The present invention provides constructed Bacillus thuringiensis (B.t.) strains that are toxic to mosquito larvae. The mosquito-toxic B.t. strains have improved characteristics, such as increased production of mosquito-toxic proteins, reduced number of spores or increased variety of mosquito-toxic proteins, compared with the well-known, mosquito-toxic strain Bacillus thuringiensis subspecies israelensis (B.t. israelensis). The present invention involves insertion of an antibiotic-resistance gene into the B.t. israelensis mosquito-toxin plasmid having a size of approximately 75 mDa such that the plasmid is “tagged” with antibiotic resistance. The advantage of the tagged mosquito-toxin plasmid is that it permits detection of the rare event in which the mosquito-toxin plasmid is transferred from the B.t. israelensis strain (the donor strain) into a non-B.t. israelensis strain (the recipient strain) that normally does not harbor the mosquito-toxin plasmid. Non-B.t. israelensis recipient cells may have improved characteristics such as reduced spore numbers or increased variety of mosquito-toxic proteins. The present invention also provides for a mechanism of inactivating the antibiotic tag so that B.t. strains harboring the mosquito-toxin plasmid do not disseminate antibiotic resistance when released into the environment.
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