Enantiomer sensing enables social avoidance by bacterial spores

The role of molecular chirality in shaping interactions among organisms remains unclear. We discovered that amino acid stereochemistry provides a social avoidance mechanism for Bacillus subtilis spores. Germination of spores is triggered by L-alanine and competitively inhibited by its enantiomer D-alanine, produced almost exclusively by bacteria. The biological role of this enantiomer sensing remains unclear. We quantified the L- and D-alanine concentrations secreted by over 20 diverse bacterial species. We find that enantiomer ratios secreted by these species are located just beyond the germination response range for B. subtilis spores. Spores thus avoid germination when sensing other species. By forcing germination in pairwise co-cultures, we show that the presence of another bacterial species is often detrimental to germinating spores. Spores thus appear to exploit the L-/D-alanine ratio as a social avoidance mechanism, revealing a benefit for enantiomer sensing and potential importance of amino acid handedness in ecosystems.