Some chemical and physical properties of nisin, a small-protein antibiotic produced by Lactococcus lactis

Kinetic studies of the action of lactacin F, a bacteriocin produced by Lactobacillus johnsonii that forms poration complexes in the cytoplasmic membrane

The bacteriocin lactacin F is bactericidal against Lactobacillus delbrueckii, Lactobacillus helveticus, and Enterococcus faecalis. Activity against L. delbrueckii was recently shown to be dependent on two peptides, LafA and LafX, which are encoded within the lactacin F operon (T. R. Klaenhammer, FEMS Microbiol. Rev. 12:39-87, 1993). It has been proposed that two peptides form an active lactacin F complex. In this study, the action of lactacin F against E. faecalis ATCC 19443 and the effects of various environmental parameters were investigated in detail. Addition of lactacin F induced the loss of K+ from cells of L. delbrueckii, Lactobacillus johnsonii 88-4, and E. faecalis, while the lactacin F producer L. johnsonii VPI 11088 was not affected by the bacteriocin. Lactacin F caused an immediate loss of cellular K+, depolarization of the cytoplasmic membrane, and hydrolysis of internal ATP in E. faecalis. Lactacin F induced loss of K+ in 3,3',4',5-tetrachlorosalicylanilide-treated cells, indicating that pores are formed in the absence of a proton motive force. ATP hydrolysis was not due to dissipation of the proton motive force but was most likely caused by efflux of inorganic phosphate, resulting in a shift of the ATP hydrolysis equilibrium. Action of lactacin F was optimal at acidic pH values and was reduced in the presence of di- and trivalent cations. The lanthanide gadolinium (Gd3+) prevented action of lactacin F completely at a concentration of 0.2 mM. Lactacin F-induced loss of cell K+ was severely reduced at low temperatures, presumably as a result of increased ordering of the lipid hydrocarbon chains in the cytoplasmic membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Response of spheroplasts and chelator-permeabilized cells of gram-negative bacteria to the action of the bacteriocins pediocin SJ-1 and nisin

We have attempted to bypass the outer membrane (OM) barrier of Escherichia coli and Salmonella typhimurium with pediocin SJ-1 (as compared with nisin) using chelating agents as OM permeabilizers. EDTA, and to less extent EGTA, enabled nisin, but not pediocin SJ-1, to permeate the cell OM of E. coli, to have access to the cytoplasmic membrane and to cause subsequent permeability changes, indicated by an increase in ANS fluorescence intensity and a shift of its emission maximum. Such spectral changes did not occur when, prior to addition, EDTA was saturated with Ca2+ and Mg2+. ANS fluorescence data indicated that, in spite of the fact that pediocin SJ-1 did traverse the EDTA-permeabilized OM of E. coli, it did not cause perturbation of its cytoplasmic membrane and was, therefore, unable to cause cell death. Spheroplasts prepared from E. coli were lysed when treated with nisin but not with pediocin SJ-1. We suggest that the resistance of Gram-negative bacteria to pediocin SJ-1 is due not only to this material's inability to permeate the OM but also (in contrast to nisin) to its inability to interact with the cytoplasmic membrane.

Nisin as a model food preservative

Nisin is a ribosomally synthesized peptide that has broad-spectrum antibacterial activity, including activity against many bacteria that are food-spoilage pathogens. Nisin is produced as a fermentation product of a food-grade bacterium, and the safety and efficacy of nisin as a food preservative have resulted in its widespread use throughout the world, including the U.S. Nisin is a member of the class of antimicrobial substances known as lantibiotics, so called because they contain the unusual amino acid lanthionine. Lantibiotics, in general, have considerable promise as food preservatives, although only nisin has been sufficiently well characterized to be used for this purpose. As the number of known natural lantibiotics has increased and their useful characteristics have been explored, it has become desirable to synthesize structural analogs of nisin and other lantibiotics that do not occur naturally. The fact that lantibiotics are gene-encoded peptides synthesized by transcription and translation allows structural variants to be generated by mutagenesis. This review focuses on the progress that has been made in the construction and biological expression of genetically engineered nisin structural analogs. For example, a host-vector pair has been engineered that permits the construction of mutants of the structural gene for subtilin, which is a naturally occurring structural analog of nisin. The vector is designed in such a way that the mutant gene can be substituted for the natural subtilin gene in the chromosome of Bacillus subtilis, which in turn directs the transcription, translation, posttranslational modifications, and secretion of the mature form of the structural analog. Several structural analogs have been constructed, and their properties have provided insight into some of the structure-function relationships in lantibiotics, as well as their mechanism of antimicrobial action. These advances are assessed together with potential problems in the future development of nisin analogs as valuable new food preservatives.

