The thnR gene is a negative transcription regulator of the thurincin H genetic cassette in Bacillus thuringiensis subsp. morrisoni

Thurincin H Is a Nonhemolytic Bacteriocin of Bacillus thuringiensis with Potential for Applied Use

Thurincin H, a bacteriocin produced by Bacillus thuringiensis, exhibits antibacterial activity against Gram-positive and Gram-negative bacteria. While much is known about its expression and antimicrobial spectrum, its hemolytic property has yet to be established. In this study, thurincin H was produced in a plasmid-free acrystalliferous strain of B. thuringiensis (Bt Cry^-B) that naturally lacked antimicrobial and hemolytic activities. When grown in Tryptic Soy Broth (TSB), the bacteriocin's maximal production in Bt Cry^-B harboring the thurincin H genetic cluster (Bt Cry^-B/pThur) was observed at 24 h. Thurincin H was purified as a sole peptide of ~5 kDa using three purification steps, i.e., salt precipitation, ultrafiltration, and gel filtration chromatography. The bacteriocin showed inhibitory activity against B. cereus (5631 U), Bt Cry^-B (8827 U), E. faecium wild type (11,197 U), and E. faecium ATCC 19,434 (6950 U), but not against Bt Cry^-B/pThurH and Bt Cry^-B/pThurHΔThnA. In addition, a minimum inhibitory concentration (MIC) of 5.0 μg/mL against B. cereus 183 was observed. In silico predictions suggested that thuricin H lacks hemolytic activity, which was validated in vitro using 4 × the MIC, i.e., 20 μg/ml. Our data lay a foundation for the potential safe use of thurincin H as an antibacterial peptide for medical use, in food products, and for expression in probiotic bacteria.

Expression of thurincin H, ChiA74 and Cry proteins at the sporulation phase in Bacillus thuringiensis HD1

AIMS

The objective of this study was to produce thurincin H, ChiA74 and Cry proteins together using B. thuringiensis subsp. kurstaki HD1 as a heterologous host.

METHODS AND RESULTS

pSTAB-ThurH and pSTAB-ChiA74 constructs were designed to produce thurincin H and chitinase respectively, at the sporulation phase. They were transformed into Bt HD1 generating the recombinant strains HD1/pSTAB-ThurH and HD1/pSTAB-ThurH/pSTAB-ChiA74. Antimicrobial and chitinolytic activity tests were performed with recombinant strains. Both strains were able to produce thurincin H up to 72 h with antibacterial activity of ~ 4000 U mg^-1 . The HD1/pSTAB-ThurH/pSTAB-ChiA74 strain also showed chitinolytic activity of ~ 23 mU mg^-1 at 72 h. All B. thuringiensis strains exhibited crystal formation at 72, and 96 h. In addition, the application of thurincin H in corn seeds increased the germination percentage and root length by 7 % and 10 %, respectively.

CONCLUSIONS

We showed that is possible to produce three proteins of biotechnological interest at the sporulation stage in B. thuringiensis, which two of them (thurincin H, and ChiA74) are naturally expressed in the vegetative stage.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results form the basis for developing of a biocontrol and biostimulator product that can be used as an alternative for chemical application.

Improving the yields of thurincin H in a native producer strain

Thurincin H is a bacteriocin produced by Bacillus thuringiensis with activity against a wide range of bacteria, including Gram positive and Gram negative. Disadvantages of producing thurincin H in B. thuringiensis is the low production level in the native strain probably due to the highly regulated mechanism of biosynthesis. The present study aimed to increase the synthesis of thurincin H produced by the native strain (Btm) through the establishment of additional copies of the structural gene (i.e. thnA) and the genes responsible for the bacterial self-immunity (thnRDE). Here, three recombinant strains of Btm were generated, harboring three, six and nine additional copies of thnA, and three with one copy of thnRDE upstream to the thnA copies. Data showed that the highest yield was obtained at 16 h using three additional copies of thnA (Btm/pHT-One) with a bacteriocin activity of 20,000 U/mg compared with the parental strain which showed 10,000 U/mg, increase of 100% in the production of the bacteriocin. Also, the addition of the genes responsible for self-immunity showed that recombinant B. thuringiensis (Btm/pHT-TwoRDE) can support six additional copies of thnA with an increase of 60% compared with the parental strain.

Chitinases of Bacillus thuringiensis: Phylogeny, Modular Structure, and Applied Potentials

The most important bioinsecticide used worldwide is Bacillus thuringiensis and its hallmark is a rich variety of insecticidal Cry protein, many of which have been genetically engineered for expression in transgenic crops. Over the past 20 years, the discovery of other insecticidal proteins and metabolites synthesized by B. thuringiensis, including chitinases, antimicrobial peptides, vegetative insecticidal proteins (VIP), and siderophores, has expanded the applied value of this bacterium for use as an antibacterial, fungicidal, and nematicidal resource. These properties allow us to view B. thuringiensis not only as an entity for the production of a particular metabolite, but also as a multifaceted microbial factory. In particular, chitinases of B. thuringiensis are secreted enzymes that hydrolyze chitin, an abundant molecule in the biosphere, second only to cellulose. The observation that chitinases increase the insecticidal activity of Cry proteins has stimulated further study of these enzymes produced by B. thuringiensis. Here, we provide a review of a subset of our knowledge of B. thuringiensis chitinases as it relates to their phylogenetic relationships, regulation of expression, biotechnological potential for controlling entomopathogens, fungi, and nematodes, and their use in generating chitin-derived oligosaccharides (ChOGs) that possess antibacterial activities against a number of clinically significant bacterial pathogens. Recent advances in the structural organization of these enzymes are also discussed, as are our perspective for future studies.