Chitosan, a hydrophilic polysaccharide derived from chitin, exhibits broad antimicrobial activity against gram-positive, gram-negative bacteria, and fungi. Three main antimicrobial mechanisms are proposed: (1) electrostatic interaction between positively charged chitosan and negatively charged microbial cell membranes, leading to membrane disruption and leakage; (2) inhibition of mRNA and protein synthesis via chitosan penetration into microbial nuclei; and (3) chelation of metals and suppression of essential nutrients for microbial growth. The effectiveness of chitosan depends on its molecular weight (MW) and degree of acetylation (DA), with lower MW and DA generally enhancing antimicrobial activity. Chitosan is more effective against gram-positive bacteria than gram-negative ones, though recent studies suggest that gram-negative bacteria are more sensitive due to higher hydrophilicity. Chitosan's antimicrobial activity is also influenced by pH, concentration, and the presence of other factors. Water-soluble chitosan derivatives, such as quaternized or hydroxypropyl chitosan, show enhanced antimicrobial activity and broader applicability. These derivatives can improve inhibitory growth of bacteria and are more effective against fungi as well. Chitosan films are used in food preservation and medical applications, such as wound dressings and tissue engineering. The antimicrobial activity of chitosan varies depending on the microorganism and its characteristics, and further research is needed to fully understand the mechanisms and optimize its use.Chitosan, a hydrophilic polysaccharide derived from chitin, exhibits broad antimicrobial activity against gram-positive, gram-negative bacteria, and fungi. Three main antimicrobial mechanisms are proposed: (1) electrostatic interaction between positively charged chitosan and negatively charged microbial cell membranes, leading to membrane disruption and leakage; (2) inhibition of mRNA and protein synthesis via chitosan penetration into microbial nuclei; and (3) chelation of metals and suppression of essential nutrients for microbial growth. The effectiveness of chitosan depends on its molecular weight (MW) and degree of acetylation (DA), with lower MW and DA generally enhancing antimicrobial activity. Chitosan is more effective against gram-positive bacteria than gram-negative ones, though recent studies suggest that gram-negative bacteria are more sensitive due to higher hydrophilicity. Chitosan's antimicrobial activity is also influenced by pH, concentration, and the presence of other factors. Water-soluble chitosan derivatives, such as quaternized or hydroxypropyl chitosan, show enhanced antimicrobial activity and broader applicability. These derivatives can improve inhibitory growth of bacteria and are more effective against fungi as well. Chitosan films are used in food preservation and medical applications, such as wound dressings and tissue engineering. The antimicrobial activity of chitosan varies depending on the microorganism and its characteristics, and further research is needed to fully understand the mechanisms and optimize its use.