Carnocin UI49, a potential biopreservative produced by Carnobacterium piscicola: large scale purification and activity against various gram-positive bacteria including Listeria sp

This paper describes a simple purification method for the purification of carnocin UI49, a potential biopreservative produced by Carnobacterium piscicola UI49. The protocol was also applicable for the isolation of nisin Z, which is a biopreservative produced by Lactococcus lactis SIK-83. The protocol consists of only two purification steps, XAD chromatography and cation exchange chromatography. It is quick, easy, and can be used for large scale purification of these lantibiotics. The bactericidal activity of carnocin UI49 against carnobacteria, lactococci and Listeria was compared with that of nisin Z. The carnobacteria showed similar sensitivity towards carnocin UI49 and nisin. The nisin producing L. lactis strains were very sensitive towards carnocin UI49, while the non-producing L. lactis strains were more sensitive to nisin. The Listeria strains were weakly sensitive to carnocin UI49, lower concentrations of nisin were needed to inhibit growth.

Molecular analysis of the lactacin F operon

Lactacin F is a nonlantibiotic, heat-stable, peptide bacteriocin produced by Lactobacillus johnsonii VPI11088. Molecular analysis of the lactacin F DNA region characterized a small operon that codes for three open reading frames, designated lafA, lafX, and ORFZ. The peptide encoded by lafA, the lactacin F structural gene, was compared with various peptide bacteriocins from lactic acid bacteria, and similarities were identified in the amino and carboxy termini of the propeptides. Site-directed mutagenesis of the LafA precursor at the two glycine residues in positions -1 and -2 defined an essential motif for processing of mature lactacin F. The involvement of the peptides encoded by lafX and ORFZ in bacteriocin expression was investigated by subcloning various fragments from the lactacin F region into the shuttle vector pGKV210. In addition to lafA, expression of lafX is essential to lactacin F activity. The lactacin F operon resembles the genetic organization of lactococcin M. Although no function has been assigned to ORFZ by genetic analysis, both peptide Z and the lactococcin M immunity protein are predicted to be integral membrane proteins with four putative transmembrane segments. Lactacin F activity, defined by bactericidal action on Lactobacillus delbrueckii, is dependent on the expression of two genes, lafA and lafX.

Antibiosis revisited: bacteriocins produced by dairy starter cultures

Well before the existence of starter bacteria was recognized, their activities were instrumental in preserving dairy foods. During growth in fermented products, dairy starters, including lactobacilli, lactococci, leuconostocs, streptococci, and propionibacteria, produce inhibitory metabolites. Inhibitors include broad-spectrum antagonists, organic acids, diacetyl, and hydrogen peroxide. Some starters also produce bacteriocins or bactericidal proteins active against species that usually are related closely to the producer culture. Several bacteriocins have been biochemically and genetically characterized. Evaluating properties of the Lactobacillus acidophilus bacteriocin, lactacin B, led to a new purification protocol. Purified lactacin B migrates in SDS-PAGE as a single 8100-Da band with inhibitory activity after Coomassie blue staining. Production of lactacin B is enhanced by cultivation of the producer with the sensitive indicator, Lactobacillus delbrueckii ssp. lactis 4797; understanding this interaction may increase knowledge of production of bacteriocins in heterogeneous cultures. Bacteriocins have been recently identified in dairy propionibacteria. Jenseniin G, a bacteriocin produced by Propionibacterium jensenii P126, has narrow activity; propionicin PLG-1 produced by Propionibacterium thoenii P127 inhibits propionibacteria, some fungi, Campylobacter jejuni, and additional pathogens. Better understanding of these antagonists may lead to targeted biocontrol of spoilage flora and foodborne pathogens.

