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

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 in complex media under different agitation conditions and cultivation times was evaluated. 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 (MMW) weights of 80,050 g mol^-1 to 116,960 g mol^-1 for B. cereus and from 75,805 to 111,000 g mol^-1 for A. vinelandii OP. TGA/DSC analysis revealed that higher agitation rates decrease crystallinity and thermal stability by altering polymer chain alignment. The volumetric oxygen transfer coefficient (k_La) confirmed the role of oxygen availability on P3HB. These results highlight two promising strains with distinct metabolic behaviors and strong potential for scale-up in P3HB production.