Resistance of Escherichia coli and Salmonella against nisin and curvacin A

Bacterial protoplast-derived nanovesicles carrying CRISPR-Cas9 tools re-educate tumor-associated macrophages for enhanced cancer immunotherapy

The CRISPR-Cas9 system offers substantial potential for cancer therapy by enabling precise manipulation of key genes involved in tumorigenesis and immune response. Despite its promise, the system faces critical challenges, including the preservation of cell viability post-editing and ensuring safe in vivo delivery. To address these issues, this study develops an in vivo CRISPR-Cas9 system targeting tumor-associated macrophages (TAMs). We employ bacterial protoplast-derived nanovesicles (NVs) modified with pH-responsive PEG-conjugated phospholipid derivatives and galactosamine-conjugated phospholipid derivatives tailored for TAM targeting. Utilizing plasmid-transformed E. coli protoplasts as production platforms, we successfully load NVs with two key components: a Cas9-sgRNA ribonucleoprotein targeting Pik3cg, a pivotal molecular switch of macrophage polarization, and bacterial CpG-rich DNA fragments, acting as potent TLR9 ligands. This NV-based, self-assembly approach shows promise for scalable clinical production. Our strategy remodels the tumor microenvironment by stabilizing an M1-like phenotype in TAMs, thus inhibiting tumor growth in female mice. This in vivo CRISPR-Cas9 technology opens avenues for cancer immunotherapy, overcoming challenges related to cell viability and safe, precise in vivo delivery.

Application of Liposome Encapsulating Lactobacillus curvatus Extract in Cosmetic Emulsion Lotion

Probiotic extracts have various positive attributes, such as antioxidant, tyrosinase inhibitory, and antimicrobial activity. Lactobacillus curvatus produces bacteriocin, which activates the lipid membrane structure and has potential as a natural preservative for cosmetic emulsions. In this study, L. curvatus extract was encapsulated in liposomes and formulated as an oil-in-water (O/W) emulsion. Radical scavenging activity, tyrosinase inhibition, and challenge tests were conducted to confirm the liposome activity and the activity of the applied lotion emulsion. The liposome-encapsulated extract had a relatively high absolute ζ-potential (52.53 > 35.43), indicating its stability, and 96% permeability, which indicates its potential as an active agent in lotion emulsions. Characterization of emulsions containing the liposomes also indicated a stable state. The liposome-encapsulated extract exhibited a higher radical scavenging activity than samples without the extract and non-encapsulated samples, and the functionality was preserved in the lotion emulsion. The tyrosinase inhibition activity of the lotion emulsion with the liposome-encapsulated extract was similar to that of the non-treated extract. Candida albicans and Aspergillus niger were also inhibited in the challenge test with the lotion emulsions during storage. Collectively, these findings indicate that the liposome-encapsulated extract and the lotion containing the encapsulated extract have potential applicability as natural preservatives.

Antimicrobial plant secondary metabolites, MDR transporters and antimicrobial resistance in cereal-associated lactobacilli: is there a connection?

Cereal-associated lactobacilli resist antimicrobial plant secondary metabolites. This study aimed to identify multi-drug-resistance (MDR) transporters in isolates from mahewu, a Zimbabwean fermented cereal beverage, and to determine whether these MDR-transporters relate to resistance against phenolic compounds and antibiotics. Comparative genomic analyses indicated that all seven mahewu isolates harbored multiple MATE and MFS MDR proteins. Strains of Lactiplantibacillus plantarum and Limosilactobacillus fermentum encoded for the same gene, termed mahewu phenolics resistance gene mprA, with more than 99% nucleotide identity, suggesting horizontal gene transfer. Strains of Lp. plantarum were more resistant than strains of Lm. fermentum to phenolic acids, other antimicrobials and antibiotics but the origins of strains were not related to resistance. The resistance of several strains exceeded EFSA thresholds for several antibiotics. Analysis of gene expression in one strain each of Lp. plantarum and Lm. fermentum revealed that at least one MDR gene in each strain was over-expressed during growth in wheat, sorghum and millet relative to growth in MRS5 broth. In addition, both strains over-expressed a phenolic acid reductase. The results suggest that diverse lactobacilli in mahewu share MDR transporters acquired by lateral gene transfer, and that these transporters mediate resistance to secondary plant metabolites and antibiotics.

