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Pathogenic\n                      <jats:italic toggle=\"yes\">Leptospira<\/jats:italic>\n                      are thought to infect humans from environmental sources; however, direct isolation from environmental samples remains technically challenging and is not yet well established. Here, we report that a ubiquitous environmental bacterium belonging to the former\n                      <jats:italic toggle=\"yes\">Massilia<\/jats:italic>\n                      group produces metabolites to promote the growth of\n                      <jats:italic toggle=\"yes\">Leptospira interrogans<\/jats:italic>\n                      , which has been encountered through an incidental contamination event and analyzed in this study. Gas chromatography\u2013tandem mass spectrometry (GC-MS\/MS) analysis demonstrated that cultivation of\n                      <jats:italic toggle=\"yes\">Massilia<\/jats:italic>\n                      sp. strain NBRC 108631 in R2A medium resulted in the accumulation of metabolites, including branched-chain amino acid (BCAA) intermediates, compared to fresh medium. By combining genome-scale metabolic modeling with experimental validation using cell-free culture supernatant supplementation assays, we demonstrate that BCAA intermediates, particularly 2-ketoisocaproic acid (4-methyl-2-oxopentanoate; 4MOP), a leucine biosynthetic intermediate produced by strain NBRC 108631, enhance\n                      <jats:italic toggle=\"yes\">Leptospira<\/jats:italic>\n                      growth. To investigate the metabolic role of 4MOP, we incorporated transcriptomic data into a genome-scale metabolic network model to generate condition-specific models. Resulted flux distributions indicated that\n                      <jats:italic toggle=\"yes\">Leptospira<\/jats:italic>\n                      catabolized imported 4MOP to produce acetyl-CoA. Our results reveal a previously unrecognized metabolic interaction where metabolites produced by environmental bacteria support the growth of pathogenic\n                      <jats:italic toggle=\"yes\">Leptospira<\/jats:italic>\n                      , offering mechanistic insight into its metabolic requirement. These findings have implications to understand the environmental persistence of\n                      <jats:italic toggle=\"yes\">Leptospira<\/jats:italic>\n                      through its metabolic dependencies on coexisting microbes, and they also help develop better strategies for this pathogen.\n                    <\/jats:p>\n                    <jats:sec>\n                      <jats:title>IMPORTANCE<\/jats:title>\n                      <jats:p>\n                        Pathogenic\n                        <jats:italic toggle=\"yes\">Leptospira<\/jats:italic>\n                        persist in environmental reservoirs, yet the mechanisms supporting their growth remain poorly defined. Here, we find that metabolites produced by common environmental bacteria belonging to the former\n                        <jats:italic toggle=\"yes\">Massilia<\/jats:italic>\n                        group can promote\n                        <jats:italic toggle=\"yes\">Leptospira<\/jats:italic>\n                        growth, suggesting a previously unrecognized metabolic dependency on coexisting microbes. Importantly, this study indicates that combining genome-scale metabolic modeling with experimental validation provides a useful framework for identifying metabolic interactions that are otherwise difficult to resolve using conventional culture-based approaches. Current strategies may facilitate the systematic identification of growth-supporting metabolites and provide a basis for improving selective cultivation for uncultured or difficult-to-culture organisms. The determination of growth-promoting metabolites advances our understanding of pathogen persistence in natural environments and offers a generalized framework to study metabolically dependent microorganisms.\n                      <\/jats:p>\n                    <\/jats:sec>\n                  <\/jats:sec>","DOI":"10.1128\/msystems.00638-26","type":"journal-article","created":{"date-parts":[[2026,6,30]],"date-time":"2026-06-30T13:07:30Z","timestamp":1782824850000},"update-policy":"https:\/\/doi.org\/10.1128\/asmj-crossmark-policy-page","source":"Crossref","is-referenced-by-count":0,"title":["Environmental former\n                    <i>Massilia<\/i>\n                    group bacteria secrete metabolites that promote\n                    <i>Leptospira<\/i>\n                    growth"],"prefix":"10.1128","volume":"11","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-5571-2182","authenticated-orcid":false,"given":"Michinobu","family":"Yoshimura","sequence":"first","affiliation":[{"id":[{"id":"https:\/\/ror.org\/04nt8b154","id-type":"ROR","asserted-by":"publisher"}],"name":"Department of Microbiology and Immunology, Faculty of Medicine, Fukuoka University","place":["Fukuoka, Japan"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0616-0190","authenticated-orcid":true,"given":"Ryo","family":"Ozuru","sequence":"additional","affiliation":[{"id":[{"id":"https:\/\/ror.org\/04nt8b154","id-type":"ROR","asserted-by":"publisher"}],"name":"Department of Microbiology and Immunology, Faculty of Medicine, Fukuoka University","place":["Fukuoka, Japan"]},{"id":[{"id":"https:\/\/ror.org\/0153tk833","id-type":"ROR","asserted-by":"publisher"}],"name":"Department of Biomedical Engineering, University of Virginia","place":["Charlottesville, USA"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8522-860X","authenticated-orcid":true,"given":"Satoshi","family":"Miyahara","sequence":"additional","affiliation":[{"name":"Department of Microbiology, Faculty of Medicine, University of Occupational and Environmental Health","place":["Kitakyushu, Japan"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Fumiko","family":"Obata","sequence":"additional","affiliation":[{"id":[{"id":"https:\/\/ror.org\/024yc3q36","id-type":"ROR","asserted-by":"publisher"}],"name":"Division of Bacteriology, Department of Microbiology and Immunology, Faculty of Medicine, Tottori University","place":["Yonago, Japan"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Mitsumasa","family":"Saito","sequence":"additional","affiliation":[{"name":"Department of Microbiology, Faculty of Medicine, University of Occupational and Environmental 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