A newly Bacillus subtilis isolate for PHB biosynthesis: optimization and thermal spectroscopic validation

Abstract Poly(3-hydroxybutyrate) (PHB) is a biodegradable polyester considered a sustainable alternative to conventional plastics. This study aimed to isolate a PHB-producing bacterium from agricultural soil and to evaluate the natural honey produced by Apis mellifera as an alternative carbon source for PHB production. A PHB-producing isolate was obtained and identified as Bacillus subtilis strain MUST-MWS1 based on morphological, biochemical, and 16 S rRNA gene analyses. Cultures were grown at pH 8.2 using LB medium, glucose, or honey as the main carbon source, and PHB was extracted by a chloroform–methanol method. The polymer was characterized by Fourier transform infrared spectroscopy, gas chromatography–mass spectrometry, X-ray diffraction, differential scanning calorimetry, and thermogravimetric analysis. Honey yielded the highest PHB titer (0.2669 ± 0.0078 g/L), compared with glucose (0.2168 ± 0.0068 g/L) and LB medium (0.1933 ± 0.0067 g/L). The structural and thermal properties of the biosynthesized PHB were consistent with those reported for commercial PHB. These findings indicate that B. subtilis MUST-MWS1 is a promising PHB producer and that honey can serve as an effective alternative carbon source for bioplastic production.