The Inhibitory Concentration of Natural Food Preservatives May Be Biased by the Determination Methods

The demand for natural antimicrobials as food preservatives has increased due to the growing interest of the population for a healthy lifestyle. The application of screening methods to identify the antimicrobial activity of natural compounds is of great importance. The in vitro determination of antimicrobial activity requires determining their minimum inhibitory concentrations to assess microbial susceptibility. This study aimed to evaluate the minimum inhibitory concentrations of three natural antimicrobial compounds—chitosan, ethanolic propolis extract, and nisin—against 37 microorganisms (different pathogens and spoilage microorganisms) by the methods of agar dilution and drop diffusion on agar. Culture media at different pH values were used for both methods to simulate different food products. Most of the microorganisms were inhibited by chitosan (0.5% w/v) and propolis (10 mg/mL), and most of the Gram-positive bacteria by nisin (25 μg/mL). Different pH values and the in vitro method used influenced the inhibition of each compound. Generally, lower minimum inhibitory concentrations were observed at lower pH values and for the agar dilution method. Furthermore, some microorganisms inhibited by the compounds on the agar dilution method were not inhibited by the same compounds and at the same concentrations on the drop diffusion technique. This study reinforces the need for using defined standard methods for the in vitro determination of minimum inhibitory concentrations. Natural compounds with potential antimicrobial action are a bet on food preservation. The use of standard techniques such as those used for antimicrobials of clinical applications are crucial to compare results obtained in different studies and different matrices.

The Locus of Heat Resistance Confers Resistance to Chlorine and Other Oxidizing Chemicals in Escherichia coli

Some chlorine-resistant Escherichia coli isolates harbor the locus of heat resistance (LHR), a genomic island conferring heat resistance. In this study, the protective effect of the LHR for cells challenged by chlorine and oxidative stress was quantified. Cloning of the LHR protected against NaClO (32 mM; 5 min), H₂O₂ (120 mM; 5 min), and peroxyacetic acid (105 mg/liter; 5 min) but not against 5.8 mM KIO₄, 10 mM acrolein, or 75 mg/liter allyl isothiocyanate. The lethality of oxidizing treatments for LHR-negative strains of E. coli was about 2 log₁₀ CFU/ml higher than that for LHR-positive strains of E. coli The oxidation of cytoplasmic proteins and membrane lipids was quantified with the fusion probe roGFP2-Orp1 and the fluorescent probe BODIPY^581/591, respectively. The fragment of the LHR coding for heat shock proteins protected cytoplasmic proteins but not membrane lipids against oxidation. The middle fragment of the LHR protected against the oxidation of membrane lipids but not of cytoplasmic proteins. The addition of H₂O₂, NaClO, and peroxyacetic acid also induced green fluorescent protein (GFP) expression in the oxidation-sensitive reporter strain E. coli O104:H4 Δstx ₂::gfp::amp Cloning of pLHR reduced phage induction in E. coli O104:H4 Δstx ₂::gfp::amp after treatment with oxidizing chemicals. Screening of 160 strains of Shiga toxin-producing E. coli (STEC) revealed that none of them harbors the LHR, additionally suggesting that the LHR and Stx prophages are mutually exclusive. Taking our findings together, the contribution of the LHR to resistance to chlorine and oxidative stress is based on the protection of multiple cellular targets by different proteins encoded by the genetic island.IMPORTANCE Chlorine treatments are used in water and wastewater sanitation; the resistance of Escherichia coli to chlorine is thus of concern to public health. We show that a genetic island termed the locus of heat resistance (LHR) protects E. coli not only against heat but also against chlorine and other oxidizing chemicals, adding to our knowledge of the tools used by E. coli to resist stress. Specific detection of the oxidation of different cellular targets in combination with the cloning of fragments of the LHR provided insight into mechanisms of protection and demonstrated that different fragments of the LHR protect different cellular targets. In E. coli, the presence of the LHR virtually always excluded other virulence factors. It is tempting to speculate that the LHR is maintained by strains of E. coli with an environmental lifestyle but is excluded by pathogenic strains that adapted to interact with vertebrate hosts.

