Multiple myeloma, or MM, is the second-most common malignancy of the blood. In MM, malignant plasma cells in the bone marrow crowd out healthy cells. The result is anemia, bone damage, and renal damage. Researchers have now discovered that part of that damage, especially to the kidneys, is caused by a microbial imbalance triggered by MM. Comparing fecal samples between 19 patients newly diagnosed with MM and 18 healthy controls revealed significant differences in the makeup of their gut microbiomes, with patients diagnosed with MM showing a high abundance of nitrogen-recycling bacteria. The accumulation of these bacteria could create a vicious cycle, where they cause a buildup of urea or ammonium (NH₄⁺) that leads to progressive kidney failure and worsening MM and cultivates more nitrogen-active bacteria. Data from a larger population could help clarify these findings and lead to new ways of diagnosing and treating multiple myeloma.