Predation Parasitism and Disease

Penetration of bacterial colonies and subsequent lysis of bacterial cells by cel-lulolytic and lignolytic basiodiomycetes, for example P. ostreatus and Lentinula edodes, has been well documented (Barron, 1988; Tsuneda and Thorn, 1994a; Barron, 2003). Nutrient-limiting conditions, in particular nitrogen limitation, seem to trigger this predatory behaviour of the fungi. Therefore, it has been proposed that the bacteria may form a valuable source of nitrogen for the fungi (Barron, 2003). Fungal predation upon bacterial colonies has been observed mostly in artificial media, but one case of fungal consumption of bacterial biomass that had developed on dead nematodes in wood has been reported (Tsuneda and Thorn, 1994b). We observed that two white-rot basidiomycetes, H. fasiculare and R. bicolor, strongly reduced the number of wood-inhabiting bacteria upon colonization of beech wood blocks (Folman et al., submitted).

Microscopy revealed that the bacteria had actually disappeared, suggesting fungal-induced lysis of the bacteria. Future research should confirm whether the lysed bacterial biomass is actually consumed by the fungi.

Besides being predators, fungi can also be the prey of bacteria. Bacterial mycophagy has, however, been studied much less intensively than fungal my-cophagy—mycoparasitism. The scarce reports on bacterial mycophagy, so far, deal almost exclusively with ascomycetes in an experimental setting. Strepto-mycetes, myxomycetes, paenibacilli, pseudomonads and the recently described genus Collimonas have been indicated as potential mycophagous bacteria (De Boer et al., 2005). However, actual proof of importance in situ is difficult as all of these bacteria can grow very well on other energy sources, that is they are facultatively mycophagous.. Hence, an equivalent of Bdellovibrio—an obligate bacterial predator of bacteria, has not been found so far.

Tsuneda and Thorn (1994b) reported on lytic activities of Agrobacterium tumefaciens and P. tolaasii against wood-degrading basidiomycetes under laboratory conditions. The potential mycophagous bacterium Collimonas fungivorans has been observed in beech wood blocks colonized by the white-rot fungus R. bicolor (Folman, Boddy and de Boer, unpublished results).

Bacterial pathogens on fruit bodies of saptrotrophic basidiomycetes may be considered to be a special case of mycophagy. For obvious reasons, these bacterial pathogens have been most intensively studied on commercially grown edible mushrooms, for example Agaricus bisporus and P. ostreatus, but they have also been found on mushrooms sampled in the field (Bessette, 1984). The best-studied example is that of P. tolaasii causing brown blotch disease (Soler-Rivas et al., 1999). When it is present at sufficient density, the bacterium induces disease by producing fungal membrane-disrupting secondary metabolites, in particular the lipodepsipeptide tolaasin. Hence, the bacterium is growing at the expense of hyphal contents that are released due to fungal membrane disruption. Similar strategies have also been found for some other Pseudomonas spp. as well as other bacterial species, for example Janthinobacterium agaracidamnosum (Lincoln et al., 1999; Godfrey et al., 2001).

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