Validation of predictive growth models describing superatmospheric oxygen effects on Pseudomonas fluorescens and Listeria innocua on fresh-cut lettuce

Two microbial growth models predicting the growth of Pseudomonas fluorescens and Listeria innocua at superatmospheric oxygen and carbon dioxide concentrations at 7 degrees C were validated on fresh-cut butterhead lettuce. Cut lettuce was inoculated with the same strain of L. innocua as the in vitro experiments. The P. fluorescens strain was tagged with a gene encoding green fluorescent protein (GFP) in order to distinguish the inoculated strain from contaminating Pseudomonaceae. Also growth of aerobic mesophilic and lactic acid bacteria was monitored during the experiments. The suggested P. fluorescens model was appropriate to predict growth on cut lettuce. L. innocua on the other hand, grew considerably slower under in vivo circumstances than predicted. CO(2) had a growth promoting effect on L. innocua growing on cut lettuce, whereas in vitro an inhibiting effect was observed. Validation parameters are calculated and hypotheses to explain the discrepancy between predicted and observed growth of L. innocua are provided.

Cell wall substrate specificity of six different lysozymes and lysozyme inhibitory activity of bacterial extracts

We have investigated the specificity of six different lysozymes for peptidoglycan substrates obtained by extraction of a number of gram-negative bacteria and Micrococcus lysodeikticus with chloroform/Tris-HCl buffer (chloroform/buffer). The lysozymes included two that are commercially available (hen egg white lysozyme or HEWL, and mutanolysin from Streptomyces globisporus or M1L), and four that were chromatographically purified (bacteriophage lambda lysozyme or LaL, bacteriophage T4 lysozyme or T4L, goose egg white lysozyme or GEWL, and cauliflower lysozyme or CFL). HEWL was much more effective on M. lysodeikticus than on any of the gram-negative cell walls, while the opposite was found for LaL. Also the gram-negative cell walls showed remarkable differences in susceptibility to the different lysozymes, even for closely related species like Escherichia coli and Salmonella Typhimurium. These differences could not be due to the presence of lysozyme inhibitors such as Ivy from E. coli in the cell wall substrates because we showed that chloroform extraction effectively removed this inhibitor. Interestingly, we found strong inhibitory activity to HEWL in the chloroform/buffer extracts of Salmonella Typhimurium, and to LaL in the extracts of Pseudomonas aeruginosa, suggesting that other lysozyme inhibitors than Ivy exist and are probably widespread in gram-negative bacteria.

N-acyl-L-homoserine lactone quorum sensing controls butanediol fermentation in Serratia plymuthica RVH1 and Serratia marcescens MG1

Butanediol fermentation in two Serratia species is shown to be affected by N-acyl-L-homoserine lactone-dependent quorum sensing. Knockout of quorum-sensing signal production caused a shift towards enhanced acid production, resulting in early growth arrest, which was reversible by the addition of synthetic signal molecules.

Inactivation of Escherichia coli and Shigella in acidic fruit and vegetable juices by peroxidase systems

AIMS

To study the bactericidal properties of the lactoperoxidase (LPER)-thiocyanate and soybean peroxidase (SBP)-thiocyanate systems at low pH, their efficiency for inactivation of Escherichia coli and Shigella in acidic fruit and vegetable juices, their effect on colour stability of the juices and interaction with ascorbic acid.

METHODS AND RESULTS

Three-strain cocktails of E. coli and Shigella spp. in selected juices were supplemented with the LPER or SBP system. Within 24 h at 20 degrees C, the LPER system inactivated both cocktails by > or = 5 log10 units in apple, 2-5 log10 units in orange and < or = 1 log10 unit in tomato juices. In the presence of SBP, browning was significant in apple juice and white grape juice, slight in pink grape juice and absent in orange or tomato juice. Ascorbic acid protected E. coli and Shigella against inactivation by the LPER system, and peroxidase systems significantly reduced the ascorbic acid content of juices.

CONCLUSIONS

Our results suggest a different specificity of LPER and SBP for SCN-, phenolic substrates of browning and ascorbic acid in acidic juices. The LPER system appeared a more appropriate candidate than the SBP system for biopreservation of juices.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work may open perspectives towards the development of LPER or other peroxidases as biopreservatives in acidic foods.

