A detailed analysis of how bacterial DNA is distributed among human internal organs is described in a paper published in Nature Metabolism.
Bacteria can be found all over the body and inside the gut. Although internal organs are typically sterile, in diseases - such as obesity - fragments of bacteria can move from the gut into the blood and trigger inflammatory responses. However, a detailed understanding of the origin of such bacterial fragments and their distribution among tissues has been lacking, in part because of the difficulty in obtaining clean tissue samples.
Andre Marette and colleagues collected blood, liver and three types of fat tissue samples (subcutaneous, mesenteric and omental fat) from 40 patients as they underwent weight-loss surgery. The authors analysed microbial DNA found within the samples and uncovered specific differences in the type of bacteria and amount of bacterial DNA in each tissue sample. The authors also identified a unique microbial DNA signature in the fat tissue of 20 patients with type 2 diabetes. Furthermore, in the tissue samples, they detected DNA not only from gut bacteria but also from bacteria commonly found in soil or water. This finding suggests that internal organs may be regularly exposed to foreign genetic material.
This study demonstrates how bacteria or bacterial DNA is distributed within the human body outside the gut. Further research is needed to determine whether the bacteria in the samples were alive, and therefore able to form an internal tissue microbiome, or whether tissues contained fragments of bacterial DNA. Future work could help to clarify how microbial DNA reached the tissues - whether it involved transportation out of the gut or occurred with the help of immune cells.
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