Effect of drying on oxidation of membrane lipids and expression of genes encoded by the Shiga toxin prophage in Escherichia coli

Drying processes do not eliminate pathogenic Escherichia coli in foods but induce sublethal injury, which may also induce the Shiga toxin (Stx) prophage. This study investigated the effect of drying on membrane lipid oxidation and stx expression in E. coli. Lipid peroxidation was probed with C₁₁-BODIPY^581/591; and stx expression was assayed by quantification of GFP in E. coli O104:H4 Δstx2a:gfp:amp^r. Treatment of E. coli with H₂O₂ oxidized the probe; probe oxidation was also observed after drying and rehydration. Lipid oxidation and the lethality of drying were reduced when cells were dried with trehalose under anaerobic condition; in addition, viability and probe oxidation differed between E. coli AW1.7 and E. coli AW1.7Δcfa. Desiccation tolerance thus relates to membrane lipid oxidation. Drying also resulted in expression of GFP in 5% of the population. Overexpression of gfp and recA after drying and rehydration suggested that the expression of Stx prophage was regulated by the SOS response. Overall, C₁₁-BODIPY^581/591 allowed investigation of lipid peroxidation in bacteria. Drying causes lipid oxidation, DNA damage and induction of genes encoded by the Stx prophage in E. coli.

Gut Microbiota and Colonization Resistance against Bacterial Enteric Infection

The gut microbiome is critical in providing resistance against colonization by exogenous microorganisms. The mechanisms via which the gut microbiota provide colonization resistance (CR) have not been fully elucidated, but they include secretion of antimicrobial products, nutrient competition, support of gut barrier integrity, and bacteriophage deployment. However, bacterial enteric infections are an important cause of disease globally, indicating that microbiota-mediated CR can be disturbed and become ineffective. Changes in microbiota composition, and potential subsequent disruption of CR, can be caused by various drugs, such as antibiotics, proton pump inhibitors, antidiabetics, and antipsychotics, thereby providing opportunities for exogenous pathogens to colonize the gut and ultimately cause infection. In addition, the most prevalent bacterial enteropathogens, including Clostridioides difficile, Salmonella enterica serovar Typhimurium, enterohemorrhagic Escherichia coli, Shigella flexneri, Campylobacter jejuni, Vibrio cholerae, Yersinia enterocolitica, and Listeria monocytogenes, can employ a wide array of mechanisms to overcome colonization resistance. This review aims to summarize current knowledge on how the gut microbiota can mediate colonization resistance against bacterial enteric infection and on how bacterial enteropathogens can overcome this resistance.

Molecular recognition of lipopolysaccharide by the lantibiotic nisin

Nisin is a lanthionine antimicrobial effective against diverse Gram-positive bacteria and is used as a food preservative worldwide. Its action is mediated by pyrophosphate recognition of the bacterial cell wall receptors lipid II and undecaprenyl pyrophosphate. Nisin/receptor complexes disrupt cytoplasmic membranes, inhibit cell wall synthesis and dysregulate bacterial cell division. Gram-negative bacteria are much more tolerant to antimicrobials including nisin. In contrast to Gram-positives, Gram-negative bacteria possess an outer membrane, the major constituent of which is lipopolysaccharide (LPS). This contains surface exposed phosphate and pyrophosphate groups and hence can be targeted by nisin. Here we describe the impact of LPS on membrane stability in response to nisin and the molecular interactions occurring between nisin and membrane-embedded LPS from different Gram-negative bacteria. Dye release from liposomes shows enhanced susceptibility to nisin in the presence of LPS, particularly rough LPS chemotypes that lack an O-antigen whereas LPS from microorganisms sharing similar ecological niches with antimicrobial producers provides only modest enhancement. Increased susceptibility was observed with LPS from pathogenic Klebsiella pneumoniae compared to LPS from enteropathogenic Salmonella enterica and gut commensal Escherichia coli. LPS from Brucella melitensis, an intra-cellular pathogen which is adapted to invade professional and non-professional phagocytes, appears to be refractory to nisin. Molecular complex formation between nisin and LPS was studied by solid state MAS NMR and revealed complex formation between nisin and LPS from most organisms investigated except B. melitensis. LPS/nisin complex formation was confirmed in outer membrane extracts from E. coli.

Exploiting synergies of sourdough and antifungal organic acids to delay fungal spoilage of bread