N-acyl-l-homoserine lactone signal interception byEscherichia coli

N-acyl-L-homoserine lactone (AHL) mediated quorum sensing is a widespread communication system in gram-negative bacteria which regulates a wide range of target genes in a cell density-dependent manner. Although Escherichia coli is not capable of synthesizing AHL molecules because it lacks an AHL synthase encoding gene, it does produce a predicted AHL receptor of the LuxR family, named SdiA. In this work, we used a promoter trap library to screen for E. coli MG1655 promoters whose expression was affected by synthetic N-hexanoyl-L-homoserine lactone (C6-HSL), and we identified six upregulated and nine downregulated promoters, which also responded to synthetic 3-oxo-N-hexanoyl-L-homoserine lactone (3-oxo-C6-HSL). The AHL responsiveness of these promoters was eliminated by knock-out of sdiA, and was temperature dependent, since the identified promoters showed a response at 30 degrees C but not, or only very weakly at 37 degrees C. In addition, in line with the observed induction of gadA encoding a glutamate decarboxylase, we could demonstrate an increased acid tolerance of E. coli upon exposure to C6-HSL. In conclusion, our work shows that E. coli has the capacity to alter its pattern of gene expression and its phenotypical properties in response to AHLs by means of the AHL responsive transcriptional regulator SdiA.

Comparison of bactericidal activity of six lysozymes at atmospheric pressure and under high hydrostatic pressure

The antibacterial working range of six lysozymes was tested under ambient and high pressure, on a panel of five gram-positive (Enterococcus faecalis, Bacillus subtilis, Listeria innocua, Staphylococcus aureus and Micrococcus lysodeikticus) and five gram-negative bacteria (Yersinia enterocolitica, Shigella flexneri, Escherichia coli O157:H7, Pseudomonas aeruginosa and Salmonella typhimurium). The lysozymes included two that are commercially available (hen egg white lysozyme or HEWL, and mutanolysin from Streptomyces globisporus or M1L), and four that were chromatographically purified (bacteriophage lambda lysozyme or LaL, bacteriophage T4 lysozyme or T4L, goose egg white lysozyme or GEWL, and cauliflower lysozyme or CFL). T4L, LaL and GEWL were highly pure as evaluated by silver staining of SDS-PAGE gels and zymogram analysis while CFL was only partially pure. At ambient pressure each gram-positive test organism displayed a specific pattern of sensitivity to the six lysozymes, but none of the gram-negative bacteria was sensitive to any of the lysozymes. High pressure treatment (130-300 MPa, 25 degrees C, 15 min) sensitised several gram-positive and gram-negative bacteria for one or more lysozymes. M. lysodeikticus and P. aeruginosa became sensitive to all lysozymes under high pressure, S. typhimurium remained completely insensitive to all lysozymes, and the other bacteria showed sensitisation to some of the lysozymes. The possible applications of the different lysozymes as biopreservatives, and the possible reasons for the observed differences in bactericidal specificity are discussed.

Investigation into the resistance of lactoperoxidase tolerantEscherichia colimutants to different forms of oxidative stress

Six lactoperoxidase tolerant Escherichia coli transposon mutants isolated and characterized in an earlier study, and some newly constructed double mutants, were subjected to peroxide, superoxide and hypochlorite stress, and their inactivation was compared to that of the wild type strain MG1655. Knock out mutants of waaQ and waaO, which owed their lactoperoxidase tolerance to an impaired outer membrane permeability due to a reduced porin content, also exhibited higher resistance to hypochlorite, as did a knock-out strain of lrp, encoding a regulatory protein affecting a wide range of cellular functions. Unlike the outer membrane mutants however, the lrp strain was also more resistant to t-butyl hydroperoxide, but more susceptible to the superoxide generating compound plumbagin. Finally, a lactoperoxidase tolerant knock-out strain of ulaA, involved in ascorbic acid uptake, did not show resistance to any of the other oxidants. The possible modes of action of these different oxidants are discussed.

