Purple-Pigmented Violacein-Producing Duganella spp. Inhabit the Rhizosphere of Wild and Cultivated Olives in Southern Spain
Bacteria have evolved mechanisms that allow them to grow and survive in highly competitive environments like soil and the rhizosphere. Using classical microbiological, physiological, and genetic analyses, we isolated and identified for the first time Duganella spp. associated with the rhizosphere of woody plants in Mediterranean environments that are able to produce violacein, a blue–purple secondary metabolite of considerable biotechnological interest. Based on physiological and biochemical characterization and phylogenetic analysis of different genes including 16S rRNA, gyrB, and vioA (implicated in the synthesis of violacein), the seven Duganella spp. strains isolated and studied were differentiated according to their host of origin (wild versus cultivated olives) and potentially might belong to new species. All the Duganella spp. strains produced violacein in vitro, with natural production levels significantly higher than that previously reported for other violacein-producing bacteria without optimizing growing conditions. The important biological, medical, and industrial applications of violacein make these bacteria good candidates for their biotechnological exploitation because low violacein yields are considered as one of the main limitations of using wild-type strains for extensive exploitation and pigment production. Independent of violacein production, purple-pigmented strains from olives showed proteolytic and lipolytic activities and a weak siderophore production. No in vitro inhibitory activity was demonstrated for bacteria or crude violacein filtrates against plant-pathogenic Gram-negative bacteria and fungi, but they did inhibit Gram-positive bacteria.
This research was supported by grants AGL2008-00344 and HA2008-0014 from ‘Ministerio de Ciencia e Innovación’ of Spain and the European Social Fund and PI2007-40I012 ‘Intramural Project’ to B. B. Landa from the Spanish National Research Council (CSIC). S. Aranda was a recipient of a Ph.D. grant from ‘Consejo Nacional de Ciencia y Tecnología’ (CONACYT) México. The authors thank F.J. Durán Gutiérrez, C. Cantalapiedra-Navarrete, and P. Castillo from IAS-CSIC for the excellent technical assistance and providing some samples from wild and commercial olives and E. Prats and A. de Haro from IAS-CSIC for HPLC and spectrophotometric analysis, respectively. We also thank the ‘Patología Agroforestal’ group from University of Córdoba, E. Montesinos from University of Girona, and M. López from IVIA, Valencia, Spain, for providing some of the fungal and bacterial isolates. Editorial improvement from anonymous reviewers is gratefully acknowledged.
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