Mode of Action of a Multi-Microbial Inoculant On Pathogen Suppression and Impact On Soil Microbial Communities.

Multi-microbial Inoculants (MI) that consist of lactic acid bacteria, yeasts and phototrophic bacteria have been in use in agriculture for several decades. The benefits of MI application have been described to include: (1) improved crop yields; (2) pathogen control; and (3) enhanced soil quality and biological activity. There is a paucity of data that describes the mode of action of MI, much of which is contradictory. For example, MI application appears to result in microbial activity enhancement (i.e. increased soil biological activity) and, at the same time, attenuation of microbial activity is also observed (i.e. decreased pathogen virulence). Our objective was to investigate possible modes of action of microorganisms in an MI including: (1) disruption of quorum sensing (QS)-mediated virulence; and (2) enhancement of soil microbial diversity. Degradation of QS autoinducers that mediate virulence in Pectobacterium carotovrum in the presence of MI microorganisms was quantified and monitored by a gas chromatography-mass spectrometry. Also, field and pot studies were conducted with MI inoculated soils and soil microbial DNA profiles were characterized as indicators of microbial diversity. Soils for pot studies were collected from field plots, removed as Intact Soil Microcosms (ISM). Subsequently ISMs were planted to tomato var. ‘Brandywine’ (Lycopersicon esculentum), treated with MI, and grown under environmental growth chamber conditions. The results of this study not only help to elucidate the possible mode of actions, but also reveal the extent of effectiveness of MI on suppression of soft rot disease and enhancement of soil microbial diversity.