The complex world of polysaccharides edited by Desiree Nedra Karunaratne

 

The Complex World of Polysaccharides 

 

16 

preventing the entry of nutrients or causing the leakage of intracellular contents [95-100]. 

Another reported mechanism involves the penetration of low- molecular weight chitosan 

into the cell, binding to DNA and the subsequent inhibition of RNA and protein synthesis 

[101]. Chitosan has also been shown to activate several defence processes in plant tissues 

and it inhibits the production of toxins and microbial growth because of its ability to chelate 

metal ions [102,103]. 

EI-Ghaouth et. al.(1992) have proposed possible antibacterial actions of chitosan and its 

derivatives [104]. They asserted that chitosan reacted with the cell surface, altered cell 

permeability, and further prevented the entry of material or caused the leakage of material. 

However, no evidence has been provided to demonstrate the relationship between the 

antibacterial activity of chitosan and the surface characteristics of the bacterial cell wall. 

Antimicrobial activity of chitosan has been demonstrated against many bacteria, 

filamentous fungi and yeasts [105-108]. Chitosan has wide spectrum of activity and high 

killing rate against Gram-positive and Gram-negative bacteria, but lower toxicity toward 

mammalian cells [109,110]. Variations in chitosan's antimicrobial efficacy arise from various 

factors. These factors can be classified into four categories as follows: (1) microbial factors, 

related to microorganism species and cell age; (2) intrinsic factors of chitosan, including 

positive charge density, Mw, concentration, hydrophilic/hydrophobic characteristic and 

chelating capacity; (3) physical state, namely water-soluble and solid state of chitosan; (4) 

environmental factors, involving ionic strength in medium, pH, temperature and reactive 

time [111]. 

Although owning a broad spectrum of antimicrobial activity, chitosan exhibits different 

inhibitory efficiency against different fungi, Gram-positive and Gram-negative bacteria. 

Chitosan exerts an antifungal effect by suppressing sporulation and spore germination [112]. 

In contrast, the mode of antibacterial activity is a complicating process that differs between 

Gram positive and Gram-negative bacteria due to different cell surface characteristics. 

Based on the available evidences, bacteria appear to be generally less sensitive to the 

antimicrobial action of chitosan than fungi. The antifungal activity of chitosan is greater at 

lower pH values [113]. For a given microbial species, age of the cell can influence 

antimicrobial efficiency. 

Chitosan has a broad spectrum of unique biological activities, including its ability to induce 

resistance to viral infections in plants, inhibit viral infection in animal cells, and prevent the 

development of phage infection in infected microbial cultures. High polymeric chitosan, 

when added to a nutrient medium, prevents the accumulation of the infectious phage 

progeny in infected cultures of Gram-positive and Gram-negative organisms. The yield of 

infectious DNA containing phage can decrease by several orders of magnitude in the 

presence of chitosan [114, 115]. The observed effect also depends on the concentration, 

degree of polymerization, and molecular structure of chitosan. Thus, chitosans with a 

polymerization degree of 250 and higher were much more effective in inhibiting coliphage 

infection than their fragments with a polymerization degree of 15–19. On the other hand, 

chitosan oligomers were more effective than their polymeric precursor in inhibiting the 

---

 

Is Chitosan a New Panacea? Areas of Application 

 

17 

replication of 1-97A phage in Bacillus thuringiensis cultures. Factors determining such strong 

differences are currently unclear. Anionic derivatives of chitosan, such as 6-O-sulfate and N-

succinate-6-O-sulfate, caused no effect on phage infection [114]. This finding showed that 

the positive charge of a chitosan molecule is important for inhibition of phage infection. 

It has been suggested that chitosan can inhibit the replication of bacteriophages by several 

mechanisms: it can (a) decrease the viability of cultured bacterial cells, (b) neutralize the 

infectivity of mature phage particles in the inoculum and/or daughter phage particles, and 

(c) block the replication of the virulent phage [114]. 

The condition of the phage culture is known to be of paramount importance for the 

development of phage infection because phages can reproduce only in viable cells. 

However, there is evidence that chitosan displays a bactericidal activity toward many 

microbial species including Escherichia coli [116, 117] and species of the genus Bacillus [118]. 

Chitosan, because of its polycationic nature, binds to the external membrane of Gram 

negative microorganisms by electrostatic forces, which is demonstrated in experiments with 

core phosphate groups of lipid A, thereby decreasing the potency of endotoxin. It was 

suggested that the antibacterial effects of chitosan and many other cationic agents are based 

on their ability to increase the permeability of the outer membrane of Gram negative 

microorganisms to an extent incompatible with their viability [116-119]. 

Great amount of literature support the essential importance of polycationic structure in 

antimicrobial activity. A higher positive charge density leads to strong electrostatic 

interaction. Therein, the positive charge is associated with DD or degree of substitution (DS) 

of chitosan or its derivatives, which affect positive charge density. Concerning chitosan 

derivatives, antimicrobial activity mostly depends on DS of the grafting groups. 

There are several works regarding the influence of the molecular weight of chitosan on its 

antimicrobial properties [120-126]. Some of them have demonstrated that COS 

(chitooligosaccharides), which are soluble in water, were the least effective in terms of 

biocide properties [124-126]. Recent work carried out by Qin et al. 2006, on the evaluation of 

chitosan solutions against the growth of Candida albicans, Escherichia coli and Staphylococcus 

aureus, it has shown that only water insoluble chitosan in organic acidic solutions, i.e. 

chitosonium salts, exhibit efficient biocide properties. On the other hand, a research 

performed by Fernandes et al, 2008 showed that the growth of E. coli was markedly 

inhibited by COS, and this inhibition decreased slightly as molecular weight increased. In 

another work performed by Fernandez-Saiz et al. 2009, changes in molecular weight of the 

chitosan materials tested, i.e. 310–375 and 50–190 kDa, did not lead to significant variations 

in biocide properties [127, 128,129]. 

Concerning DD, there are several works that consider this feature, and there is no doubt that 

the antimicrobial properties of chitosan increased with this variable. The antimicrobial 

performance tends to increase upon an increase in the DD of chitosan, which is related to an 

increase in the positive charge of the polymer [130-133].  

---