Arab Journal of Nuclear Science and Applications, 50 (1), (120-130) 2017 021 Characterization of Chitosan Produced from Fermented Shrimp Shell Waste by Bacillus subtilis NA12 Using Gamma Radiation A.S. Bashandy 1, *, E.I. Raffat 1 , H.M.M. Ibrahim 1 , T. S. El Tayeb 2 and R.F. Gamal 2 (1) Department of Radiation Microbiology, National Center for Radiation Research and Technology (NCRRT), Atomic Energy Authority, Cairo, Egypt (2) Department of Agricultural Microbiology, Faculty of Agriculture, Ain Shams University, Egypt Received: 15/3/2016 Accepted: 20/6/2016 ABSTRACT This study focused on characterization of chitosan obtained in a previous s tudy from fe rme nte d s hrimp s hell was te (SSW) by Bacillus subtilis NA12. Extracted chitin was exposed to different gamma radiation doses (5-35 kGy). The molecular weight of the resultant chitosan decreased constantly with increasing radiation dose from 1.9 × 10 6 (g/mol) (non-irradiated) to 3.7 × 10 4 (g/mol) (at 35 kGy). The degree of deacetylation (DDA%) was determined by using potentiometric titration. The structural properties of chitin and chitosan were characterized by Fourier transform infrare d spectroscopy (FT-IR), X-ray diffraction analysis (XRD), and thermogravimetric analysis (TGA). The prepared chitosan has higher solubility and DDA% compared to the standard chitosan. FT-IR analysis clearly confirmed the successful extraction of pure chitin and chitosan. TGA showed that chitin exhibited a stable structure toward thermal decomposition than chitosan. XRD analysis revealed that extracted chitin was more crystalline than prepared chitosan. Chitosan with different molecular weights was evaluated as an antibacterial agent against representative pathogenic bacterial strains. Chitosan obtained at 35 kGy, with molecular weight 3.7 × 10 4 (g/mol) and DDA of 87.9 %, showed the highest antimicrobial activity against Escherichia coli, Klebsiella sp., Staphylococcus aureus and Pseudomonas aeruginosa . It revealed inhibition zone diameter of 5.4 ± 0.2, 5.4 ± 0.12, 3.5 ± 0.21 and 1.4 ± 0.06 cm, respectively. Keywords: Bacillus subtilis / Shrimp Shell Waste / γ-Irradiation / Chitosan / Antibacterial Agent. INTRODUCTION Chitin is the most important natural polysaccharide after cellulose, it is found in exoskeleton of crustaceous shells as well as in cell walls of fungi. Ideally, chitin is a linear polysaccharide consisting of β-(1-4)-2-acetamido-2- deoxy-d-glucopyranose repeating units where the amine groups are entirely acetylated. The ideal structure of chi tosan, the primary derivative of chitin, is comprised of linear β -(1- 4)-2-amino-2-deoxy-d-glucopyranose repeating units where the N-acetylglucosamine residues in chitin macromolecular chain are fully deacetylated to become N-glucosamine residues. However, chitin is not widely used for industrial applications untill now because it is insoluble in many solvents, relatively difficult to isolate from natural sources in pure form, and difficult to be reproduced under economic conditions. That is why it is also difficult to characterize this polysaccharide (1) . Therefore, the chemical modifications of chitin were performed. The most common derivative is chitosan, derived by partial deacetylation of chitin (2) . After deacetylation of chitin, generally, the obtained chitosan has high molecular weight and viscosity, which limit its solubility. The use of chitosan in diverse areas is directly related to the polymer’s molecular weight and degree of deacetylation (3) . Under these circumstances, reducing the molecular weight is very crucial for its use. According to the results reported by Wasikiewicz et al. (4) , the most efficient way for the reduction of molecular weight of chitosan is gamma-irradiation. In a recent study, the degradation of chitosan by γ -irradiation to reduce its molecular weight was