Chitosan, a polysaccharide biopolymer, combines a group of physicochemical and biological characteristics, which allow for a wide range of applications. In this study, we investigated the in vitro antimicrobial activity of chitosan. Our goals were to (i) identify factors influencing its antimicrobial activity and study its interaction with bacterial systems; (ii) explore possible mechanisms of its action against staphylococci; as well as (iii) discover potential resistance mechanisms developed by bacteria against this compound. Chitosan exhibited an adequate strain- and dose-dependent in vitro growth-inhibitory activity against Gram-positive bacteria. Its activity was influenced by a number of factors, including its chain length, culture medium and the presence of metal ions. Chitosan’s ability to flocculate bacterial cells was clearly evidenced; on the other hand, its anti-biofilm property was only partly documented. Although several bacteria were capable of degrading chitosan, this did not influence their susceptibility to the antimicrobial activity of this biopolymer. Our study demonstrated that the site of action of chitosan is at the microbial cell envelope, but we do contend that there probably is not a single classical target that would explain its antimicrobial action. The cationic nature of chitosan plays a pivotal role in its antimicrobial activity, allowing its interaction with the anionic cell surface polymers, which leads to a generalized destabilization of the cytoplasmic membrane and subsequent disruption of membrane function. A simultaneous permeabilization of the physically intact cell membrane to small cellular components was detected, coupled with a significant membrane depolarization. Analysis of transcriptional response data revealed that chitosan treatment lead to multiple changes in the expression profile of S. aureus SG511 genes involved in the regulation of stress and autolysis, as well as genes associated with energy metabolism, resulting in impairment of oxygen consumption and forcing cells to shift to anaerobic respiration. Several pathways via which staphylococci may develop resistance against chitosan have been recognized, including: (i) increased positive surface charge resulting in reduced chitosan binding, (ii) modest increase in hydrophobicity; (iii) enhanced production of cell wall polymers; and (iv) elevated levels of positively-charged membrane lipids, thus increasing electrostatic cell surface repulsion of chitosan. There is still much to be learned, but a stage has been reached at which it is becoming possible to present a general account of the main processes involved in chitosan’s antimicrobial activity in terms of basic molecular findings