In vitro pore-forming activity of the lantibiotic nisin. Role of protonmotive force and lipid composition

Nisin is a lantibiotic produced by some strains of Lactococcus lactis subsp. lactis. The target for nisin action is the cytoplasmic membrane of Gram-positive bacteria. Nisin dissipates the membrane potential (delta psi) and induces efflux of low-molecular-mass compounds. Evidence has been presented that a delta psi is needed for nisin action. The in vitro action of nisin was studied on liposomes loaded with the fluorophore carboxyfluorescein. Nisin-induced efflux of carboxyfluorescein was observed in the absence of a delta psi from liposomes composed of Escherichia coli lipids or dioleoylglycerophosphocholine (Ole2GroPCho) at low nisin/lipid ratios. The initial rate of carboxyfluorescein efflux is dependent on the nisin/lipid ratio and saturates at high ratios. Both delta psi (inside negative) and delta pH (inside alkaline) enhance the action of nisin, while nisin is more potent at acidic external pH values. Efficient carboxyfluorescein efflux is observed with the zwitterionic phospholipid Ole2GroPCho or mixtures of Ole2GroPCho with dioleoylglycerophosphoethanolamine and neutral glycolipids, while anionic phospholipids are strongly inhibitory. It is concluded that a delta psi is not essential, but that the total protonmotive force stimulates the action of nisin.

The antimicrobial effect of a structural variant of subtilin against outgrowing Bacillus cereus T spores and vegetative cells occurs by different mechanisms

Subtilin is a ribosomally synthesized antimicrobial peptide that contains several unusual amino acids as a result of posttranslational modifications. Site-directed mutagenesis was employed to construct a structural variant of subtilin in which the unusual dehydroalanine (Dha) residue at position 5 was changed to alanine. Proton nuclear magnetic resonance spectroscopy, amino acid composition, and N-terminal sequence analysis established that the mutation did not disrupt posttranslational processing of the precursor peptide. This mutant subtilin was devoid of antimicrobial activity as assessed by its lack of inhibitory effects on outgrowth of Bacillus cereus T spores. However, this same mutant subtilin was fully active with respect to its ability to induce lysis of vegetative B. cereus T cells. Because an intact Dha-5 residue is required in the one instance but not in the other, it was concluded that the molecular mechanism by which subtilin inhibits (without lysis) spore outgrowth is not the same as the mechanism by which it inhibits (with lysis) vegetative cells.

Properties of nisin Z and distribution of its gene, nisZ, in Lactococcus lactis

Two natural variants of the lantibiotic nisin that are produced by Lactococcus lactis are known. They have a similar structure but differ in a single amino acid residue at position 27; histidine in nisin A and asparagine in nisin Z (J.W.M. Mulders, I.J. Boerrigter, H.S. Rollema, R.J. Siezen, and W.M. de Vos, Eur. J. Biochem, 201:581-584, 1991). The nisin variants were purified to apparent homogeneity, and their biological activities were compared. Identical MICs of nisin A and nisin Z were found with all tested indicator strains of six different species of gram-positive bacteria. However, at concentrations above the MICs, with nisin Z the inhibition zones obtained in agar diffusion assays were invariably larger than those obtained with nisin A. This was observed with all tested indicator strains. These results suggest that nisin Z has better diffusion properties than nisin A in agar. The distribution of the nisin variants in various lactococcal strains was determined by amplification of the nisin structural gene by polymerase chain reaction followed by direct sequencing of the amplification product. In this way, it was established that the nisZ gene for nisin Z production is widely distributed, having been found in 14 of the 26 L. lactis strains analyzed.

