Bacteriocins of gram-positive bacteria

In recent years, a group of antibacterial proteins produced by gram-positive bacteria have attracted great interest in their potential use as food preservatives and as antibacterial agents to combat certain infections due to gram-positive pathogenic bacteria. They are ribosomally synthesized peptides of 30 to less than 60 amino acids, with a narrow to wide antibacterial spectrum against gram-positive bacteria; the antibacterial property is heat stable, and a producer strain displays a degree of specific self-protection against its own antibacterial peptide. In many respects, these proteins are quite different from the colicins and other bacteriocins produced by gram-negative bacteria, yet customarily they also are grouped as bacteriocins. Although a large number of these bacteriocins (or bacteriocin-like inhibitory substances) have been reported, only a few have been studied in detail for their mode of action, amino acid sequence, genetic characteristics, and biosynthesis mechanisms. Nevertheless, in general, they appear to be translated as inactive prepeptides containing an N-terminal leader sequence and a C-terminal propeptide component. During posttranslational modifications, the leader peptide is removed. In addition, depending on the particular type, some amino acids in the propeptide components may undergo either dehydration and thioether ring formation to produce lanthionine and beta-methyl lanthionine (as in lantibiotics) or thio ester ring formation to form cystine (as in thiolbiotics). Some of these steps, as well as the translocation of the molecules through the cytoplasmic membrane and producer self-protection against the homologous bacteriocin, are mediated through specific proteins (enzymes). Limited genetic studies have shown that the structural gene for such a bacteriocin and the genes encoding proteins associated with immunity, translocation, and processing are present in a cluster in either a plasmid, the chromosome, or a transposon. Following posttranslational modification and depending on the pH, the molecules may either be released into the environment or remain bound to the cell wall. The antibacterial action against a sensitive cell of a gram-positive strain is produced principally by destabilization of membrane functions. Under certain conditions, gram-negative bacterial cells can also be sensitive to some of these molecules. By application of site-specific mutagenesis, bacteriocin variants which may differ in their antimicrobial spectrum and physicochemical characteristics can be produced. Research activity in this field has grown remarkably but sometimes with an undisciplined regard for conformity in the definition, naming, and categorization of these molecules and their genetic effectors. Some suggestions for improved standardization of nomenclature are offered.

Vibrio parahaemolyticus infections in the United States, 1973-1998

Vibrio parahaemolyticus infections are associated with consumption of raw or undercooked shellfish, contaminated food, and exposure of wounds to warm seawater. Foodborne outbreaks and sporadic infections from Vibrio species in 4 Gulf Coast states are reported routinely to the Centers for Disease Control and Prevention (CDC). Between 1988 and 1997, 345 sporadic V. parahaemolyticus infections were reported: 59% were gastroenteritis, 34% were wound infections, 5% were septicemia, and 2% were from other exposures. Forty-five percent of patients suffering from these conditions were hospitalized for their infections, and 88% of persons with acute gastroenteritis reported having eaten raw oysters during the week before their illness occurred. Between 1973 and 1998, 40 outbreaks of V. parahaemolyticus infections were reported to the CDC, and these outbreaks included >1000 illnesses. Most of these outbreaks occurred during the warmer months and were attributed to seafood, particularly shellfish. The median attack rate among persons who consumed the implicated seafood was 56%. To prevent V. parahaemolyticus infections, persons should avoid consumption of raw or undercooked shellfish and exposure of wounds to seawater.

Novel method to extract large amounts of bacteriocins from lactic acid bacteria

Antimicrobial peptides, bacteriocins, produced by lactic acid bacteria were adsorbed on the cells of producing strains and other gram-positive bacteria. pH was a crucial factor in determining the degree of adsorption of these peptides onto cell surfaces. In general, between 93 and 100% of the bacteriocin molecules were adsorbed at pHs near 6.0, and the lowest (< or = 5%) adsorption took place at pH 1.5 to 2.0. On the basis of this property, a novel isolation method was developed for bacteriocins from four genera of lactic acid bacteria. By using this method we made preparations of pediocin AcH, nisin, sakacin A, and leuconocin Lcm1 that were potent and concentrated. This method produced a higher yield than isolation procedures, which rely on precipitation of the bacteriocins from the cell-free culture liquor. It is simple and can be used to produce large quantities of bacteriocins from lactic acid bacteria to be used as food biopreservatives.

