| Xenorhabdus and Photorhabdus are entomopathogenic bacteria symbiotically associated with insect pathogenic nematodes. They are highly virulent when injected into the hemocoels of insects. Estimations of LD50 are generally 1-10 cells. Bowen reported that Phoorhabdus luminescens W-14 could produce a protein complex that was lethal when given orally or injected. This discovery arouses general concern. At present, more and more researches focus on insecticidal toxins and insecticidal mechanism of Xenorhabdus and Photorhabdus. However, little work has been done in China. The work in this thesis will fill the gap in the study of insecticidal toxins from these bacteria in China. In this paper, eleven representative strains were isolated from their nematode symbionts. Insecticidal activities of their extracellular secretions were examined by the means of oral and injection bioassay. Xenorhabdus bovienii A54 was found highly toxic to insects, and it can produce two different toxins, a polysaccharide oral toxin and an injectable protein toxin. It was the first time that the high- molecular insecticidal toxins from Xenor/zabdus bovienii were studied. This work will provide fresh insecticidal toxins and is helpful to reveal the insecticidal mechanism of Xenorhabdus. Insecticidal activities of toxins from eleven strains were compared. It was found that different toxins were produced by different swains. Materials from five strains were lethal to insects whether injected or given orally. Materials from three strains had only injection toxicity. Materials from other three swains were avirulent to insects. In addition, toxicity of the same toxin among the eleven strains was different. Injectable toxicity of strain 15-2 and HZL was the highest, with the mortality of 1 00%of G. mellonella larvae within 48hr. Injection of toxins produced by strain A54, BJ, ALL, CB-8 and Meg killed over 80% of insects within 96hr. Materials from strain A54 were highly toxic when given orally. Mortalities of 0. furnacalis larvae and H. armigera larvae treated with A54 toxin were 100% and 91.7% within 1 2Ohr respectively. Toxins from strain BJ, ALL, D43, and 15-2 could inhibit the growth of H arm igera larvae, but not killed the larvae. These results showed that strain A54 was highly toxic to insects.Therefore, strain A54 was used to further study the insecticidal mechanism. The metabolic products produced by phase I and II of A54 were studied. Results of PAGE showed there was great difference in extracellular proteins between phase I and phase II. The varieties and contents of extracellular proteins produced by phase I were more than those produced by phase II. All the extracellular proteins produced by phase II could produce by phase I. Materials secreted by phase I had toxicity whether injected or fed to insects, while phase II had no toxicity. Two different toxins, oral toxin and injectable toxin were found to be produced simultaneously by the bacteria of A54. It was the first report that a polysaccharide oral toxin was produced by Xenorhabdus. The column chromatography demonstrated that oral toxin produced by A54 was different from injectable toxin. For further research, the oral toxin was isolated and purified. Along with examinations on oral toxicity against the first instar of H armigera larvae, a purification procedure was applied in isolating the toxin, such as precipitation with ammonium sulfate, DEAE-52 chromatography and Sp- Sepharose chromatography. Through these steps, the oral...
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