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Pseudomonas chlororaphis strain PA23 is a biocontrol agent capable of suppressing disease caused by the fungal pathogen Sclerotinia sclerotiorum. This bacterium produces the diffusible antibiotics phenazine-1-carboxylic acid, 2-hydroxyphenazine and pyrrolnitrin (PRN). Because the individual contribution of these antibiotics to PA23 biocontrol has not been defined, mutants deficient in the production of phenazine (PHZ), PRN or both antibiotics were created. Analysis of the PHZ mutant revealed enhanced antifungal activity in vitro and wild-type levels of Sclerotinia disease suppression. Conversely, the PRN- and the PRN/PHZ-deficient strains exhibited decreased antifungal activity in vitro and markedly reduced the ability to control Sclerotinia infection of canola in the greenhouse. These findings suggest that PRN is the primary antibiotic mediating biocontrol of this pathogen. Analysis of prnA-lacZ and phzA-lacZ transcriptional fusions revealed that PRN and PHZ are not subject to autoregulation; moreover, they do not cross-regulate each other. However, HPLC showed a twofold increase in PRN levels in the PHZ(-) background. Finally, PHZ, but not PRN production, is involved in biofilm development in P. chlororaphis PA23.

Examines the role and expression patterns of quorom-sensing genes in Pseudomonas aeruginosa biofilms. Role of the las and rhl quorum-sensing systems in static biofilm formation; Effect of quorum-sensing on lipopolysaccharide expression; lasI and rhlI expression during PAO1 biofilm formation; Effect of media on biofilm formation in a flowing system.

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. [ABSTRACT FROM AUTHOR]

Abstract: Pseudomonas sp. DF41 is a biocontrol agent capable of suppressing Sclerotinia sclerotiorum-mediated stem rot of canola. Using transposon mutagenesis, we identified two mutants with greatly reduced antifungal (AF) activity. The first mutant had an insertion in gacS, forming part of the GacS/GacA regulatory system. The second mutation was in a gene involved in lipopeptide (LP) synthesis suggesting LPs are essential for biocontrol. Strain DF41 and the gacS (DF41-469) and lp (DF469-1278) mutants were characterized with respect to their extracellular metabolite production, antifungal (AF) properties and ability to form biofilms. We discovered that DF41 produces a number of compounds that may contribute to biocontrol including hydrogen cyanide (HCN), protease, alginate, and LP molecules. All of these products were found to be under Gac control. In addition, DF41 produces autoinducers, suggesting this bacterium employs quorum sensing as part of its lifestyle. In the greenhouse, the gacS and lp mutants were unable to protect canola from disease incited by S. sclerotiorum. Both mutants were able to sustain themselves in the canola phyllosphere; therefore, the loss of biocontrol activity can be attributed to reduced production of AF compounds and not a declining population size. We conclude that suppression of Sclerotinia stem rot of canola by Pseudomonas strain DF41 depends upon LP production and a functional Gac system. [ABSTRACT FROM AUTHOR]