0.5% being mature. This study demonstrated that MDV morphogenesis is complete upon reactivation in T-lymphocytes, albeit with poor efficiency, with a defect in the exit of virions from the nucleus and secondary envelopment, as occurs in infected fibroblasts.

Functional expression, purification, characterization, and membrane reconstitution of non-structural protein 2 from hepatitis C virus

Non-structural protein 2 (NS2) of the hepatitis C virus (HCV) is an integral membrane protein that contains a cysteine protease and that plays a central organizing role in assembly of infectious progeny virions. While the crystal structure of the protease domain has been solved, the NS2 full-length form remains biochemically and structurally uncharacterized because recombinant NS2 could not be prepared in sufficient quantities from cell-based systems. We show here that functional NS2 in the context of the NS2-NS3pro precursor protein, ensuring NS2-NS3 cleavage, can be efficiently expressed by using a wheat germ cell-free expression system. In this same system, we subsequently successfully produce and purify milligram amounts of a detergent-solubilized form of full-length NS2 exhibiting the expected secondary structure content. Furthermore, immuno-electron microscopy analyses of reconstituted proteoliposomes demonstrate NS2 association with model membranes.

Correlative scanning-transmission electron microscopy reveals that a chimeric flavivirus is released as individual particles in secretory vesicles

The intracellular morphogenesis of flaviviruses has been well described, but flavivirus release from the host cell remains poorly documented. We took advantage of the optimized production of an attenuated chimeric yellow fever/dengue virus for vaccine purposes to study this phenomenon by microscopic approaches. Scanning electron microscopy (SEM) showed the release of numerous viral particles at the cell surface through a short-lived process. For transmission electron microscopy (TEM) studies of the intracellular ultrastructure of the small number of cells releasing viral particles at a given time, we developed a new correlative microscopy method: CSEMTEM (for correlative scanning electron microscopy - transmission electron microscopy). CSEMTEM analysis suggested that chimeric flavivirus particles were released as individual particles, in small exocytosis vesicles, via a regulated secretory pathway. Our morphological findings provide new insight into interactions between flaviviruses and cells and demonstrate that CSEMTEM is a useful new method, complementary to SEM observations of biological events by intracellular TEM investigations.

The Arabidopsis thaliana-Alternaria brassicicola pathosystem: A model interaction for investigating seed transmission of necrotrophic fungi

BACKGROUND

Seed transmission constitutes a major component of the parasitic cycle for several fungal pathogens. However, very little is known concerning fungal or plant genetic factors that impact seed transmission and mechanisms underlying this key biological trait have yet to be clarified. Such lack of available data could be probably explained by the absence of suitable model pathosystem to study plant-fungus interactions during the plant reproductive phase.

RESULTS

Here we report on setting up a new pathosystem that could facilitate the study of fungal seed transmission. Reproductive organs of Arabidopsis thaliana were inoculated with Alternaria brassicicola conidia. Parameters (floral vs fruit route, seed collection date, plant and silique developmental stages) that could influence the seed transmission efficiency were tested to define optimal seed infection conditions. Microscopic observations revealed that the fungus penetrates siliques through cellular junctions, replum and stomata, and into seed coats either directly or through cracks. The ability of the osmosensitive fungal mutant nik1Δ3 to transmit to A. thaliana seeds was analyzed. A significant decrease in seed transmission rate was observed compared to the wild-type parental strain, confirming that a functional osmoregulation pathway is required for efficient seed transmission of the fungus. Similarly, to test the role of flavonoids in seed coat protection against pathogens, a transparent testa Arabidopsis mutant (tt4-1) not producing any flavonoid was used as host plant. Unexpectedly, tt4-1 seeds were infected to a significantly lower extent than wild-type seeds, possibly due to over-accumulation of other antimicrobial metabolites.

CONCLUSIONS

The Arabidopsis thaliana-Alternaria brassicicola pathosystem, that have been widely used to study plant-pathogen interactions during the vegetative phase, also proved to constitute a suitable model pathosystem for detailed analysis of plant-pathogen interactions during the reproductive phase. We demonstrated that it provides an excellent system for investigating the impact of different fungal or plant mutations on the seed transmission process and therefore paves the way towards future high-throughput screening of both Arabidopsis and fungal mutant.

Cell surface properties of two differently virulent strains of Acinetobacter baumannii isolated from a patient

The aim of this study was to unravel, by focusing on cell surface properties, the underlying virulence factors contributing to the difference in the pathogenicity observed in two Acinetobacter baumannii strains isolated from the same patient. The two strains were phenotypically different: (i) a mucoid strain (AB-M), highly virulent in a mouse model of pneumonia, and (ii) a nonmucoid strain (AB-NM), moderately virulent in the same model. The study of the cell surface properties included the microbial adhesion to solvents method, the measurement of the electrophoretic mobility of bacteria, the analysis of biofilm formation by calcofluor white staining, the adherence to silicone catheters, and scanning electron microscopy. The AB-NM strain was more hydrophobic, more adherent to silicone catheters, and produced more biofilm than the AB-M strain. Scanning electron microscopy showed bacterial cells with a rough surface and the formation of large cell clusters for AB-NM whereas the AB-M strain had a smooth surface and formed only a few cell clusters. Contrary to the results of most previous studies, cell surface properties were not correlated to the virulence described in our experimental model, indicating that mechanisms other than adherence may be involved in the expression of A. baumannii virulence.

Impact of the unfolded protein response on the pathogenicity of the necrotrophic fungus Alternaria brassicicola

The unfolded protein response (UPR) is an important stress signalling pathway involved in the cellular development and environmental adaptation of fungi. We investigated the importance of the UPR pathway in the pathogenicity of the plant necrotrophic fungus Alternaria brassicicola, which causes black spot disease on a wide range of Brassicaceae. We identified the AbHacA gene encoding the major UPR transcription regulator in A. brassicicola. Deletion of AbHacA prevented induction of the UPR in response to endoplasmic reticulum stress. Loss of UPR in mutants resulted in a complete loss of virulence and was also associated with a cell wall defect and a reduced capacity for secretion. In addition, our results showed that the UPR was triggered by treatment of mycelia with camalexin, i.e. the major Arabidopsis thaliana phytoalexin, and that strains lacking functional AbHacA exhibited increased in vitro susceptibility to antimicrobial plant metabolites. We hypothesize that the UPR plays a major role in fungal virulence by altering cell protection against host metabolites and by reducing the ability of the fungus to assimilate nutrients required for growth in the host environment. This study suggests that targeting the UPR pathway would be an effective plant disease control strategy.