Bactericidal and fungicidal activities of different molecular weight chitosan samples

The antimicrobial activity of different molecular weight chitosan samples (with approximately the same degree of deacetylation 83 ± 2 %) against some common of plant pathogenic bacteria and fungi was investigated. Average molecular weight of chitosan samples was determined by measurements of intrinsic viscosity and was found to be 3.60×105 Da for low molecular weight (LMW), 6.11×105 Da for medium molecular weight (MMW) and 9.53×105 Da for high molecular weight (HMW) chitosan samples. The antibacterial assessment of the chitosan samples was performed with agar dilution method against cultures of six plant pathogenic bacteria Agrobacterium tumefaciens, Corynebacterium fascians, Erwinia amylovolora, Erwinia carotovora, Pseudomonas solanacearum and Sarcina lutea. The results are expressed in term of Minimum Inhibitory Concentration (MIC). The results indicated that MMW and HMW were more potent in bactericidal activity than LMW chitosan and a HMW chitosan exhibited a good antibacterial potency especially against C. fascians with MIC 500 mg.L-1. The fungicidal assessment was assessed using a mycelial radial growth inhibition technique against six plant pathogenic fungi Alternaria alternata, Botrytis fabae, Fusarium oxysporum, Penecillium digitatum, Pythium debrianum and Rhizoctonia solani and the results are expressed as Minimum Effective Concentration of 50% of mycelial growth (EC50). The data also demonstrated that the fungicidal activity was increased with the increasing of the molecular weight and a HMW chitosan was the most potent one against all the tested fungi especially P. digitatum with EC50 of 510 mg.L-1.