Regulation of nisin biosynthesis by continuous cultures and by resting cells of Lactococcus lactis subsp. lactis

Nisin production by Lactococcus lactis subsp. lactis has been investigated using lactose as carbon source. Whether or not continuous cultures were lactose-limited, maximum nisin titre was observed at an intermediate mu value with a sharp peak of activity between 0.2 and 0.3/h. The maximum specific growth rate obtained in the medium used was 0.6/h and the maximum titre of nisin at mu = 0.25/h (160 AU/ml) was about nine-fold higher as compared with activity obtained at a dilution rate of 0.05/h or 0.4/h. With a constant dilution rate of 0.25/h and varying initial lactose concentrations from 3 to 40 g/l, there is an increase in nisin biosynthesis with increasing lactose concentration correlated with higher rates of sugar consumption. A Ymax value of 0.2 g bacterial dry weight and a maintenance coefficient of 124 mg lactose/g bacterial dry weight/h were determined. Lactose consumption increased from 1 to 3.28 g of lactose/g (dry wt) of cell mass/h and the nisin titre from 12.5 to 164.2 AU/ml. At higher values, nisin production declined. This implies that biosynthesis of nisin is regulated by a system of repression and derepression. Addition of lanthionine and beta-methyllanthionine precursors to the medium decreased the nisin titre when either threonine, threonine-cysteine, or cysteine-serine-threonine was added at the optimal dilution rate of 0.25/h; however, simultaneous addition of serine and cysteine elicited a slight increase in nisin activity. Studies with resting cells confirm that the biosynthesis of nisin is tightly regulated, since the production rate can be 5.6-fold higher than in cells grown in continuous culture. In addition, cell-adhered nisin appears to play a role in the production of the enzyme: low levels of cell-adhered nisin elicited high production rates, whereas high levels were not associated with nisin biosynthesis. In addition to pH, magnesium sulphate and lactose concentrations, nitrogen sources were also able to interfere in cell-adherence nisin.

Identification and characterization of two bacteriocin-producing strains of Lactococcus lactis isolated from vegetables

Isolated from mixed salad and fermented carrots, 123 strains of lactic acid bacteria were screened for bacteriocin production. Two strains, D53 and 23, identified as Lactococcus lactis by DNA-DNA hybridizations, produced heat stable bacteriocins which were resistant to trypsin and pepsin, but were inactivated by alpha-chymotrypsin and proteinase K. The bacteriocins were active from pH 2 to 9 and inhibited species of Listeria, Lactobacillus, Lactococcus, Pediococcus, Leuconostoc, Carnobacterium, Bacillus and Staphylococcus. Strain D53 produced bacteriocin at pH values of 4.5-8.0 and from 10 to 37 degrees C.

Purification and primary structure of pediocin PA-1 produced by Pediococcus acidilactici PAC-1.0

The plasmid-encoded bacteriocin pediocin PA-1, produced by the gram-positive bacterium Pediococcus acidilactici strain PAC-1.0, was purified to homogeneity. The purified product exhibited antibacterial activity against several gram-positive bacterial strains, including the food pathogen Listeria monocytogenes. Pediocin PA-1 is a 4629-Da peptide with 44 amino acids and two disulfide bonds. The amino acid sequence and arrangement of the disulfide bonds were determined. Sequence data were used to calculate an isoelectric point of 10.0. The small and basic nature of PA-1 is comparable to several other bacteriocins produced by gram-positive bacteria. Reported sequences of other bacteriocins and of other antimicrobial peptides from diverse origins bear no resemblance to the sequence reported here.

Antimicrobial action of nisin against Salmonella typhimurium lipopolysaccharide mutants

The antimicrobial activity of nisin against outer membrane lipopolysaccharide mutants of Salmonella typhimurium LT2 was investigated. Nisin sensitivity was associated with the extent of saccharide deletions from the outer membrane core oligosaccharide. The results indicated that the core oligosaccharide in lipopolysaccharide plays a role in nisin sensitivity.

