Physico-chemical interactions between nanoparticles and cell membranes play a crucial role in determining the cytotoxicity of nanoparticles, which may thereby vary depending on the nature of the target microorganisms. We investigated the responses of two different models of unicellular bacteria to cerium oxide (CeO₂) nanoparticles. These organisms are: Synechocystis PCC6803 a representative of environmentally important cyanobacterial organisms (producer of biomass for aquatic food chains), and Escherichia coli a representative of intestine-colonizing bacteria. Coupling physico-chemical (adsorption isotherms and electrophoretic mobility), biological (survival tests), microscopical (SEM, TEM and EDS) and spectroscopic (XANES) methods, we enlightened two distinct mechanisms for the CeO₂ nanoparticles toxicological impact: A ‘direct’ mechanism that requires a close contact between nanoparticles and cell membranes, and an ‘indirect’ influence elicited by the acidity of nanoparticles stabilizing agents. We showed that E. coli is sensitive to the ‘direct’ effects of nanoparticles, whereas Synechocystis being protected by extracellular polymeric substances preventing direct cellular contacts is sensitive only to the ‘indirect’ mechanism. Consequently, our findings demonstrate the importance of the ‘direct/indirect’ effects of nanoparticles on cell fitness, a phenomenon that should be systematically investigated with appropriate techniques and dose metrics to make meaningful environmental and/or health recommendations.

Little is known about the production of exopolysaccharides (EPS) in cyanobacteria, and there are no genetic and physiological evidences that EPS are involved in cell protection against the frequently encountered environmental stresses caused by salt and metals. We studied four presumptive EPS production genes, sll0923, sll1581, slr1875 and sll5052, in the model cyanobacterium Synechocystis PCC6803, which produces copious amounts of EPS attached to cells (CPS) and released in the culture medium (RPS) as shown here. We show that sll0923, sll1581, slr1875 and sll5052 are all dispensable to the growth of all corresponding single and double deletion mutants in absence of stress. Furthermore, we report that sll0923, sll1581 and slr1875 unambiguously operate in the production of both CPS and RPS. Both sll1581 and slr1875 are more important than sll0923 for CPS production, whereas the contrary is true for RPS production. We show that the most EPS-depleted mutant, doubly deleted for sll1581 and slr1875, lacks the EPS mantle that surrounds WT cells and sorbs iron in their vicinity. Using this mutant, we demonstrate for the first time that cyanobacterial EPS directly operate in cell protection against NaCl, CoCl₂, CdSO₄ and Fe-starvation. We believe that our EPS-depleted mutants will be useful tools to investigate the role of EPS in cell-to-cell aggregation, biofilm formation, biomineralization and tolerance to environmental stresses. We also suggest using the fast sedimenting mutants as biotechnological cell factories to facilitate the otherwise expensive harvest of the producer cell biomass and/or its separation from products excreted in the growth media.

The facultative heterotrophic cyanobacterium Synechocystis sp. strain PCC 6803 was transformed by HaeII Cmr fragments ligated at random to HaeII DNA fragments of the host genome. A similar transformation was done with an AvaII Kmr marker ligated to AvaII host DNA fragments. Integration of the resistance markers into the host genome led to a high frequency of stable Kmr and Cmr transformants. Physical analysis of individual transformants indicated that this result was due to homologous recombination by conversionlike events leading to insertion of the Cmr (or Kmr) gene between two HaeII (or AvaII) sites of the host genome, with precise deletion of the host DNA between these sites. In contrast, integrative crossover of circular DNA molecules with homology to the host DNA is very rare in this cyanobacterium. Strain PCC 6803 was shown to have about 12 genomic copies per cell in standard growth conditions, which complicates the detection of recessive mutations induced by chemical or UV mutagenesis. Random disruption of the host DNA by insertional transformation provides a convenient alternative to transposon mutagenesis in cyanobacteria and may help to overcome the difficulties encountered in generating recessive mutants by classical mutagenesis.

Summary: The cyanobacterial genes lexA, recA and ruvB were analysed in Synechocystis PCC6803, which is shown here to be more radiation resistant than the other unicellular model strain Synechococcus PCC7942. We found that cyanobacteria do not have an Escherichia coli‐type SOS regulon. The Synechocystis lexA and recA promoters were found to be strong and UV insensitive, unlike the ruvB promoter, which is weak and UV‐C inducible. Yet, lexA and recA are regulated by UV‐C, but the control is negative and occurs at the post‐transcriptional level. Two novel conserved elements were characterized in the lexA promoter: (i) an unusually long crucial box 5′‐TAAAATTTTGTATCTTTT‐3′ (−64, −47); and (ii) a negatively acting motif 5′‐TAT GAT‐3′ (−42, −37). These elements were not found in the recA promoter, which appeared to be unusually simple in harbouring only a single crucial element (i.e. the canonical −10 box). RuvB, operating in recombination‐dependent cellular processes, was found to be dispensable to cell growth, whereas LexA and RecA appeared to be critical to cell viability. Using DNA microarrays, we have identified 57 genes with expression that is altered, at least twofold, in response to LexA depletion. None of these genes is predicted to operate in DNA metabolism, arguing against the involvement of LexA in the regulation of DNA repair. Instead, most of the LexA‐responsive genes were known to be involved in carbon assimilation or controlled by carbon availability. Consistently, the growth of the LexA‐depleted strain was found to be strongly dependent on the availability of inorganic carbon.

From bacteria to plants and humans, the glutathione system plays a pleiotropic role in cell defense against metabolic, oxidative and metal stresses. Glutathione (GSH), the γ-L-glutamyl-L-cysteinyl-glycine nucleophile tri-peptide, is the central player of this system that acts in redox homeostasis, detoxification and iron metabolism in most living organisms. GSH directly scavenges diverse reactive oxygen species (ROS), such as singlet oxygen, superoxide anion, hydrogen peroxide, hydroxyl radical, nitric oxide and carbon radicals. It also serves as a cofactor for various enzymes, such as glutaredoxins (Grxs), glutathione peroxidases (Gpxs), glutathione reductase (GR) and glutathione-S-transferases (GSTs), which play crucial roles in cell detoxication. This review summarizes what is known concerning the GSH-system (GSH, GSH-derived metabolites and GSH-dependent enzymes) in selected model organisms (Escherichia coli, Saccharomyces cerevisiae, Arabidopsis thaliana and human), emphasizing cyanobacteria for the following reasons. Cyanobacteria are environmentally crucial and biotechnologically important organisms that are regarded as having evolved photosynthesis and the GSH system to protect themselves against the ROS produced by their active photoautotrophic metabolism. Furthermore, cyanobacteria synthesize the GSH-derived metabolites, ergothioneine and phytochelatin, that play crucial roles in cell detoxication in humans and plants, respectively. Cyanobacteria also synthesize the thiol-less GSH homologs ophthalmate and norophthalmate that serve as biomarkers of various diseases in humans. Hence, cyanobacteria are well-suited to thoroughly analyze the role/specificity/redundancy of the players of the GSH-system using a genetic approach (deletion/overproduction) that is hardly feasible with other model organisms (E. coli and S. cerevisiae do not synthesize ergothioneine, while plants and humans acquire it from their soil and their diet, respectively).