Results of the in vitro antibiosis test on KB medium suggest that,
the inhibitory activity of all isolates except B. cereus BC1AW and
BC4SS against R. solanacearum could be due to production of siderophores.
Since KB is an iron deficient medium, it is optimal for
siderophore production (Lim and Kim, 1997). Also Chen et al.
(2003) reported that the type of culture medium had an impact
on the inhibitory activity of antagonists by mediating the production
of growth inhibiting substances. This was further supported
by Muleta et al. (2007) who found growth inhibition of fungal
pathogens of coffee on KB medium by a Pseudomonas strain
through production of siderophores. Though, B. cereus strains
BC1AW and BC4SS were unable to produce siderophores, they still
showed inhibitory activity against the pathogen, suggesting the
production of antibiotics. Although there is a general agreement that in vitro antibiosis does not necessarily relate to the same effect
under in vivo conditions (Kloepper, 1993), in vitro screening is a
common method in the initial screening of antagonists against bacterial
and fungal pathogens.