Bacillus sphaericus as a mosquito pathogen: properties of the organism and its toxins

Efficient expression of a 100-kilodalton mosquitocidal toxin in protease-deficient recombinant Bacillus sphaericus

The expression of the 100-kDa mosquitocidal toxin (Mtx) during vegetative growth and sporulation in nine different mosquito-larvicidal strains of Bacillus sphaericus has been analyzed. In five out of the nine strains the 100-kDa toxin was found to be expressed predominantly in the vegetative phase of growth, and in all nine strains the level of the toxin in sporulated cells was very low or undetectable. Strains in four out of the six DNA homology groups of B. sphaericus produced intracellular and extracellular proteases, which degraded the 100-kDa toxin, during sporulation. The 100-kDa toxin gene was expressed by using its native promoter on a multicopy number plasmid in B. sphaericus 1693 (protease negative) and B. sphaericus 13052 (protease positive). High levels of the 100-kDa toxin were produced in vegetative cells of both strains as well as in sporulated cells of protease-negative strain 1693, which is in contrast to the low levels of the 100-kDa toxin produced in sporulated cells of protease-positive strain 13052. Thus, the small amount of the 100-kDa toxin in sporulated cells of the nine mosquito-larvicidal strains is probably due to degradation of the 100-kDa toxin synthesized during vegetative growth by a protease(s) produced during sporulation. B. sphaericus 1693 transformed with the 100-kDa toxin gene was as toxic to mosquito larvae during both vegetative growth and sporulation as the natural high-toxicity strains of sporulated B. sphaericus.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of mosquitocidal toxin synthesis in Bacillus sphaericus

Translational lacZ fusions to the promoters of the parasporal, crystal protein (binary toxin) and 100-kDa ADP-ribosylating mosquitocidal toxin genes of Bacillus sphaericus were prepared and expression of the toxin genes monitored as beta-galactosidase activity. Transcription of the crystal protein gene fusion began immediately before the end of exponential growth and continued into stationary phase in both B. sphaericus and Bacillus subtilis but accompanied exponential-phase growth in Escherichia coli. Expression of this fusion was severely delayed in a B. subtilis spo0A mutant and decreased relative to the wild type in a B. subtilis spoIIAC background. beta-Galactosidase activity from the 100-kDa toxin gene fusion was restricted to early exponential phase in B. sphaericus, but in B.subtilis it continued into late exponential phase. Expression was about eightfold lower in B. sphaericus than B. subtilis suggesting an element of negative control in the native host.

Polymerase chain reaction and non-radioactive gene probe based identification of mosquito larvicidal strains of Bacillus sphaericus and monitoring of B. sphaericus 1593M, released in the environment

In attempts towards an accurate monitoring in the environment, the status of the deliberate release of Bacillus sphaericus, a powerful mosquito larvicidal agent, as well as to evolve a rapid method of screening for potent isolates of B. sphaericus, we have developed a polymerase chain reaction (PCR) method of analysis. Using specific primers spanning the 5' and 3' ends of the coding sequences of these two mosquito larvicidal genes, a 1.3 and a 1.1 kb product from gene A and a 2.6 kb product from gene B have been amplified by PCR. The primers and the products amplified from them in PCR are highly specific for B. sphaericus. We used digoxigenin based non-radioactive chemiluminescence method for the detection of PCR products and the sensitivity of the method was high enough to detect the presence of 1 to 5 cells of B. sphaericus. A simple and inexpensive sample processing procedure has also been developed for direct PCR amplification of B. sphaericus DNA from field samples collected from areas where it had been applied. Several highly toxic to non-toxic strains of B. sphaericus were screened with these primers by PCR for the presence of either or both of these toxin genes. The results indicate that there is a good correlation between the presence of both genes with higher toxicity of the strains.

Expression of mosquitocidal toxin genes in a gas-vacuolated strain of Ancylobacter aquaticus

A series of plasmids bearing the binary toxin genes of Bacillus sphaericus 2297 or 2317.3, the 100-kDa toxin gene of B. sphaericus SSII-1, or the 130-kDa (cryIVB) toxin gene of Bacillus thuringiensis subsp. israelensis were constructed and introduced into Ancylobacter aquaticus by electroporation. The transformed A. aquaticus cells exhibited significant toxicity towards mosquito larvae, demonstrating a potential use of recombinant A. aquaticus for biological control of mosquitoes.

