<i>Bacillus subtilis</i> and <i>Bacillus velezensis</i> population dynamic and quantification of spores after inoculation on ornamental plants.

<i>Bacillus subtilis</i> and <i>Bacillus velezensis</i> 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 if they remain active or enter a dormant state. Nonetheless, these characteristics are strikingly important to determine the usage of <i>B. subtilis</i> and <i>B. velezensis</i> and their efficacy in environmental conditions. Here, we investigate the population dynamic on plants of <i>B. subtilis</i> NCIB3610 and <i>B. velezensis</i> QST713 when applied as spores on different ornamental plants. We report that on all plants studied (<i>Echinacea purpurea</i> cv. Salsa red, <i>Echinacea purpurea</i> cv. Fatal attraction and <i>Lavandula angustifolia</i> cv. Hidecote blue) spores rapidly germinated and colonized the rhizoplane, maintaining a relatively low proportion of spores in the population over time, whereas bacterial population on 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.