Plasmid Transfer between Bacillus thuringiensis subsp. israelensis Strains in Laboratory Culture, River Water, and Dipteran Larvae

Abstract

Plasmid transfer between strains of Bacillus thuringiensis subsp. israelensis was studied under a range of environmentally relevant laboratory conditions in vitro, in river water, and in mosquito larvae. Mobilization of pBC16 was detected in vitro at a range of temperatures, pH values, and available water conditions, and the maximum transfer ratio was 10 ؊3 transconjugant per recipient under optimal conditions. Transfer of conjugative plasmid pXO16ϻTn5401 was also detected under this range of conditions. However, a maximum transfer ratio of 1.0 transconjugant per recipient was attained, and every recipient became a transconjugant. In river water, transfer of pBC16 was not detected, probably as a result of the low transfer frequency for this plasmid and the formation of spores by the introduced donor and recipient strains. In contrast, transfer of plasmid pXO16ϻTn5401 was detected in water, but at a lower transfer ratio (ca. 10 ؊2 transconjugant per donor). The number of transconjugants increased over the first 7 days, probably as a result of new transfer events between cells, since growth of both donor and recipient cells in water was not detected. Mobilization of pBC16 was not detected in killed mosquito larvae, but transfer of plasmid pXO16::Tn5401 was evident, with a maximum rate of 10 ؊3 transconjugant per donor. The reduced transfer rate in insects compared to broth cultures may be accounted for by competition from the background bacterial population present in the mosquito gut and diet or by the maintenance of a large population of B. thuringiensis spores in the insects.

Role of Receptors in Bacillus thuringiensis Crystal Toxin Activity

“…Many cry genes are associated with transposable elements that may facilitate gene amplification, leading to the evolution of new toxins (29). In addition, most cry genes are found on plasmids, and horizontal transfer by conjugation may result in the creation of new strains with a novel complement of cry genes (166,167).…”

Conjugative Transfer of Insecticidal Plasmid pHT73 from Bacillus thuringiensis to B. anthracis and Compatibility of This Plasmid with pXO1 and pXO2

“…However, plasmid transfer from other B. cereus group strains to B. anthracis, and vice versa, may occur under some specific conditions. It is well documented that the spores of insecticidal B. thuringiensis can germinate and then multiply in the midguts of dead target insects, and plasmid transfer at different frequencies under these conditions have been recorded previously (31,34). The large-scale application of commercial B. thuringiensis for agricultural pest control may facilitate gene exchange among B. cereus group strains in target insects and then endow the strains with insecticidal properties and wider host ranges.…”

“…Some reports have confirmed that strains of B. thuringiensis and B. cereus exhibit low degrees of clonality and that exchange of genetic material between them occurs frequently in river water (31), soil (35), food preparation environments (33), and even the guts of insects (31,37). Previous reports revealed that the toxin genes or whole toxin plasmids of B. anthracis can transfer to other Bacillus sp.…”

High-Salt Stress Conditions Increase the pAW63 Transfer Frequency in Bacillus thuringiensis

“…In this context, conjugation has been extensively studied in different niches, including insect gut (20), gnotobiotic rats (28), the rhizosphere of grass plant (18), soil (24), water (21), and, most recently, dairy products (15,23). These habitats were found to variably affect the overall process of plasmid conjugation and mobilization, suggesting adaptative transfer mechanisms.…”