Evaluation of peracetic acid sanitizers efficiency against spores isolated from spoiled cans in suspension and on stainless steel surfaces

The aim of this study was to determine the inactivation effect of industrial formulations of peracetic acid biocides on bacterial spores adhering to stainless steel surfaces. A standardized protocol was used to validate biocide activity against spores in suspension. To validate sporicidal activity under practical conditions, we developed an additional protocol to simulate industrial sanitization of stainless steel surfaces with a foam sanitizer. Spores of three spore-forming bacteria, Clostridium sporogenes PA3679, Geobacillus stearothermophilus, and Moorella thermoacetica/thermoautotrophica, were sprayed onto stainless steel as bioaerosols. Sporicidal activity was high against the C. sporogenes spore suspension, with more than 5 log CFU ml(-1) destroyed at all liquid biocide contact times. Sporicidal activity also was high against G. stearothermophilus and M. thermoacetica/thermoautotrophica spores after 30 min of contact, but we found no population reduction at the 5-min contact time for the highest sporicide concentration tested. The foam biocide effectively inactivated C. sporogenes spores adhered to stainless steel but had a reduced decontamination effect on other species. For G. stearothermophilus spores, sanitization with the foam sporicide was more efficient on horizontal steel than on vertical steel, but foam sanitization was ineffective against M. thermoacetica/thermoautotrophica whatever the position. These results highlight that decontamination efficiency may differ depending on whether spores are suspended in an aqueous solution or adhered to a stainless steel surface. Biocide efficiency must be validated using relevant protocols and bacteria representative of the microbiological challenges and issues affecting each food industry.

Peptidases specific for proline-containing peptides and their unusual peptide-dependent regulation in Oenococcus oeni

AIMS

Growth of the lactic acid bacterium (LAB) Oenococcus oeni, which is involved in malolactic fermentation during the winemaking process, is stimulated by peptides originating from yeast. In this study, we investigated the impact of peptides on O. oeni growth, peptidase activity and the expression of genes encoding the studied peptidases.

METHODS AND RESULTS

Low levels of PepN activity and very high levels of PepI activity were observed in O. oeni, whereas levels of PepX activity were intermediate. The level of biosynthesis of these O. oeni peptidases was shown to depend on peptides present in the culture medium. These results were confirmed by transcriptional analyses of putative pep genes. The mechanism of repression by peptides did not involve a CodY-like regulator.

CONCLUSIONS

Peptides from yeast decrease the levels of enzymatic activity and relative gene expression of O. oeni peptidases. Peptidases specific for proline-containing peptides are important for O. oeni nitrogen metabolism.

SIGNIFICANCE AND IMPACT OF THE STUDY

We report here for the first time that the enzymes involved in the assimilation of proline-containing peptides by O. oeni differ from the well-described proteolytic system of milk LAB. This may reflect a specific adaptation to the wine environment.

Oligopeptide assimilation and transport by Oenococcus oeni

AIMS

Oenococcus oeni is a slow-growing wine bacterium with a low growth yield. It thrives better on complex nitrogen sources than on free amino-acid medium. We aimed to characterize the oligopeptide use of this micro-organism.

METHODS AND RESULTS

Several peptides of two to eight amino-acid residues were able to provide essential amino acids. The disappearance of various peptides from extracellular medium was assessed with whole cells. Initial rates of utilization varied with the peptide, and free amino acids were released into the medium.

CONCLUSIONS

Oenococcus oeni was able to transport the oligopeptides with two to five amino-acid residues tested and to hydrolyse them further.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study has clear implications for the relationship between wine nitrogen composition and the ability of O. oeni to cope with its environment.

Examination of Lactobacillus plantarum lactate metabolism side effects in relation to the modulation of aeration parameters

AIMS

The characterization of global aerobic metabolism of Lactobacillus plantarum LP652 under different aeration levels, in order to optimize acetate production kinetics and to suppress H2O2 toxicity.

METHODS AND RESULTS

Cultures of L. plantarum were grown on different aeration conditions. After sugar exhaustion and in the presence of oxygen, lactate was converted to acetate, H2O2 and carbon dioxide with concomitant ATP production. Physiological assays were performed at selected intervals in order to assess enzyme activity and vitality of the strain during lactic acid conversion. The maximal aerated condition led to fast lactate-to-acetate conversion kinetics between 8 and 12 h, but H2O2 immediately accumulated, thus affecting cell metabolism. Pyruvate oxidase activity was highly enhanced by oxygen tension and was responsible for H2O2 production after 12 h of culture, whereas lactate oxidase and NADH-dependent lactate dehydrogenase activities were not correlated to metabolite production. Limited NADH oxidase (NOX) and NADH peroxidase (NPR) activities were probably responsible for toxic H2O2 levels in over-aerated cultures.

CONCLUSION

Modulating initial airflow led to the maximal specific activity of NOX and NPR observed after 24 h of culture, thus promoting H2O2 destruction and strain vitality at the end of the process.

SIGNIFICANCE AND IMPACT OF THE STUDY

Optimal aeration conditions were determined to minimize H2O2 concentration level during growth on lactate.

Effect of nitrogen limitation and nature of the feed upon Oenococcus oeni metabolism and extracellular protein production

AIMS

The aim of the study was to characterize the effect of various nitrogen sources on Oenococcus oeni growth, carbon source utilization, extracellular protease activity and extracellular proteins. More generally, the goal is to understand how nitrogen-based additives might act to enhance malolactic fermentation in wine.

