Genomic analysis of cyclic-di-GMP-related genes in rhizobial type strains and functional analysis in Rhizobium etli.

Rhizobia are soil bacteria that can fix nitrogen in symbiosis with leguminous plants or exist free living in the rhizosphere. Crucial to their complex lifestyle is the ability to sense and respond to diverse environmental stimuli, requiring elaborate signaling pathways. In the majority of bacteria, the nucleotide-based second messenger cyclic diguanosine monophosphate (c-di-GMP) is involved in signal transduction. Surprisingly, little is known about the importance of c-di-GMP signaling in rhizobia. We have analyzed the genome sequences of six well-studied type species (Bradyrhizobium japonicum, Mesorhizobium loti, Rhizobium etli, Rhizobium leguminosarum, Sinorhizobium fredii, and Sinorhizobium meliloti) for proteins possibly involved in c-di-GMP signaling based on the presence of four domains: GGDEF (diguanylate cyclase), EAL and HD-GYP (phosphodiesterase), and PilZ (c-di-GMP sensor). We find that rhizobia possess a high number of these proteins. Conservation analysis suggests that c-di-GMP signaling proteins modulate species-specific pathways rather than ancient rhizobia-specific processes. Two hybrid GGDEF-EAL proteins were selected for functional analysis, R. etli RHE_PD00105 (CdgA) and RHE_PD00137 (CdgB). Expression of cdgA and cdgB is repressed by the alarmone (p)ppGpp. cdgB is significantly expressed on plant roots and free living. Mutation of cdgA, cdgB, or both does not affect plant root colonization, nitrogen fixation capacity, biofilm formation, motility, and exopolysaccharide production. However, heterologous expression of the individual GGDEF and EAL domains of each protein in Escherichia coli strongly suggests that CdgA and CdgB are bifunctional proteins, possessing both diguanylate cyclase and phosphodiesterase activities. Taken together, our results provide a platform for future studies of c-di-GMP signaling in rhizobia.

Microscopic and serological survey of Trypanosoma evansi infection in Tunisian dromedary camels (Camelus dromedarius).

Camels are the most adapted species to the harsh ecosystems of the African continent where they represent 80% of world's population. However, they can be susceptible to many pathogens such as Trypanosoma evansi. This parasite is widely distributed in Africa and represents a real threat to animal health and production. In Tunisia, despite its importance, this disease is underestimated and only a few data are available. The present study aims to investigate the prevalence of the disease by combining microscopic and serological tests. A cross-sectional study was conducted on 307 dromedary camels randomly selected from Southern Tunisia. Collected blood samples were microscopically examined and tested with Card Agglutination Trypanosomiasis Test to detect anti-T. evansi antibodies. The microscopic and serological prevalence were estimated at 2.93 and 62.54%, respectively. Microscopic examination showed morphological features of T. evansi. All positives samples were collected from aged males (>5 years old). Seroprevalence was similar for both sexes but it was higher in dry season and in the southwest of Tunisia. The present study revealed high prevalence of T. evansi infection among dromedary camels. This highlights the need for adequate control measures based on the detection and treatment of infected animals and vector control. We recommend investigating the prevalence of infection in other domestic animals living in the same environment.

Agrobacterium strains isolated from root nodules of common bean specifically reduce nodulation by Rhizobium gallicum.

In a previous work, we showed that non-nodulating agrobacteria strains were able to colonize root nodules of common bean. Both rhizobia and agrobacteria co-existed in the infected nodules. No impact on symbiosis was found in laboratory conditions when using sterile gravel as a support for growth. In this study, soil samples originating from different geographic and agronomic regions in Tunisia were inoculated with a mixture of agrobacteria strains isolated previously from root nodules of common bean. A significant effect on nodulation and vegetal growth of common bean was observed. Characterization of nodulating rhizobia and comparison with non-inoculated controls showed a biased genetic structure. It seemed that Rhizobium gallicum was highly inhibited, whereas nodulation by Sinorhizobium medicae was favored. Co-inoculation of non-sterile soils with R. gallicum and agrobacteria confirmed these findings. In vitro antibiosis assays indicated that agrobacteria exercised a significant antagonism against R. gallicum.