Screening and optimization of polyhydroxybutyrate (PHB) by Lysinibacillus fusiformis from diverse environmental sources

Abstract

The need to explore microbial strains inhabiting ecosystems capable of yielding bioplastics with high production capacity and environmental sustainability is particularly of noteworthy due to the negative impacts of petrochemical plastics on humans and the environment. This study thus focused on the identification of novel polyhydroxybutyrate (PHB) producers, coupled with the screening and optimization of PHB or bioplastics for maximum yield. Bacterial strains were isolated from incinerator dump-site soil, hydrocarbon soil, rhizosphere and bulk soil samples of tomato and okra biochar plants. The identification of isolates was carried out based on biochemical and molecular methods. Screening of bacterial isolates for PHB was conducted using Sudan stain and Bromothymol techniques. Optimization analysis for maximum PHB production conditions by potential PHB strains was carried out via response surface methodology using Box-Behnken design at pH (2–11); substrates: rice bran (1–5 g); and time (24–96 h). Results showed that 112 bacterial isolates were recovered from all the samples collected. Tomato plant biochar soil recorded the highest number of bacterial isolates (37), while hydrocarbon soil yielded the lowest number of bacterial isolates (16). Predominant genera are Micrococcus, Bacillus and Staphylococcus. Lysinibacillus fusiformis was revealed to be the most potent PHB producer. Among the 112 bacteria isolated, nine isolates (5 isolates from tomato plant biochar soil, 2 bacteria from okra plant biochar soil and a single bacterial isolate from an incinerator dump site and hydrocarbon soil) exhibited positive results in both Sudan black and bromothymol tests. Box-Behnken design indicated the optimal experimental conditions for PHB synthesis to be pH 7, 72 h and substrates of rice bran of 3 g. In conclusion, Lysinibacillus fusiformis obtained from a tomato biochar plant demonstrates a high potential for bioplastic synthesis utilizing agricultural waste (rice bran) as a cost-effective as well as sustainable carbon source and could be utilized for commercial-scale PHB production.