We investigated the in vitro influence of humic substances (HS) extracted from de-inking paper sludge compost on the inhibition
of Pythium ultimum by two compost bacteria, Rhizobium radiobacter (Agrobacterium radiobacter) and Pseudomonas aeruginosa.
When low concentrations (5 or 50 mgl1
) of HS were added to the culture medium, fungal inhibition by R. radiobacter significantly
increased (P < 0.01) by 2–3%. In contrast, these low levels of HS had no effect on P. ultimum inhibition by P. aeruginosa. The Fe,
chelated by HS, was in part responsible for the decrease of P. ultimum inhibition by the bacteria when increasing amounts of HS
were added in the culture medium. The addition of 500 mgl1 of humic acids isolated from de-inking paper sludge compost or from
fossil origin completely eliminated the inhibition of P. ultimum by R. radiobacter. This Fe effect also stimulated growth of R. radiobacter
and reduced its siderophore production in a minimal medium supplemented with HS as sole source of Fe. The results showed
that HS influence microbial antagonism when added to a culture medium. However, this effect varies with different factors such as
the type of bacteria, concentration of HS, molecular weight and Fe content.
2004 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.