Nematocidal activity of bacillus-thuringiensis isolates

Abstract

Summary - The nematicidal activity of the spore-crystal mixtures of three Bacillus thuringiensis isolates against hatched juveniles and adlUts of Caenorhabditiselegans was investigated. Toxicity was detennined bl' adding 50-f.L! aliquots of the spore-crystal mixtures to microtitre plate wells containing 50-f.l.1 aqueous suspensions of 200-400 hatched juveniles and adults of C. elegans. Nematode mortality was observed from 8 hours incubation onwards; after 24 hOUTS incubation no more significant increases in nematode mortality occurred. Nematode mortality varied from about 50 to 60 % when the nematicidal activity was tested in distilled water and was usually somewhat higher (but less than 10 %) when tested in axenic medium. Toxicity varied between the three isolates. Concentrations of at least 108 particles/ml were necessary to cause a nematode mortality higher than 30 %. Nematicidal activity was ooll' observed when spore-crystal mixtures from at least 2-day-old ClUtureS, consisting of about 50 % of vegetative cells, often containing a spore, and for about 50 % of a mixture of spores and crystals, were used. Heating to 75 oC and higher for 24 hours and autoclaving at 120 oC for 20 min destroyed the nematicidal activity of all three isolates. Differences in stability of the nematicidal activity were observed between the three isolates. In two isolates the nematicidal activity did not decline after storage at 28 oC for 15 days; in the third isolate the nematicidal activity declined after storage at 28 oC for 7 days. MlUtiple freezing at - 20 oC or - 70 oC and thawing had no effect on the nematicidal activity of two isolates but decreased the nematicidal activity of the third isola te. pH changes resulted in differences in stability of the nematicidal activity between the three isolates. These results may indicate the presence of different toxins. Resume - Activite nbnaticide de trois isolats de Bacillus thuringiensis - L'activite nematicide de melanges de spores et d'inclusions cristallines de trois isolats de BG.(;illus thul1'ngiensis envers les juveniles et les adultes de Caenorhabditis elegans est etudiee. La toxicite est determinee en ajoutant 50 f.l.l d'un melange de spores et d'inclusions cristallines a 50 f.l.l d'une suspension de 200 a 400 juveniles et adultes de C. elegans. La mortalite des nematodes est observee apres 8 heures d'incubation; aucune augmentation significative du pourcentage de mortalite n'est observee apres 24 heures d'incubation. Ce pourcentage varie d'environ 50 a 60 % lorsque les tests sont realises avec de l'eau distillee et il augmente legerement (moins de 10%) si les tests sont effectues en milieu axenique. La toxicite varie selon les isolats. Un minimum de 10 8 particules/ml est necessaire pour provoquer un effet letal superieur a 30 %. L'activite nematicide n'est observee qu'avec des melanges de spores et d'inclusions cristallines provenant de clUtures âgees d'au moins 2 jours et constituees d'environ 50 % de celllUes vegetatives - contenant souvent une spore - et d'environ 50 % d'une mixture de spores et d'inclusions cristallines. Le chauffage a 75 oC ou plus pendant 24 heures ou un autoclavage a 120 oC pendant 20 minutes detruit l'activite nematicide des trois isolats. Des differences de stabilite de l'activite nematicide sont observees entre les trois isolats. Celle-ci ne diminue pas pour deux isolats maintenus a 28 oC durant quinze jours, mais elle diminue pour le troisieme s'il est stocke sept jours a 28 oc. De meme, l'activite nematicide de deux des isolats n'est pas modifiee apres plusieurs congelations a - 20 oC ou - 70 oC suivies de decongelations, mais eUe diminue pour le troisieme isolat. Les variations du pH des melanges de particules entrainent des variations dans la stabilite des activites nematicides des trois isolats. Ces reslUtats pourraient indiquer la presence de toxines differentes.