Endophytic bacterial communities from Lavandula officinalis: a new source of biotechnologically relevant natural bioactive compounds

Bacterial endophytes are fundamental to plant life: they facilitate the uptake of nutrients such as nitrogen, phosphorus, sulphur, magnesium, and calcium and show plant growth-promoting activity (PGPR) related to the production of phytohormones and enzymes involved in plant regulatory metabolism. Moreover, endophytic bacteria are important players to decrease or prevent many pests and diseases that affect plants, or to increase plant tolerance to pollution or stresses. Therefore, endophytes are very promising from a biotechnological perspective, as they produce bioactive molecules such as novel antimicrobial compounds; they can be defined as “chemical synthesizer inside plants”, able to produce a plethora of bioactive compounds that can be used, for example, in tackling the emergence of antibiotic resistance and other human health and wellbeing issues. Indeed it is likely that the therapeutic properties of medicinal plants are also influenced by their endophytic communities as bacterial strains can directly produce bioactive compounds and/or elicit plant metabolism to produce them. The present work is aimed at the isolation and molecular characterization of bacterial communities from the widely used aromatic plants Lavandula officinalis, building a collection of bacterial isolates, and checking the inhibitory capacity of endophytic isolates toward human pathogens. To this purpose, heterotrophic cultivable bacteria were isolated from surface-sterilized tissues (roots stems, and leaves) and from the rhizosphere; sequencing of the 16S rRNA genes from 400 randomly selected isolated (100 isolates for each sample) was carried out, followed by taxonomic identification in databases. Antimicrobial activity testing was performed with the cross-streak method against Cystic Fibrosis opportunistic pathogens belonging to the Burkholderia cepacia complex (Bcc). The total lavender aerobic heterotrophic cultivable endophytic community is made up by a total of 11 genera; the majority of the isolated strains (88%) belonged to Proteobacteria, with a strong prevalence of Gammaproteobacteria (74%) with Pseudomonas being the most abundant genus accounting for the 51% of total community, followed by Stenotrophomonas (13%) and Pantoea (9%). Rhizobium is also frequently found in the internal tissue of lavender, representing the 14% of the total community. Actinomycetales are also represented (8%) with the genus Microbacterium being the most abundant (6%). Other genera were found: Bacillus, Plantibacter, Psychrobacter, Sanguibacter, Salinibacterium, Jeotgalibacillus representing together the 6% of the endophytic population. Overall the data showed that there is a strong variation in the composition of the communities between the different plant tissues, the lowest diversity was registered in the stem whose communities is dominated by Pseudomonadales; ii) a low overall diversity in the composition was found; iii) many known PGPR species were identified; iv) several isolates exhibited a strong antimicrobial activity against Bcc bacteria. In conclusion our results suggested that medicinal plants appear to be an important source of interesting isolates with biotechnological applications potential.