Synthesis, characterization and biological activity of Schiff bases based on chitosan and arylpyrazole moiety

 
4.2. The antimicrobial activity 
4.2.1. The antibacterial activity 
The antibacterial activity of chitosan and chitosan Schiff bases (ChBs) against S. pneumonia, B. 
subtilis and E. coli are shown in Table 3. It was found that the diameter of the inhibition zone varied 
according to the substitution and also the microorganisms tested. Compared with chitosan, the ChBs 
derivatives showed a better antibacterial Activity toward the tested bacterial species in the low 
concentrations. Furthermore, chitosan and ChBs have greater antibacterial activity against E. coli 
than that of B. subtilis and S. pneumonia at the same concentration of the tested materials. The 
growth inhibitory effect of the ChBs against both gram negative and gram positive examined 
bacteria increases in the following order: Ch-H 
>
Ch-NO
2
≈ Ch-CH
3 
>
Ch-OH
3 
≈ Ch-Cl. Among the 
derivatives, Ch-H has the highest potential to suppress the growth of bacteria and showed lower 
MIC value (3.9 
µg mL
-1
). 
Several mechanisms elucidating the antimicrobial activity of chitosan have been postulated. The 
first mechanism is the interaction between positively charged chitosan molecules and negatively 

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charged microbial cell membrane. The interaction is mediated by the electrostatic forces between 
the protonated NH
3
+
groups of chitosan and the electronegative charges on the microbial cell 
surface [57]. Another proposed mechanism is the binding of chitosan with microbial DNA, which 
leads to the inhibition of the mRNA and protein synthesis via penetration of chitosan into the nuclei 
of the microorganisms [58]. The third mechanism which is considered to be the most acceptable 
mechanism is the chelating of metals, suppression of spore elements and binding to essential 
nutrients to microbial growth [59]. The investigated derivatives have excellent metal-binding 
capacity, due to the changing of the amino groups of the chitosan to imine –N=CH- groups giving 
the chance for more binding sites. Furthermore, the derivatives of ChBs might interact with cellular 
components of bacteria via hydrophobic interactions due to the hydrophobic moiety of the 
substituted groups.
Table 3 
4.2.2. The antifungal activity 
The capability of chitosan and ChBs derivatives in inhibiting the growth of A. fumigatus, G. 
candidum and S. recemosum fungi are listed in Table 4. Compared with the antifungal activity of 
chitosan, the ChBs derivatives had better activity. It is obvious from Table 4 that antifungal activity 
of Ch-H against investigated fungi is 
the best
. The growth inhibitory effects of the ChBs increases 
in the following order: Ch-H 
>
Ch-CH
3
>
Ch-OCH
3 
≈ Ch-Cl
>
Ch-NO
2
. All the samples showed 
antifungal activity in the order S. recemosum 
>
A. fumigatus 
>
G. candidum. The minimum 
inhibitory concentrations (MICs) of the Schiff bases were found to be in the range 1.0- 31.3 
µg mL
-
1
. 
Table 4