Mrr instigates the SOS response after high pressure stress in Escherichia coli

The bacterial SOS response is not only a vital reply to DNA damage but also constitutes an essential mechanism for the generation of genetic variability that in turn fuels adaptation and resistance development in bacterial populations. Despite the extensive depiction of the SOS regulon itself, its activation by stresses different from typical DNA damaging treatments remains poorly characterized. Recently, we reported the RecA- and LexA-dependent induction of the SOS response in Escherichia coli MG1655 after exposure to high hydrostatic pressure (HP, approximately 100 MPa), a physical stress of which the cellular effects are not well known. We now found this HP mediated SOS response to depend on RecB and not on RecF, which is a strong indication for the involvement of double strand breaks. As the pressures used in this work are thermodynamically unable to break covalent bonds in DNA, we hypothesized the involvement of a cellular function or pathway in the formation of this lesion. A specialized screening allowed us to identify the cryptic type IV restriction endonuclease Mrr as the final effector of this pathway. The HP SOS response and its corresponding phenotypes could be entirely attributed to the HP triggered activation of Mrr restriction activity. Several spontaneously occurring alleles of mrr, incapable of triggering the HP-induced SOS response, were isolated and characterized. These results provide evidence for a specific pathway that transmits the perception of HP stress to induction of the SOS response and support a role for Mrr in bacterial stress physiology.

CorA affects tolerance of Escherichia coli and Salmonella enterica serovar Typhimurium to the lactoperoxidase enzyme system but not to other forms of oxidative stress

The enzyme lactoperoxidase is part of the innate immune system in vertebrates and owes its antimicrobial activity to the formation of oxidative reaction products from various substrates. In a previous study, we have reported that, with thiocyanate as a substrate, the lactoperoxidase system elicits a distinct stress response in Escherichia coli MG1655. This response is different from but partly overlapping with the stress responses to hydrogen peroxide and to superoxide. In the current work, we constructed knockouts in 10 lactoperoxidase system-inducible genes to investigate their role in the tolerance of E. coli MG1655 to this antimicrobial system. Five mutations resulted in a slightly increased sensitivity, but one mutation (corA) caused hypersensitivity to the lactoperoxidase system. This hypersensitive phenotype was specific to the lactoperoxidase system, since neither the sensitivity to hydrogen peroxide nor to the superoxide generator plumbagin was affected in the corA mutant. Salmonella enterica serovar Typhimurium corA had a similar phenotype. Although corA encodes an Mg2+ transporter and at least three other inducible open reading frames belonged to the Mg2+ regulon, repression of the Mg stimulon by Mg2+ did not change the lactoperoxidase sensitivity of either the wild-type or corA mutant. Prior exposure to 0.3 mM Ni2+, which is also transported by CorA, strongly sensitized MG1655 but not the corA mutant to the lactoperoxidase system. Furthermore, this Ni2+-dependent sensitization was suppressed by the CorA-specific inhibitor Co(III) hexaammine. These results indicate that CorA affects the lactoperoxidase sensitivity of E. coli by modulating the cytoplasmic concentrations of transition metals that enhance the toxicity of the lactoperoxidase system.

Predictive modelling and validation of Listeria innocua growth at superatmospheric oxygen and carbon dioxide concentrations

The effect of superatmospheric oxygen and carbon dioxide concentrations on the growth of Listeria innocua, which was used as a model organism for the pathogen Listeria monocytogenes, was evaluated. The bacteria were grown on a nutrient agar surface at 7 degrees C. Three carbon dioxide levels (0%, 12.5% and 25%) were combined with different levels of high oxygen concentrations (above 20%) based on a mixture design. The applied oxygen concentrations did not significantly influence the growth. High CO2 concentrations, on the contrary, reduced the maximum specific growth rate and prolonged the lag time. An overall model to describe the growth of L. innocua under high carbon dioxide conditions was constructed based on nine growth experiments, using a weighted one-step regression procedure. The influence of carbon dioxide on lag time and maximum specific growth rate was described using Ratkowsky-type models and inserted in the Baranyi equation. The model described the growth very well. To assess the validity of the model, 14 additional experiments were carried out. There was a good correlation of the model predictions and observed validation data.

