Sergey Kulikov

Antifungal activity of oligochitosans (short chain chitosans) against some Candida species and clinical isolates of Candida albicans: molecular weight-activity relationship.

A series of oligochitosans (short chain chitosans) prepared by acidic hydrolysis of chitosan and characterized by their molecular weight, polydispersity and degree of deacetylation were used to determine their anticandidal activities. This study has demonstrated that oligochitosans show a high fungistatic activity (MIC 8-512 μg/ml) against Candida species and clinical isolates of Candida albicans, which are resistant to a series of classic antibiotics. Flow cytometry analysis showed that oligochitosan possessed a high fungicidal activity as well. For the first time it was shown that even sub-MIC oligochitosan concentration suppressed the formation of C. albicans hyphal structures, cause severe cell wall alterations, and altered internal cell structure. These results indicate that oligochitosan should be considered as a possible alternative/additive to known anti-yeast agents in pharmaceutical compositions.

Influence of glucosamine on oligochitosan solubility and antibacterial activity

Light scattering studies indicate that oligochitosan (short-chain chitosan) solutions contain aggregates at pH values below the critical pH of phase separation, while at or above this point the gel phase coexists with the aggregate solution. This work demonstrates for the first time that the presence of D-glucosamine in an oligochitosan solution shifts the critical pH to a higher value and improves the oligochitosan antibacterial activity against Escherichia coli, Staphylococcus aureus, and Staphylococcus epidermis in neutral and slightly alkaline aqueous media. By comparing the results of light scattering studies and antimicrobial assays one can conclude that the antimicrobial activity of oligochitosan is dependent on its unimolecular form, not its supramolecular structures. The widening of the homogeneity region of an oligochitosan solution could lead to promising biomedical applications.

Evaluation of a method for the determination of antibacterial activity of chitosan

A method for the determination of the antimicrobial activity of chitosan with the use of organic salts for the production of pH in the range of 5.5–8.2 was studied. The double-dilution method demonstrated the effectiveness of the determination of the antimicrobial activity of chitosan samples with different molecular weights and solubilities. It was found that the antibacterial activity increased at low pH values with increasing molecular weight, but chitosans with a molecular weight of 5–6 kDa showed higher activity at neutral and slightly alkaline pH levels. Determination of the antimicrobial activity of various chitosan samples at different pH values allowed a more reliable assessment of the potential biological activity of chitosan.

N-Reacetylated Oligochitosan: pH Dependence of Self-Assembly Properties and Antibacterial Activity

Oligochitosan (short chain chitosan) is more soluble in acidic aqueous media than a high molecular weight (MW) chitosan, but its antimicrobial activity decreases with increase in degree of acetylation (DA) and increase in pH above a critical pH threshold point. In the present study, oligochitosans varying in MW were additionally N-acetylated and their self-assembly properties and antibacterial activity toward Staphylococcus aureus and Escherichia coli were investigated in a wide pH range as a function of MW and DA. Light scattering studies reveals that reacetyleted oligochitosan with Mw ≤ 11 kDa is completely soluble in alkaline media (up to pH 12.5), if its DA is not less than 16%. Reacetylated chitosans with DA ∼ 30% are solubile in the entire pH range up to 12.5, if their Mw is not higher than 25 kDa, but they aggregate and precipitate from the solution at pH ≥ 8 when their Mw is above 25 kDa. Considering the influence of DA and MW, the antibacterial activity of reacetylated oligochitosans is maximal in the short interval of DA 16-28% at pH 7.4. These results are promising for expanding practical application of oligochitosan in pharmaceutical, cosmetic, and food compositions.

Comparative evaluation of antimicrobial activity of oligochitosans against Klebsiella pneumoniae

The Antibacterial activity of chitosan of different molecular weights was studied against gramnegative Klebsiella pneumoniae at different pH values. It was found that the dependence of the inhibitory activity of chitosan on its molecular weight was undergoes inversion when increasing the pH of the medium above 7.0. In acidic media, chitosan of the higher molecular weight had the higher antibacterial activity, while in weàk alkaline media, oligomeric forms of chitosan displayed only the inhibition effect. Our results showed that the antibacterial activity of chitosan against Klebsiella pneumoniae was closely associated with its polycationic nature, and depended on the degree of protonation of the chitosan amino groups, which, in turn was the function of the degree of polymerization and the pH values of the medium. The results allow one to explain, in part, the contradictory literature data concerning the relationship between the antibacterial activity and molecular weight of chitosan.

Consequences of chitosan decomposition by nitrous acid: Approach to non-branched oligochitosan oxime

It is well known that chitosan degradation by nitrous acid leads to oligochitosan (oligoCHIt-ahm) bearing reactive 2,5-anhydromannose (3,4-dihydroxy-5-hydroxymethyl-tetrahydrofuran-2-aldehyde) units at the new reducing ends of macromolecules. Standard protocol requires reduction of oligoCHIt-ahm with NaBH4 to corresponding oligoCHIt-hml bearing unreactive hydroxymethyl group instead of reactive aldehyde group. For the first time, HP SEC as well as UV and CD spectroscopy methods have revealed that the reduction leads to an indefinite side modification and the formation of a branched oligoCHIt-hml with increased molecular weight. Here, it is shown that the branching and modification can be prevented by means of the simple and reproducible reaction of oligoCHIt-ahm with hydroxylamine that allows preparation of a stable linear oligochitosan oxime, oligoCHIt-oxm. Cytotoxicity tests show that oligoCHIt-ahm, oligoCHIt-hml and oligoCHIt-oxm are non-toxic at concentration below 2.5 mg/ml, and the cytotoxicity is concentration dependent and decreases in the order oligoCHIt-ahm > oligoCHIt-hml > oligoCHIt-oxm at higher concentrations both before and after long shelf-storage. The elaborated approach and cytotoxicity data give an opportunity to use the non-branched oligoCHIt-oxm for biomedical applications.