Purification, characterization and antimicrobial spectrum of a bacteriocin produced by Pediococcus acidilactici

An antimicrobial peptide designated pediocin AcH was isolated from Pediococcus acidilactici strain H. The pediocin AcH was purified by ion exchange chromatography. The molecular weight of pediocin AcH was determined by SDS-PAGE to be about 2700 daltons. Pediocin AcH was sensitive to proteolytic enzymes resistant to heat and organic solvents, and active over a wide range of pH. Pediocin AcH exhibited inhibition against several food spoilage bacteria and foodborne pathogens including Staphylococcus aureus, Clostridium perfringens and Listeria monocytogenes. It was bactericidal to sensitive cells and acted very rapidly. The bactericidal effect was not produced by either cell lysis or apparent loss of membrane permeability.

Influence of Growth Conditions on the Production of a Bacteriocin, Pediocin AcH, by Pediococcus acidilactici H

The influence of growth parameters on the production of pediocin AcH by Pediococcus acidilactici H was studied. This strain produced large quantities of pediocin AcH in TGE broth (Trypticase [1%], glucose [1%], yeast extract [1%], Tween 80 [0.2%], Mn [0.033 mM], Mg [0.02 mM] [pH 6.5]) within 16 to 18 h at 30 to 37 degrees C (final pH, 3.6 to 3.7). Pediocin AcH production was negligible when the pH of the medium was maintained at 5.0 or above, even in the presence of high cell mass.

Variation in resistance to hydrostatic pressure among strains of food-borne pathogens

Among food-borne pathogens, some strains could be resistant to hydrostatic pressure treatment. This information is necessary to establish processing parameters to ensure safety of pressure-pasteurized foods (N. Kalchayanand, A. Sikes, C. P. Dunne, and B. Ray, J. Food Prot. 61:425-431, 1998). We studied variation in pressure resistance among strains of Listeria monocytogenes, Staphylococcus aureus, Escherichia coli O157:H7, and Salmonella species at two temperatures of pressurization. Early-stationary-phase cells in 1% peptone solution were pressurized at 345 MPa either for 5 min at 25 degrees C or for 5, 10, or 15 min at 50 degrees C. The viability loss (in log cycles) following pressurization at 25 degrees C ranged from 0.9 to 3.5 among nine L. monocytogenes strains, 0.7 to 7.8 among seven S. aureus strains, 2.8 to 5.6 among six E. coli O157:H7 strains, and 5.5 to 8.3 among six Salmonella strains. The results show that at 25 degrees C some strains of each species are more resistant to pressure than the others. However, when one resistant and one sensitive strain from each species were pressurized at 345 MPa and 50 degrees C, the population of all except the resistant S. aureus strain was reduced by more than 8 log cycles within 5 min. Viability loss of the resistant S. aureus strain was 6.3 log cycles even after 15 min of pressurization. This shows that strains of food-borne pathogens differ in resistance to hydrostatic pressure (345 MPa) at 25 degrees C, but this difference is greatly reduced at 50 degrees C. Pressurization at 50 degrees C, in place of 25 degrees C, will ensure greater safety of foods.

Interactions of high hydrostatic pressure, pressurization temperature and pH on death and injury of pressure-resistant and pressure-sensitive strains of foodborne pathogens

The objective of this study is to determine the interactions between high hydrostatic pressure, pressurization temperature, time and pH during pressurization on death and injury of pressure-resistant and pressure-sensitive strains of four foodborne pathogens: Staphylococcus aureus 485 and 765, Listeria ,monocytogenes CA and OH2, Escherichia coli O157:H7 933 and 931, Salmonella enteritidis FDA and Salmonella typhimurium E21274. Among these strains S. aureus 485, L. monocytogenes CA, E. coli O157:H7 933 and S. enteritidis FDA were reported to be more pressure-resistant than the respective strain of the same species (Alpas et al., 1999). In general, viability loss of all pathogens was enhanced significantly as the level of pressure and temperature were increased (P < 0.05). All the strains except S. aureus 485 demonstrated more than 8 log cycle reduction when pressurized at 345 MPa at 50 degrees C for 5 min. This strain seemed to be the most pressure-resistant strain within the conditions of the study. Pressurization in the presence of either citric or lactic acid increased the viability loss by an additional 1.2-3.9 log cycles at pH 4.5 for both acids at 345 MPa. This study has indicated that high hydrostatic pressure applied in conjunction with mild heat and acidity can be an effective method for inactivating pressure-resistant and pressure-sensitive strains of four foodborne pathogens in organic acid solutions. This combination treatment indicates possible pressure pasteurization applications to liquid foods that have low pH. reserved.

