# Diversity of Potential (Bio)Technological Applications of Amino Acid-Based Ionic Liquids
## 2 Bioapplications of AA ILs: An Overview
### Antimicrobial Activity, Antibacterial Coatings, and Antibiotic Formation  
[Ch][AAs] have been extensively studied for their antimicrobial activity. In a previous study, we demonstrated that [Ch][Arg] and [Ch][Lys] effectively inhibited the growth of Gram-positive bacteria (_Staphylococcus aureus_ 29213, _Staphylococcus saprophyticus_ 15305, and _Bacillus subtilis_ 168) and Gram-negative bacteria (_Escherichia coli_ 420, _Escherichia coli_ 25922, and _Pseudomonas aeruginosa_ PAO1), with estimated IC50 values in the millimolar range [71]. In contrast, [Ch][Glu] and [Ch][Asp] showed no cytotoxic effect nor did they even promote the growth of all tested bacterial strains except _Staphylococcus aureus_ 29213 [71]. Polyionic membranes based on bacterial cellulose and [Ch][AA] exhibited good biocompatibility and strong antimicrobial activity against _Staphylococcus aureus_, _Escherichia coli_, and _Candida albicans_[72]. Within this series, membranes containing aromatic and hydrophobic AAs, such as Phe, Trp, and Ile, were more active than those containing Gly or polar AAs such as Ser and Pro [72]. In addition, [Ch][AA]-polyionic liquid hydrogels containing hydrophobic (Phe) or charged AA anions (Asp, Arg) were found to be homcompatible and demonstrated good antimicrobial activity [73]. The hydrogels with Asp showed excellent antibacterial activity against _Escherichia coli_ and _Staphylococcus aureus_, while those with Phe exhibited higher cytotoxicity against the fungus _Crytococcus neoformans_[73]. In another experiment, it was demonstrated that a tri-component antibacterial coating composed of loofah fiber, epoxy resin, and positively charged choline AA ILs (Arg or Lys) was shown to have strong antibacterial activity against the Gram-positive bacteria _Staphylococcus aureus_[74]. [PERSON] et al. found that bis-ammonium or bis-phosphonium-based ILs with Pro or His anions and short- to medium-chain linkers exhibited moderate antimicrobial activity against numerous strains, including Gram-positive bacteria such as _Staphylococcus aureus_ ATCC 33862, _Staphylococcus epidermidis_ ATCC 12228, _Enterococcus faecalis_ ATCC 19433, _Bacillus subtilis_ ATCC 11774, _Microococcus luteus_ ATCC 4698, _Clostridium perfringens_ ATCC 13124, and _Lactiplantiobacillus plantarum_ DKK 003, as well as Gram-negative bacteria, including _Escherichia coli_ ATCC 25922, _Pseudomonas aeruginosa_ ATCC 9027, _Serratia marcescens_ ATCC 8100, _Proteus vulgaris_ ATCC 49132, _Moraxella catarrhalis_ ATCC 25238, and _Salmonella enteridis_ ATCC 13076, and fungi such as _Candida albicans_ ATCC 10231, _Rhodotorula mucilaginous_ DKK 040, _Botrytis cinerea_ BPR 187, and _Fusarium graninearum_ KZF 1 [68]. It should be noted that changing the linker in the cation from an alkyl to a two-ester linker significantly reduces the antimicrobial activity of the ILs. The antibacterial activity of ILs containing cations such as ValOiPr, IleOiPr, ThrOiPr, or MetOiPr in combination with NSAID anions was tested against three bacteria commonly associated with skin infections: _Staphylococcus epidermidis_, _Escherichia coli_, and _Microococcus luteus_[61]. The ILs demonstrated a significant inhibitory effect on the growth of Gram-negative bacteria, outperforming the acid forms of the NSAIDs. Among all the salts, the MetOiPr-based ILs exhibited the highest antibacterial activity [61]. Interestingly, the [L-ThrOiPr] salts of Keto and Nap did not inhibit the growth of Staphylococcus epidermidis, while their Ibu and SA salts effectively suppressed its growth. However, conversion of the NSAIDs to AAOiPr salts had no effect on their activity against _Microococcus luteus_[61]. AA ILs containing EmPyr cations exhibit antimicrobial activity against human pathogenic bacteria, including _Aeronomas hydrophilic_, _Escherichia coli_, _Listeria monocytogenes_, and _Staphylococcus aureus_[75]. The EC\({}_{50}\) values are comparable among the four strains, ranging from 18 to 66 mM, and are strongly influenced by the structure of the anion [75].