ABSTRACT Natural isolates coming from chemically untreated environmental samples can be a source of new nematocidal bacterial strains. Little is known about the abundance of such isolates; the diversity of nematocidal proteins that they produce, or how their nematocidal activity changes in a growth phase-dependent manner. In this study, a collection of 17 Bacillus thuringiensis (Bt) natural isolates was screened for nematocidal activity and the presence of nematocidal genes (cry1, cry5, cry6, cry12, cry13, cry14, and cry21). Most strains were positive for cry1 and at least one other cry gene. Further, nematocidal activity against Caenorhabditis elegans was detected in 82.35% of strains. This percentage is significantly higher than percentages found in previously reported studies on similar strain collections. We next sought to investigate the correlation between bacterial growth phase and nematocidal activity. All strains clustered into two growth phase-dependent nematocidal activity profiles. 28.57% of nematocidal strains were active from the exponential and throughout the stationary phase. 42.85% of them were active only in the exponential phase. Strains active only in the stationary phase were not observed. A combination of strains active in different growth phases was then used to investigate the possibility of killing entire nematode populations. Complete clearance of nematodes was accomplished after two days of incubation when a mixed culture of two such strains (SS-9.4 and SS-16.2) was used. Therefore, combining Bt strains, which are nematocidal in different growth phases, could ameliorate their application in the biological control of nematodes.

中文翻译：

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来自天然样品的苏云金芽孢杆菌菌株的生长相依赖性杀线虫活性

摘要来自未经化学处理的环境样品的天然分离物可能是新的杀线虫细菌菌株的来源。人们对此类分离株的丰富程度知之甚少。它们产生的杀线虫蛋白的多样性，或者它们的杀线虫活性如何以依赖于生长阶段的方式变化。在这项研究中，对 17 个苏云金芽孢杆菌 (Bt) 天然分离株的杀线虫活性和杀线虫基因（cry1、cry5、cry6、cry12、cry13、cry14 和 cry21）的存在进行了筛选。大多数菌株对cry1 和至少一个其他cry 基因呈阳性。此外，在 82.35% 的菌株中检测到对秀丽隐杆线虫的杀线虫活性。该百分比显着高于先前报告的类似菌株集合研究中发现的百分比。我们接下来试图研究细菌生长期与杀线虫活性之间的相关性。所有菌株聚集成两个生长阶段依赖性杀线虫活性谱。28.57% 的杀线虫菌株在指数期和整个稳定期都具有活性。其中42.85%仅在指数期活跃。没有观察到仅在固定相中有活性的菌株。然后使用在不同生长阶段有活性的菌株组合来研究杀死整个线虫种群的可能性。当使用两种此类菌株（SS-9.4 和 SS-16.2）的混合培养物时，在培养两天后完成线虫的完全清除。因此，结合在不同生长阶段杀线虫的 Bt 菌株，