Hydrostatic pressure and electroporation have increased bactericidal efficiency in combination with bacteriocins

We report here that both ultrahigh hydrostatic pressure (UHP) and electroporation (EP) induced sublethal injury to bacterial cells surviving the treatments. The injured cells of both gram-positive and -negative bacteria became sensitive to the bacteriocins pediocin AcH and nisin. Bacteriocins in combination with either UHP or EP have greater antibacterial effectiveness than UHP or EP alone.

The role of Gulf Coast oysters harvested in warmer months in Vibrio vulnificus infections in the United States, 1988-1996. Vibrio Working Group

Vibrio vulnificus infections are highly lethal and associated with consumption of raw shellfish and exposure of wounds to seawater. V. vulnificus infections were reported to the Centers for Disease Control and Prevention from 23 states. For primary septicemia infections, oyster trace-backs were performed and water temperature data obtained at harvesting sites. Between 1988 and 1996, 422 infections were reported; 45% were wound infections, 43% primary septicemia, 5% gastroenteritis, and 7% from undetermined exposure. Eighty-six percent of patients were male, and 96% with primary septicemia consumed raw oysters. Sixty-one percent with primary septicemia died; underlying liver disease was associated with fatal outcome. All trace-backs with complete information implicated oysters harvested in the Gulf of Mexico; 89% were harvested in water >22 degrees C, the mean annual temperature at the harvesting sites (P < .0001). Control measures should focus on the increased risk from oysters harvested from the Gulf of Mexico during warm months as well as education about host susceptibility factors.

Interaction of hydrostatic pressure, time and temperature of pressurization and pediocin AcH on inactivation of foodborne bacteria

High hydrostatic pressure, because it can kill microorganisms, is being investigated for potential use as a nonthermal food preservation method. The objective of this study was to determine the hydrostatic pressurization parameters, pressure, time, and temperature, and a bacteriocin that in combination would destroy 7 to 8 log cycles of pathogenic and spoilage bacterial populations. We suspended cells of Staphylococcus aureus, Listeria monocytogenes, Salmonella typhimurium, Escherichia coli O157:H7, Lactobacillus sake, Leuconostoc mesenteroides, Serratia liquefaciens, and Pseudomonas fluorescens in peptone solution and exposed them to the combination of treatments. The combined parameters used were hydrostatic pressure (138 to 345 MPa), time (5 to 15 min), temperature (25 to 50 degrees C), and pediocin AcH (3,000 AU/ml, final concentration). In general, cell death increased as the pressure, time, or temperature increased; however, the cells developed proportionately greater sensitivity as the pressure increased to 276 MPa and higher and the temperature increased above 35 degrees C. Pressurization for longer than 5 min, especially at lower pressure and temperature ranges, had very little added benefit. Among the four gram-negative species, E. coli O157:H7 was the most resistant to pressurization while among the four gram-positive species, L. sake and L. mesenteroides had greater resistance. The death rate at high pressure (345 MPa) and high temperature (50 degrees C) in combination followed first-order kinetics; at lower pressure and temperature combination it showed a late tailing effect. Estimated D value data indicated that even at 345 MPa and 50 degrees C an 8-log-cycle viability loss could not be achieved within 5 min for all eight species. However, when pediocin AcH was included during pressurization this loss was achieved.

Human male germ cell tumor resistance to cisplatin is linked to TP53 gene mutation