Fungal spoilage of bread remains an unsolved issue in bread making. This work aims to identify alternative strategies to conventional preservatives in order to prevent or delay fungal spoilage of bread. The minimum inhibitory concentration (MIC) of bacterial metabolites and chemical preservatives was evaluated in vitro, and compared to their in situ activity in baking trials. Calcium propionate, sorbic acid, 3-phenyllactic acid, ricinoleic acid, and acetic acid were tested both individually and in combination at their MIC values against Aspergillus niger and Penicillium roqueforti. The combination of acetic acid with propionate and sorbate displayed additive effects against the two fungi. For these reasons, we introduced sourdough fermentation with specific strains of lactobacilli, using wheat or flaxseed, in order to generate acetate in bread. A combination of Lactobacillus hammesii and propionate reduced propionate concentration required for shelf life extension of wheat bread 7-fold. Flaxseed sourdough bread fermented with L. hammesii, excluding any preservative, showed a shelf life 2 days longer than the control bread. The organic acid quantification indicated a higher production of acetic acid (33.8 ± 4.4 mM) when compared to other sourdough breads. Addition of 4% of sucrose to sourdough fermentation with L. brevis increased the mould free shelf-life of bread challenged with A. niger by 6 days. The combination of L. hammesii sourdough and the addition of ricinoleic acid (0.15% or 0.08%) prolonged the mould free shelf-life by 7-8 days for breads produced with wheat sourdoughs. In conclusion, the in vitro MIC of bacterial metabolites and preservatives matched the in situ antifungal effect. Of the different bacterial metabolites evaluated, acetic acid had the most prominent and consistent antifungal activity. The use of sourdough fermentation with selected strains able to produce acetic acid allowed reducing the use of chemical preservatives.

Mechanisms of pressure-mediated cell death and injury in Escherichia coli: from fundamentals to food applications

High hydrostatic pressure is commercially applied to extend the shelf life of foods, and to improve food safety. Current applications operate at ambient temperature and 600 MPa or less. However, bacteria that may resist this pressure level include the pathogens Staphylococcus aureus and strains of Escherichia coli, including shiga-toxin producing E. coli. The resistance of E. coli to pressure is variable between strains and highly dependent on the food matrix. The targeted design of processes for the safe elimination of E. coli thus necessitates deeper insights into mechanisms of interaction and matrix-strain interactions. Cellular targets of high pressure treatment in E. coli include the barrier properties of the outer membrane, the integrity of the cytoplasmic membrane as well as the activity of membrane-bound enzymes, and the integrity of ribosomes. The pressure-induced denaturation of membrane bound enzymes results in generation of reactive oxygen species and subsequent cell death caused by oxidative stress. Remarkably, pressure resistance at the single cell level relates to the disposition of misfolded proteins in inclusion bodies. While the pressure resistance E. coli can be manipulated by over-expression or deletion of (stress) proteins, the mechanisms of pressure resistance in wild type strains is multi-factorial and not fully understood. This review aims to provide an overview on mechanisms of pressure-mediated cell death in E. coli, and the use of this information for optimization of high pressure processing of foods.

Potential of bacteriocin-producing lactic acid bacteria for safety improvements of traditional Thai fermented meat and human health

Lactic acid bacteria (LAB) are very important in converting of agricultural products into safe, delicious and shelf stable foods for human consumption. The preservative activity of LAB in foods is mainly attributed to the production of anti-microbial metabolites such as organic acids and bacteriocins which enables them to grow and control the growth of pathogens and spoilage microorganisms. Besides ensuring safety, bacteriocin-producing LAB with their probiotic potentials could also be emerging as a means to develop functional meat products with desirable health benefits. Nevertheless, to be qualified as a candidate probiotic culture, other prerequisite probiotic properties of bacteriocin-producing LAB have to be assessed according to regulatory guidelines for probiotics. Nham is an indigenous fermented sausage of Thailand that has gained popularity and acceptance among Thais. Since Nham is made from raw meat and is usually consumed without cooking, risks due to undesirable microorganisms such as Salmonella spp., Staphylococcus aureus, and Listeria monocytogenes, are frequently observed. With an ultimate goal to produce safer and healthier product, our research attempts on the development of a variety of new Nham products are discussed.

Nitropropenyl benzodioxole, an anti-infective agent with action as a protein tyrosine phosphatase inhibitor

We report on the activities of a broad spectrum antimicrobial compound,nitropropenyl benzodioxole (NPBD) which are of relevance to its potential as an anti-infective drug. These investigations support the proposal that a major mechanism of NPBD is action as a tyrosine mimetic, competitively inhibiting bacterial and fungal protein tyrosine phosphatases (PTP). NPBD did not affect major anti-bacterial drug targets, namely, ATP production, cell wall or cell membrane integrity, or transcription and translation of RNA. NPBD inhibited bacterial YopH and human PTP1B and not human CD45 in enzyme assays. NPBD inhibited PTP-associated bacterial virulence factors, namely, endospore formation in Bacillus cereus, prodigiosin secretion in Serratia marcescens , motility in Proteus spp., and adherence and invasion of mammalian cells by Yersinia enterocolitica . NPBD acts intracellularly to inhibit the early development stages of the Chlamydia trachomatis infection cycle in mammalian cells known to involve sequestration of host cell PTPs. NPBD thus both kills pathogens and inhibits virulence factors relevant to early infection, making it a suitable candidate for development as an anti-infective agent, particularly for pathogens that enter through, or cause infections at, mucosal surfaces. Though much is yet to be understood about bacterial PTPs, they are proposed as suitable anti-infective targets and have been linked to agents similar to NPBD. The structural and functional diversity and heterogeneous distribution of PTPs across microbial species make them suitably selective targets for the development of both broadly active and pathogen-specific drugs.

