Pyrrolnitrin and Hydrogen Cyanide Production by Pseudomonas chlororaphis Strain PA23 Exhibits Nematicidal and Repellent Activity against Caenorhabditis elegans

Pseudomonas chlororaphis strain PA23 is a biocontrol agent able to suppress growth of the fungal pathogen Sclerotinia sclerotiorum. This bacterium produces an arsenal of exometa-bolites including pyrrolnitrin (PRN), phenazine (PHZ), hydrogen cyanide (HCN), and degradative enzymes. Production of these compounds is controlled at both the transcriptional and posttranscriptional levels by the Gac-Rsm system, RpoS, PsrA, and the Phz quorum-sensing system. Beyond pathogen-suppression, the success of a biocontrol agent is dependent upon its ability to establish itself in the environment where predation by bacterivorous organisms, including nematodes, may threaten persistence. The focus of this study was to investigate whether PA23 is able to resist grazing by Caenorhabditis elegans and to define the role played by exoproducts in the bacterial-nematode interaction. We discovered that both PRN and HCN contribute to fast- and slow-killing of C. elegans. HCN is well-established as having lethal effects on C. elegans; however, PRN has not been reported to be nematicidal. Exposure of L4 stage nematodes to purified PRN reduced nematode viability in a dose-dependent fashion and led to reduced hatching of eggs laid by gravid adults. Because bacterial metabolites can act as chemoattractants or repellents, we analyzed whether PA23 exhibited attractant or repulsive properties towards C. elegans. Both PRN and HCN were found to be potent repellents. Next we investigated whether the presence of C. elegans would elicit changes in PA23 gene activity. Co-culturing the two organisms increased expression of a number of genes associated with biocontrol, including phzA, hcnA, phzR, phzI, rpoS and gacS. Exoproduct analysis showed that PHZ and autoinducer signals were upregulated, consistent with the gene expression profiles. Collectively, these findings indicate that PA23 is able to sense the presence of C. elegans and it is able to both repel and kill the nematodes, which should facilitate environmental persistence and ultimately biocontrol.