Male germ cell tumors (GCTs) are uniquely sensitive to cisplatin-based chemotherapy, with more than 90% of newly diagnosed cases cured. The underlying cause for resistance to treatment in 20-30% of metastatic lesions remains to be identified. Unlike other solid tumors, no mutations in the TP53 gene have been identified to date in random panels of GCT specimens, which could account for the exquisite sensitivity of these tumors to genotoxic insult. However, in a panel of resistant GCTs that did either not respond to cisplatin-based chemotherapy or subsequently relapsed and resulted in the death of the patient, we have now identified a subset of tumors to contain TP53 mutations within exons 6-9. A cell line derived from one of these tumors (228A) displayed the same TP53 mutation as the tumor specimen, expressed only mutant TP53 mRNA, and exhibited a relative resistance to cisplatin in vitro in comparison to a cell line (218A) derived from a responsive tumor with wild-type TP53. The resistant cell line displayed a much reduced apoptotic cell death and did not exhibit an induction of transcription of the p53-responsive genes WAF1 and MDM2 following cisplatin treatment, compared to that observed in the sensitive cell line. The levels of bax, an agonist of apoptosis, were found to be reduced in the resistant cell line. The simplest explanation for the resistance of this subset of GCTs that are resistant to cisplatin-based chemotherapy, is the inability of the cells to mount an apoptotic response following exposure due to a functionally inactivating mutation in the TP53 gene.

Autism, Alzheimer disease, and fragile X: APP, FMRP, and mGluR5 are molecular links

The present review highlights an association between autism, Alzheimer disease (AD), and fragile X syndrome (FXS). We propose a conceptual framework involving the amyloid-β peptide (Aβ), Aβ precursor protein (APP), and fragile X mental retardation protein (FMRP) based on experimental evidence. The anabolic (growth-promoting) effect of the secreted α form of the amyloid-β precursor protein (sAPPα) may contribute to the state of brain overgrowth implicated in autism and FXS. Our previous report demonstrated that higher plasma sAPPα levels associate with more severe symptoms of autism, including aggression. This molecular effect could contribute to intellectual disability due to repression of cell-cell adhesion, promotion of dense, long, thin dendritic spines, and the potential for disorganized brain structure as a result of disrupted neurogenesis and migration. At the molecular level, APP and FMRP are linked via the metabotropic glutamate receptor 5 (mGluR5). Specifically, mGluR5 activation releases FMRP repression of APP mRNA translation and stimulates sAPP secretion. The relatively lower sAPPα level in AD may contribute to AD symptoms that significantly contrast with those of FXS and autism. Low sAPPα and production of insoluble Aβ would favor a degenerative process, with the brain atrophy seen in AD. Treatment with mGluR antagonists may help repress APP mRNA translation and reduce secretion of sAPP in FXS and perhaps autism.

Emergence of a new Vibrio parahaemolyticus serotype in raw oysters: A prevention quandary

CONTEXT

In May and June 1998, reported Vibrio parahaemolyticus infections increased sharply in Texas.

OBJECTIVE

To determine factors that contributed to the increase in V parahaemolyticus infections.

DESIGN, SETTING, AND PARTICIPANTS

Cross-sectional survey of persons reporting gastroenteritis after eating seafood in Texas; survey of environmental conditions in Galveston Bay.

MAIN OUTCOME MEASURES

Traceback of oysters, water quality measures in harvest areas, presence of V parahaemolyticus in stool cultures; comparison of median values for environmental conditions before and during the outbreak compared with during the previous 5 years.

RESULTS

Between May 31 and July 10, 1998, 416 persons in 13 states reported having gastroenteritis after eating oysters harvested from Galveston Bay. All 28 available stool specimens from affected persons yielded V parahaemolyticus serotype O3:K6 isolates. Oyster beds met current bacteriologic standards during harvest and fecal coliform counts in water samples were within acceptable limits. Median water temperature and salinity during May and June 1998 were 30.0 degrees C and 29.6 parts per thousand (ppt) compared with 28.9 degrees C and 15.6 ppt for the previous 5 years (P<.001).

CONCLUSIONS

This is the first reported outbreak of V parahaemolyticus serotype O3:K6 infection in the United States. The emergence of a virulent serotype and elevated seawater temperatures and salinity levels may have contributed to this large multistate outbreak of V parahaemolyticus. Bacteriologic monitoring at harvest sites did not prevent this outbreak, suggesting that current policy and regulations regarding the safety of raw oysters require reevaluation. Consumers and physicians should understand that raw or undercooked oysters can cause illness even if harvested from monitored beds. In patients who develop acute gastroenteritis within 4 days of consuming raw or undercooked oysters, a stool specimen should be tested for Vibrio species using specific media. JAMA. 2000;284:1541-1545.

