oligospora. CT and MT were observed in all the nematode-trapping Doramapimod fungal species tested.

However, the extent of trap formation differed between species. Arthrobotrys oligospora strains isolated from different soils could form CT and MT frequently as A. oligospora ATCC 24927 (data not shown). Monacrosporium ellipsosporum also formed many sticky knobs, most frequently at short intervals on young hyphae (data not shown). Arthrobotrys dactyloides developed fewer constricting rings and Arthrobotrys musiformis formed fewer traps than the above mentioned species, and traps were all on the long germination hyphae (data not shown). Several previous studies have shown that traps of the nonspontaneous trap formers are induced either by organic compounds or by nematodes (Dijksterhuis et al., 1994). Jaffee et al. (1992) questioned the need for special trap-inducing compounds in soil as they found that more traps were produced from nematodes infected with nematode-trapping fungi when BYL719 placed in soil extracts compared with those placed in a KCl solution. Persmark & Nordbring-Hertz (1997) also indicated that soil microorganisms

might be involved in the formation of CT. Furthermore, the presence of bacteria increased trap formation in four nematode-trapping fungi more than nematodes by themselves (Rucker & Zachariah, 1987). These studies indicate that bacteria play an important role in the transition of the fungi into a parasitic habit, although this transition was thought to be the result of a certain level of competition for nutrients between fungi and bacteria (Persmark & Nordbring-Hertz, 1997). Our study indicates that the formation of MT and CT in nematode-trap fungi in soil is related to specific bacteria and their metabolites. Induction was clearly due to bacterial cells with its metabolites simultaneously, as bacteria alone induced a few traps and their metabolites did not induce traps. This is the first study demonstrating soil bacteria as being responsible for MT and CT formation in nematophagous fungi. Trap formation in A. oligospora could be caused by bacterial metabolites that are released into the environment. To test whether diffusible low-molecular-weight

signalling molecules triggered fungal trap formation, we treated the fungal culture with the supernatant of the bacterial culture as well as heat-inactivated bacteria. In no case was the fungal trap formation ADP ribosylation factor observed. Obviously, the induction of traps in fungi depends on the direct contact between the fungus and the bacterium. This assumption was unambiguously confirmed by SEM of fungal hyphae obtained from cocultivation. In soil, many bacteria and fungi will often occupy a shared microhabitat, called the bacterial–fungal interface (Johansson et al., 2004). Traditional studies have shown the presence of bacterial cells at the interface, for example on top of fungal hyphae and spores, on mycorrhized roots and in association with fungal fruiting bodies (de Boer et al., 2005).