## 4 Functional Properties of Chitosan and Derivatives
### Antimicrobial Functions
\begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Effect** & **Medium/Method** & \begin{tabular}{c} **Chitosan Form or** \\ **Derivative** \\ \end{tabular} & \begin{tabular}{c} **Microbial** \\ **Species** \\ **Targeted** \\ \end{tabular} & **References** \\ \hline Microbial growth & \begin{tabular}{c} Liquid model \\ medium (MIC) \\ Beer slices \\ Beer, wine \\ Solid agar plates \\ \end{tabular} & \begin{tabular}{c} Nanoparticles, \\ many Mw/DA \\ \end{tabular} & Many species & [93] \\  & & & & [94; 95; 96] \\  & & & & [81; 83; 92; 97; 89] \\ \cline{2-5}  & \begin{tabular}{c} Medical catheter \\ \end{tabular} & Diverse viscosity & \begin{tabular}{c} _K. pneumoniae_ \\ _E. coli_ \\ \end{tabular} & [99] \\ \cline{2-5}  & Liquid media & \begin{tabular}{c} Distinct \\ concentrations \\ \end{tabular} & \begin{tabular}{c} Microbials \\ cultures \\ \end{tabular} & [95; 100] \\ \hline Metabolism modification & Liquid medium & \begin{tabular}{c} Distinct \\ concentrations \\ \end{tabular} & _S cerevisiae_ & [101] \\ \hline Biofilm inhibition & Liquid medium & Nanoparticles & _S. aureus_ & [81; 102] \\ \hline Microbial elimination & \begin{tabular}{c} Liquid medium, \\ minimal lethal \\ concentration \\ \end{tabular} & Many Mw/DA & Many species & [87; 88; 89; 103; 83; 84; 85; 86; 87; 88; 89; 103] \\ \hline Biofilm elimination & \
\begin{tabular}{c} Elimination of \\ biofilms, in flow \\ cells/polystyrene \\ wells \\ \end{tabular} & Nanoparticles &
\begin{tabular}{c} _S. mutans_ \\ _S. aureus_ \\ \end{tabular} & [102; 104] \\ \hline Floculation/sedimentation & Liquid medium & Many Mw/DA & Distinct species & [77; 87; 101; 105; 106; 107] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Studies on antimicrobial activity of chitosan: diversity of target microbes, test media, and final aim of the treatment. (COS), which only displayed a bacteriostatic effect and the bacteria could rapidly grow after separation from the CS by membrane filtration [108; 119]. In other studies, high Mw and low DD insoluble CS fractions were shown to act as finsing agents, which eliminate such cell aggregates [105; 106];
* The diffusion of low molecular weight CS into the cell and its interaction with DNA, RNA, and proteins is also suggested to contribute to the global mechanism [120; 121; 122];
* At sublethal doses, an induction of genes involved in stress regulation, arginine or glucose metabolism (energy), protein glycosylation, membrane synthesis, ion transport, wall construction, and autolysis is reported [84; 85; 110; 122; 123; 124]. _S. cerevisiae_ cells treated with sub-lethal doses of CS strengthen their wall and become resistant to beta-glucanase treatment [122; 124];
* Disruption of the membrane and release of cellular components are often reported, especially for Gram negative bacteria and for some yeasts [111], but depending on the dose used, this can or cannot be observed with some Gram positive bacteria, such as _S. aureus_[113; 119; 125; 126; 127; 128; 87; 68; 88];
* The chelation and sequestration of metal ions and other nutrients in the broth have also been proposed [125]. In addition, several studies have focused on the parameters that modulate the antimicrobial activity of CS. Figure 7 gives an overview of the main parameters modulating the antimicrobial activity of CS. Figure 6: Inventory of the different molecular processes that may contribute to the chitosan antimicrobial activity (adapted from [79]). The numbers (_i_) to (_vii_) correspond to those used in the text (see above). Regarding the intrinsic parameters, the CS Mw and DA are important parameters, more than the origin of the CS. Regarding the size of the active fractions, no consensus can be reached from the literature. The optimal active Mw may be species or even strain specific, and the opposite results are reported for various _E. coli_ strains [83; 108; 120; 129; 130; 131; 132]. On the other hand, the antimicrobial activity is directly proportional to DD and inversely proportional to DA [83; 84; 129; 133].