Bioactivities of Chitosan and Its Derivatives - Chitosan Based Hydrogels: Functions and Applications - page 140

3.3.3.2 AFM Characterization
AFM was used to assess the relationship between the MW of chitosan and its antimicro-
bial activity on both the vegetative and resistance forms of Bacillus cereus ( B. cereus ). Higher
MW chitosan (628 and 100 kDa) surrounded both forms of B. cereus cells by forming a
polymer layer. This eventually led to the death of the vegetative form by preventing the
uptake of nutrients, yet did not affect the spores because these can survive for extended
periods without nutrients. COSs (<3 kDa), on the other hand, produced more visible dam-
age in the B. cereus vegetative form—most probably due to the penetration of the cells
by COS. The use of COS by itself on B. cereus spores was not enough for the destruction
of a large number of cells, but it may well weaken the spore structure and its ability to con-
taminate, by inducing exosporium loss [138].
The effect of low, medium, and high MW chitosan was evaluated on the development of
three isolates of Rhizopus stolonifer . Image analysis and electronic microscopy observations
were performed in spores of this fungus. Germination of R. stolonifer in potato dextrose
broth with chitosan was also evaluated. The results indicated that LMWC was more effec-
tive in the inhibition of mycelial growth whereas HMW chitosan affected spore shape, spo-
rulation, and germination. Studies with scanning and transmission electron microscopy
revealed numerous and deeper ridge ornamentations of the chitosan-treated spore [139].
The antibacterial activity of chitosan was investigated by assessing the mortality rates
of Escherichia coli and Staphylococcus aureus based on the extent of damaged or missing
cell walls and the degree of leakage of enzymes and nucleotides from different cellular
locations. Chitosan was found to react with both the cell wall and the cell membrane, but
not simultaneously, indicating that the inactivation of E. coli by chitosan occurs by a two-
step sequential mechanism: an initial separation of the cell wall from its cell membrane,
followed by destruction of the cell membrane. The similarity between the antibacterial
profiles and patterns of chitosan and those of two control substances, polymyxin and
EDTA, verified this mechanism. The antibacterial activity of chitosan could be altered by
blocking the amino functionality through coupling of the chitosan to active agarose
derivatives. These results verify the status of chitosan as a natural bactericide.
3.3.3.3 Antibacterium Activity of the Schiff Base of Chitosan and Metallic Ion-Loaded
Chitosan Nanoparticles
The Schiff base of chitosan was synthesized by the reaction of chitosan with citral working
under high-intensity ultrasound. The antimicrobial activities of chitosan and the Schiff
base of chitosan were investigated against E. coli , S. aureus , and Aspergillus niger . The results
indicate that the antimicrobial activity of Schiff base increases with an increase in the con-
centration. It was also found that the antimicrobial activity of Schiff base was stronger
than that of chitosan [140].
Chitosan nanoparticles were prepared based on ionic gelation between chitosan and
sodium tripolyphosphate. Then, Ag + , Cu 2+ , Zn 2+ , Mn 2+ , or Fe 2+ was individually loaded onto
chitosan nanoparticles. Their antibacterial activities were evaluated by the determination of
MIC and minimum bactericidal concentration (MBC) against E. coli 25922, Salmonella choler-
aesuis ATCC 50020, and S. aureus 25923 in vitro . The results showed that antibacterial activity
was significantly enhanced by the metal ions loaded, except for Fe 2+ . Especially for chitosan
nanoparticle-loaded Cu 2+ , the MIC and MBC against E. coli 25922, S. choleraesuis ATCC 50020,
and S. aureus 25923 were 21-42 times lower than that of Cu 2+ , respectively. Moreover, it was
found that antibacterial activity was directly proportional to zeta potential [141].