The intestinal microbiota is known to regulate host energy homeostasis and can be influenced by high-calorie diets. However, changes affecting the ecosystem at the functional level are still not well characterized. We measured shifts in cecal bacterial communities in mice fed a carbohydrate or high-fat (HF) diet for 12 weeks at the level of the following: (i) diversity and taxa distribution by high-throughput 16S ribosomal RNA gene sequencing; (ii) bulk and single-cell chemical composition by Fourier-transform infrared- (FT-IR) and Raman micro-spectroscopy and (iii) metaproteome and metabolome via high-resolution mass spectrometry. High-fat diet caused shifts in the diversity of dominant gut bacteria and altered the proportion of Ruminococcaceae (decrease) and Rikenellaceae (increase). FT-IR spectroscopy revealed that the impact of the diet on cecal chemical fingerprints is greater than the impact of microbiota composition. Diet-driven changes in biochemical fingerprints of members of the Bacteroidales and Lachnospiraceae were also observed at the level of single cells, indicating that there were distinct differences in cellular composition of dominant phylotypes under different diets. Metaproteome and metabolome analyses based on the occurrence of 1760 bacterial proteins and 86 annotated metabolites revealed distinct HF diet-specific profiles. Alteration of hormonal and anti-microbial networks, bile acid and bilirubin metabolism and shifts towards amino acid and simple sugars metabolism were observed. We conclude that a HF diet markedly affects the gut bacterial ecosystem at the functional level.

Because of their beneficial impact on plants, the highly diverse mycorrhizal fungi grouped in the order Sebacinales lay claim to high ecological and agricultural significance. Here, we describe for the first time associations of Sebacinoid members with bacteria. Using quantitative PCR, denaturating gradient gel electrophoresis and fluorescence in situ hybridization, we detected an intimate association between Piriformospora indica and Rhizobium radiobacter, an α‐Proteobacterium. The stability of the association, vertical transmission of the bacteria during asexual fungal reproduction and fungal plant colonization was monitored using R. radiobacter‐specific primers. Treatment of mycelium or fungal protoplasts with antibiotics highly efficient against the free bacteria failed to cure the fungus. Barley seedlings dip‐inoculated with R. radiobacter showed growth promotion and systemic resistance to the powdery mildew fungus Blumeria graminis comparable to P. indica inoculation. By screening additional isolates of the Sebacina vermifera complex, three species‐specific associations with bacteria from the genera Paenibacillus, Acinetobacter and Rhodococcus were found. These findings suggest that Sebacinales species regularly undergo complex interactions involving host plants and bacteria reminiscent of other ectomycorrhizal and endomycorrhizal associations.

Gluconacetobacter diazotrophicus is an aerobic diazotrophic plant-growth-promoting bacterium isolated from different gramineous plants. We showed that reactive oxygen species (ROS) were produced at early stages of rice root colonization, a typical plant defense response against pathogens. The transcription of the pathogen-related-10 gene of the jasmonic acid (JA) pathway but not of the PR-1 gene of the salicylic acid pathway was activated by the endophytic colonization of rice roots by G. diazotrophicus strain PAL5. Quantitative polymerase chain reaction analyses showed that, at early stages of colonization, the bacteria upregulated the transcript levels of ROS-detoxifying genes such as superoxide dismutase (SOD) and glutathione reductase (GR). To proof the role of ROS-scavenging enzymes in the colonization and interaction process, transposon insertion mutants of the SOD and GR genes of strain PAL5 were constructed. The SOD and GR mutants were unable to efficiently colonize the roots, indicated by the decrease of tightly root-associated bacterial cell counts and endophytic colonization and by fluorescence in situ hybridization analysis. Interestingly, the mutants did not induce the PR-10 of the JA-pathway, probably due to the inability of endophytic colonization. Thus, ROS-scavenging enzymes of G. diazotrophicus strain PAL5 play an important role in the endophytic colonization of rice plants.

Bacterial quorum sensing compounds are important modulators of microbe-plant interactions