Growth phase-dependent nematocidal activity of Bacillus thuringiensis strains from natural samples

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.