The antibacterial spectrum of Nisin and its mode of action

Nisin is a natural antimicrobial peptide produced by Lactococcus lactis. Classified as a GRAS (Generally Recognized as Safe) food preservative, it exhibits significant specificity toward Gram-positive bacteria. Its core mechanism of action revolves around disrupting bacterial cell membrane structure and function, complemented by synergistic inhibition of cell wall synthesis. Widely used in food preservation and biological control, its key properties are detailed as follows:

I. Antimicrobial Spectrum of Nisin

Nisin’s antimicrobial activity is primarily directed against Gram-positive bacteria (G⁺), with inherently weak inhibitory effects on Gram-negative bacteria (G⁻) (requiring auxiliary treatments to disrupt the outer membrane). It has no direct inhibitory activity against fungi or viruses. The specific antimicrobial range is as follows:

1. Main Targets: Gram-Positive Bacteria (Core Substrates)

Lacking an outer membrane barrier, these bacteria are directly accessible to Nisin, resulting in potent inhibitory effects that cover both food spoilage bacteria and pathogens:

Bacillus spp.: Including Bacillus subtilis, Bacillus cereus (a major food spoilage pathogen producing emetic toxins), and Clostridium botulinum (a lethal pathogen producing botulinum toxin). Nisin inhibits both vegetative cells and spores—particularly disrupting membrane integrity during spore germination to reduce viability.

Streptococcus spp.: Such as Streptococcus pneumoniae, Streptococcus pyogenes (human pathogens), and Streptococcus lactis (some strains cause dairy spoilage).

Staphylococcus spp.: Including Staphylococcus aureus (a common foodborne pathogen producing enterotoxins) and Staphylococcus epidermidis.

Lactobacillus spp. (selected strains): Inhibits lactobacilli that overproliferate and cause food deterioration (e.g., certain Lactobacillus acidophilus strains), but exerts weak effects on beneficial lactobacilli essential for fermentation (e.g., yogurt cultures)—requiring precise dosage control.

Other G⁺ bacteria: Such as Listeria monocytogenes (a foodborne pathogen viable at low temperatures), Clostridium perfringens (causes meat spoilage and food poisoning), and pathogenic Mycobacterium spp.

2. Weakly Inhibited or Auxiliary-Dependent Bacteria

Gram-negative bacteria: Including Escherichia coli, Salmonella spp., Shigella spp., and Pseudomonas aeruginosa. The outer membrane and lipopolysaccharide (LPS) layer form a physical barrier that blocks Nisin penetration, resulting in weak standalone activity. Enhanced inhibition is achievable via: ① Combination with EDTA (ethylenediaminetetraacetic acid), which chelates Ca²⁺ and Mg²⁺ in the outer membrane to disrupt its structure, enabling Nisin access to the cell membrane; ② Acidic conditions (pH < 5.0) or low temperatures to increase outer membrane permeability; ③ Physical treatments (e.g., high pressure, ultrasound) to disrupt the outer membrane, after which Nisin exerts significant antimicrobial effects.

Acid-tolerant and anaerobic bacteria: Weakly inhibits some acid-tolerant G⁺ bacteria (e.g., Lactobacillus acidophilus) and has no obvious effect on anaerobes (e.g., Bifidobacterium spp.), making it suitable for fermented food preservation.

3. Non-Inhibited Microorganisms

Nisin has no direct inhibitory activity against fungi (e.g., yeasts, molds), viruses (e.g., norovirus, rotavirus), or parasites. Its mechanism targets the phospholipid bilayer of bacterial cell membranes, while fungal membranes are primarily composed of ergosterol and viruses lack cell membranes—neither can serve as Nisin targets.

II. Mechanisms of Action (Core: Membrane Disruption + Synergistic Inhibition)

Nisin acts with "high specificity and rapid efficacy," centered on disrupting the structure and function of Gram-positive bacterial cell membranes. The process involves three key steps:

1. Targeted Binding to Cell Membrane Receptor (Lipid II)

Nisin contains unique amino acid residues (e.g., lanthionine, methyllanthionine) that enable specific recognition and binding to Lipid II— a key precursor for bacterial cell wall peptidoglycan synthesis, widely distributed on the surface of G⁺ bacterial cell membranes and serving as Nisin’s core target. This specific binding ensures targeted antimicrobial activity while avoiding damage to human cells (which lack Lipid II receptors), ensuring high safety.

