الفهرس 
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Abstract
Bacterial poly-β-hydroxybutyrate (PHB) is a natural, biodegradable polymer, which is accumulated in the cells as an energy reserve material due to depletion of nitrogen or phosphorous in the presence of excess carbon source. This polymer is foreseen to possess high industrial potentiality and excellent alternative to the non-degradable petroleum-based plastics. Albeit the high production cost; Microbial Poly-β-hydroxybutyrate (PHB) was proven to be the most potential polymer to be used in the biodegradable polymer industry. In this study, we isolated and characterized a local bacterial strain WA81 which accumulated 18mg/L PHB after 72 h growth in mineral salt medium under nitrogen deficiency. The PHB granules were detected in the cells using TEM and the genes encode for this polymer were detected by oligonucleotide primers using PCR technology. The 16S rRNA gene nucleotide sequence for this isolate was used to construct a phylogenetic tree against all available sequences in the GenBank. The phylogenetic tree data suggested that the closest type strain to the local bacterium is the Microbacterium paraoxydans CF36T and hence we named it Microbacterium sp. strain WA81. Moreover, the set of enzymes responsible for the PHB biosynthetic pathway and their controlling elements were detected in this local isolate using PCR. The genes encode for the biosynthesis enzymes are phbA (β-ketothiolase), phbB (acetoacetly CoA reductase), phbC (PHB polymerase), while the genes encode for the controlling elements are phbP (phasin), phbZ (PHB depolymerase). The novelty of this local bacterium lies in its ability to accumulate huge amounts of PHB in its cytoplasm and the presence of a whole set of genes encode for the PHB biosynthetic and catabolic pathways of this polymer. The produced polymer was extracted, purified and fully characterized to detect its chemical structure, molecular weight and thermal properties. In order to reduce the production cost, the second cheap materials medium was generated. A number of local cheap materials were screened using 2-level fractional factorial design and found that molasses, whey and ground sesame to be the most potential media components for maximizing the PHB productivity by the local bacterial isolate. A subsequent response surface methodology algorithm was applied to determine the exact optimum combination to maximize the PHB productivity by converting industrial by-products into value-added ones. Depending on the generated mathematical model, the optimum combination was determined to be 9.21 g/L , 8.49g /L and 8.12g/L for molasses , whey and ground sesame respectively leading to a remarkable increase in PHB productivity (up to 690 mg/L). On the transfer of this process to the bioreactor cultivation level, the amount of the polymer produced by the local bacterial isolate under investigation was increased up to 2.5 g/L achieving more than 139% increase compared to the amount produced prior to optimization. The data concluded that application of statistical optimization via DOE tools are very useful in optimizing PHB by this novel local bacterial isolate.