Investigation of the physiological relationship between the cyanide-insensitive oxidase and cyanide production in Pseudomonas aeruginosa

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Abstract
Pseudomonas aeruginosais an opportunistic pathogen which demonstrates considerable respiratory versatility, possessing up to five terminal oxidases. One oxidase, the cyanide-insensitive oxidase (CIO), has been previously shown to be resistant to the potent respiratory inhibitor cyanide, a toxin that is synthesized by this bacterium. This study investigated the physiological relationship between hydrogen cyanide production and the CIO. It was found that cyanide is produced inP. aeruginosaat similar levels irrespective of its complement of CIO, indicating that the CIO is not an obligatory electron sink for cyanide synthesis. However, MICs for cyanide and growth in its presence demonstrated that the CIO providesP. aeruginosawith protection against the effects of exogenous cyanide. Nevertheless, the presence of cyanide did not affect the viability ofciomutant strains compared to the wild-type during prolonged incubation in stationary phase. The detection of the fermentation end products acetate and succinate in stationary-phase culture supernatants suggests thatP. aeruginosa, irrespective of its CIO complement, may in part rely upon fermentation for energy generation in stationary phase. Furthermore, the decrease in cyanide levels during incubation in sealed flasks suggested that active breakdown of HCN by the culture was taking place. To investigate the possibility that the CIO may play a role in pathogenicity, wild-type andciomutant strains were tested in the paralytic killing model ofCaenorhabditis elegans, a model in which cyanide is the principal toxic agent leading to nematode death. The CIO mutant had delayed killing kinetics, demonstrating that the CIO is required for full pathogenicity ofP. aeruginosain this animal model.