Isolation and characterization of Bacillus thuringiensis from coastal intertidal brackish sediments and from farmlands in Machakos District Kenya

Abstract

Bacillus thuringiensis (Bt) delta endotoxins represent the most successful use of biological control agents targeting crop pests to date. Studies have shown that Bt produces a wider range of toxins targeting a variety of unrelated pests than was initially documented. This further increases the prospects for its wider use as a microbial biopesticide. In this view the study sought to isolate and characterize native Bt from two different ecological regions; Kenya coastal intertidal brackish sediments and farmlands in Machakos district. Twenty eight Bt isolates were isolated and identified. Distinctions between the isolates were based on their morphological appearances, presence of parasporal inclusions and biochemical characteristics. Results for staining tests revealed that the isolates were Gram positive, rod shaped cells, with the spores terminally located. The cells from the two ecological regions did not have significant variation in average size. The rods had length ranging between 2.0 μm and 3.0 μm on average while the width was approximately 1 μm on average. Parasporal inclusions had varied shapes (i.e. bipyramidal, circular, squared, oval and rhomboid). Both biochemical tests (Proskauer (VP) test and catalase test) were positive. Average optimum temperature range for growth was between 28°C and 35°C while the optimal pH growth range was between 5.5 and 7.5. Both the morphological and biochemical results provided evidence that the isolates were different forms of Bacillus thuringiensis. Out of the fifty six soil and sediment samples, 28 samples (50%) yielded Bacillus thuringiensis. 53.6% of the isolates were recovered from samples from farms in Machakos district and 46.4% from the intertidal brackish sediments from the Gazi coastal mangroves of Kenya. This shows that Bt is also present in soils which are under constant cultivation and those lying fallow within the intertidal brackish sediments.