Characterizing the microbiome of “sterile” organs in experimental mice and evidence of translocation of bacteria from the gut to other internal organs

Using culturomics and metagenomics, we demonstrate the existence of non-pathogenic microbiota in the internal organs of healthy experimental mice, challenging the traditional dogma of organ sterility. Based on the analysis of 104 commercially sourced mice (C57BL/6J, BALB/c, ICR), the study reveals that over 20% of the analyzed mice harbored a high microbial burden in the internal organs and identified a total of 463 microbial species. Several species, including <i>Ligilactobacillus murinus</i>, <i>Alcaligenes faecalis</i>, <i>Micrococcus luteus</i>, <i>Pseudochrobactrum asaccharolyticum</i>, <i>Escherichia coli</i>, and <i>Microbacterium</i> sp., were frequently identified and were abundant in the mouse tissues. Further investigation implies that microorganisms in the "sterile" tissues could be associated with the gut microbiota. Given the wide use of experimental mice in medical and biological research, these findings of resident microorganisms in the animal's internal organs raise concerns about potential variability in experimental outcomes.