<i>Lactobacillus gallinarum</i> modulates the gut microbiota and produces anti-cancer metabolites to protect against colorectal tumourigenesis.

Abstract

Using faecal shotgun metagenomic sequencing, we identified the depletion of <i>Lactobacillus gallinarum</i> in patients with colorectal cancer (CRC). We aimed to determine the potential antitumourigenic role of <i>L. gallinarum</i> in colorectal tumourigenesis. The tumor-suppressive effect of <i>L. gallinarum</i> was assessed in murine models of CRC. CRC cell lines and organoids derived from patients with CRC were cultured with <i>L. gallinarum</i> or <i>Escherichia coli</i> MG1655 culture-supernatant to evaluate cell proliferation, apoptosis and cell cycle distribution. Gut microbiota was assessed by 16S ribosomal DNA sequencing. Antitumour molecule produced from <i>L. gallinarum</i> was identified by liquid chromatography mass spectrometry (LC-MS/MS) and targeted mass spectrometry. <i>L. gallinarum</i> significantly reduced intestinal tumour number and size compared with <i>E. coli</i> MG1655 and phosphate-buffered saline in both male and female murine intestinal tumourigenesis models. Faecal microbial profiling revealed enrichment of probiotics and depletion of pathogenic bacteria in <i>L. gallinarum</i>-treated mice. Culturing CRC cells with <i>L. gallinarum</i> culture-supernatant (5%, 10% and 20%) concentration-dependently suppressed cell proliferation and colony formation. <i>L. gallinarum</i> culture-supernatant significantly promoted apoptosis in CRC cells and patient-derived CRC organoids, but not in normal colon epithelial cells. Only <i>L. gallinarum</i> culture-supernatant with fraction size <3 kDa suppressed proliferation in CRC cells. Using LC-MS/MS, enrichments of indole-3-lactic acid (ILA) was identified in both <i>L. gallinarum</i> culture-supernatant and the gut of <i>L. gallinarum</i>-treated mice. ILA displayed anti-CRC growth <i>in vitro</i> and inhibited intestinal tumourigenesis <i>in vivo</i>. <i>L. gallinarum</i> protects against intestinal tumourigenesis by producing protective metabolites that can promote apoptosis of CRC cells.