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Applied and Environmental Microbiology, November 2006, p. 7083-7090, Vol. 72, No. 11
0099-2240/06/$08.00+0     doi:10.1128/AEM.00557-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Secondary Metabolites Help Biocontrol Strain Pseudomonas fluorescens CHA0 To Escape Protozoan Grazing

Alexandre Jousset, Enrique Lara, Luis G. Wall, and Claudio Valverde^*

Programa Prioritario sobre Interacciones Biológicas, Departamento de Ciencia y Tecnología, Universidad Nacional de Quilmes, Roque Sáenz Peña 352, Bernal B1876BXD, Provincia de Buenos Aires, Argentina

Received 8 March 2006/ Accepted 6 August 2006

In soil ecosystems, bacteria must cope with predation activity, which is attributed mainly to protists. The development of antipredation strategies may help bacteria maintain higher populations and persist longer in the soil. We analyzed the interaction between the root-colonizing and biocontrol strain Pseudomonas fluorescens CHA0 and three different protist isolates (an amoeba, a flagellate, and a ciliate). CHA0 produces a set of antibiotics, HCN, and an exoprotease. We observed that protists cannot grow on CHA0 but can multiply on isogenic regulatory mutants that do not produce the extracellular metabolites. The in vitro responses to CHA0 cells and its exoproducts included growth inhibition, encystation, paralysis, and cell lysis. By analyzing the responses of protists to bacterial supernatants obtained from different isogenic mutants whose production of one or more exometabolites was affected and also to culture extracts with antibiotic enrichment, we observed different contributions of the phenolic antifungal compound 2,4-diacetylphloroglucinol (DAPG) and the extracellular protease AprA to CHA0 toxicity for protists and to the encystation-reactivation cycle. The grazing pressure artificially produced by a mixture of the three protists in a microcosm system resulted in reduced colonization of cucumber roots by a regulatory isogenic CHA0 mutant unable to produce toxins. These results suggest that exometabolite production in biocontrol strain CHA0 may contribute to avoidance of protist grazing and help sustain higher populations in the rhizosphere, which may be a desirable and advantageous trait for competition with other bacteria for available resources.

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* Corresponding author. Mailing address: Programa Prioritario sobre Interacciones Biológicas, Departamento de Ciencia y Tecnología, Universidad Nacional de Quilmes, Roque Sáenz Peña 352, Bernal B1876BXD, Provincia de Buenos Aires, Argentina. Phone: 54 (11) 4365 7100, ext. 313. Fax: 54 (11) 4365 7132. E-mail: cvalver{at}unq.edu.ar.

Present address: Rhizosphere Ecology Group, Institut für Zoologie, Technische Universität Darmstadt, D-64287 Darmstadt, Germany.

Present address: Laboratoire de Biotechnologie Environnementale, ISTE, ENAC, Polytechnical School of Lausanne, CH-1015 Lausanne, Switzerland.

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Applied and Environmental Microbiology, November 2006, p. 7083-7090, Vol. 72, No. 11
0099-2240/06/$08.00+0     doi:10.1128/AEM.00557-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.