More than 500 kinds with number of 100–1000 trillion of bacteria colonize intestinal tract and form intestinal microbiota (flora). Recently, researches on intestinal microbiota have advanced progressively due to development of 16S ribosomal RNA analysis and meta-genome analysis. Novel findings in the correlation between intestinal microbiota and medical diseases including Clostridium difficile infection, inflammatory bowel diseases, irritable bowel syndrome, colorectal cancer, liver diseases (liver cirrhosis, nonalcoholic steatohepatitis, hepatic cancer), obesity, type 2 diabetes, atherosclerosis and multiple sclerosis have been reported. In addition to the correlation between intestinal microbiota and the medical diseases, the researches to determine how intestinal microbiota cause the diseases are expected to be done in the near future.

We report a case of septic pulmonary embolization due to Mycobacterium mucogenicum. A 6-year-old boy was diagnosed B precursor cell type acute lymphoblastic leukemia a year ago, and thereafter he had been receiving chemotherapy. During the 6^th chemotherapy treatment, he had fever and blood culture was positive. The Gram staining of his blood culture sample did not show any bacteria but Ziehl-Neelsen staining showed bacteria. We did a rapid identification test using mass spectrometer, which proved that the results were highly positive for Mycobacterium phocaicum. Computed tomography of his chest revealed multiple small nodules images, so we diagnosed his infection as a septic pulmonary embolization due to rapidly growing mycobacteria. We removed the central venous catheter in an early stage and started multiple antibiotic agents. After that his fever rapidly subsided. The pathogen was finally identified as M. mucogenicum by gene analysis. M. mucogenicum is one of the rapidly growing mycobacteria which could be found in water and soil. It causes opportunistic infectious disease including abscess in lung and skin. In the case of immunocompromised patient with positive blood culture and negative Gram staining, we should consider the possibility of bloodstream infection due to rapidly growing mycobacteria.