Sensitisation of Escherichia coli to antibacterial peptides and enzymes by high-pressure homogenisation

The sensitisation of Escherichia coli towards lysozyme, nisin and lactoperoxidase by high-pressure homogenisation (HPH) at pressures ranging from 100 to 300 MPa was studied. At above 150 MPa, E. coli became sensitive to lysozyme and nisin when these compounds were added before the HPH treatment, but cells treated by HPH remained insensitive for lysozyme and nisin added after that treatment. No sensitisation to the lactoperoxidase enzyme system was observed, and no evidence of cytoplasmic membrane damage could be found in cells surviving HPH treatment using SYTO 9/propidium iodide staining. A previously isolated spontaneous E. coli mutant that is resistant to lysozyme under high hydrostatic pressure, was also found to be resistant to lysozyme upon HPH treatment. Further, lysozyme resistance in this mutant was not due to enhanced levels of periplasmic lysozyme inhibitor (Ivy), since knock-out of the ivy gene did not increase the sensitivity of the mutant to lysozyme upon HPH treatment. These results indicate that HPH treatment at pressures up to 300 MPa, unlike treatment with high hydrostatic pressure, does not cause sublethal injury, and thus does not sensitise bacteria to antimicrobial compounds that disturb cellular homeostasis such as might be the case for the lactoperoxidase system. However, like high hydrostatic pressure treatment, HPH may sensitise E. coli to lysozyme and nisin by inducing a transient permeabilisation of the outer membrane that does not involve a physical disruption and that is immediately repaired after the process.

Genotypic and phenotypic characterization of a biofilm-forming Serratia plymuthica isolate from a raw vegetable processing line

Recently, we isolated from a raw vegetable processing line a Serratia strain with strong biofilm-forming capacity and which produced N-acyl-L-homoserine lactones (AHLs). Within the Enterobacteriaceae, strains of the genus Serratia are a frequent cause of human nosocomial infections; in addition, biofilm formation is often associated with persistent infections. In the current report, we describe the detailed characterization of the isolate using a variety of genotypic and phenotypic criteria. Although the strain was identified as Serratia plymuthica on the basis of its small subunit ribosomal RNA (16S rRNA) gene sequence, it differed from the S. plymuthica type strain in production of pigment and antibacterial compounds, and in AHL production profile. Nevertheless, the identification as S. plymuthica could be confirmed by gyrB phylogeny and DNA:DNA hybridization.

Construction and use of an stx1 transcriptional fusion to gfp

Shiga toxins (Stxs), also termed Vero toxins, are cytotoxic ribosome inactivating proteins that are produced by a number of gastrointestinal pathogens and that contribute to the severity of the associated diseases. In this work, we constructed and validated a transcriptional fusion of the stx1AB promoter to the gfp reporter gene. The cloned promoter region encompasses both the proximal and the distal promoter regions of stx1AB, mediating control by the host's iron-responsive Fur repressor and the Stx prophage's Q antiterminator protein, respectively. The probe was validated by demonstrating its responsiveness towards mitomycin C and EDTA, and the contribution of host and phage encoded factors could be separated by studying stx1 expression in either wild-type or isogenic lysogenic cells. Moreover, stx1AB expressing populations could be visualized by flow cytometry. The potential use of such a probe for non-destructive online detection of stx1AB expression and visualization of stx1AB expressing populations is further discussed.

Role of porins in sensitivity of Escherichia coli to antibacterial activity of the lactoperoxidase enzyme system

Lactoperoxidase is an enzyme that contributes to the antimicrobial defense in secretory fluids and that has attracted interest as a potential biopreservative for foods and other perishable products. Its antimicrobial activity is based on the formation of hypothiocyanate (OSCN-) from thiocyanate (SCN-), using H2O2 as an oxidant. To gain insight into the antibacterial mode of action of the lactoperoxidase enzyme system, we generated random transposon insertion mutations in Escherichia coli MG1655 and screened the resultant mutants for an altered tolerance of bacteriostatic concentrations of this enzyme system. Out of the ca. 5,000 mutants screened, 4 showed significantly increased tolerance, and 2 of these had an insertion, one in the waaQ gene and one in the waaO gene, whose products are involved in the synthesis of the core oligosaccharide moiety of lipopolysaccharides. Besides producing truncated lipopolysaccharides and displaying hypersensitivity to novobiocin and sodium dodecyl sulfate (SDS), these mutants were also shown by urea-SDS-polyacrylamide gel electrophoresis analysis to have reduced amounts of porins in their outer membranes. Moreover, they showed a reduced degradation of p-nitrophenyl phosphate and an increased resistance to ampicillin, two indications of a decrease in outer membrane permeability for small hydrophilic solutes. Additionally, ompC and ompF knockout mutants displayed levels of tolerance to the lactoperoxidase system similar to those displayed by the waa mutants. These results suggest that mutations which reduce the porin-mediated outer membrane permeability for small hydrophilic molecules lead to increased tolerance to the lactoperoxidase enzyme system because of a reduced uptake of OSCN-.