Influence of fat and emulsifiers on the efficacy of nisin in inhibiting Listeria monocytogenes in fluid milk

The recent FDA affirmation of nisin, an antimicrobial peptide, as a GRAS (generally recognized as safe) additive in pasteurized cheese spreads has renewed interest in its potential application in US dairy products. Fluid milks were prepared with varying concentrations of milk fat (0 to 12.9%) and of nisin (0 to 50 U/ml). Biological activity assays using a sensitive indicator microorganism in a well diffusion system indicated that initial nisin activity (50 U/ml) decreased by about 33% when it was added to skim milk and by more than 88% when added to milk containing 12.9% fat. Nisin activity decreased by ca. 50% in milk containing 1.29% fat. Milks containing 0, 10, or 50 U/ml of nisin and varying fat percentages were challenged with approximately log10 7 to 7.5 cfu/ml of log phase Listeria monocytogenes Scott A or Jalisco. At 2 h after inoculation, the viable count of L. monocytogenes Scott A decreased to log10 .30 cfu/ml in skim milk with 50 U/ml of nisin, decreased to log10 2.90 cfu/ml in skim milk with 10 U/ml of nisin, and increased slightly (log10 7.8 cfu/ml) in skim milk without nisin. In half-and-half (12.9% milk fat), nisin was far less effective in inhibiting Listeria with populations decreasing to log10 6.57 cfu/ml for 10 U/ml of nisin and log10 5.87 cfu/ml for 50 U/ml. Similar results were obtained with L. monocytogenes Jalisco. The nonionic emulsifier, Tween 80, partially counteracted decreases of nisin activity in milks, whereas the anionic emulsifier, lecithin, had no effect. Addition of Tween 80 significantly increased the activity of nisin against L. monocytogenes in milk regardless of fat content.

NMR studies of lantibiotics. The structure of nisin in aqueous solution

Nisin is a posttranslationally modified protein of 34 amino acids, and is a member of the class of bacteriocidal polypeptides known as lantibiotics, that contain the unusual amino acid lanthionine. Its structure in aqueous solution has been determined on the basis of NMR data, i.e. interproton distance constraints derived from nuclear Overhauser enhancement spectroscopy and torsion angle constraints derived from double-quantum-filtered correlated spectroscopy. Translation of the NMR constraints into a three-dimensional structure was carried out with the distance-geometry program DISMAN, followed by restrained energy minimization using CHARMm. The internal mobility of the peptide chain prohibited the determination of a precise overall folding of the molecule, but parts of the structure could be obtained, albeit sometimes with low resolution. The structure of nisin can best be defined as follows. The outermost N-terminal and C-terminal regions of nisin appear quite flexible, the remainder of the molecule consists of an amphiphilic N-terminal fragment (residues 3-19), joined by a flexible 'hinge' region to a rigid double-ring fragment formed by residues 23-28. The latter fragment has the appearance of a somewhat overwound alpha-helix. It is suggested, by assuming the presence of a (transient) alpha-helical structure in this part of prenisin, that the coupling between residues 23 and 26, as well as between 25 and 28, by thioether bridges, and the inversion of the C alpha chiralities at positions 23 and 25, can be rationalized.

Nisin treatment for inactivation of Salmonella species and other gram-negative bacteria

Nisin, produced by Lactococcus lactis subsp. lactis, has a broad spectrum of activity against gram-positive bacteria and is generally recognized as safe in the United States for use in selected pasteurized cheese spreads to control the outgrowth and toxin production of Clostridium botulinum. This study evaluated the inhibitory activity of nisin in combination with a chelating agent, disodium EDTA, against several Salmonella species and other selected gram-negative bacteria. After a 1-h exposure to 50 micrograms of nisin per ml and 20 mM disodium EDTA at 37 degrees C, a 3.2- to 6.9-log-cycle reduction in population was observed with the species tested. Treatment with disodium EDTA or nisin alone produced no significant inhibition (less than 1-log-cycle reduction) of the Salmonella and other gram-negative species tested. These results demonstrated that nisin is bactericidal to Salmonella species and that the observed inactivation can be demonstrated in other gram-negative bacteria. Applications involving the simultaneous treatment with nisin and chelating agents that alter the outer membrane may be of value in controlling food-borne salmonellae and other gram-negative bacteria.

