Polylysine is a natural antimicrobial agent with diverse applications. It enhances product safety and longevity across various industries.
What is Polylysine?
Polylysine is a natural, positively charged polymer derived from the amino acid lysine. Epsilon-poly-L-lysine (ε-poly-L-lysine) is the predominant type used commercially. This specific form is a homopolymer of L-lysine, consisting of repeating units of the same amino acid. These units are linked by an unusual amide bond between the α-carboxyl group and the ε-amino group of adjacent lysine residues, rather than the more common α-amino bond found in typical proteins.
The positive charge of ε-poly-L-lysine is key to its antimicrobial function. This cationic nature allows it to interact strongly with negatively charged components of microbial cell membranes, such as phospholipids. This interaction disrupts the membrane’s integrity, leading to leakage of intracellular substances and ultimately inhibiting microbial growth or causing cell death. This natural compound is considered a microbial metabolite, produced by microorganisms as part of their metabolic processes.
How Polylysine is Made
The primary method for producing ε-poly-L-lysine involves natural fermentation using specific microorganisms. The bacterium Streptomyces albulus is widely recognized for its ability to synthesize this compound.
During fermentation, Streptomyces albulus converts simple carbon sources, such as glucose, into ε-poly-L-lysine under aerobic conditions. The compound is produced as a secondary metabolite, accumulating when the fermentation broth reaches an acidic pH, typically around 3.2. After fermentation, purification steps like ion exchange chromatography, decolorizing, ultrafiltration, and ethanol sedimentation are used to obtain high-quality ε-poly-L-lysine. The resulting product is a highly uniform polymer, often consisting of 25-35 L-lysine residues.
Common Uses of Polylysine
Polylysine, particularly ε-poly-L-lysine, is widely recognized for its application as a natural food preservative. It exhibits strong antimicrobial activity against a broad spectrum of microorganisms, including various Gram-positive and Gram-negative bacteria, yeasts, and molds. Its mechanism involves electrostatic adsorption to the microbial cell surface, which can disrupt the outer membrane and lead to abnormal distribution of the cytoplasm, causing cell damage or death. This makes it particularly effective against certain Gram-negative bacteria like Escherichia coli and Salmonella, which are often challenging for other natural preservatives to control.
This natural preservative is commonly used in various food products, especially in Asian cuisine, and its use is expanding globally. Examples include cooked rice, noodles, soups, baked goods, and seafood. It can also extend the shelf life of milk and is sometimes combined with other food additives like glycine, organic acids, or alcohol to enhance its antimicrobial effects.
Beyond food preservation, polylysine is finding emerging applications in other sectors. In cosmetics, it functions as a hair conditioning agent, forming an invisible film that smooths the hair cuticle, reduces static, and eases combing. It is found in hair treatments, serums, milky lotions, and foundations, offering a hypoallergenic alternative to petrochemical-derived ingredients.
Polylysine also shows promise in biomedical research and applications. Its biocompatibility and antimicrobial properties make it suitable for use in tissue engineering, where it can provide scaffolds that support cell adhesion, proliferation, and differentiation. In drug delivery systems, polylysine-based composites serve as platforms for targeted and controlled release of therapeutic agents, boasting high loading capacity and minimal cytotoxicity. Furthermore, its antimicrobial action aids in wound healing by preventing infections and promoting cell migration.
Safety Profile and Approvals
Polylysine is recognized for its safety profile, particularly ε-poly-L-lysine, which is produced through microbial fermentation. It is considered a highly safe substance, supported by various studies, including acute toxicity, subacute toxicity, chronic toxicity, mutagenicity, and ADME (Absorption, Distribution, Metabolism, and Excretion) studies. These studies indicate that it is practically non-toxic in acute oral toxicity tests and is not mutagenic.
A key aspect of its safety recognition is its Generally Recognized as Safe (GRAS) status in the United States, granted by the FDA. This designation signifies that polylysine is considered safe under specified conditions of use. Beyond the United States, it is approved as a food additive in several other countries, including Japan, South Korea, and China. When consumed, polylysine is biodegradable and can be completely digested into L-lysine, an amino acid, by the human body, contributing to its non-toxic nature. Approximately 94% of an administered dose passes unabsorbed through the gastrointestinal tract, with the absorbed portion being rapidly metabolized.