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

Abstract

Pseudomonas brassicacearum DF41 is a biocontrol agent that suppresses disease caused by the fungal pathogen Sclerotinia 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 by Caenorhabditis elegans. We discovered that this bacterium is capable of killing C. 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 for Yersinia spp. and Xenorhabdus nematophila, is dependent upon the Gac system. Biofilms were not observed when bacteria were grown on NaCl-containing medium or on C. elegans biofilm-resistant mutants. Coculturing with nematodes led to the increased expression of the pdfRI-rfiA QS genes and hcnA, 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 of C. elegans, and through two distinct mechanisms, it can escape predation. IMPORTANCEPseudomonas 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 nematode Caenorhabditis 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 the C. elegans surface. When grown with C. 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.

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“…Therefore it is not surprising that bacteria have evolved defensive mechanisms to resist predation. A study investigating the interaction of DF41 and the model organism Caenorhabditis elegans revealed that DF41 was capable of killing the nematodes through two distinct mechanisms [25]. The first involved exposure to toxic bacterial metabolites, in particular HCN, which induced nematode death [25].…”

“…A study investigating the interaction of DF41 and the model organism Caenorhabditis elegans revealed that DF41 was capable of killing the nematodes through two distinct mechanisms [25]. The first involved exposure to toxic bacterial metabolites, in particular HCN, which induced nematode death [25]. HCN is known to cause lethal paralysis by inhibiting mitochondrial cytochrome oxidase in C. elegans [71,128].…”

“…HCN is known to cause lethal paralysis by inhibiting mitochondrial cytochrome oxidase in C. elegans [71,128]. Addition of FeCl 3 to the media, which elevates HCN production, increased the lethality of DF41 even further [25]. Interestingly, the LP-minus derivative exhibited only a modest decrease in nematicidal activity compared to wildtype, suggesting that sclerosin is only slighly toxic for the nematodes [25].…”

“…There are currently no metadata associated with BS3663 and to the best of our knowledge, there are no peer-reviewed publications available on this or strain 51MFCVI2.1. The second group includes DF41, a well-characterized biocontrol isolate from canola root [22,23,9,24,25], as well as two strains isolated from Wyoming soil, 36B7 and 36D4 which have high (98.3 and 98.5 %) pairwise amino acid identities (AAIs) with DF41 [20]. Two other soil isolates from Wyoming, 38D4 and 38D7, form the third group, while the fourth group consists of 37D10 and 48H11, isolated from soils in Wyoming and Wisconsin, respectively.…”

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“…Other studies have raised the possibility that bacterial biofilm formation is important in the process of biocontrol (Al-Ali et al, 2017;Chen et al, 2013). Indeed, the formation of a biofilm in the buccal cavity of the nematode, Caenorhabditis elegans, has been cited as an active virulence factor for the biocidal effect of Pseudomonas brassicacearum (Nandi et al, 2016). This process could be relevant to the use of the bacterium as a biocontrol agent for nematodes.…”