Conversion of Bacillus subtilis 168 to a subtilin producer by competence transformation

Subtilin is a ribosomally synthesized peptide antibiotic produced by Bacillus subtilis ATCC 6633. B. subtilis 168 was converted to a subtilin producer by competence transformation with chromosomal DNA from B. subtilis ATCC 6633. A chloramphenicol acetyltransferase gene was inserted next to the subtilin structural gene as a selectable marker. The genes that conferred subtilin production were derived from a 40-kb region of the B. subtilis ATCC 6633 chromosome that had flanking homologies to the B. subtilis 168 chromosome. The subtilin produced by the mutant was identical to natural subtilin in its biological activity, chromatographic behavior, amino acid composition, and N-terminal amino acid sequence.

Mechanism of action of the peptide antibiotic nisin in liposomes and cytochrome c oxidase-containing proteoliposomes

The interaction of the peptide antibiotic nisin with liposomes has been studied. The effect of this interaction was analyzed on the membrane potential (inside negative) and the pH gradient (inside alkaline) in liposomes made from Escherichia coli phosphatidylethanolamine and egg phosphatidylcholine (9:1, wt/wt). The membrane potential and pH gradient were generated by artificial ion gradients or by the oxidation of ascorbate, N,N,N',N'-tetramethyl-p-phenylenediamine, and cytochrome c by the beef heart cytochrome c oxidase incorporated in the liposomal membranes. Nisin dissipated the membrane potential and the pH gradient in both types of liposomes and inhibited oxygen consumption by cytochrome c oxidase in proteoliposomes. The dissipation of the proton motive force in proteoliposomes was only to a minor extent due to a decrease of the oxidase activity by nisin. The results in these model systems show that a membrane potential and/or a pH gradient across the membrane enhances the activity of nisin. Nisin incorporates into the membrane and makes the membrane permeable for ions. As a result, both the membrane potential and pH gradient are dissipated. The activity of nisin was found to be influenced by the phospholipid composition of the liposomal membrane.

Public health implication of refrigerated pasteurized ('sous-vide') foods

Food that upon pasteurization is stored in hermetically sealed containers at food temperatures not exceeding 3 degrees C could be designated by the generic term Refrigerated Pasteurized Foods of Extended Durability, REPFEDs. If not properly processed or protected against recontamination, or if temperature-abused, REPFEDs may present serious health risks. However, control is readily available. Sound microbial ecology, supported by expert risk assessment, allows the design and introduction of longitudinally integrated manufacture, distribution, handling by outlets and consumers and culinary preparation, which result in the assurance of the wholesomeness of the commodity as eaten. Recent progress, including intrinsic preservation by the incorporation of starter cultures, bacteriocins or particular enzymes, opens vistas for attractive future developments. Once microbiological safety has been built into the REPFED-line, monitoring can be limited to (i) real-time tests particularly applied to the factory environment; and (ii) rapid, simple examination for marker organisms of freshly manufactured products versus those approaching expiration dates. Such audits will allow rapid retrieval of incidental process failure and its rectification. It also serves to substantiate measurements of food temperature and spot checks on intrinsic inhibitory attributes. The application of scientific knowledge and technological expertise should primarily be entrusted to the industry itself, heeding Lord Plumb's strategy of "partnership along the food production chain from farm to fork." It should be supported and validated by Public Health Authorities. At all stages safety communication with the public should be ensured.

Inhibition of food-borne bacterial pathogens by bacteriocins from lactic acid bacteria isolated from meat

Ten strains of bacteriocin-producing lactic acid bacteria were isolated from retail cuts of meat. These 10 strains along with 11 other bacteriocin-producing lactic acid bacteria were tested for inhibitory activity against psychotrophic pathogens, including four strains of Listeria monocytogenes, two strains of Aeromonas hydrophila, and two strains of Staphylococcus aureus. Inhibition due to acid, hydrogen peroxide, and lytic bacteriophage were excluded. The proteinaceous nature of the inhibitory substance was confirmed by demonstration of its sensitivity to proteolytic enzymes. Eight of the meat isolates had inhibitory activity against all four L. monocytogenes strains. Bacteriocin activity against L. monocytogenes was found in all of the strains obtained from other sources. Activity against A. hydrophila and S. aureus was also common.