Cell-wall polysaccharides from the marine green alga Ulva "rigida" (Ulvales, Chlorophyta)--NMR analysis of ulvan oligosaccharides

In order to obtain information on sugar sequences in the water soluble polysaccharides from Ulva "rigida", oligosaccharides were purified by anion exchange and gel permeation chromatography from the partial acid hydrolysate of the native or desulfated ulvan. The chemical structure and sugar sequence of these oligomers were determined by 1H and 13C NMR spectroscopy to be alpha-L-rhamnosyl (1-->4) D-xylose, beta-D-glucuronosyluronic acid (1-->2)-alpha-L-rhamnosyl (1-->4) D-xylose, beta-D-glucuronosyluronic acid (1-->4) L-rhamnose 3 sulfate, beta-D-glucuronosyluronic acid (1-->4) [beta-D-glucuronosyluronic acid (1-->2)] L-rhamnose and beta-D-glucuronosyluronic acid (1-->4) [beta-D-glucuronosyluronic acid (1-->2)] alpha-L-rhamnosyl (1-->4) D-xylose. The sugar linkages and particularly the branching and the sulfate position were in total agreement with previous chemical results.

Nucleotide and amino acid sequence of pap-gene (pediocin AcH production) in Pediococcus acidilactici H

N-terminal analysis of purified pediocin AcH produced a partial sequence of 23 amino acids. This sequence matched perfectly with a segment of 23 amino acids in a 62 amino acid molecule generated from the 186 nucleotide sequence open reading frame in a Hind III fragment in pSMB74 encoding pap-gene (pediocin AcH production). It is suggested that the molecule is translated as inactive prepediocin AcH of 62 amino acids. Then through enzymatic modifications the leader segment of 18 amino acids is removed from the NH2-terminal. The remaining segment of 44 amino acids is active pediocin AcH of 4628 M(r).

Enhanced recovery of injured Escherichia coli by compounds that degrade hydrogen peroxide or block its formation

Escherichia coli LSUFS was injured either by freezing at -10 degrees C or by heating at 57 degrees C for 12 min. Surviving cells were recovered on nonselective tryptone-glucose extract agar and selective violet red bile agar supplemented with compounds that degrade hydrogen peroxide or block its formation. Various concentrations of the following compounds were tested: sodium pyruvate, 3,3'-thiodipropionic acid, catalase, ascorbic acid, potassium permanganate, sodium thioglycolate, dimethylsulfoxide, ethoxyquin, n-propyl gallate, alpha-tocopherol sodium metabisulfite, and ferrous sulfate. Sodium pyruvate and 3,3'-thiodipropionic acid, when added to either medium, significantly (P greater than 0.01) increased recovery of injured cells. More than 90% of the heat-injured cells and 40 to 90% of the freeze-injured cells failed to grow on unsupplemented tryptone-glucose extract agar. Supplementation of violet red bile agar increased recovery, but the counts remained considerably lower than the tryptone-glucose extract agar counts. The repair detection procedure of Speck et al. (M. Speck, B. Ray, R. Read, Jr., Appl. Microbiol. 29:549-550, 1975) was greatly improved by the addition of pyruvate or 3,3'-thiodipropionic acid. However, when this improved repair detection procedure was applied to foods, pyruvate-supplemented media showed some false-positives. We therefore recommend that 3,3'-thiodipropionic acid be used to supplement media in the repair detection procedure.

International Nosocomial Infection Control Consortium (INICC) report, data summary of 45 countries for 2012-2017: Device-associated module

BACKGROUND

We report the results of International Nosocomial Infection Control Consortium (INICC) surveillance study from January 2012 to December 2017 in 523 intensive care units (ICUs) in 45 countries from Latin America, Europe, Eastern Mediterranean, Southeast Asia, and Western Pacific.

METHODS

During the 6-year study period, prospective data from 532,483 ICU patients hospitalized in 242 hospitals, for an aggregate of 2,197,304 patient days, were collected through the INICC Surveillance Online System (ISOS). The Centers for Disease Control and Prevention-National Healthcare Safety Network (CDC-NHSN) definitions for device-associated health care-associated infection (DA-HAI) were applied.

