Bacillus subtilis and Bacillus velezensis population dynamics and quantification of spores after inoculation on ornamental plants.

Bacillus subtilis and Bacillus velezensis are used in organic agriculture as an alternative to chemical pesticides to fight against phytopathogen organisms. These Gram-positive soil-dwelling bacteria are able to resist harsh conditions and survive by differentiating into endospores. Few studies have examined how bacterial populations change on plants over time, and whether they remain active or enter a dormant state. Nonetheless, these characteristics are strikingly important to determine the usage of B. subtilis and B. velezensis and their efficacy in environmental conditions. Here, we investigated the population dynamics of B. subtilis NCIB3610 and B. velezensis QST713 when applied as spores on different ornamental plants. We report that on all the plants studied (Echinacea purpurea 'Salsa red', Echinacea purpurea 'Fatal attraction', and Lavandula angustifolia 'Hidecote blue'), spores rapidly germinated and colonized the rhizoplane, maintaining a relatively low proportion of spores in the population over time, whereas the bacterial population on the leaves rapidly declined. Bacteria in the surrounding soil did not germinate and persisted as spores. Taken together, these results suggest that only cells found at the rhizosphere remain metabolically active to allow the formation of a lasting relationship with the plant, making possible beneficial effects from the inoculated bacteria.

Surfactin production is not essential for pellicle and root-associated biofilm development of Bacillus subtilis.

Secondary metabolites have an important impact on the biocontrol potential of soil-derived microbes. In addition, various microbe-produced chemicals have been suggested to impact the development and phenotypic differentiation of bacteria, including biofilms. The non-ribosomal synthesized lipopeptide of Bacillus subtilis, surfactin, has been described to impact the plant promoting capacity of the bacterium. Here, we investigated the impact of surfactin production on biofilm formation of B. subtilis using the laboratory model systems; pellicle formation at the air-medium interface and architecturally complex colony development, in addition to plant root-associated biofilms. We found that the production of surfactin by B. subtilis is not essential for pellicle biofilm formation neither in the well-studied strain, NCIB 3610, nor in the newly isolated environmental strains, but lack of surfactin reduces colony expansion. Further, plant root colonization was comparable both in the presence or absence of surfactin synthesis. Our results suggest that surfactin-related biocontrol and plant promotion in B. subtilis strains are independent of biofilm formation.