Inhibitory spectra and modes of antimicrobial action of gallotannins from mango kernels (Mangifera indica L.)

This study investigated the antimicrobial activities and modes of action of penta-, hexa-, hepta-, octa-, nona-, and deca-O-galloylglucose (gallotannins) isolated from mango kernels. The MICs and minimum bactericidal concentrations (MBCs) against food-borne bacteria and fungi were determined using a critical dilution assay. Gram-positive bacteria were generally more susceptible to gallotannins than were Gram-negative bacteria. The MICs of gallotannins against Bacillus subtilis, Bacillus cereus, Clostridium botulinum, Campylobacter jejuni, Listeria monocytogenes, and Staphylococcus aureus were 0.2 g liter(-1) or less; enterotoxigenic Escherichia coli and Salmonella enterica were inhibited by 0.5 to 1 g liter(-1), and lactic acid bacteria were resistant. The use of lipopolysaccharide mutants of S. enterica indicated that the outer membrane confers resistance toward gallotannins. Supplementation of LB medium with iron eliminated the inhibitory activity of gallotannins against Staphylococcus aureus, and siderophore-deficient mutants of S. enterica were less resistant toward gallotannins than was the wild-type strain. Hepta-O-galloylglucose sensitized Lactobacillus plantarum TMW1.460 to hop extract, indicating inactivation of hop resistance mechanisms, e.g., the multidrug resistance (MDR) transporter HorA. Carbohydrate metabolism of Lactococcus lactis MG1363, a conditionally respiring organism, was influenced by hepta-O-galloylglucose when grown under aerobic conditions and in the presence of heme but not under anaerobic conditions, indicating that gallotannins influence the respiratory chain. In conclusion, the inhibitory activities of gallotannins are attributable to their strong affinity for iron and likely additionally relate to the inactivation of membrane-bound proteins.

Application of natural antimicrobials for food preservation

In this review, antimicrobials from a range of plant, animal, and microbial sources are reviewed along with their potential applications in food systems. Chemical and biochemical antimicrobial compounds derived from these natural sources and their activity against a range of pathogenic and spoilage microorganisms pertinent to food, together with their effects on food organoleptic properties, are outlined. Factors influencing the antimicrobial activity of such agents are discussed including extraction methods, molecular weight, and agent origin. These issues are considered in conjunction with the latest developments in the quantification of the minimum inhibitory (and noninhibitory) concentration of antimicrobials and/or their components. Natural antimicrobials can be used alone or in combination with other novel preservation technologies to facilitate the replacement of traditional approaches. Research priorities and future trends focusing on the impact of product formulation, intrinsic product parameters, and extrinsic storage parameters on the design of efficient food preservation systems are also presented.

Multilayer polyelectrolyte films functionalized by insertion of defensin: a new approach to protection of implants from bacterial colonization

Infection of implanted materials by bacteria constitutes one of the most serious complications following prosthetic surgery. In the present study, we developed a new strategy based on the insertion of an antimicrobial peptide (defensin from Anopheles gambiae mosquitoes) into polyelectrolyte multilayer films built by the alternate deposition of polyanions and polycations. Quartz crystal microbalance and streaming potential measurements were used to follow step by step the construction of the multilayer films and embedding of the defensin within the films. Antimicrobial assays were performed with two strains: Micrococcus luteus (a gram-positive bacterium) and Escherichia coli D22 (a gram-negative bacterium). The inhibition of E. coli D22 growth at the surface of defensin-functionalized films was found to be 98% when 10 antimicrobial peptide layers were inserted in the film architecture. Noticeably, the biofunctionalization could be achieved only when positively charged poly(l-lysine) was the outermost layer of the film. On the basis of the results of bacterial adhesion experiments observed by confocal or electron microscopy, these observations could result from the close interaction of the bacteria with the positively charged ends of the films, which allows defensin to interact with the bacterial membrane structure. These results open new possibilities for the use of such easily built and functionalized architectures onto any type of implantable biomaterial. The modified surfaces are active against microbial infection and represent a novel means of local host protection.

