Comparison of poly-3-hydroxybutyrate (P3HB) synthesis by Bacillus cereus and Azotobacter vinelandii OP: effect of agitation on the accumulation and physicochemical properties of the biopolymer

Abstract Poly-3-hydroxybutyrate (P3HB) is a biodegradable thermoplastic polyester with mechanical and thermal properties comparable to those of petrochemical-based plastics. In this study, the synthesis of P3HB by Bacillus cereus ATCC 14579 and Azotobacter vinelandii OP ATCC 13705 was evaluated in complex media under varying agitation conditions and cultivation times to assess their impact on biopolymer properties. The growth kinetics of each microorganism responded differently to changes in agitation patterns. Maximum cell concentrations of 2.4 g L⁻ ¹ and 4.3 g L⁻ ¹ were achieved at 200 rpm (24 h) for B. cereus and 150 rpm (48 h) for A. vinelandii, respectively. While B. cereus reached an accumulation of 31.3% (0.37 g P3HB L⁻ ¹), A. vinelandii OP achieved 55.8% (2.3 g P3HB L⁻ ¹). The biopolymer was characterized by ATR-FTIR, with a prominent carbonyl (C = O) stretching vibration observed at 1724 cm⁻ ¹. SEC-HPLC analysis revealed mean molecular weights ranging from 80,050 g mol⁻ ¹ to 116,960 g mol⁻ ¹ for B. cereus and from 75,805 g mol⁻ ¹ to 111,000 g mol⁻ ¹ for A. vinelandii OP. TGA/DSC analysis was used to evaluate thermal stability and transition properties, revealing that higher agitation rates decrease crystallinity and thermal stability by altering polymer chain alignment. The volumetric oxygen transfer coefficient (kLa) was estimated to examine the relationship between agitation speed and P3HB production, confirming the influence of oxygen availability in the medium. This study highlights two promising strains with distinct metabolic behaviors and strong potential for scale-up in P3HB production.