The antibacterial activity of Nisin and its effects

Nisin, a natural antimicrobial peptide produced by lactic acid bacteria, exhibits significant inhibitory effects primarily against Gram-positive bacteria (such as Staphylococcus, Streptococcus, and Bacillus). Its antimicrobial activity stems from a unique mechanism of action, while being influenced by various environmental and matrix factors, as detailed below:

I. Antimicrobial Mechanism of Nisin

Nisin exerts its antimicrobial effects mainly by disrupting bacterial cell membrane integrity and interfering with cellular metabolism. Its molecular structure contains multiple lanthionine residues, which can specifically bind to lipid II (a key precursor in bacterial cell wall synthesis) on the cell membrane of Gram-positive bacteria, forming transmembrane channels. This leads to the outflow of small molecules such as intracellular potassium ions and protons, disrupting the osmotic balance of the cell membrane and ultimately causing bacterial lysis. Additionally, nisin inhibits the synthesis of bacterial cell wall peptidoglycan, blocking cell division—this effect is particularly potent against bacteria in the logarithmic growth phase and cells in the spore germination stage. Since spore germination requires the synthesis of new membrane components, their sensitivity to nisin increases significantly, making nisin widely used to inhibit the proliferation of Bacillus in food.

II. Key Factors Affecting Nisin’s Antimicrobial Activity

Environmental pH

Nisin’s antimicrobial activity is highly sensitive to pH. Under acidic conditions (pH 2.0–6.0), its molecules carry a positive charge, facilitating binding to the negatively charged bacterial cell membrane, and its structural stability is high, resulting in significant antimicrobial effects. In neutral or alkaline conditions (pH > 7.0), however,nisin molecules tend to undergo conformational changes. The protonation state of certain amino acid residues (e.g., histidine) alters, reducing their binding capacity to lipid II. Additionally, nisin may be degraded by specific proteases, leading to a substantial loss of activity. For example, at pH 5.0, the minimum inhibitory concentration (MIC) of Nisin against Staphylococcus aureus is approximately 0.1 μg/mL, while at pH 8.0, the MIC may increase to over 1.0 μg/mL, representing a nearly 10-fold decrease in activity.

Temperature and Heat Treatment

Nisin’s thermal stability is related to the pH of its environment. Under acidic conditions, it can withstand relatively high temperatures (e.g., retaining over 50% activity after sterilization at 121°C), making it suitable for heat-sterilized foods such as canned goods and beverages. In neutral or alkaline environments, however, high temperatures accelerate structural damage, resulting in activity loss. Furthermore, repeated freeze-thaw cycles or long-term storage at high temperatures (above 60°C) can reduce activity due to molecular polymerization or oxidation.

Food Matrix Components

Components in food such as fats, proteins, and carbohydrates may affect nisin’s effectiveness through adsorption or complexation. For instance, fat molecules in milk fat or meat can encapsulate nisin, hindering its contact with bacterial cell membranes; high concentrations of proteins (e.g., whey protein) may bind to nisin via hydrogen bonds, reducing its free concentration. Conversely, certain metal ions (e.g., Ca²⁺, Mg²⁺) can enhance nisin’s stability—these ions bind to negatively charged regions in nisin molecules, maintaining their spatial conformation and indirectly improving antimicrobial activity. Additionally, food-borne proteases (e.g., cathepsins in meat) may degrade nisin, particularly under neutral pH where protease activity is high, significantly shortening nhalf-life.

Microbial Characteristics

Bacterial sensitivity to nisin varies by species and physiological state. Gram-negative bacteria are naturally insensitive to nisin due to the protective lipopolysaccharide layer outside their cell membranes, but their sensitivity can be enhanced by disrupting the outer membrane with substances like EDTA. Among Gram-positive bacteria, differences in lipid II content and membrane protein composition between strains lead to significant variations in sensitivity (e.g., Streptococcus is generally more sensitive to nisin than Staphylococcus). Furthermore, when bacteria form biofilms, extracellular matrices block nisin penetration, reducing its antimicrobial effect—higher concentrations of Nisin or combination with other antimicrobials are required in such cases.

Synergistic Effects with Other Substances

Nisin often synergizes with other antimicrobial factors to enhance activity. For example, combining it with organic acids (e.g., lactic acid, citric acid) can improve stability by lowering environmental pH; combining with chelating agents (e.g., EDTA) can disrupt the outer membrane of Gram-negative bacteria, expanding the antimicrobial spectrum; combining with plant extracts (e.g., carvacrol) enhances inhibition of drug-resistant strains through multiple mechanisms (e.g., membrane disruption + DNA damage). Such synergies not only reduce nisin usage but also minimize the risk of bacterial resistance to single antimicrobials.

Nisin exerts antimicrobial effects by targeting bacterial cell membranes and cell wall synthesis, with its activity regulated by factors such as pH, temperature, food matrix, microbial characteristics, and synergistic substances. In practical applications, conditions must be optimized for specific scenarios to maximize its antimicrobial efficacy.