J. Dijksterhuis et al., Antibiosis plays a role in the context of direct interaction during antagonism of Paenibacillus polymyxa towards Fusarium oxysporum, J APPL MICR, 86(1), 1999, pp. 13-21
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.