Tracking the origin of bacterial DNA in blood: Indication of localized and sporadic transfer from other body sites.

Recent studies propose the existence of a blood microbiome, but its composition, origin, and dynamics remain largely unresolved. In this pilot study, we analyzed the bacterial DNA present in the blood of 10 volunteers by comparing the taxonomic profiles of 16S rRNA gene sequences from skin, vaginal, oral, and fecal samples. After applying stringent decontamination protocols, we detected bacterial DNA in all blood samples, predominantly from the Pseudomonas genus. A key finding was the identification of 32 unique Amplicon Sequence Variants (ASVs) that were identical between blood and a single body site within individual participants, with no overlap between multiple body sites or across different participants. This participant-specific overlap suggests a true biological origin of bacterial DNA in blood, likely stemming from localized bacterial migration, such as from the skin. Additionally, 27.4% of the ASVs in blood were found in other body sites, with the highest overlap observed in skin samples. Furthermore, 25.3% of blood ASVs persisted after three months, suggesting a consistent pattern in the bacterial DNA composition detected in blood over time. These findings deepen our understanding of the blood microbiome and provide a basis for future research linking blood microbiota to health and disease phenotypes.