Cell wall

A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mechanism. Cell walls are absent in many eukaryotes, including animals, but are present in some other ones like fungi, algae and plants, and in most prokaryotes (except mollicute bacteria). A major function is to act as pressure vessels, preventing over-expansion of the cell when water enters. The composition of cell walls varies between taxonomic group and species and may depend on cell type and developmental stage. The primary cell wall of land plants is composed of the polysaccharides cellulose, hemicelluloses and pectin. Often, other polymers such as lignin, suberin or cutin are anchored to or embedded in plant cell walls. Algae possess cell walls made of glycoproteins and polysaccharides such as carrageenan and agar that are absent from lan
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Related publications (100)

The double-edged toxins

Mario Gonzalo Garcia Arraez

Many insect species are associated with endosymbiotic bacteria which have the particularity of living within host tissues. Endosymbionts benefit from this stable and nutritious environment, while providing ecological advantages to their host, such as protection against parasites or thermal tolerance. Spiroplasma poulsonii is an endosymbiotic bacterium that infects natural populations of Drosophila melanogaster. Spiroplasma poulsonii lacks a cell wall, a fact that renders it invisible to the host immune system and allows it to thrive in the host hemolymph. It invades the female germline by co-opting the host’s yolk transport and uptake machinery, which ensures its vertical transmission. Its efficient transmission is associated with a phenotype called male-killing, whereby infected male embryos die during their early development while infected females survive. The mechanisms that ensure the stability of Drosophila-Spiroplasma symbiosis are increasingly well understood, but the bacterial genes involved remain poorly known because of the intractability of Spiroplasma. This project aims at better characterizing the Drosophila-Spiroplasma interaction, with particular focus on the bacterial side. In the first part, I developed a method to cultivate Spiroplasma poulsonii in vitro by optimizing a commercially available medium. This culture method allowed comparing the transcriptome of in vitro grown versus host-grown Spiroplasma, enabling us to identify putative genes involved in the interaction with the host. Interestingly, inside its insect host, S. poulsonii up-regulates genes coding for toxins of the Ribosomal Inactivating Protein (RIP) family. RIPs were previously known for their role in host protection against macro-parasites, such as wasps and nematodes. Their up-regulation in unparasitized hosts compared to culture was thus peculiar, raising the question what effect these RIPs have on host biology. Thus, in the second part, I studied the function of S. poulsonii RIPs in the absence of parasites. I showed that two of them are constantly expressed within the host and provide evidence that these toxins shorten host life span and increase embryonic mortality. Interestingly, the expression of RIPs was more toxic to male embryos than females, suggesting that RIPs contribute to S. poulsonii-induced male-killing. Last, I studied the D. melanogaster response to RIPs, and how the host mitigates the deleterious effects of these toxins by up-regulating the cytosolic chaperone Heat-Shock-Protein 70B (HSP70B). This protein carries out essential functions in protein homeostasis under normal and stressful conditions such as folding, refolding, or increasing the half-life of proteins. Interestingly, up-regulation of Hsp70B in the presence of RIPs results in an increased lifespan and in a better tolerance to heat stress, which may be ecologically advantageous. Altogether, this work illustrates how Spiroplasma-derived RIP toxins can differentially affect Drosophila depending on the ecological context, ranging from beneficial upon parasite infection or heat shock to detrimental in the absence of such environmental pressures.

