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.