NEMATOCIDAL ACTIVITY OF BACILLUS-THURINGIENSIS ISOLATES

The nematicidal activity of the spore-crystal mixtures of three Bacill us thuringiensis isolates against hatched juveniles and adults of Caen orhabditis elegans was investigated. Toxicity was determined by adding 50-mu l aliquots of the spore-crystal mixtures to microtitre plate we lls containing 50-mu l aqueous suspensions of 200-400 hatched juvenile s and adults of C. elegans. Nematode mortality was observed from 8 hou rs incubation onwards; after 24 hours incubation no more significant i ncreases in nematode mortality occurred. Nematode mortality varied fro m about 50 to 60 % when the nematicidal activity was tested in distill ed water and was usually somewhat higher (but less than 10 %) when tes ted in axenic medium. Toxicity varied between the three isolates. Conc entrations of at least 10(8) particles/ml were necessary to cause a ne matode mortality higher than 30 %. Nematicidal activity was only obser ved when spore-crystal mixtures from at least 2-day-old cultures, cons isting of about 50 % of vegetative cells, often containing a spore, an d for about 50 % of a mixture of spores and crystals, were used. Heati ng to 75 degrees C and higher for 24 hours and autoclaving at 120 degr ees C for 20 min destroyed the nematicidal activity of all three isola tes. Differences in stability of the nematicidal activity were observe d between the three isolates. In two isolates the nematicidal activity did not decline after storage at 28 degrees C for 15 days; in the thi rd isolate the nematicidal activity declined after storage at 28 degre es C for 7 days. Multiple freezing at -20 degrees C or -70 degrees C a nd thawing had no effect on the nematicidal activity of two isolates b ut decreased the nematicidal activity of the third isolate. pH changes resulted in differences in stability of the nematicidal activity betw een the three isolates. These results may indicate the presence of dif ferent toxins.