Biomedical Engineering Reference
In-Depth Information
dii cult to determine the most optimal MW for the maximal antimicrobial
activity.
h e antimicrobial activity of chitosan varies depending on the micro-
organisms tested. h e relationship between MW of chitosan and the anti-
microbial activity too is af ected by the test organisms. Shimojoh,
et al.
,
treated several oral bacteria with the same concentration of chitosans of
dif erent molecular weights [67]. It was found that the chitosan with MW
2,20,000 was most ef ective and MW 10,000 was the least ef ective in their
bactericidal activities. h e antimicrobial activity of chitosan with MW of
70,000 was better than MW of 4,26,000 for some bacteria, but for the oth-
ers, the ef ectiveness was reversed. Yalpani,
et al.
, reported that chitosans
(medium and high MW) showed higher antimicrobial activities against
Bacillus circulans
than chitooligosaccharides (DP 2-30), whereas they
were less ef ective against
E. coli
than chitooligosaccharides [68]. A posi-
tive correlation between the MW of chitosan and its activity against the
Gram-positive bacteria,
Staph. aureus
, and the negative correlation for
Gram-negative bacteria has been reported by Zheng and Zhu also [69].
Similar trends were rel ected in inspection of antimicrobial activity by
Atomic Force Microscopy (AFM), where specii cally, cell lysis, surface
roughening and cell clustering were observed [70]. In the case of
E. coli
,
AFM results displayed clustering due to the ionic interaction between chi-
tosan oligosaccharide and cell wall (or due to the production of extracel-
lular polymeric substance). h is apparent response strategy presumably
protected the bacilli in the interior of the clusters from the action of chito-
san oligosaccharide and led to only a short-lived ef ect on the cell-viability,
i.e., bacteriostatic action. However, for the high-MW chitosan, the poly-
mer prevented this clustering mechanism (possibly due to comparatively
lower ionic inl uence v/s the chitosan oligosaccharide); keeping bacilli iso-
lated from each other and consequently was a more ef ective antibacterial
agent. AFM images for
Staph. aureus
showed much less intense cellular
morphological changes on chitosan and oligosaccharide treatment than
for
E. coli
. h is was in accordance to presence of much thicker peptidogly-
can layer of the cell wall of Gram-positive bacteria; but nanoindentation
studies revealed that even so, the cells were weakened by treatment with
the chitooligosaccharides.
It has been proven that lower MW leads to longer persistence length (PL)
at the same deacetylation degree [71, 72] and the persistence length decides
whether the chitosan molecule will penetrate into the bacteria cell or not.
Scherrer and Gerhardt found that the minimum persistence length to pass
the cell wall of Gram-positive bacteria is 11วบ [73]. Decad and Nikaido found
that the minimum persistence length to pass the cell wall of Gram-negative
