Effects of media components and agricultural by-products on γ-polyglutamic acid production by Bacillus toyonensis As8.

Abstract

Background: Poly-γ‑glutamic acid (γ‑PGA) provides an environmentally friendly alternative to plastic materials which have widely polluted the environment.

Objectives: The microbial production of γ‑PGA, an amino acid biopolymer with glutamic acid subunits, was investigated using renewable agricultural residues in an attempt to find cheaper substitutes for conventional synthetic media components.

Material and methods: Bacteria which produce γ‑PGA were isolated through depolymerizing Coix lacryma-jobi, a cellulosic grass, and the effects of various carbon and nitrogen sources, temperature, inoculant load, incubation period, and pH on γ‑PGA yield were determined after submerged fermentation. Bacterial growth was measured turbidimetrically at 550 nm. The γ‑PGA produced was characterized using Fourier transform infrared (FT-IR) spectroscopy and the polymer shape was determined using scanning electron microscopy (SEM).

Results: The best γ‑PGA producer was molecularly identified as Bacillus toyonensis As8. The conditions which produced the highest γ‑PGA yield were glucose, ammonium sulfate, 25°C, a pH of 5.5, and an incubation period of 48 h. This bacterium yielded the most γ‑PGA (26.45 g/L) on cassava peels, while other agro-wastes (corn cob, sorghum leaves, Coix noir leaves, and rice bran) also supported bacterial growth with lower γ‑PGA yields than conventional carbon sources. The wrinkled γ‑PGA had absorbance peaks of hydroxyl, amide, carbonyl, and amine groups comparable with the ranges of those found in commercial γ‑PGA.

Conclusions: The use of agricultural by-products as fermentation substrates increased γ‑PGA yield and may therefore be used as substitute components in γ‑PGA production.