Screening for Bacillus subtilis mutants deficient in pressure induced spore germination: identification of ykvU as a novel germination gene

Exposure to high pressure induces germination in spores of Bacillus subtilis. To investigate the mechanisms of this process and to compare the pressure and nutrient induced germination pathways, a random transposon knock-out library of B. subtilis was constructed and screened for clones with a compromised pressure induced germination at 100 MPa. Two mutants were isolated and their transposon insertion was mapped to gerAC and ykvU respectively. While GerAC is required for production of the l-alanine receptor which has been implicated in pressure-induced germination before, YkvU is shown here to be a novel germination determinant in B. subtilis, affecting germination by high (100 MPa) and very high (600 MPa) pressure, by nutrients and by dodecylamine, but not by Ca(2+)-dipicolinic acid.

Induction of oxidative stress by high hydrostatic pressure in Escherichia coli

Using leaderless alkaline phosphatase as a probe, it was demonstrated that pressure treatment induces endogenous intracellular oxidative stress in Escherichia coli MG1655. In stationary-phase cells, this oxidative stress increased with the applied pressure at least up to 400 MPa, which is well beyond the pressure at which the cells started to become inactivated (200 MPa). In exponential-phase cells, in contrast, oxidative stress increased with pressure treatment up to 150 MPa and then decreased again, together with the cell counts. Anaerobic incubation after pressure treatment significantly supported the recovery of MG1655, while mutants with increased intrinsic sensitivity toward oxidative stress (katE, katF, oxyR, sodAB, and soxS) were found to be more pressure sensitive than wild-type MG1655. Furthermore, mild pressure treatment strongly sensitized E. coli toward t-butylhydroperoxide and the superoxide generator plumbagin. Finally, previously described pressure-resistant mutants of E. coli MG1655 displayed enhanced resistance toward plumbagin. In one of these mutants, the induction of endogenous oxidative stress upon high hydrostatic pressure treatment was also investigated and found to be much lower than in MG1655. These results suggest that, at least under some conditions, the inactivation of E. coli by high hydrostatic pressure treatment is the consequence of a suicide mechanism involving the induction of an endogenous oxidative burst.

Inactivation of Escherichia coli by high-pressure homogenisation is influenced by fluid viscosity but not by water activity and product composition

The inactivation of Escherichia coli MG1655 by high-pressure homogenisation (HPH) at pressures ranging from 100 to 300 MPa was studied in buffered suspensions adjusted to different relative viscosities (1.0, 1.3, 1.7, 2.7 and 4.9) with polyethylene glycol 6000 (PEG 6000). The water activity of these suspensions was not significantly affected by this high molecular weight solute. Bacterial inactivation was found to decrease with increasing viscosity of the suspensions, an effect that was more pronounced at higher pressures. To study the effect of water activity, series of E. coli suspensions having a different water activity (0.953-1.000) but the same relative viscosity (1.3, 1.7, 2.7 and 4.9) were made using PEG of different molecular weights (400, 600, 1000 and 6000), and subjected to HPH treatment. The results indicated that water activity does not influence inactivation. Finally, inactivation of E. coli MG1655 by HPH in skim milk, soy milk and strawberry-raspberry milk drink was found to be the same as in PEG containing buffer of the corresponding viscosity. These results identify fluid viscosity as a major environmental parameter affecting bacterial inactivation by HPH, as opposed to water activity and product composition, and should contribute to the development of HPH applications for the purpose of bacterial inactivation.

