The influence of Nisin on the gut microbiota

Nisin, a natural antibacterial peptide, its impact on intestinal flora and safety evaluation are important contents in the safety study of food additives. From the existing research, Nisin has a certain specificity in the impact on intestinal flora, and it is relatively safe within the reasonable use range. The specific analysis is as follows:

I. Mechanism and research evidence of Nisin's effect on intestinal flora

Selectivity of action scope

The antibacterial mechanism of Nisin mainly destroys the cell membrane of Gram-positive bacteria (such as Staphylococcus aureus, Listeria, etc.), but has weak inhibitory effect on Gram-negative bacteria (such as Escherichia coli, Bifidobacterium). This characteristic makes its effect on intestinal flora selective:

In vitro research: In the fermentation model simulating the intestinal environment, Nisin has a significant inhibitory effect on Gram-positive bacteria such as Enterococcus faecalis and Enterococcus faecium, but has little effect on the number of Bifidobacterium, Lactobacillus (strains with low sensitivity to Nisin among Gram-positive bacteria) and Bacteroides (Gram-negative bacteria).

Animal experiments: After rats were gavaged with high-dose Nisin (1000 mg/kg body weight), the detection of intestinal flora showed that the number of Gram-positive bacteria (such as Enterococcus) decreased transiently, while the abundance of beneficial bacteria such as Bifidobacterium and Lactobacillus did not change significantly, and the flora structure could recover after drug withdrawal.

Short-term and long-term effects on intestinal flora balance

Short-term exposure: In human clinical trials, healthy volunteers consumed Nisin-containing foods daily (the dose is equivalent to the upper limit of ADI) for 28 days. Fecal flora analysis showed that the flora diversity (such as Shannon index) was not different from that of the control group, and the number of main bacterial genera such as Bifidobacterium and Escherichia coli remained stable.

Long-term impact: Because Nisin is easily degraded by proteases (such as pepsin, trypsin) in the digestive tract, the effective concentration in the intestine is maintained for a short time. Long-term feeding experiments (such as 90-day feeding test in mice) did not find persistent changes in the structure of intestinal flora, nor did they observe excessive proliferation of harmful bacteria (such as Salmonella).

II. Safety evaluation: from mechanism to practical application

Low toxicity to beneficial bacteria

The main beneficial bacteria in the intestine (such as Bifidobacterium, most Lactobacillus) have low sensitivity to Nisin, which is related to their cell membrane structure - Nisin needs to bind to Lipid Ⅱ on the cell membrane to play a role, and the Lipid Ⅱ structure or expression level of some probiotics makes them not easy to be recognized by Nisin. For example, the tolerance concentration of Lactobacillus acidophilus to Nisin can reach more than 1000 IU/mL, which is much higher than the common Nisin concentration in food (10-1000 ppm, about 1-10 IU/mL).

Safety of metabolism and excretion

As a polypeptide composed of 34 amino acids, Nisin can be rapidly degraded into amino acids (such as alanine, lysine, etc.) by proteases in the digestive tract after entering the human body. Its metabolites can participate in normal physiological processes without accumulation risk. Urine and blood tests show that the residual amount of Nisin and its degradation products in the body is extremely low, mainly excreted through the kidneys, and no effect on liver and kidney function is found.

Population exposure risk assessment

The Joint Expert Committee on Food Additives (JECFA) set the acceptable daily intake (ADI) of Nisin as 0-33,000 IU/kg body weight (about 20 mg/kg body weight) based on Nisin's antibacterial spectrum, metabolic characteristics and toxicological data. For a 60 kg adult, about 1.2 g Nisin needs to be ingested daily to reach the upper limit of ADI, while the Nisin ingested through food (such as cheese, yogurt, meat) in actual diet is usually less than 10 mg/day, which is far lower than the safety threshold.

III. Controversies and considerations for special populations

Controversial points of potential risks

A few studies have pointed out that high-dose Nisin may inhibit some symbiotic Gram-positive bacteria (such as some Clostridium) in the intestine, thus affecting the production of short-chain fatty acids (such as butyric acid). However, most of these studies are based on in vitro high-concentration models (such as 1000 IU/mL), which are very different from the actual dietary exposure, and no significant changes in intestinal metabolites are observed in animal experiments.

Safety of special populations

Infants and pregnant women: At present, there is a lack of direct research on special populations, but Nisin naturally exists in breast milk (content about 1-10 IU/L), and Nisin is allowed to be added to infant formula (such as the EU approves it for use in infant milk powder), suggesting that its risk to special populations is low.

Immunocompromised populations: Theoretically, Nisin's inhibition of intestinal Gram-positive bacteria may affect immune function, but clinical data show that Nisin at conventional doses does not increase the infection risk of immunocompromised patients.

IV. Conclusion and prospect

Nisin has selectivity in the impact on intestinal flora, mainly inhibiting pathogenic bacteria (such as Gram-positive bacteria), and has limited impact on beneficial bacteria and the overall balance of flora. Its safety has been verified by toxicological research, metabolic analysis and long-term population exposure data, and there is no significant risk to intestinal health within the scope of food additive use. Future research can further focus on the interaction between Nisin and other food components (such as dietary fiber), as well as the potential impact in intestinal disease models, so as to provide more comprehensive evidence for its safety evaluation.