Hydrogen cyanide is a volatile modulator of bacterial behaviour

Bacteria communicate with each other and with other organisms in a chemical language comprising both diffusible and volatile molecules, and volatiles have recently gained increasing interest as mediators of bacterial interactions1. One of the first volatile compounds discovered to play a role in biotic interactions is hydrogen cyanide (HCN)2, a well-known toxin, which irreversibly binds to the key respiratory enzyme cytochrome C oxidase3. The main ecological function of this molecule was so far thought to lie in the inhibition of competing microorganisms4. Here we show that HCN is much more than a respiratory toxin and should be considered a major regulator of bacterial behaviour rather than a solely defensive secondary metabolite. Cyanogenesis occurs in both environmental and clinical Pseudomonas strains5,6. Using cyanide-deficient mutants in two Pseudomonas strains, we demonstrate that HCN functions as an intracellular and extracellular volatile signalling molecule, which leads to global transcriptome reprogramming affecting growth, motility, and biofilm formation, as well as the production of other secondary metabolites such as siderophores and phenazines. Our data suggest that bacteria are not only using endogenous HCN to control their own cellular functions, but are also able to remotely influence the behaviour of other bacteria sharing the same environment.