Nisin A. Because its residues are not standard amino acids, these values come from the literature rather than from the calculators on this site.
A peptide in the food supply
Nisin is an antimicrobial peptide made by the bacterium Lactococcus lactis, the same organism used to ferment milk into cheese. It was first described in 1928, when researchers noticed that some lactococcal strains inhibited others, and it has been used commercially as a food preservative since 1953. It is permitted in many countries and carries the additive number E234 in the European Union.
That makes nisin unusual among the peptides on this site. It is not a hormone, a toxin or a laboratory reagent but an ingredient produced on an industrial scale by fermentation and added to processed cheese, canned vegetables, dairy desserts and some meat products, where it suppresses spore-forming bacteria such as Clostridium and Bacillus.
Decades of use without meaningful resistance developing in practice has made nisin a frequently cited counterexample to the assumption that antimicrobial resistance always follows widespread use. Resistance can be selected in the laboratory, so the reasons for this are debated, but the observation is striking.
What a lantibiotic is
Nisin is the best-known member of the lantibiotics, a class named for the unusual amino acid lanthionine that they contain. Lanthionine is two alanine units joined by a thioether bridge, essentially a disulfide bond with one sulfur replaced by a carbon. Methyllanthionine has an extra methyl group.
These bridges are not made by the ribosome. Nisin is translated as an ordinary 57-residue precursor from a normal gene, and dedicated enzymes then rebuild it:
- A dehydratase removes water from serine and threonine residues, producing the unsaturated residues dehydroalanine and dehydrobutyrine.
- A cyclase adds the thiol of a nearby cysteine across each double bond, forming a thioether ring: lanthionine from dehydroalanine, methyllanthionine from dehydrobutyrine.
- A protease removes the leader sequence, releasing the mature 34-residue peptide with five rings.
Peptides made this way, on the ribosome and then heavily modified, are called RiPPs (ribosomally synthesised and post-translationally modified peptides), a route described in the guide to cyclic peptides. The thioether rings give nisin the rigidity and protease resistance that a disulfide-bridged peptide would have, but without the redox sensitivity: a thioether cannot be reduced by the thiols of the cell, unlike the disulfide bonds that hold most other small bioactive peptides in shape.
Two mechanisms at once
Nisin kills bacteria in a way that distinguishes it from the cationic membrane-disrupting peptides described in the guide to antimicrobial peptides. It does not simply attack the membrane by charge attraction. Instead it binds a specific molecular target.
That target is lipid II, the carrier molecule that shuttles cell-wall building blocks across the bacterial membrane. Structural work published in 2001 and 2004 showed that the first two rings of nisin form a cage around the pyrophosphate group of lipid II. The consequences are twofold:
- Cell wall synthesis stops, because lipid II is sequestered and cannot deliver its cargo. This is the same target that vancomycin attacks, though at a different part of the molecule.
- Pores form. Once anchored to lipid II, several nisin molecules assemble with it into a pore that punctures the membrane. Because the peptide is concentrated at its target rather than distributed over the membrane, it is effective at nanomolar concentrations, orders of magnitude lower than peptides that act by charge alone.
Nisin acts on Gram-positive bacteria. Gram-negative bacteria are protected by their outer membrane, which nisin cannot cross, although treatments that permeabilise that outer layer make them susceptible.
Chemistry and stability
Nisin is cationic, with three lysines and a histidine and only a single acidic group, so it is strongly positively charged at neutral pH. It is much more soluble and more stable in acid: solubility is high around pH 2 and falls markedly towards neutrality, and it survives autoclaving at low pH, which is why it is used in acidic and heat-processed foods.
Its dehydro residues are also its chemical weak point. Dehydroalanine reacts readily with nucleophiles, and in solution, especially above pH 6, nisin undergoes chemical modification and loses activity over time. The commercial preparation is a standardised powder, and activity is expressed in international units rather than by mass, because the material is not pure peptide.
Because of the modified residues, the tools on this site cannot calculate nisin's properties from its sequence. Lanthionine, methyllanthionine, dehydroalanine and dehydrobutyrine are outside the 20-letter alphabet, and each ring formation removes a water molecule. The values in the fact sheet come from the literature rather than from the MW calculator. This is a general limitation for RiPPs and other heavily modified peptides, described in the guide to post-translational modifications.
Variants and relatives
Several natural nisin variants are known, designated by letters: nisin A is the most used, and nisin Z, which differs by a single residue, is more soluble at neutral pH. Others have been found in Streptococcus and Blautia species.
Because nisin is gene-encoded, its sequence can be altered by mutating the gene and letting the bacterium's own modification enzymes do the rest, an approach not available for nonribosomal peptides. Many engineered variants have been made this way to study which residues matter and to improve solubility or stability.
Other lantibiotics include subtilin from Bacillus subtilis, epidermin from a skin bacterium, and mersacidin, which also binds lipid II but does not form pores.
Frequently asked questions
Is nisin an antibiotic?
Chemically yes: it is a bacterially produced antimicrobial peptide. It is not used as a medicine in humans, which is one reason its use in food has been considered acceptable, since it does not overlap with clinical antibiotic classes.
Does nisin survive digestion?
It is degraded by digestive proteases, in particular by pancreatic enzymes in the small intestine, which is part of why it has been regarded as suitable for food use for decades.
Why is nisin's activity given in units rather than milligrams?
Because commercial nisin is a standardised fermentation preparation containing salt and milk solids rather than pure peptide. Activity is measured biologically and expressed in international units.
Why can't I calculate nisin's mass from its sequence?
Its five thioether rings and its dehydro residues are not standard amino acids, and each ring costs a water molecule. Sequence-based calculators cover only the 20 standard residues with free termini.
References
- Gross E, Morell JL (1971) The structure of nisin. Journal of the American Chemical Society 93:4634–4635.
- Brötz H, Josten M, Wiedemann I, et al. (1998) Role of lipid-bound peptidoglycan precursors in the formation of pores by nisin, epidermin and other lantibiotics. Molecular Microbiology 30:317–327.
- Hsu S-TD, Breukink E, Tischenko E, et al. (2004) The nisin–lipid II complex reveals a pyrophosphate cage that provides a blueprint for novel antibiotics. Nature Structural & Molecular Biology 11:963–967.
- Delves-Broughton J, Blackburn P, Evans RJ, Hugenholtz J (1996) Applications of the bacteriocin, nisin. Antonie van Leeuwenhoek 69:193–202.