Fermentation and toxin studies of the molluscicidal strains of Bacillus brevis

Strain SS86-4 was one of 40 Bacillus brevis strains shown to be molluscicidal to the schistosomiasis snail vector Biomphalaria glabrata. When grown in mB4 medium in 2-L fermentors, SS86-4 was molluscicidal only if fructose or phenylalanine was present in the medium. This is reminiscent of secondary fermentation factor effects, in this case an antioxidant effect. In vivo proteases also were capable of reducing molluscicidal activity. The molluscicidal toxin has an LC50 of 1 microgram toxin protein ml-1 (approx. 1 p.p.m.) and may be described as a small proteinaceous, heat-stable, oxygen-sensitive entity associated with the particulate portion of the cell wall fraction of B. brevis that is formed prior to sporulation. Initial information indicates that its HPLC signature shows major peaks at 148.37 and 163.96 s and consists of two bands of approximately 5.3 kDa and 8.7 kDa on PAGE gel.

Mosquitocidal toxins of bacilli and their genetic manipulation for effective biological control of mosquitoes

The identification, cloning, and characterization of protein toxins from various species of bacilli have demonstrated the existence of mosquitocidal toxins with different structures, mechanisms of action, and host ranges. A start has been made in understanding the polypeptide determinants of toxicity and insecticidal activity, and the purification of toxins from recombinant organisms may lead to the elucidation of their X-ray crystal structures and the cloning of brush border membrane receptors. The results of cloning mosquitocidal toxins in heterologous microorganisms show the potential of expanding the range of susceptible mosquito species by combining several toxins of different host specificity in one cell. Toxins have been expressed in new microorganisms with the potential for increasing potency by persisting at the larval feeding zone. The powerful tools of bacterial genetics are being applied to engineer genetically stable, persistent toxin expression and expand the insecticidal host ranges of Bacillus sphaericus and Bacillus thuringiensis strains. These techniques, together with modern formulation technology, should eventually lead to the construction of mosquitocidal microorganisms which are effective enough to have a real impact on mosquito-borne diseases.

New high-toxicity mosquitocidal strains of Bacillus sphaericus lacking a 100-kilodalton-toxin gene

Five new high-toxicity mosquitocidal strains of Bacillus sphaericus were isolated in Singapore. They all belong to phage group 8 and have binary toxin (51.4- plus 41.9-kDa) genes located on the chromosome but lack a 100-kDa-toxin gene. These strains of B. sphaericus constitute a new subgroup, as only two weakly toxic strains in phage group 8 have previously been described and all the known high-toxicity strains have both binary toxin and 100-kDa-toxin genes.

Proteolytic processing of the mosquitocidal toxin from Bacillus sphaericus SSII-1

The 97-kDa protein Mtx21, derived from the 100-kDa mosquitocidal protein (Mtx) from Bacillus sphaericus SSII-1 by the deletion of the putative signal sequence, was expressed as a fusion protein with glutathione S-transferase in Escherichia coli, and the fusion protein was purified by affinity chromatography. The fusion protein bound to glutathione agarose was cleaved with thrombin to release the Mtx21 protein. The 97-kDa Mtx21 protein was found to be toxic to Culex quinquefasciatus larvae with a 50% lethal concentration of 15 ng/ml. Treating Mtx21 with crude mosquito larval gut extracts gave rise to two major peptides of 70 and 27 kDa. Treating the 97-kDa Mtx21 protein with trysin also gave rise to a similar proteolytic cleavage pattern. N-terminal sequencing showed that the 27-kDa peptide was derived from the N-terminal region of the 97-kDa protein and that the 70-kDa protein was from the C-terminal region of the 97-kDa protein. The 27-kDa peptide has all the previously identified regions of homology with the catalytic peptides of the ADP-ribosyltransferase toxins, such as pertussis toxin S1 peptide, while the 70-kDa peptide has three internal regions of homology.