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Pseudomonas brassicacearum DF41 is a biocontrol agent that suppresses disease caused by the fungal pathogenSclerotinia sclerotiorum . A number of exometabolites are produced by DF41, including the lipopeptide sclerosin, hydrogen cyanide (HCN), and degradative enzymes. The production of these compounds is controlled at both the transcriptional and posttranscriptional levels by quorum sensing (QS) and the Gac two-component regulatory system. In order to be successful, a biocontrol agent must persist in the environment at levels sufficient for pathogen control. Bacterivorous predators, including nematodes, represent a challenge to the establishment of introduced microorganisms. In the current study, DF41 was investigated for its ability to resist predation byCaenorhabditis elegans . We discovered that this bacterium is capable of killingC. elegans through two different mechanisms: the first involves exposure to toxic metabolites, and the second entails biofilm formation on the nematode head blocking the buccal cavity. Biofilm formation on nematodes, which has been reported only forYersinia spp. andXenorhabdus nematophila , is dependent upon the Gac system. Biofilms were not observed when bacteria were grown on NaCl-containing medium or onC. elegans biofilm-resistant mutants. Coculturing with nematodes led to the increased expression of thepdfRI-rfiA QS genes andhcnA , which is under QS control. HCN was the most nematicidal of the exometabolites, suggesting that this bacterium can respond to predator cues and upregulate expression of toxins accordingly. In summary, DF41 is able to respond to the presence ofC. elegans , and through two distinct mechanisms, it can escape predation.IMPORTANCE Pseudomonas brassicacearum DF41 can suppress fungal pathogens through a process known as biocontrol. To be successful, a biocontrol agent must be able to persist in the environment at levels sufficient for pathogen control. Predators, including the nematodeCaenorhabditis elegans , represent a threat to persistence. The aim of the current study was to investigate the DF41-C. elegans interaction. We discovered that DF41 is able to escape predation through two distinct mechanisms. The first involves exposure to toxic bacterial metabolites, and the second entails the formation of a sticky coating on the nematode head, called a biofilm, which blocks feeding and causes starvation. We report here a pseudomonad forming biofilms on theC. elegans surface. When grown withC. elegans , DF41 exhibits altered gene expression and metabolite production, indicating that this bacterium can sense the presence of these predators and adjust its physiology accordingly.

Pseudomonas brassicacearum Strain DF41 Kills Caenorhabditis elegans through Biofilm-Dependent and Biofilm-Independent Mechanisms