Induction of Shiga toxin-converting prophage in Escherichia coli by high hydrostatic pressure

Since high hydrostatic pressure is becoming increasingly important in modern food preservation, its potential effects on microorganisms need to be thoroughly investigated. In this context, mild pressures (<200 MPa) have recently been shown to induce an SOS response in Escherichia coli MG1655. Due to this response, we observed a RecA- and LexA-dependent induction of lambda prophage upon treating E. coli lysogens with sublethal pressures. In this report, we extend this observation to lambdoid Shiga toxin (Stx)-converting bacteriophages in MG1655, which constitute an important virulence trait in Stx-producing E. coli strains (STEC). The window of pressures capable of inducing Stx phages correlated well with the window of bacterial survival. When pressure treatments were conducted in whole milk, which is known to promote bacterial survival, Stx phage induction could be observed at up to 250 MPa in E. coli MG1655 and at up to 300 MPa in a pressure-resistant mutant of this strain. In addition, we found that the intrinsic pressure resistance of two types of Stx phages was very different, with one type surviving relatively well treatments of up to 400 MPa for 15 min at 20 degrees C. Interestingly, and in contrast to UV irradiation or mitomycin C treatment, pressure was not able to induce Stx prophage or an SOS response in several natural Stx-producing STEC isolates.

Role of bacterial cell surface structures in Escherichia coli biofilm formation

Various cell surface molecules and structures have been implicated in biofilm formation in Escherichia coli. This review presents an overview of the occurrence, production and interaction of these components, their influence at one or more developmental stages of biofilm formation, and their potential role as virulence factors in pathogenic E. coli strains.

SulA-dependent hypersensitivity to high pressure and hyperfilamentation after high-pressure treatment of Escherichia coli lon mutants

High-pressure treatment (>100 MPa) is known to induce several heat shock proteins as well as an SOS response in Escherichia coli. In the current work, we have investigated properties with respect to high-pressure treatment of mutants-deficient in Lon, a pressure-induced ATP-dependent protease that belongs to the heat shock regulon but that also has a link to the SOS regulon. We report that lon mutants show increased pressure sensitivity and exhibit hyperfilamentation during growth after high-pressure treatment. Both phenotypes could be entirely attributed to the action of the SOS protein SulA, a potent inhibitor of the cell division ring protein FtsZ and a specific target of the Lon protease, since they were suppressed by knock-out of SulA. Introduction of the lexA1 allele, which effectively blocks the entire SOS response, also suppressed the high pressure hypersensitivity of lon mutants, but not their UV hypersensitivity. These results indicate the existence of a SulA-dependent pathway of high-pressure-induced cell filamentation, and suggest involvement of the SOS response, and particularly of SulA, in high-pressure-mediated cell death in E. coli strains which are compromised in Lon function.

Inactivation of Escherichia coli by high hydrostatic pressure at different temperatures in buffer and carrot juice

The inactivation of Escherichia coli MG1655 was studied at 256 different pressure (150-600 MPa)-temperature (5-45 degrees C) combinations under isobaric and isothermal conditions in Hepes-KOH buffer (10 mM, pH 7.0) and in fresh carrot juice. A linear relationship was found between the log10 of inactivation and holding time for all pressure-temperature combinations in carrot juice, with R2-values>or=0.91. Decimal reduction times (D-values), calculated for each pressure-temperature combination, decreased with pressure at constant temperature and with temperature at constant pressure. Further, a linear relationship was found between log10D and pressure and temperature. A first order kinetic model, describing log10D in carrot juice as a function of pressure and temperature was formulated that allows to identify process conditions (pressure, temperature, holding time) resulting in a desired level of inactivation of E. coli. For Hepes-KOH buffer, the Weibull model more accurately described the entire set of inactivation curves of E. coli MG1655 compared to the log-linear or the biphasic model. Several secondary models (first and second order polynomial and Weibull) were evaluated, but all had poor fitting capacities. When the Hepes-KOH dataset was limited to 22 of the 34 pressure-temperature combinations, a first order model was appropriate and enabled us to use the same model structure as for carrot juice, for comparative purposes. The major difference in kinetic behaviour of E. coli in buffer and in carrot juice was that inactivation rate as a function of temperature showed a minimum around 20-30 degrees C in buffer, whereas it increased with temperature over the entire studied temperature range in carrot juice.