Inside Echinacea purpurea endophytes bacterial community: new sources of bioactive compounds with medical and/or pharmaceutical applications

Microorganisms are naturally associated with plants in various ways: as beneficial symbionts , pathogens or commensal endophytes. Endophytic bacteria (or fungi) internally colonize host tissues, sometimes at high concentration, without damaging the host or eliciting symptoms of plant disease. There are evidences that many endophytic bacteria have beneficial effects on plants, like growth promotion and a role in nitrogen fixation or mobilization of other nutrients; endophytes also provide for plant protection against pathogens, by induction of plant defence mechanisms, production of pathogen-antagonistic substances or through competition for colonization sites and nutrients. Therefore, the therapeutic properties of medicinal plants could also be related to their endophytic communities. Consequently, endophytes might provide an abundant reservoir of bioactive metabolites for medicinal/biotechnological application. In this study bacterial communities isolated from the medicinal plant Echinacea purpurea were analysed. Aims of this work were to: a) study the structure (via RAPD analysis) and composition (via 16S rRNA analysis) of endophytic communities; b) check their ability to grow in presence of antibiotics, heavy metals, H2O2 and diesel fuel; c) identify strains with antimicrobial activity. Data obtained revealed that bacterial communities isolated from distinct compartments of the plant exhibited a different degree of biodiversity at genus/species and/or strain level. The vast majority of bacteria analysed is Gram- (73%) and overall the community is composed by γ-proteobacteria (45%), whereas among them the most represented genus is Pseudomonas (43%). The situation is different analysing the distribution of the communities in different plant compartments. In particular in soil and in roots the most represented group is γ-proteobacteria, whereas in the steam/leaf compartment the most represented are α-proteobacteria. Concerning physiology, bacteria exhibited different characteristics: strains from roots were much resistant to antibiotics than those from steam/leaf, which showed a better growth rate in presence of H2O2. Interestingly, cross-streak tests using different Cystic Fibrosis (opportunistic) pathogens as targets (Burkholderia sp., Staphylococcus aureus and Pseudomonas aeruginosa) revealed that several endophytic strains were able to inhibit the growth of such pathogens. Data highlighted that endophytes from E. purpurea might represent a new source of natural bioactive compounds with medical/pharmaceutical and/or biotechnological applications.