Cooperative adsorption of critical metal ions using archaeal poly-γ-glutamate

The poly-gamma-glutamate of Bacillus subtilis interacts specifically with silver nanoparticles

For many years, silver nanoparticles, as with other antibacterial nanoparticles, have been extensively used in manufactured products. However, their fate in the environment is unclear and raises questions. We studied the fate of silver nanoparticles in the presence of bacteria under growth conditions that are similar to those found naturally in the environment (that is, bacteria in a stationary phase with low nutrient concentrations). We demonstrated that the viability and the metabolism of a gram-positive bacteria, Bacillus subtilis, exposed during the stationary phase is unaffected by 1 mg/L of silver nanoparticles. These results can be partly explained by a physical interaction of the poly-gamma-glutamate (PGA) secreted by Bacillus subtilis with the silver nanoparticles. The coating of the silver nanoparticles by the secreted PGA likely results in a loss of the bioavailability of nanoparticles and, consequently, a decrease of their biocidal effect.

Engineering antimicrobial coating of archaeal poly-γ-glutamate-based materials using non-covalent crosslinkages

We are now entering a new age of intelligent material development using fine, sustainable polymers from extremophiles. Herein we present an innovative (but simple) means of transforming archaeal poly-γ-glutamate (PGA) into extremely durable polyionic complexes with potent antimicrobial performance. This new supra-polymer material (called PGA/DEQ) was subjected to nuclear magnetic resonance and X-ray diffraction spectroscopies to characterize in structural chemistry. Calorimetric measurements revealed its peculiar thermal properties; to the best of our knowledge, it is one of the most heat-resistant biopolymer-based polyionic complexes developed to date. PGA/DEQ is particularly useful in applications where surface functionalization is important, e.g., antimicrobial coatings. The spontaneously assembled PGA/DEQ coatings (without any additional treatments) were remarkably resistant to certain organic solvents (including chloroform), even at high salt concentrations (theoretically greater than those found in sea water), and various pH values. However, the pH-response tests also implied that the PGA/DEQ coatings could be removed only when concentrated citrate di-salts were used, whereas most crosslinked polymer composites (e.g., thermoset matrices) are difficult to recycle and treat downstream. We also discuss PGA/DEQ-immobilized surfaces that exhibit enigmatic microbicidal mechanisms.

Investigation of poly(γ-glutamic acid) production via online determination of viscosity and oxygen transfer rate in shake flasks

BACKGROUND

Poly(γ-glutamic acid) (γ-PGA) is a biopolymer with many useful properties making it applicable for instance in food and skin care industries, in wastewater treatment, in biodegradable plastics or in the pharmaceutical industry. γ-PGA is usually produced microbially by different Bacillus spp. The produced γ-PGA increases the viscosity of the fermentation broth. In case of shake flask fermentations, this results in an increase of the volumetric power input. The power input in shake flasks can be determined by measuring the torque of an orbitally rotating lab shaker. The online measurement of the volumetric power input enables to continuously monitor the formation or degradation of viscous products like γ-PGA. Combined with the online measurement of the oxygen transfer rate (OTR), the respiration activity of the organisms can be observed at the same time.

RESULTS

Two different Bacillus licheniformis strains and three medium compositions were investigated using online volumetric power input and OTR measurements as well as thorough offline analysis. The online volumetric power input measurement clearly depicted changes in γ-PGA formation due to different medium compositions as well as differences in the production behavior of the two investigated strains. A higher citric acid concentration and the addition of trace elements to the standard medium showed a positive influence on γ-PGA production. The online power input signal was used to derive an online viscosity signal which was validated with offline determined viscosity values. The online measurement of the OTR proved to be a valuable tool to follow the respiration activity of the cultivated strains and to determine its reproducibility under different cultivation conditions.

CONCLUSIONS

The combination of the volumetric power input and the OTR allows for an easy and reliable investigation of new strains, cultivation conditions and medium compositions for their potential in γ-PGA production. The power input signal and the derived online viscosity directly reflect changes in γ-PGA molecular weight and concentration, respectively, due to different cultivation conditions or production strains.