Antibiosis plays a role in the context of direct interaction during antagonism of Paenibacillus polymyxa towards Fusarium oxysporum

Interaction of Fusarium oxysporum and Paenibacillus polymyxa starts with po lar attachment of bacteria to the fungal hyphae followed by the formation o f a large cluster of non-motile cells embedded in an extracellular matrix i n which the bacteria develop endospores. Enumeration of fungal viable count s showed that less than one of 36 000 colony-forming units survived in pair ed cultures for 71 h. Effective antagonism was not observed below pH 5 and was specific for the bacterial species. Development off. oxysporum was inhi bited in cell-free filtrates derived from cultures of P. polymyxa, but was much more strongly repressed in the presence of living bacteria. Furthermor e, recovery of fungal growth started immediately after addition of antibiot ics to paired cultures. Restoration of fungal growth was enhanced in filtra tes that were supplemented with MgCl2, which suggests that anti-fungal comp ounds produced by the bacteria were counteracted by magnesium ions. In pair ed cultures, fungal counts remained very low, even in the presence of the m agnesium salt. This study clearly showed that P. polymyxa antagonizes the p lant pathogenic fungus F. oxysporum in liquid medium by means of an interac tion process in which the presence of living bacteria is a prerequisite for continuous suppression of fungal growth.