Ability of converting sugarcane bagasse hydrolysate into polyhydroxybutyrate (PHB) by bacteria isolated from stressed environmental soils

Abstract

We isolated 109 potential polyhydroxybutyrate (PHB)-producing bacteria from various soil sources. Using Sudan Black B staining, lipid granules were observed in 65 isolates. UV spectrophotometry-based quantification revealed that 14 isolates effectively produced PHB, and all isolates carried the phaC gene encoding polyhydroxyalkanoate synthase required for PHB polymerization. After cultivation at 30 °C for 72 h in E2 medium containing 3.5% (w/v) glucose as the carbon source and 0.354% (w/v) NH4Y as the nitrogen source, the highest PHB content (%-CDW) and yield (g-PHB/g-glucose) with values of 20.4% and 0.28 and 19.7% and 0.07 were obtained for Priestia megaterium KKR5 and Burkholderia cepacia ASL22, respectively. TEM analysis revealed PHB granules with diameters of 0.2–0.7 μm inside KKR5 and ASL22 cells. We achieved the bioconversion of sugarcane bagasse hydrolysate (BGH) into PHB by these strains via separate hydrolysis and fermentation. H3PO4 steam-pretreated BGH generated reducing sugars containing glucose and xylose without toxic chemicals. Using the reducing sugars of BGH as the carbon source, ASL22 exhibited a higher PHB yield (0.22 g-PHB/g-reducing sugar) than KKR5 (0.12 g-PHB/g-reducing sugar). PHB from KKR5 and ASL22 had FTIR and NMR spectra that were similar to those of standard PHB. A GC/MS analysis confirmed the molecular structure of PHB as methyl hydroxybutyrate with a mass value of 43–103. By DSC analysis, the melting temperatures of PHB from KKR5 and ASL22 and standard PHB were 167.8, 168.5, and 176.6 °C, respectively. Thus, PHB-producing bacteria isolated from stressed soils successfully utilize a low-cost BGH substrate for PHB accumulation.