Effects of Thermal Stress on Sporulation, Crystallization, and Virulence of Bacillus thuringiensis

다국어 초록 (Multilingual Abstract)

Bacillus thuringiensis (Bt) is an insect-pathogenic bacterium that produces insecticidal δ-endotoxins during sporulation and is widely used as biopesticide. In this study, a new Bt strain was isolated from the soil of a Cnidium officinale field in Korea. The isolate was identified as B. thuringiensis subsp. kurstaki based on 16S rRNA gene sequencing and biochemical nutrient utilization profiling, and it was named as B. thuringiensis strain IPTL3 (Bt IPTL3). Bt IPTL3 exhibited pathogenicity against lepidopteran insect species, Spodoptera frugiperda, Spodoptera exigua, Tuta absoluta, Plutella xylostella, and Ostrinia furnacalis. However, there is no pathogenicity against 1 Coleopteran and 2 Hymenopteran species. The Bt IPTL3 increased pathogenicity to 5 Lepidoptera larvae by cold-induced stress during culture. Highest virulence was observed in P. xylostella larvae, with a median lethal concentration (LC₅₀) of 2.7 × (10^3) spores/mL for 2 days post-treatment, while the lowest virulence was recorded in S. frugiperda larvae with LC₅₀ of 4.4 × (10^3) spores/mL for 4 days post-treatment. These results indicate that the Bt strain isolated in this study has promising potential for biopesticide applications in Korea.

다국어 초록 (Multilingual Abstract)

Bacillus thuringiensis (Bt) has been used for a widely used biopesticide that produces insecticidal crystal (Cry) proteins and spores. This study investigated the effects of temperature stress on sporulation, crystallization, and insecticidal efficacy of Bt strains, BtA and BtK, to Plutella xylostella larvae. Additionally, the temperature-dependent virulence was compared using Spodoptera frugiperda to assess species-specific susceptibility. Exposure of the bacteria to 0℃ for 72 hours significantly enhanced both spore and crystal yields compared to 28℃ and 70℃ as temperature stress following culture at 28℃ for 48 hours and resulted in enhanced insecticidal action.

목차 (Table of Contents)

• GENERAL INTRODUCTION 1
• LITERATURE REVIEW 3
• CHAPTER I 14
• Abstract 15
• Introduction 16
• Materials and methods 19
• 1. Isolation of Bt strains 19
• 2. Identification of isolated Bt strains. 19
• 3. Measurement of growth curves of BtA and BtK strains. 20
• 4. Measurement of sporulation in BtA and BtK 21
• 5. Effect of temperature stress on Bt sporulation and crystallization 22
• 6. Insecticidal effect of BtA and BtK by temperature stress 22
• 7. Statistical analysis. 23
• Results 24
• 1. Identification of BtA and BtK 24
• 2. Growth curves of Bt strains 30
• 3. Optimal sporulation conditions of Bt strains under different temperatures and incubation times 34
• 4. Sporulation of Bt strains at different incubation times under high and low temperatures. 39
• 5. Crystallization of Bt strains under different temperatures 43
• 6. Insecticidal effect of Bt strains under different temperatures. 47
• Discussion 54
• Literature cited 58
• CHAPTER Ⅱ 64
• Abstract 65
• Introduction 66
• Materials and methods 70
• 1. Isolation of bacteria from soil 70
• 2. Identification of bacterial isolate 70
• 3. Rearing of test insects 73
• 4. Virulence of new Bt isolate 74
• 5. Cold stress enhances insecticidal activity of new Bt isolate 75
• 6. Virulence bioassays of new Bt isolate: LC₅₀ and LT₅₀ determination. 75
• 7. Statistical analysis. 76
• Results 77
• 1. Identification of bacterium isolated from soil. 77
• 2. Virulence screening of Bt IPTL3 against test insects. 82
• 3. Enhancement of Bt IPTL3 pathogenicity by cold-induced stress. 85
• 4. Virulence assessment of Bt IPTL3 against lepidopteran insects 88
• Discussion 99
• Literature cited 103
• GENERAL SUMMARY 112
• SUMMARY (KOREAN) 114
• LITERATURE CITED 116
• ACKNOWLEDGEMENT 123