Effects of different spices used in production of fermented sausages on growth of and curvacin A production by Lactobacillus curvatus LTH 1174

Lactobacillus curvatus LTH 1174, a fermented sausage isolate, produces the listericidal bacteriocin curvacin A. The effect of different spices relevant for the production of fermented sausages was investigated in vitro through laboratory fermentations with a meat simulation medium and an imposed pH profile relevant for Belgian-type fermented sausages. The influence on the growth characteristics and especially on the kinetics of curvacin A production with L. curvatus LTH 1174 was evaluated. Pepper, nutmeg, rosemary, mace, and garlic all decreased the maximum specific growth rate, while paprika was the only spice that increased it. The effect on the lag phase was minor except for nutmeg and especially for garlic, which increased it, yet garlic was stimulatory for biomass production. The maximum attainable biomass concentration (X(max)) was severely decreased by the addition of 0.40% (wt/vol) nutmeg, while 0.35% (wt/vol) garlic or 0.80% (wt/vol) white pepper increased X(max). Nutmeg decreased both growth and bacteriocin production considerably. Garlic was the only spice enhancing specific bacteriocin production, resulting in higher bacteriocin activity in the cell-free culture supernatant. Finally, lactic acid production was stimulated by the addition of pepper, and this was not due to the manganese present because an amount of manganese that was not growth limiting was added to the growth medium. Addition of spices to the sausage mixture is clearly a factor that will influence the effectiveness of bacteriocinogenic starter cultures in fermented-sausage manufacturing.

Resistance of spheroplasts and whole cells of Pseudomonas aeruginosa to bactericidal activity of various biocides: evidence of the membrane implication

To emphasise the role of outer and inner membranes in the resistance of Pseudomonas aeruginosa to bactericidal activity of various disinfectants, spheroplasts and whole cells were compared. Spheroplasts are more sensitive than whole cells to quaternary ammonium compounds such as didecyl dimethyl ammonium bromide (DDAB) and C16-benzalkonium chloride. The outer membrane acts as a barrier to prevent these disinfectants from entering the cell. It seems to have no influence on activities of smaller molecules such as C12, C14-benzalkonium chlorides and sodium dichloroisocyanurate. For tri-sodium phosphate, the presence of outer membrane emphasized the action of the molecule. Moreover, resistance of DDAB-adapted spheroplasts to bactericidal activity of DDAB is higher than the resistance of non-adapted spheroplasts. This suggests that the inner membrane could also play a role in resistance to DDAB.

Sodium chloride reduces production of curvacin A, a bacteriocin produced by Lactobacillus curvatus strain LTH 1174, originating from fermented sausage

Lactobacillus curvatus LTH 1174, a strain originating in fermented sausage, produces the antilisterial bacteriocin curvacin A. Its biokinetics of cell growth and bacteriocin production as a function of various concentrations of salt (sodium chloride) were investigated in vitro during laboratory fermentations using modified MRS medium. A model was set up to describe the effects of different NaCl concentrations on microbial behavior. Both cell growth and bacteriocin activity were affected by changes in the salt concentration. Sodium chloride clearly slowed down the growth of L. curvatus LTH 1174, but more importantly, it had a detrimental effect on specific curvacin A production (k(B)) and hence on overall bacteriocin activity. Even a low salt concentration (2%, wt/vol) decreased bacteriocin production, while growth was unaffected at this concentration. The inhibitory effect of NaCl was mainly due to its role as an a(w)-lowering agent. Further, it was clear that salt interfered with bacteriocin induction. Additionally, when 6% (wt/vol) sodium chloride was added, the minimum biomass concentration necessary to start the production of curvacin A (X(B)) was 0.90 g (cell dry mass) per liter. Addition of the cell-free culture supernatant or a protein solution as a source of induction factor resulted in a decrease in X(B), an increase in k(B), and hence an increase in the maximum attainable bacteriocin activity.