RESULTS

Although device use in INICC ICUs was similar to that reported from CDC-NHSN ICUs, DA-HAI rates were higher in the INICC ICUs: in the medical-surgical ICUs, the pooled central line-associated bloodstream infection rate was higher (5.05 vs 0.8 per 1,000 central line-days); the ventilator-associated pneumonia rate was also higher (14.1 vs 0.9 per 1,000 ventilator-days,), as well as the rate of catheter-associated urinary tract infection (5.1 vs 1.7 per 1,000 catheter-days). From blood cultures samples, frequencies of resistance, such as of Pseudomonas aeruginosa to piperacillin-tazobactam (33.0% vs 18.3%), were also higher.

CONCLUSIONS

Despite a significant trend toward the reduction in INICC ICUs, DA-HAI rates are still much higher compared with CDC-NHSN's ICUs representing the developed world. It is INICC's main goal to provide basic and cost-effective resources, through the INICC Surveillance Online System to tackle the burden of DA-HAIs effectively.

The contribution of subtle oocyte or sperm dysfunction affecting fertilization in endometriosis-associated or unexplained infertility: a controlled comparison with tubal infertility and use of donor spermatozoa

This study aims to determine the relative contribution of oocyte and/or sperm dysfunction to the reduction of fertilization rates in vitro in cases of minor endometriosis and prolonged unexplained infertility. The results of in-vitro fertilization (IVF) treatment with ovarian stimulation have been compared between couples with the above conditions and women with tubal infertility (as control for oocyte function) and the use of donor spermatozoa (as control for sperm function). Fertilization and cleavage rates using husband's spermatozoa were significantly reduced in endometriosis couples (56%, n = 194, P < 0.001) and further significantly reduced in couples with unexplained infertility (52%, n = 327, P < 0.001) compared with tubal infertility (60%, n = 509). Using donor spermatozoa the rates were the same as using husband's spermatozoa in tubal infertility (61%, n = 27) or endometriosis (55%, n = 21) but significantly though only partly improved with unexplained infertility (57%, n = 60, P < 0.02). In unexplained infertility, a significantly increased proportion of couples experienced complete failure of fertilization and cleavage in a cycle (5-6% versus 2-3%). However, complete failure was not usually repetitive, and the affected couples did not account for the overall reduction in fertilization and cleavage rates, which remained significantly lower in the rest of the unexplained and endometriosis groups. Implantation and pregnancy rates appeared similar in all groups. The benefit of IVF treatment in cases of minor endometriosis and prolonged unexplained infertility is due to superabundance of oocytes obtained by stimulation. The reduction in natural fertility associated with endometriosis appears to be at least partly due to a reduced fertilizing ability of the oocyte. In unexplained infertility, there is distinct impairment due to otherwise unsuspected sperm dysfunction but probably also oocyte dysfunction.

Isolation and characterization of pediocin AcH chimeric protein mutants with altered bactericidal activity

A collection of pediocin AcH amino acid substitution mutants was generated by PCR random mutagenesis of DNA encoding the bacteriocin. Mutants were isolated by cloning mutagenized DNA into an Escherichia coli malE plasmid that directs the secretion of maltose binding protein-pediocin AcH chimeric proteins and by screening transformant colonies for bactericidal activity against Lactobacillus plantarum NCDO955 (K. W. Miller, R. Schamber, Y. Chen, and B. Ray, 1998. Appl. Environ. Microbiol. 64:14-20, 1998). In all, 17 substitution mutants were isolated at 14 of the 44 amino acids of pediocin AcH. Seven mutants (N5K, C9R, C14S, C14Y, G37E, G37R, and C44W) were completely inactive against the pediocin AcH-sensitive strains L. plantarum NCDO955, Listeria innocua Lin11, Enterococcus faecalis M1, Pediococcus acidilactici LB42, and Leuconostoc mesenteroides Ly. A C24S substitution mutant constructed by other means also was inactive against these bacteria. Nine other mutants (K1N, W18R, I26T, M31T, A34D, N41K, H42L, K43N, and K43E) retained from <1% to approximately 60% of wild-type activity when assayed against L. innocua Lin11. One mutant, K11E, displayed approximately 2. 8-fold-higher activity against this indicator. About one half of the mutations mapped to amino acids that are conserved in the pediocin-like family of bacteriocins. All four cysteines were found to be required for activity, although only C9 and C14 are conserved among pediocin-like bacteriocins. Several basic amino acids as well as nonpolar amino acids located within the hydrophobic C-terminal region also were found to be important. The mutations are discussed in the context of structural models that have been proposed for the bacteriocin.