2. Formation of Transmembrane Pores to Disrupt Membrane Permeability

Upon binding to Lipid II, Nisin molecules polymerize to form transmembrane pores (pore size ~1–2 nm): The hydrophobic regions of Nisin insert into the phospholipid bilayer of the cell membrane, while the hydrophilic regions face the pore interior, forming a "barrel-stave" channel structure. These pores disrupt membrane integrity and permeability, causing massive leakage of intracellular small molecules (e.g., K⁺, Na⁺, amino acids, ATP) and influx of extracellular water and harmful substances. This leads to intracellular osmotic imbalance, ultimately resulting in bacterial swelling and rupture. The process is rapid and irreversible, typically inactivating bacteria within minutes of binding.

3. Synergistic Inhibition of Cell Wall Synthesis

In addition to membrane disruption, Nisin’s binding to Lipid II indirectly inhibits bacterial cell wall synthesis: Bound Lipid II is unavailable for peptidoglycan cross-linking, halting cell wall formation. For dividing bacteria, impaired cell wall synthesis prevents morphological maintenance, further exacerbating membrane rupture. For spores, Nisin binds to Lipid II synthesized during germination, blocking spore cell wall formation and inhibiting germination into vegetative cells—achieving a dual effect of "bactericidal + spore-inhibitory."

4. Key Characteristics and Synergistic Effects

Rapid action: Nisin kills bacteria directly by disrupting membranes without entering cells, acting faster than antibiotics (which interfere with cellular metabolism)—typically reducing bacterial counts significantly within 1–2 hours of addition.

Concentration dependence: Antimicrobial efficacy increases with Nisin concentration. The effective dosage in foods is usually 0.01–0.1 g/kg (10–100 mg/kg), compliant with standards such as GB 2760.

Synergistic enhancement: Combinations with organic acids (e.g., lactic acid, citric acid), EDTA, high-temperature processing, or high-pressure sterilization improve efficacy by lowering bacterial pH tolerance, disrupting outer membranes, and enhancing membrane permeability. This reduces Nisin dosage while expanding the antimicrobial spectrum (e.g., covering some G⁻ bacteria).

III. Key Factors Influencing Antimicrobial Efficacy

pH value: Nisin is stable and highly active under acidic conditions (pH 2.0–6.0), but degrades easily under neutral or alkaline conditions (pH > 7.0), leading to significant efficacy loss. It is therefore more suitable for acidic foods (e.g., yogurt, fruit juice, pickles) or low-pH processing scenarios.

Temperature: Nisin exhibits strong heat resistance, retaining over 80% activity after autoclaving at 121℃ for 15 minutes—suitable for high-temperature processed foods such as canned goods and sterilized milk. However, prolonged exposure to temperatures > 130℃ damages its structure and reduces activity.

Food composition: Fats and proteins in foods bind to Nisin, reducing its free concentration and antimicrobial efficacy. Higher Nisin dosages are therefore required in high-fat, high-protein foods (e.g., meat, dairy products).

Bacterial growth phase: Bacteria in the logarithmic growth phase are more sensitive to Nisin, while spores require higher concentrations for inhibition. Adding Nisin early in food processing (during initial bacterial proliferation) yields optimal antimicrobial effects.

Nisin’s antimicrobial spectrum is centered on Gram-positive bacteria, covering major food spoilage and pathogenic bacteria. It requires auxiliary treatments to act on Gram-negative bacteria and has no activity against fungi or viruses. Its core mechanism—"targeted binding to Lipid II → transmembrane pore formation → membrane permeability disruption"—is complemented by synergistic cell wall synthesis inhibition, offering rapid action, high specificity, and safety. In the food industry, optimizing pH, temperature, dosage, or combining with other preservation methods maximizes its antimicrobial efficacy, extends food shelf life, and aligns with consumer demand for natural, healthy products.