In vitro assessment of the cytotoxicity of nisin, pediocin, and selected colicins on simian virus 40-transfected human colon and Vero monkey kidney cells with trypan blue staining viability assays

Gram-positive bacterial bacteriocins (nisin and pediocin) and gram-negative bacterial bacteriocins (colicins [Col] E1, E3, E6, E7, and K) were evaluated for cytotoxicity against cultured simian virus 40-transfected human colon (SV40-HC) and Vero monkey kidney (Vero) cells. Bacteriocin-treated cells were assessed for viability by trypan blue staining. Monolayers of SV40-HC and Vero cells were cultured in tissue culture plates (35 degrees C, 10% CO2 in humidified air) with the use of Dulbecco's modified Eagle's medium supplemented with 10% (vol/vol) calf serum. Actively growing cells in the log phase (ca. 10(4) cells per ml) were treated with individual partially purified bacteriocin preparations at 170, 350, and 700 activity units per ml. Duplicate culture plates for each bacteriocin treatment and untreated controls were withdrawn after 16, 32, and 48 h of incubation. Cells were dissociated with trypsin and treated with trypan blue and were then counted in a hemocytometer with the use of a phase-contrast microscope. Viability assays indicated dose-dependent toxicity for some bacteriocins. Nisin, pediocin, and Col E6 were the most cytotoxic bacteriocins; SV40-HC cells demonstrated greater sensitivity than Vero cells did. Some bacteriocins can be toxic to mammalian cells; therefore, bacteriocins intended for use as biopreservatives must be evaluated for toxicity to mammalian cells and for other toxicities. Col E1, Col E3, Col E7, and Col K demonstrated little toxicity at the activities tested, indicating that they are safe and thus have potential for use as food biopreservatives.

Application of nontraditional meat starter cultures in production of Hungarian salami

Listeria monocytogenes and Escherichia coli O111 have been implicated in several outbreaks of food-borne disease linked to smallgoods products. Traditional meat starter cultures, containing a mixture of lactic acid bacteria (LAB) and staphylococci, are used to maintain safety and sensory properties of Hungarian salami. The present study investigated if nontraditional meat starter (NTMS) cultures can be used for improving the safety of Hungarian salami. Salami batter was inoculated with List. monocytogenes and E. coli and subsequently fermented with NTMS cultures and a commercially available meat starter. A total of 15 NTMS cultures were tested. The salami was monitored for levels of pathogen, LAB and pH. When used in conjunction with the commercial meat starter, 9 NTMS cultures reduced the E. coli O111 count by more than 2.5 log units, whereas 10 of the NTMS cultures reduced List. monocytogenes by more than 2.5 log units. The commercial meat starter alone reduced E. coli and List. monocytogenes by 1.2 and 1.3 log units, respectively. Some NTMS cultures reduced the pathogen count without affecting pH of the salami batter. All NTMS cultures survived in salami throughout fermentation and maturation. It was concluded that NTMS cultures, including Lactobacillus acidophilus LAFTI L10, L. paracasei LAFTI L26, L. paracasei 5119, Lactobacillus sp. L24 and Bifidobacterium lactis LAFTI B94, may be used to increase the safety of Hungarian salami because these cultures gave strong inhibition of both E. coli O111 and List. monocytogenes.

On-line fluorescence determination of pressure mediated outer membrane damage in Escherichia coli

The outer membrane (OM) of Gram-negative bacteria provides a protective barrier for natural occurring inhibitors. Pressure mediated OM permeabilisation therefore contributes to the elimination of Escherichia coli and Salmonella by pressure preservation processes. Pressure mediated inactivation, sublethal injury, and membrane permeabilisation of E. coli were determined using two strains differing in their barotolerance. Pressure treatment of E. coli TMW 2.427 at 300, 500 and 600 MPa for 40 min resulted in a 0, 1, and greater 6 log decrease of viable cell counts, respectively. The kinetics of OM and cytoplasmic membrane permeabilisation after pressure treatment were determined by staining of pressure treated cells with the fluorescent dyes propidium iodide (PI) and 1-N-phenylnaphtylamine (NPN), respectively. A slight increase of PI fluorescence was observed at conditions resulting in a greater 6 log decrease of viable cell counts only. In contrast, increased NPN fluorescence indicating OM permeabilisation was observed prior to cell death and sublethal injury. An on-line assay for determination of pressure mediated OM damage based on NPN fluorescence was established to distinguish between reversible and irreversible OM damage. Generally, the same degree of outer membrane damage was observed by either on line or off line determinations. However, whereas reversible membrane damage occurred fast and in thermodynamic equilibrium with pressure conditions, irreversible outer membrane damage was a time dependent process.

