Host preference and invasiveness of commensal bacteria in the Lotus and Arabidopsis root microbiota.

Roots of different plant species are colonized by bacterial communities, that are distinct even when hosts share the same habitat. It remains unclear to what extent the host actively selects these communities and whether commensals are adapted to a specific plant species. To address this question, we assembled a sequence-indexed bacterial culture collection from roots and nodules of Lotus japonicus that contains representatives of most species previously identified using metagenomics. We analysed taxonomically paired synthetic communities from L. japonicus and Arabidopsis thaliana in a multi-species gnotobiotic system and detected signatures of host preference among commensal bacteria in a community context, but not in mono-associations. Sequential inoculation experiments revealed priority effects during root microbiota assembly, where established communities are resilient to invasion by latecomers, and that host preference of commensal bacteria confers a competitive advantage in their cognate host. Our findings show that host preference in commensal bacteria from diverse taxonomic groups is associated with their invasiveness into standing root-associated communities.

A Lotus japonicus E3 ligase interacts with the Nod Factor Receptor 5 and positively regulates nodulation.

BACKGROUND: Post-translational modification of receptor proteins is involved in activation and de-activation of signalling systems in plants. Both ubiquitination and deubiquitination have been implicated in plant interactions with pathogens and symbionts. RESULTS: Here we present LjPUB13, a PUB-ARMADILLO repeat E3 ligase that specifically ubiquitinates the kinase domain of the Nod Factor receptor NFR5 and has a direct role in nodule organogenesis events in Lotus japonicus. Phenotypic analyses of three LORE1 retroelement insertion plant lines revealed that pub13 plants display delayed and reduced nodulation capacity and retarded growth. LjPUB13 expression is spatially regulated during symbiosis with Mesorhizobium loti, with increased levels in young developing nodules. CONCLUSION: LjPUB13 is an E3 ligase with a positive regulatory role during the initial stages of nodulation in L. japonicus.

A legume genetic framework controls infection of nodules by symbiotic and endophytic bacteria.

Legumes have an intrinsic capacity to accommodate both symbiotic and endophytic bacteria within root nodules. For the symbionts, a complex genetic mechanism that allows mutual recognition and plant infection has emerged from genetic studies under axenic conditions. In contrast, little is known about the mechanisms controlling the endophytic infection. Here we investigate the contribution of both the host and the symbiotic microbe to endophyte infection and development of mixed colonised nodules in Lotus japonicus. We found that infection threads initiated by Mesorhizobium loti, the natural symbiont of Lotus, can selectively guide endophytic bacteria towards nodule primordia, where competent strains multiply and colonise the nodule together with the nitrogen-fixing symbiotic partner. Further co-inoculation studies with the competent coloniser, Rhizobium mesosinicum strain KAW12, show that endophytic nodule infection depends on functional and efficient M. loti-driven Nod factor signalling. KAW12 exopolysaccharide (EPS) enabled endophyte nodule infection whilst compatible M. loti EPS restricted it. Analysis of plant mutants that control different stages of the symbiotic infection showed that both symbiont and endophyte accommodation within nodules is under host genetic control. This demonstrates that when legume plants are exposed to complex communities they selectively regulate access and accommodation of bacteria occupying this specialized environmental niche, the root nodule.

Antibody response to the 45 kDa Candida albicans antigen in an animal model and potential role of the antigen in adherence.

The Candida antigen CR3-RP (complement receptor 3-related protein) is supposed to be a 'mimicry' protein because of its ability to bind antibody directed against the alpha subunit of the mammalian CR3 (CD11b/CD18). This study aimed to (i) investigate the specific humoral isotypic response to immunization with CR3-RP in vivo in a rabbit animal model, and (ii) determine the role of CR3-RP in the adherence of Candida albicans in vitro using the model systems of buccal epithelial cells (BECs) and biofilm formation. The synthetic C. albicans peptide DINGGGATLPQ corresponding to 11 amino-acids of the CR3-RP sequence DINGGGATLPQALXQITGVIT, determined by N-terminal sequencing, was used for immunization of rabbits to obtain polyclonal anti-CR3-PR serum and for subsequent characterization of the humoral isotypic response of rabbits. A significant increase of IgG, IgA and IgM anti-CR3-RP specific antibodies was observed after the third (P<0.01) and the fourth (P<0.001) immunization doses. The elevation of IgA levels suggested peptide immunomodulation of the IgA1 subclass, presumably in coincidence with Candida epithelial adherence. Blocking CR3-RP with polyclonal anti-CR3-RP serum reduced the ability of Candida to adhere to BECs, in comparison with the control, by up to 35 % (P<0.001), and reduced biofilm formation by 28 % (P<0.001), including changes in biofilm thickness and integrity detected by confocal laser scanning microscopy. These properties of CR3-RP suggest that it has potential for future vaccine development.

Terminal complement complex (C5b-9) in children with recurrent hemolytic uremic syndrome.

Recurrent hemolytic uremic syndrome (recHUS) is a heterogeneous group of disorders. The pathogenesis of recHUS is not fully understood. recHUS has a high risk of development of terminal renal insufficiency and other sequelae. Abnormalities in complement factor H or in membrane-bound complement inhibitors with consecutive complement activation can be found in approximately 30 to 50% of the patients. Starting in 2001, we evaluated 42 patients with recHUS from five European countries (Germany, Austria, Hungary, Switzerland, and the Czech Republic). We measured the terminal complement complex (TCC) by an enzyme-linked immunosorbent assay using a neoepitope-specific anti-C9 antibody in 17 patients in plasma (native complement activation), serum (after coagulation), and zymosan-activated serum (Z-serum; after stimulation of coagulation). We compared the results to those of 16 healthy persons. In patients with recHUS (eight males, nine females) with a median age of 10.8 years, the TCC values were higher in plasma (0.57 versus 0.48 microg/mL; P = 0.04) and serum (3.1 versus 2.2 microg/mL) than in those of the control group, with a median age of 28.6 years (six males, 10 females) The TCC values in patients with low C3 compared with patients with normal C3 levels were even higher in plasma and serum, and the ratio was much lower. Children with recHUS have higher concentrations of TCC in plasma and serum. The ratio of Z-serum to serum showed significantly lower values in children with recHUS (96.01 versus 150.3; P = 0.01). These findings indicate a higher grade of complement activation and consumption in recHUS, suggesting that TCC may mediate cell toxicity. This may play an important role in the inferior outcome of these patients. The isolated substitution of factor H, or other complement inhibitors to block TCC formation, may represent useful therapies for these patients.