Summary: We report here a new screening method based on the fluorescence of colonies on calcofluor agar plates to identify transposon insertion mutants of Salmonella enteritidis that are defective in biofilm development. The results not only confirmed the requirement of genes already described for the modulation of multicellular behaviour in Salmonella typhimurium and other species, but also revealed new aspects of the biofilm formation process, such as two new genetic elements, named as bcsABZC and bcsEFG operons, required for the synthesis of an exopolysaccharide, digestible with cellulase. Non‐polar mutations of bcsC and bcsE genes and complementation experiments demonstrated that both operons are respon‐sible for cellulose biosynthesis in both S. enteritidis and S. typhimurium. Using two different growth media, ATM and LB, we showed that the biofilm produced by S. enteritidis is made of different constituents, suggesting that biofilm composition and regulation depends on environmental conditions. Bacterial adherence and invasion assays of eukaryotic cells and in vivo virulence studies of cellulose‐deficient mutants indicated that, at least under our experimental conditions, the production of cellulose is not involved in the virulence of S. enteritidis. However, cellulose‐deficient mutants were more sensitive to chlorine treatments, suggesting that cellulose production and biofilm formation may be an important factor for the survival of S. enteritidis on surface environments.

Summary: In environmental settings, biofilms represent the common way of life of microorganisms. Salmonella enterica serovar Enteritidis, the most frequent cause of gastroenteritis in developed countries, produces a biofilm whose matrix is mainly composed of curli fimbriae and cellulose. In contrast to other bacterial biofilms, no proteinaceous compound has been reported to participate in the formation of this matrix. Here, we report the discovery of BapA, a large cell‐surface protein required for biofilm formation by S. Enteritidis. Deletion of bapA caused the loss of the capacity to form a biofilm whereas the overexpression of a chromosomal copy of bapA increased the biofilm biomass formation. We provide evidence that overproduction of curli fimbriae and not cellulose can compensate for the biofilm deficiency of a bapA mutant strain. BapA is secreted through a type I protein secretion system (BapBCD) situated downstream of the bapA gene and was found to be loosely associated with the cell surface. Experiments with mixed bacterial populations positive or negative for BapA showed that BapA minus cells are not recruited into the biofilm matrix. The expression of bapA is coordinated with that of genes encoding curli fimbriae and cellulose, through the action of csgD. Studies on the contribution of BapA to S. Enteritidis pathogenesis revealed that orally inoculated animals with a bapA‐deficient strain survived longer than those inoculated with the wild‐type strain. Also, a bapA mutant strain showed a significantly lower colonization rate at the intestinal cell barrier and consequently a decreased efficiency for organ invasion compared with the wild‐type strain. Taken together, these data demonstrate that BapA contributes both to biofilm formation and invasion through the regular Salmonella infection route.

Background: The specific targeting of nanomedicines to mannose receptors, highly expressed in cells of the immune system, performs a useful strategy for improving the efficacy of vaccines and chemotherapy. Objective: This review discusses the potential of mannose-targeted drug/antigen delivery systems for vaccination and treatment of diseases localized in macrophages and other antigen-presenting cells. Methods: The first part of the review describes the characteristics, localization and functions of mannose receptors. The following sections are devoted to the description of different methods used to deliver therapeutic agents, including mannose conjugates and mannosylated carriers or particulates (i.e., liposomes, nanoparticles and niosomes). Results/conclusions: A general overview of published reports confirms the effectiveness of mannosylation strategies, although the optimization and full exploitation of mannose-targeted drug delivery systems would require a deeper understanding of the structure–activi...

Summary: Salmonella enterica serovar Typhimurium is capable of producing cellulose as the main exopolysaccharide compound of the biofilm matrix. It has been shown for Gluconacetobacter xylinum that cellulose biosynthesis is allosterically regulated by bis‐(3′,5′) cyclic diguanylic acid, whose synthesis/degradation depends on diguanylate cyclase/phosphodiesterase enzymatic activities. A protein domain, named GGDEF, is present in all diguanylate cyclase/phosphodiesterase enzymes that have been studied to date. In this study, we analysed the molecular mechanisms responsible for the failure of Salmonella typhimurium strain SL1344 to form biofilms under different environmental conditions. Using a complementation assay, we were able to identify two genes, which can restore the biofilm defect of SL1344 when expressed from the plasmid pBR328. Based on the observation that one of the genes, STM1987, contains a GGDEF domain, and the other, mlrA, indirectly controls the expression of another GGDEF protein, AdrA, we proceeded on a mutational analysis of the additional GG[DE]EF motif containing proteins of S. typhimurium. Our results demonstrated that MlrA, and thus AdrA, is required for cellulose production and biofilm formation in LB complex medium whereas STM1987 (GGDEF domain containing protein A, gcpA) is critical for biofilm formation in the nutrient‐deficient medium, ATM. Insertional inactivation of the other six members of the GGDEF family (gcpB‐G) showed that only deletion of yciR (gcpE) affected cellulose production and biofilm formation. However, when provided on plasmid pBR328, most of the members of the GGDEF family showed a strong dominant phenotype able to bypass the need for AdrA and GcpA respectively. Altogether, these results indicate that most GGDEF proteins of S. typhimurium are functionally related, probably by controlling the levels of the same final product (cyclic di‐GMP), which include among its regulatory targets the cellulose production and biofilm formation of S. typhimurium.