Polylysine is a small natural homopolymer of the essential amino acid L-lysine

Lysine

Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....

 that is produced by bacterial fermentation. ε-Poly-L-lysine is used as a natural preservative in food products.

Chemical structure & function

ε-Polylysine is typically produced as a homo-polypeptide of approximately 25-30 L-lysine residues. Epsilon (ε) refers to the linkage of the lysine molecules. In contrast to normal peptide bond

Peptide bond

This article is about the peptide link found within biological molecules, such as proteins. A similar article for synthetic molecules is being created...

 that is linked by the alpha-carbon group, the lysine amino acids are molecularly linked by the epsilon amino group and the carboxyl group.

ε-Polylysine belongs to the group of cationic polymer

Polymer

A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s. In water, ε-polylysine contains a positively charged hydrophilic amino group . According to research, ε-polylysine is adsorbed electrostatically to the cell surface of the bacteria, followed by a stripping of the outer membrane

Outer membrane

The bacterial outer membrane is found in Gram-negative bacteria. Its composition is distinct from that of the cytoplasmic membrane - among other things, the outer leaflet of the membrane includes a complex lipopolysaccharide whose lipid portion acts as an endotoxin - and it is linked to the cell's...

. This eventually leads to the abnormal distribution of the cytoplasm

Cytoplasm

The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...

 causing damage to the bacterial cell.

Production

Production of polylysine by natural fermentation is only observed in strains of bacteria in the genus Streptomyces

Streptomyces

Streptomyces is the largest genus of Actinobacteria and the type genus of the family Streptomycetaceae. Over 500 species of Streptomyces bacteria have been described. As with the other Actinobacteria, streptomycetes are gram-positive, and have genomes with high guanine and cytosine content...

. Streptomyces albulus is most often used in scientific studies and is also used for the commercial production of ε-polylysine.

History

The production of ε-polylysine by natural fermentation was first described by researchers Shoji Shima and Heiichi Sakai in 1977. Since the late 1980s, polylysine has been approved by the Japanese Ministry of Health, Labour and Welfare as a preservative in food. In January 2004, polylysine became generally recognized as safe

Generally recognized as safe

Generally recognized as safe is an American Food and Drug Administration designation that a chemical or substance added to food is considered safe by experts, and so is exempted from the usual Federal Food, Drug, and Cosmetic Act food additive tolerance requirements.-History:On January 1, 1958,...

 (GRAS) certified in the United States.

Polylysine in food

Polylysine is used commercially as a food preservative

Preservative

A preservative is a naturally occurring or synthetically produced substance that is added to products such as foods, pharmaceuticals, paints, biological samples, wood, etc. to prevent decomposition by microbial growth or by undesirable chemical changes....

 in Japan, Korea and the United States. Food products containing polylysine are mainly found in Japan. The use of polylysine is common in food applications such as boiled rice, cooked vegetables, soups, noodles and sliced fish (sushi

Sushi

is a Japanese food consisting of cooked vinegared rice combined with other ingredients . Neta and forms of sushi presentation vary, but the ingredient which all sushi have in common is shari...

).

Literature studies have reported an antimicrobial

Antimicrobial

An anti-microbial is a substance that kills or inhibits the growth of microorganisms such as bacteria, fungi, or protozoans. Antimicrobial drugs either kill microbes or prevent the growth of microbes...

 effect of ε-polylysine against yeast

Yeast

Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...

, fungi, Gram-positive bacteria and Gram-negative bacteria.

Polylysine has a light yellow appearance and is slightly bitter in taste whether in powder or liquid form.

Polylysine in tissue culture

α-Polylysine is commonly used to coat tissue cultureware as an attachment factor which improves cell adherency.

Chemical modification

In 2010, hydrophobically modified ε-polylysine was synthesized by reacting EPL with octenyl succinic anhydride (OSA). It was found that OSA-g-EPLs had glass transition temperatures lower than EPL. They were able to form polymer micelles in water and to lower the surface tension of water, confirming their amphiphilic properties. The antimicrobial activities of OSA-g-EPLs were also examined, and the minimum inhibitory concentrations of OSA-g-EPLs against Escherichia coli O157:H7 remained the same as that of EPL. Therefore, modified EPLs have the potential of becoming bifunctional molecules, which can be used either as surfactants or emulsifiers in the encapsulation of water-insoluble drugs or as antimicrobial agents.