| Nematicidal properties of some secondary metabolites of Xenorhabdus spp. and Photorhabdus spp. (Enterobacteriaceae), bacterial symbionts of the entomopathogenic nematodes, Steinernema spp. and Heterorhabditis spp., respectively, were identified and evaluated.; Cell-free (CF) culture filtrates of X. bovienii, X. nematophilus and P. luminescens isolates were shown to be nematicidal against Meloidogyne incognita and Bursaphelenchus xylophilus. The degree of activity varied with the bacterial isolate/species and the culture conditions, such as media composition, age, pH and aeration of the cultures.; Three metabolites, namely ammonia, 3,5-dihydroxy-4-isopropylstilbene (ST) and indole, were identified from CF filtrates as having nematicidal properties. Ammonia was common to all bacterial cultures tested, but ST was produced (10–30 μg/ml) by only P. luminescens and indole was produced (10–50 μg/ml) by some strains/species of both Xenorhabdus and Photorhabdus.; ST and indole affected viability, mobility, egg hatch and dispersal behaviour of nematodes in vitro. ST was active against Aphelenchoides rhytium. Bursaphelenchus spp. and Caenorhabditis elegans, but was not lethal to infective juveniles (Us) of H. megidis 90, or second stage juveniles (J2s) of M. incognita at 200 μg/ml) Indole was active in immersion tests against J2s (100–400 μg/ml) of M. incognita, but failed to prevent infection of tomato seedlings by M. incognita following a soil (≤200 μg/ml) or foliar application (≤1,000 μg/ml) Indole repelled IJs of some species of both Steinernema and Heterorhabditis whereas ST repelled only some species of Steinernema.; ST, but not indole, was detected in variable quantities (∼665.2 to 4,182 μg/g wet insect) in larval Galleria mellonella infected with Heterorhabditis spp. ST was produced after 24 h of infection (25°C) of the larvae, increased rapidly in quantity by 48 h to 5 d, and remained at a relatively high and constant level even after the nematode symbiont had completed its reproduction. Bacterial symbionts built up high populations (∼109 cells/g insect) within 24 h of entering G. mellonella larvae, and increased the cadaver pH to 7.4–7.7.; The early production and relatively large amount of ST in nematode-infected insect hosts, and the antibiotic, nematicidal and nematode-repelling properties of ST suggest that it play a significant role in the symbiotic nematodebacterium association. The potential commercial application of these nematicidal metabolites may be limited by their relatively narrow spectrum and low activity. |
