Chitosan: antibacterial activity and perspectives of the biomedical

Abstract

In the last decades, serious attention is attracted by the use of natural antimicrobial drugs instead of the usual ones because of pathogens resistance to antibiotics. Chitosan (CS) is widely used as an antimicrobial agent owing to its high biodegradability, nontoxicity and antimicrobial properties. CS is a cationic polysaccharide obtained by partial deacetylation of chitin, the major component of crustacean shells. In last time cultivation of fungi provides an alternative source of the CS obtaining: Chitin makes up 45 % of the A. niger and M. rouxii cell wall content and up to 20 % of the P. notatum cell wall content In contrast to other polymers, chitosan is a hydrophilic polymer with positive charge and has three types of functional groups: amino group at position C-2 in each deacetylated structural unit, as well as primary and secondary hydroxyl groups at C-6 and C-3 positions respectively. This causes its ability to form new hydrofilic medicals on the basis of known drugs, as well as the formation of drug release systems. CS is unique adsorbent and it is possible to combine it with another drugs. The natural ability of CS for gelation is used  in the preparation of the hemostatic agent "Celox", that is effective for preventing fatalities when arterial bleeding occurs on the battlefield. The clotting of "Celox" occurs much faster than other hemostatic agents. Antimicrobial activity of chitosan against many Gram-positive and Gramnegative bacteria, filamentous fungi and yeasts has been  widely demonstrated in the scientific literature.There are some reported mechanisms for antibacterial activity: positively charged due to NH3 + groups Chitosan interact with negatively charged functional groups at the cell surface and compromise the cell wall or outer membrane. In the case of Gram-positive bacteria, lipoteichoic acid could provide a molecular linkage for chitosan at the cell
surface, allowing it to disturb membrane functions. Lipopolysaccharides in the Gram-negative bacteria outer membrane are held together by electrostatic interactions with divalent metals. This cations may compete with CS, that also disturb the cell functions. Some authors reported that CS binds to DNA and inhibits RNA synthesis. Significant role in  antibacterial activity belongs to the physical and chemical properties of Chitosan, including its cationic structure, molecular weight, degree of the
deacetylation, concentration. Owing to the high content of amino and carboxyl groups, Chitosan can form chelate complexes with metals. Silver (Ag) ion antimicrobial activity against Gram-negative and Gram-positive bacteria is well known. Complexes Chitosan-Silver are used in medicine for example as part of the protective coatings on 
metal implants in dentistry and orthopedics in order to reduce the risk of postoperative infection. The antibacterial activity of the Silver-Chitosan-doped hydroxyapatite (HA) coating was examined using spectrophotometry by measuring the optical density of the culture medium E.coli АТСС 25922 containing the experimental samples. After 48 hours immersion of the substrate in medium, concentration of microbial cells (C, CFU/ml) was decreased from log 7 to log 4,8, what is evidence of the coating antibacterial activity.It was studied the ability of the biomaterials based on HA with Chitosan and Silver content to influence the adhesive properties of E. coli АТСС 25922 and S. aureus АТСС 25923. It was proved that under Ag+ ions action, added to the coating material, the adhesive index for E.coli decreases in relation to formalinized ram erythrocytes on 17 % as compared to control sample (pure HA) and the adhesive index for S. aureus – on 13 %. Also was  found that chitosan as a component of bioactive coating decreases the adhesive index E. coli on 29 %, and those for S. aureus on 22 %.Thus, from this short overview follows the conclusion that CS can be used in medicine as a very perspective antimicrobial agent. Also, application CS in combination with HA-Ag coatings on medical metal implants, using biomimetic technology should be taken to the attention. CS has a great potential for its using as a component of the composite biomaterial with all necessary properties (porosity, biodegradation, nontoxicity) in nanomedicine, particular for bone regeneration and stomatology. Once more direction is connected with a property of CS to bind with DNA, RNA and that open the possibility to create novel materials for gene therapy. But for more effective using all CS and its derivatives properties in practical medicine it’s necessary to perform further deeper investigations.