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Background The microbiome has emerged as an environmental factor contributing to obesity and type 2 diabetes (T2D). Increasing evidence suggests links between circulating bacterial components (i.e., bacterial DNA), cardiometabolic disease, and blunted response to metabolic interventions. In this aspect, thorough next-generation sequencing-based and contaminant-aware approaches are lacking. To address this, we tested whether bacterial DNA could be amplified in the blood of subjects with obesity and high metabolic risk under strict experimental and analytical control and whether a putative bacterial signature is related to metabolic improvement after bariatric surgery. Methods Subjects undergoing bariatric surgery were recruited into sex- and BMI-matched subgroups with (n = 24) or without T2D (n = 24). Bacterial DNA in the blood was quantified and prokaryotic 16S rRNA gene amplicons were sequenced. A contaminant-aware approach was applied to derive a compositional microbial signature from bacterial sequences in all subjects at baseline and at 3 and 12 months after surgery. We modeled associations between bacterial load and composition with host metabolic and anthropometric markers. We further tested whether compositional shifts were related to weight loss response and T2D remission. Lastly, bacteria were visualized in blood samples using catalyzed reporter deposition (CARD)-fluorescence in situ hybridization (FISH). Results The contaminant-aware blood bacterial signature was associated with metabolic health. Based on bacterial phyla and genera detected in the blood samples, a metabolic syndrome classification index score was derived and shown to robustly classify subjects along their actual clinical group. T2D was characterized by decreased bacterial richness and loss of genera associated with improved metabolic health. Weight loss and metabolic improvement following bariatric surgery were associated with an early and stable increase of these genera in parallel with improvements in key cardiometabolic risk parameters. CARD-FISH allowed the detection of living bacteria in blood samples in obesity. Conclusions We show that the circulating bacterial signature reflects metabolic disease and its improvement after bariatric surgery. Our work provides contaminant-aware evidence for the presence of living bacteria in the blood and suggests a putative crosstalk between components of the blood and metabolism in metabolic health regulation.

Helium‐ion microscopy (HIM) has so far rarely been employed to image microbial interactions. Here, the visualization of the life‐cycle of the bacterial predator Bdellovibrio bacteriovorus HD100 with Escherichia coli and Pseudomonas putida, respectively, as prey is presented. The predator is brought in contact with prey and samples are taken at selected times. The system is monitored by phase‐contrast microscopy and HIM. For HIM imaging, a sample preparation protocol is established that preserves the structure of Bdellovibrio, prey, and bdelloplasts. The micrographs show the attachment of the predator to its prey, the evolution of bdelloplasts, their lysis, and the release of predator progeny. The combination of HIM with two more approaches allows for investigating predator–prey interactions from different angles: First, phase‐contrast micrographs provide quantitative information for the numbers of predator, prey, and bdelloplasts. Second, a numerical model solving the retarded differential equations that describe the system's time‐evolution is developed and fits the experimentally determined cell numbers. In conclusion, the high resolution, the large depth of focus, and surface sensitivity of HIM hold promise to expand future studies on so far neglected ecological interactions within the microbial food web, in particular in samples with pronounced topography such as bacterial biofilms.

Objective: Increasing evidence suggests links between circulating bacterial components and cardiometabolic disease. Nevertheless, thorough next generation sequencing based and contaminant aware approaches are lacking. We hypothesized that blood bacterial signature is associated with the individual metabolic risk and outcome of bariatric surgery in patients with high metabolic burden. Design: Bacterial DNA in the blood was quantified and 16S rRNA gene amplicons were sequenced in bariatric surgery candidates recruited into sex and BMI matched subgroups with (n=24) or without T2D (n=24). A stringent experimental and bioinformatic contaminant aware approach was applied to derive a compositional microbial signature from bacterial sequences at baseline, three and twelve months post-bariatric surgery. Associations between bacterial load and composition and host metabolic state were modeled. Lastly, Catalyzed Reporter Deposition (CARD) - Fluorescence In Situ Hybridization (FISH) was employed to visualize bacteria in blood samples. Results: A metabolic syndrome classification score was derived based on bacterial phyla and genera detected in the blood, which robustly classified subjects along their actual clinical group. Blood bacterial signature in T2D was characterized by decreased bacterial richness and a loss of health-associated genera and was further impacted by medication. Weight loss and improvement of glucose tolerance following bariatric surgery were associated with an early and stable increase of these genera in parallel with improvements in key metabolic parameters. CARD-FISH allowed the detection of living bacteria in blood samples in obesity. Conclusions: Our work provides contaminant aware evidence for a putative crosstalk between bacterial components of the blood, metabolism and medication in metabolic health regulation.