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An investigation was made to find out an optimal molecular weight of chitosan exhibiting the better anti-bacterial and antifungal activities. Chitosan, whose molecular weights are 54,36,23 and 13 kDa, were subjected to minimum growth inhibitory concentration (MIC) assay against pathogenic bacteria, Escherichia coli, Staph-ylococcus aureus, and a human pathogenic yeast, Condida albicans. MICs of chitosan were varied depending upon pH of chitosan preparation, either pH 3.6 ~ 3.8 or 5.9 ~ 6.0. At lower pH, MICs of chitosan were 1.4 ~ 1.7 times lower than those at high pH. The lowest MICs for E. coli, S. aureus, and C . albicans obtained with the 54 kDa chitosan in the range of pH 3.6 ~ 3.8, were 0.033%, 0.025%, and 0.001%, respectively. It was suggested that C. albicans was about 30 times more susceptible to chitosan compared with bacteria. When the growth of Fusarium culmorom, a plant pathogenic fungus, was monitored at various concentrations of chitosan (M.W 54 kDa), growth of the fungus was significantly inhibited by chitosan, a scanty fungal growth was observed at 8 day of culture with 0.025% of chitosan. Moreover, no growth was appeared at 0.0375 % of chitosan upto 12 days. As a part of understanding the antibiotic mechanism of chitosan on bacteria, scanning electron microscophy (SEM) and transmission electron microscophy (TEM) were made with E. coli and S. aureus grown on LB medium contained a sublethal dose of chitosan (M.W 54 kDa). SEM of E. coli and S. aureus revealed severe damage of cells resulted in irregular shapes. TEM of S. aureus showed heavy swelling of bacterial cell wall and disintegration of cytoplasm. TEM of E. coli showed irregularity of cell, such as swelling of cell wall and dissociation of outer membrane from cell body.
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