Combined effect of water activity and pH on the inhibition of Escherichia coli by nisin

The Doehlert design and surface response methodology were used to study the influence of pH and water activity (aw) on Escherichia coli inhibition by nisin. Combining stress factors at levels where they are not inhibitory by themselves, a reduction of E. coli survival fraction can be achieved with lower nisin doses than in a single nisin treatment. For all the pH values assayed, a synergistic effect of aw and nisin concentration was detected, and the isoresponse lines showed the existence of an area of maximum inhibition. Factors that reduced viable cell counts by 4 to 5 log cycles were 1,000 to 1,400 IU of nisin per ml at pH 5.5 to 6.5 and a water activity of 0.97 and 0.98. The addition of different ionic and nonionic solutes to control aw suggested that the effect of aw in the inhibitory action of nisin on E. coli cells was not solute-specific. The use of the Doehlert experimental design was effective to determine the optimal combination of stress factors, as well as to point out the most important variables that affected E. coli inhibition.

Characterization of reutericyclin produced by Lactobacillus reuteri LTH2584

Lactobacillus reuteri LTH2584 exhibits antimicrobial activity that can be attributed neither to bacteriocins nor to the production of reuterin or organic acids. We have purified the active compound, named reutericyclin, to homogeneity and characterized its antimicrobial activity. Reutericyclin exhibited a broad inhibitory spectrum including Lactobacillus spp., Bacillus subtilis, B. cereus, Enterococcus faecalis, Staphylococcus aureus, and Listeria innocua. It did not affect the growth of gram-negative bacteria; however, the growth of lipopolysaccharide mutant strains of Escherichia coli was inhibited. Reutericyclin exhibited a bactericidal mode of action against Lactobacillus sanfranciscensis, Staphylococcus aureus, and B. subtilis and triggered the lysis of cells of L. sanfranciscensis in a dose-dependent manner. Germination of spores of B. subtilis was inhibited, but the spores remained unaffected under conditions that do not permit germination. The fatty acid supply of the growth media had a strong effect on reutericyclin production and its distribution between producer cells and the culture supernatant. Reutericyclin was purified from cell extracts and culture supernatant of L. reuteri LTH2584 cultures grown in mMRS by solvent extraction, gel filtration, RP-C(8) chromatography, and anion-exchange chromatography, followed by rechromatography by reversed-phase high-pressure liquid chromatography. Reutericyclin was characterized as a negatively charged, highly hydrophobic molecule with a molecular mass of 349 Da. Structural characterization (A. Höltzel, M. G. Gänzle, G. J. Nicholson, W. P. Hammes, and G. Jung, Angew. Chem. Int. Ed. 39:2766-2768, 2000) revealed that reutericyclin is a novel tetramic acid derivative. The inhibitory activity of culture supernatant of L. reuteri LTH2584 corresponded to that of purified as well as synthetic reutericyclin.

Permeability barrier of the gram-negative bacterial outer membrane with special reference to nisin

The effect of nisin pretreatment on organic acid-induced permeability increase in strains of Escherichia coli, Pseudomonas aeruginosa, P. marginalis, and Salmonella enterica sv. Typhimurium was investigated, using assays based on the uptake of a fluorescent dye 1-N-phenylnaphthylamine (NPN) and on the bacterial susceptibility to detergent-induced bacteriolysis. The outer membrane of bacteria which had been pretreated with nisin was shown to be less stable against 1 mM EDTA, as indicated by their significantly higher NPN uptake levels as compared to untreated bacteria. Upon challenge with a tenfold lower concentration of EDTA (0.1 mM) some nisin-treated strains (Typhimurium, P. marginalis) exhibited, however, NPN uptake levels which were lower than those seen in control bacteria, suggesting that nisin had stabilized their outer membrane. Nisin pretreatment also decreased the NPN uptake induced by citric or lactic acid or both in E. coli, P. marginalis, and Typhimurium, whereas in P. aeruginosa the pretreatment resulted in increased NPN uptake in response to citric and lactic acid. These results suggest that, with the exception of P. aeruginosa, nisin could protect bacteria from the outer membrane-disrupting effect caused by the acids. P. aeruginosa was, however, shown to be protected against bacteriolysis induced by the detergents sodium dodecylsulfate and Triton X-100. With a pair of isogenic mutants of Typhimurium differing in their cell surface charge it was shown that the NPN uptake response to I mM EDTA of the abnormally cationic strain was not significantly affected by nisin, whereas in the normal anionic strain nisin strongly strengthened the uptake. Our hypothesis based on these findings is that the normally anionic cell surface of Gram-negative bacteria has a tendency to bind the cationic nisin. The binding of nisin to the surface does not proceed to the cytoplasmic membrane, but in the outer membrane the bound nisin actually stabilizes its structure through electrostatic interactions. With the exception of EDTA, the organic acids at pH 4 did not cause leakage of cell contents from Typhimurium, indicating that these acids do not permeabilize the outer membrane to an extent required for cytoplasmic pore formation by nisin.