Comparative studies of the mosquito-larval toxin of Bacillus sphaericus SSII-1 and 1593

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Abstract
Two B. sphaericus strains, SSII-1 and 1593, were bioassayed for toxic activity against 2nd-instar larvae of the mosquito Culex pipiens quinquefasciatus. Strain 1593 developed a level of toxicity 3000 times that of strain SSII-1. Although to toxic activity of B. sphaericus SSII-1 was relatively unchanged throughout growth. Activity increase of strain 1593 occurred as the bacteria began to sporulate. Strain differences were examined by growth cycle experiments, bioassays of the toxicity of oligosporogenous mutants and Mn limitation experiments. The toxin of strain 1593 was more stable than that of strain SSII-1. Unlike strain SSII-1 spores, B. sphaericus 1593 spores were highly toxic. Thin sections of SSII-1 or 1593 cells did not reveal presence of any inclusion body that might be related to toxicity.

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