Dong‐Chan Oh

In all known organisms, amino acids are predominantly thought to be synthesized and used as their L-enantiomers. Here, we found that bacteria produce diverse D-amino acids as well, which accumulate at millimolar concentrations in supernatants of stationary phase cultures. In Vibrio cholerae, a dedicated racemase produced D-Met and D-Leu, whereas Bacillus subtilis generated D-Tyr and D-Phe. These unusual D-amino acids appear to modulate synthesis of peptidoglycan, a strong and elastic polymer that serves as the stress-bearing component of the bacterial cell wall. D-Amino acids influenced peptidoglycan composition, amount, and strength, both by means of their incorporation into the polymer and by regulating enzymes that synthesize and modify it. Thus, synthesis of D-amino acids may be a common strategy for bacteria to adapt to changing environmental conditions.

Host-microbe symbioses play a critical role in the evolution of biological diversity and complexity. In a notably intricate system, southern pine beetles use symbiotic fungi to help overcome host-tree defenses and to provide nutrition for their larvae. We show that this beetle-fungal mutualism is chemically mediated by a bacterially produced polyunsaturated peroxide. The molecule's selective toxicity toward the beetle's fungal antagonist, combined with the prevalence and localization of its bacterial source, indicates an insect-microbe association that is both mutualistic and coevolved. This unexpected finding in a well-studied system indicates that mutualistic associations between insects and antibiotic-producing bacteria are more common than currently recognized and that identifying their small-molecule mediators can provide a powerful search strategy for therapeutically useful antimicrobial compounds.