Interactions of silver and polystyrene nanoparticles with algae

Xiaomei Li

Nanoparticles have various physicochemical properties attributed by their small dimensions. The global production of nanoparticles lead to a raised concern for their environmental impacts. Algae are of highly ecological importance functioning as primary producers for almost all aquatic life. In the present thesis, interactions of nanoparticles with different fresh water algal strains were examined. First, the toxicity and uptake of citrate-coated silver nanoparticles (AgNP) and AgNO3 were examined in the alga Euglena gracilis, which has no cell wall but a pellicle. Secondly, to examine the role of algal cell wall in determining the particles interactions with algae, four strains were selected, including Euglena gracilis, Haematococcus pluvialis, and Chlamydomonas reinhardtii wild type and a cell wall free mutant. Their interactions with polystyrene nanoparticles (PSNP) of two sizes, 50 nm (PSNP50) and 500 nm (PSNP500), were investigated. Third, interactions of three differently coated AgNP with alkaline phosphatase (AP), an extracellular enzyme used for phosphorus acquisition, were assessed. The selected coatings were citrate (CIT), polyvinylpyrrolidone (PVP) and gelatin (GEL). Exposure to AgNP and AgNO3 for 1-2 hours led to decrease in photosynthetic yield, in a concentration-dependent manner, and changes in cell morphology in E. gracilis. Based on total silver added, AgNP were less toxic than AgNO3. Concentrations causing a 50% reduction in photosynthetic yield were 1.9 µM and 85 for AgNP and AgNO3, respectively. Damaging effects of AgNP were completely prevented by cysteine, suggesting that the toxicity of AgNP was mediated by Ag+ ions. Uptake studies showed that the maximal cell-associated silver was higher in AgNP compared to AgNO3, and the higher silver level was shown to correspond to particles adsorbed to the pellicle. By examining four algal stains, it was found that no strain internalized PSNP, emphasizing the role of algal cell walls as barrier for nanoparticle uptake. Interactions of PSNP with algae were found to be unique for each strain, and depend on particle size. PSNP50 were associated with E. gracilis cells displaying a non-homogenous distribution on pellicle. In H. pluvialis, PSNP50 distributed homogenously around the cells. The wild type and cell wall free mutant of C. reinhardtii cells exposed to PSNP50 were found to clump together packed in the extracellular polymeric substances (EPS). The particles were associated with the EPS. The larger PSNP500 were observed to interact only with the two C. reinhardtii strains. Taken together, these results indicate that the algal cell walls hinder the crossing of nanoparticles. Assessing the sorption of AP to AgNPCIT, AgNPPVP, and AgNPGEL showed that the physiochemical properties of both the particle coatings and the enzyme were determinant for the binding. The enzyme adsorbed to AgNPCIT and AgNPPVP, leading to a 10% and 70% coverage of the particle surface area, respectively. No adsorption was found in the case of AgNPGEL. The three types of AgNP decreased AP activity, however, the inhibitory effects only occurred when the AgNP were added after addition of the substrate to the enzyme, not vice versa. AgNO3 did not affect the AP activity. Thus, the results of this study indicate particle-specific effects due to interactions with the AP-substrate intermediate.

Iron-oxides and iron-citrate as new photocatalysts in solar inactivation of Escherichia coli in water

Cristina del Socorro Lonfat

This study addresses the bacterial inactivation mechanism by photo-Fenton process at near-neutral pH, focusing on iron-oxides and iron-citrate as photocatalysts for solar water disinfection and using E. coli as a bacteria model. Cell envelope damage during bacterial inactivation by photo-Fenton and TiO2 photocatalysis were investing providing evidence for lipid peroxidation and cell permeability. TiO2 photocatalysis induced significant cell membrane damage, in contrast to the photo-Fenton process, but the inactivation kinetics for both disinfection processes was similar. A higher efficiency of photo-generation of reactive oxygen species (ROS) in the presence of TiO2 photocatalyst compared with the photo-Fenton system was observed. The bactericidal effect of Fe3+/hv seems possible due to the adsorption of Fe3+ ions on the bacterial cell wall followed by photosensitization of iron-bacteria exciplexes oxidizing the cell membrane. In contrast, the effect of Fe2+/hv was associated with diffusion into the cell giving raise to intracellular dark Fenton¿s reactions. We suggest that cell envelope damage might not necessarily be a unique pathway in bacterial inactivation by photo-Fenton treatment. In particular, the enhancement of an internal (photo)-Fenton process by the synergistic action of UVA and the external Fenton's reactants appears to be an important contribution to bacterial inactivation. Bacterial inactivation by the heterogeneous photo-Fenton process was carried out via iron (hydr)oxide particles, i.e. hematite, goethite, wüstite and magnetite. We found that, the iron (hydr)oxides act as photocatalytic semiconductors and catalysts in the heterogeneous photo-Fenton process with the exception of magnetite, which needs H2O2 as electron acceptors. The Hydroxyl radical and superoxide radical were the principal ROS produced by iron (hydr)oxide particles under light in the absence or presence of H2O2. Natural organic matter (NOM) and inorganic substances did not interfere with the photocatalytic semiconducting action of hematite during bacterial inactivation, but enhanced bacterial inactivation mediated by hematite used as the photo-Fenton reagent. Our results demonstrated, for the first time, that low concentration of iron (hydr)oxides (0.6 mg/L) under sunlight, acting both as semiconductors or catalysts of the heterogeneous photo-Fenton process, may serve as a disinfection method for waterborne bacterial pathogens. Bacterial inactivation by the homogeneous photo-Fenton process was carried out using Fe¿citrate complex as a source of iron. The efficiency of the homogeneous photo-Fenton process using Fe-citrate complex strongly improved bacterial inactivation as compared with the FeSO4 and goethite as sources of iron. The bacterial inactivation rate increased in the order of goethite < FeSO4 < Fe-citrate, which agreed with the ¿OH radicals detected by ESR. Encouraging results were also obtained while applying this treatment for bacterial inactivation in natural water samples at pH 8.5. No bacterial reactivation and/or growth were observed showing that Fe-citrate-based photo-Fenton process efficiently inactivate bacteria using a low iron concentration of Fe-citrate, while avoiding precipitation of ferric hydroxides. The application of the photo-Fenton process at near-neutral pH is a promising technique for bacterial inactivation, due to its simplicity, the use of the sun, the low concentration of reagents and does not produce toxic waste.
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