Genetic engineering in Bacillus subtilis probiotic: expression of fungal phytase improves zootechnical parameters, immune system, inflammatory response and antioxidant defense in zebrafish (Danio rerio) fed a diet rich in plant matter
The phytase enzyme catalyzes the phytate hydrolysis, which represents the form of storage of phosphorus in vegetables. Phytate is an antinutritional factor for monogastric animals, since they are inefficient in the production of intestinal phytases. Therefore, the increment of plant ingredients in the feed in animal production systems is limited. The objective of the present thesis was to obtain a genetically modified Bacillus subtilis capable of expressing a fungal phytase and to evaluate the efficacy of this enzyme in the diet of zebrafish (Danio rerio). In chapter I we developed a replicative plasmid-based genetic construct containing the phytase gene from the fungus Aspergillus fumigatus (Phy-Af) fused to the secretory signal of the B. subtilis levansacarase (SacB) gene. The phytase activity produced by transgenic B. subtilis in the culture supernatant was 2.7 fold higher than the controls. After determining phytase activity, we performed a zebrafish feeding experiment for 30 days on a plant-rich diet. The experiment consisted of two groups: Phy-Af (with diet supplemented with B. subtilis expressing phytase) and the control group (diet with non-transgenic B. subtilis). The initial and final biometrics were used to evaluate the performance of the animals and to analyze the genes related to peptide transport (slc15a1b and slc15a2), appetite (ghrl), muscle growth factors (igf1, myod and myog) and mineral metabolism (bglap). The fish fed with transgenic B. subtilis presented better performance indices when compared to the control. In the same sense, all the genes analyzed were induced in the group treated with the probiotic expressing fungal phytase. In chapter II, the objective was to evaluate the effect of transgenic B. subtilis on the immune system and the inflammatory response in zebrafish fed with the feed rich in vegetable matter. In this case, genes related to the production of lymphocytes (cd4 and ikzf1), inflammatory response (ifn1, ifng, tnf-alpha and il1b) and the response to oxidative stress (sod1) were analyzed. Again, all genes analyzed were induced in zebrafish fed a diet supplemented with the transgenic probiotic. Considering that the replicative plasmid may be unstable and lead to loss of trait throughout the bacterial generations, in chapter III we approached the application of CRISPR-Cas9 technology for editing the genome of B. subtilis using the fungal phytase construct developed in Chapter I. As a result, we obtained the strain BsJJ14 with fungal phytase integrated into the genome of B. subtilis and compared maintenance capacity of antibiotic resistance with the strain Phy-Af, which in chapter III we call BsJJ5. The results showed that after 65 generations, only 11% of the BsJJ5 cells maintained the antibiotic resistance phenotype, and the strain transformed by the CRISPR-Cas9 method did not lose the phenotype at the end of the experiment. The activity profile of the BsJJ14 strain was 20% greater than in the BsJJ5 strain transformed with the replicative plasmid. In conclusion, the present thesis demonstrated that the techniques used for the transformation of B. subtilis both episomal and integrated into the genome were efficient and, in addition, recombinant phytase showed enzymatic activity and promoted beneficial health effects of zebrafish fed on a diet rich in vegetable ingredient.