Synthesis of Hydrogels Made of Poly-γ-Glutamic Acid (γ-PGA) for Potential Applications as Probiotic-Delivery Vehicles

Abstract

Numerous hydrogels made of poly-γ-glutamic acid (γ-PGA) and various cross-linkers have been explored, but only limited data on hydrogels made of γ-PGA and poly(ethylene glycol) (PEG) are available. In this study, γ-PGA, a biodegradable and edible biopolyamide, was successfully cross-linked with selected PEGs to obtain a series of hydrogels. The swelling behaviour of these hydrogels was investigated under various pH conditions. It was also found that the structure of the cross-linker (linear or branched) affected the hydrogels’ swelling behaviour. In addition, in disc diffusion assay, hydrogel discs loaded with antibiotic were tested against Staphylococcus aureus and Escherichia coli. Prolonged activity of hydrogels loaded with antibiotics in comparison to paper discs containing antibiotics was observed. Moreover, the protective effect of hydrogels on entrapped probiotic cells subjected to low pH was investigated. The hydrogel swelling ratio and amount influenced the survival rate of the protected bacteria. Considering potential biomedical applications of hydrogels, cytotoxicity was evaluated towards two cell lines, MSTO and PANC 1.

Hydrogels Made of Poly-γ-Glutamic Acid and Sugar Alcohols for Enhanced Survival of Probiotic Strains Subjected to Low pH and Freeze Drying

“…It was previously established that increasing the hydrogel amount (from 25 to 50 mg per sample) visibly improved the cell survival rate. The viable cell count of Lactobacillus cells entrapped in 50 mg of hydrogels made of γ-PGA and PEG, after 4 h of incubation under pH = 1.5, was higher than 4.0 Log CFU/mL (4.16 ± 0.37 Log CFU/mL for PEG400 or 4.29 ± 0.24 Log CFU/mL for PEG1000-3arm) [8]. Based on the obtained results, it was established that the smaller amount (25 mg) of hydrogels made of γ-PGA and sugar alcohol were able to maintain a higher viable cell count of Lactobacillus strains subjected to low pH than a larger amount (50 mg) of γ-PGA-PEG hydrogels.…”

“…Numerous hydrogels have been developed constantly to obtain materials that are able to improve the survivability of entrapped probiotic bacteria under gastric conditions, during heat treatment or storage at various temperatures [20]. Previously, we have established that γ-PGA-based hydrogels, developed by some of us, were able to improve the viability of entrapped strains of probiotic bacteria during exposure to low pH [8]. Those outcomes encouraged further research on γ-PGA-based hydrogels.…”

“…Hydrogels were synthesized in a manner similar to the method previously described [8]. Briefly, the 1.5 g of γ-PGA was placed in a round bottom flask equipped with a magnetic stirring bar and dissolved in 80 mL of DMSO.…”

“…In our previous work, we described hydrogels based on poly-γ-glutamic acid (γ-PGA), which were successfully tested as probiotic-delivery vehicles able to improve the viability of entrapped strains of probiotic bacteria under acidic conditions. Those hydrogels were obtained in a reaction between a γ-PGA biopolymer and poly(ethylene glycol)s (PEGs) cross-linkers [8]. Poly-γ-glutamic acid, a naturally occurring polymer, is water-soluble, edible, biodegradable and non-toxic toward humans or environment [9].…”

“…Previously, different PEGs were selected as cross-linkers due to their availability, relatively low price and widespread applications in the pharmaceutical and food industries [8]. However, it seemed reasonable to develop fully biobased hydrogels for probiotic oral-delivery systems.…”