Abstract

The potential use of genetically modified microorganisms under controlled fermentation conditions may result in the production of new biopolymers having novel and superior properties which will open up new areas of industrial applications and thus increase their demand. Among the biopolymers applied industrially, polyhydroxybutyrate (PHB) and exopolysaccharides (EPS) stand out because of its applications, mainly in biodegradable plastic production and in wide range of applications, including food products, pharmaceuticals, bioemulsifiers, bioflocculants, chemical products and the biosorption of heavy metals, respectively. In this context, the capacity of wild-type and mutants strains from Rhizobium tropici SEMIA 4077 and SEMIA 4080 to produce EPS and PHB was evaluated, as well as the stability of emulsions formed with soybean oil, diesel oil and toluene was studied at different concentration of EPS by these bacteria. Among the 5 strains evaluated, the majority presented fast growth. Regarding the biopolymer production, only three strains (SEMIA 4080, MUTPA7 and MUTZC3) produced more than 100 mg PHB per liter of culture medium. The MUTPA7 and MUTZC3 mutant strains of R. tropici and wild-type (SEMIA 4080) produced 3.94 ± 0.41, 5.52 ± 0.36 and 2.52 ± 0.45 g/L EPS, respectively, whereas the maximum exopolysaccharide production was 7.45 g/L and 7.67 g/L for the wild-type R. tropici SEMIA 4077 and mutant 4077::Z04, respectively, in PSYL medium with 3% sucrose as a carbon source. The R. tropici strains combines the advantages of non-pathogenicity and rapid productivity and thus has proved to be a very promising model organism and cell factory for microbial EPS production. The R. tropici 4077::Z04 mutant was PHB-negative mutant strain because was unable to synthesize PHB and produced the largest quantity of EPS when compared to the wild-type and other mutant strains, which it achieved through greater efficiency. The EPS produced by the wild-type R. tropici resulted in more stable emulsions with the tested toluene than xanthan gum, and the emulsification indexes with hydrocarbons and soybean oil were higher than 50 %, indicating strong emulsion-stabilizing capacity. These results demonstrate that the EPS of R. tropici strains could be attractive for use in industrial and environmental applications, as it had higher intrinsic viscosity and good emulsification activity.