Anyone for d ? The chemistry of amino acids comes in two chirally distinct flavors—so-called l - and d -enantiomers. By far the most commonly used form of amino acids in all kingdoms of life is the l -form. Now Lam et al. (p. 1552 ; see the Perspective by Blanke ) present the unanticipated observation that diverse bacteria release large amounts of various d -amino acids into the environment in a population density–dependent fashion and that d -amino acids act as extracellular effectors that regulate the composition, structure, amount, and strength of peptidoglycan, the major stress-bearing component of the bacterial cell wall.