Anti-fungal properties of chitinolytic dune soil bacteria

Abstract

Anti-fungal properties of chitinolytic soil bacteria may enable them to compete successfully for chitin with fungi. Additionally, the production of chitinase may be part of a lytic system that enables the bacteria to use living hyphae rather than chitin as the actual growth substrate, since chitin is an important constituent of most fungal cell walls. Lysis of living fungal hyphae by chitinolytic bacteria has been reported frequently; however, these reports nearly always deal with bacteria that had been selected because of their mycolytic properties. Our main objective was to get a better understanding of the relationship between chitinolytic and anti-fungal properties of bacteria that occur naturally in soils, i.e. without artificial selection. Three inner dune sites, two of which were lime-poor and one lime-rich, along the Dutch coast were selected for this study. Bacteria that were able to degrade colloidal chitin in water–agar comprised 0.2–5.7% of the total amount of culturable bacteria of these dune sites. Pseudomonas spp. were the most abundant culturable, chitin-degrading bacteria at the lime-poor sites, whereas Xanthomonas spp. and Cytophaga spp. were important at the lime-rich site. Chitinolytic actinomycetes were relatively abundant at all three sites. Chitinolytic and non-chitinolytic bacteria were randomly selected and tested for the possession of antagonistic activities against fungal dune strains [ Chaetomium globosum, Fusarium culmorum, F. oxysporum, Idriella (Microdochium) bolleyi, Mucor hiemalis, Phoma exigua, Ulocladium sp.]. The tests were done using water–agar to simulate the energy-limiting conditions that bacteria will encounter in dune soils. The percentage of bacterial isolates that were antagonistic against these fungi was considerably higher for chitinolytic strains than for non-chitinolytic ones. Therefore, the possible involvement of chitinase with respect to the inhibition of fungal growth was studied in more detail. It appeared that in many cases the inhibition of fungal growth was not accompanied by bacterial chitinase production. There was also no clear relationship between the activity of other cell wall degrading enzymes ( β-1,3-glucanase and protease) and antagonism. Chitinolytic bacteria had selective rather than general anti-fungal properties, which were not necessarily related to differences in general susceptibility of the fungi towards antagonism. These results may indicate that antibiotics were involved in the antagonistic activities of chitinolytic bacteria against fungi. Only growing fungi were antagonized by the chitinolytic bacteria; none of the chitinolytic bacteria were able to lyse existing mycelium of any of the fungi. The relevance of the results for the ecology of chitinolytic soil bacteria is discussed.