Biomedical Engineering Reference
In-Depth Information
between the antibacterial proi les and patterns of chitosan and those of
control substances, polymyxin and EDTA.
h e other proposed mechanism involves the binding of positively
charged chitosan with DNA to inhibit its functioning [38, 55]. However,
this mechanism is still controversial due to considerations of MW and
size. h e possibility of chitosan's direct access to the intracellular targets
is unlikely because of its high MW (upto 1000 kDa) and size. It would not
normally be able to reach these targets unless it could overcome the bar-
rier of the plasma membrane such as by getting hydrolysed to a lower MW
polymer. Nakae and Nikaido have reported inablilty of chitosan to pass the
outer membrane of Gram-negative bacteria [56].
h e chelating activity of chitosan and deprivation of metals, trace ele-
ments or essential nutrients limiting the production of toxins, growth of
microorganisms have ot en been implicated as a possible modes of action
[37, 57]. Otherwise chitosan draws ions as Ca
2+
or Mg
2+
from the cell wall
by its chelating ability [44].
Yet, chelation of metals does not seem to be
important for the antibiotic activity of chitosan; in contrast, the forma-
tion of complexes with metal ions appears to abrogate this activity [50].
Chitosan is also able to interact with and l occulate proteins, but this action
is highly pH-dependent. Moreover, growth inhibition through blockage
of nutrient l ow has been suggested by several researchers who attribute
the antibacterial activity of chitosan to its deposition (stacking) onto the
surface of bacteria, thereby impeding mass transfer and suppressing the
metabolic activity of bacteria [24, 58, 59].
6.4.2 Factors Af ecting Antimicrobial Activity
6.4.2.1
Molecular Weight (MW)
Tanigawa,
et al.
, reported that D-glucosamine hydrochloride (chitosan
monomer) did not show any growth inhibition against several bacteria,
whereas chitosan was ef ective [60]. h is suggests that the antimicrobial
activity of chitosan is related not only to its cationic nature, but also to
its chain length and MW. Kendra and Hadwiger examined the antifungal
ef ect of chitosan oligomers on
Fusarium solani f. sp. pisi
and
Fusarium
solani f. sp
.
phaseoli
[61]. In the assessment of minimum inhibitory con-
centration, the antifungal activity was found to increase as the polymer size
increased. Monomer and dimer units did not show any antifungal activ-
ity at the concentration of 1000 μg/ml, whereas, heptamer (DP 7) showed
maximal antifungal activity and the minimum concentrations were identi-
cal for native chitosan and heptamers.