METHODS AND RESULTS

Five yeast extracts were used. As the amino acid and nitrogen analyses revealed, they were similar in global amino acid composition, except for arginine level. Nevertheless the ratio of amino acids between free/bound, and low/high molecular weight fractions were highly different. One of the yeast extracts led to a significant protease activity in the supernatant and to a poor final biomass of the IOB84.13 strain compared to the other ones. For the IOB84.13 strain specifically, arginine addition to the arginine poor yeast extract did not restore growth. 35S-methionine-labelled extracellular proteins were separated by SDS-PAGE. Signals were detected in all media early in the growth phase and were maintained during 48 h of culture.

CONCLUSIONS

A significant protease activity was detected for O. oeni supernatants during growth under nitrogen limitation but only for certain nitrogen sources. Moreover, the activity was strain dependent. Peptides (0.5-10 kDa) seemed to be more favourable for growth of wine bacteria than <0.5 kDa nitrogen sources. The extracellular protein signal patterns differed more greatly between the bacterial strains tested than between the nitrogen molecules in the medium.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first study extensively considering the role of the nitrogen source composition and level upon O. oeni growth and metabolism.

Glycerol formation during wine fermentation is mainly linked to Gpd1p and is only partially controlled by the HOG pathway

Glycerol 3-phosphate dehydrogenase, a key enzyme in the production of glycerol, is encoded by GPD1 and GPD2. The isoforms encoded by these genes have different functions, in osmoregulation and redox balance, respectively. We investigated the roles of GPD1, GPD2 and HOG1-the kinase involved in the response to osmotic stress-in glycerol production during wine fermentation. We found that the deletion of GPD2 in a wine yeast-derived strain did not affect growth or fermentation performance and reduced glycerol production by only 20%. In contrast, a gpd1delta mutant displayed a prolonged lag phase, and produced 40% less glycerol than the wild-type strain. The deletion of HOG1 resulted in a slight decrease in growth rate and a 20% decrease in glycerol production, indicating that the HOG pathway operates under wine fermentation conditions. However, the hog1delta mutant was not as severely affected as the gpd1delta mutant during the first few hours of fermentation, and continued to express GPD1 strongly. The hog1delta mutant was able to increase glycerol production in response to high sugar concentration (15-28% glucose), to almost the same extent as the wild-type, whereas this response was totally abolished in the gpd1delta mutant. These data show that Gpd1p plays a major role in glycerol formation, particularly during the first few hours of exposure to high sugar concentration, and that GPD2 is only of little significance in anaerobic fermentation by wine yeast. The results also demonstrate that the HOG pathway exerts only limited control over GPD1 expression and glycerol production during wine fermentation.

Glycerol export and glycerol-3-phosphate dehydrogenase, but not glycerol phosphatase, are rate limiting for glycerol production in Saccharomyces cerevisiae

Glycerol, one of the most important by-products of alcoholic fermentation, has positive effects on the sensory properties of fermented beverages. It was recently shown that the most direct approach for increasing glycerol formation is to overexpress GPD1, which encodes the glycerol-3-phosphate dehydrogenase (GPDH) isoform Gpd1p. We aimed to identify other steps in glycerol synthesis or transport that limit glycerol flux during glucose fermentation. We showed that the overexpression of GPD2, encoding the other isoform of glycerol-3-phosphate dehydrogenase (Gpd2p), is equally as effective as the overexpression of GPD1 in increasing glycerol production (3.3-fold increase compared to the wild-type strain) and has similar effects on yeast metabolism. In contrast, overexpression of GPP1, encoding glycerol 3-phosphatase (Gpp1p), did not enhance glycerol production. Strains that simultaneously overexpress GPD1 and GPP1 did not produce higher amounts of glycerol than a GPD1-overexpressing strain. These results demonstrate that GPDH, but not the glycerol 3-phosphatase, is rate-limiting for glycerol production. The channel protein Fps1p mediates glycerol export. It has recently been shown that mutants lacking a region in the N-terminal domain of Fps1p constitutively release glycerol. We showed that cells producing truncated Fps1p constructs during glucose fermentation compensate for glycerol loss by increasing glycerol production. Interestingly, the strain with a deregulated Fps1 glycerol channel had a different phenotype to the strain overexpressing GPD genes and showed poor growth during fermentation. Overexpression of GPD1 in this strain increased the amount of glycerol produced but led to a pronounced growth defect.

The Saccharomyces cerevisiae Sko1p transcription factor mediates HOG pathway-dependent osmotic regulation of a set of genes encoding enzymes implicated in protection from oxidative damage

A major part of the transcriptional response of yeast cells to osmotic shock is controlled by the HOG pathway and several downstream transcription factors. Sko1p is a repressor that mediates HOG pathway-dependent regulation by binding to CRE sites in target promoters. Here, we report five target genes of Hog1p-Sko1p: GRE2, AHP1, SFA1, GLR1 and YML131w. The two CREs in the GRE2 promoter function as activating sequences and, hence, bind (an) activator protein(s). However, the two other yeast CRE-binding proteins, Aca1p and Aca2p, are not involved in regulation of the GRE2 promoter under osmotic stress. In the absence of the co-repressor complex Tup1p-Ssn6p/Cyc8p, which is recruited by Sko1p, stimulation by osmotic stress is still observed. These data indicate that Sko1p is not only required for repression, but also involved in induction upon osmotic shock. All five Sko1p targets encode oxidoreductases with demonstrated or predicted roles in repair of oxidative damage. Altered basal expression levels of these genes in hog1Delta and sko1Delta mutants may explain the oxidative stress phenotypes of these mutants. All five Sko1p target genes are induced by oxidative stress, and induction involves Yap1p. Although Sko1p and Yap1p appear to mediate osmotic and oxidative stress responses independently, Sko1p may affect Yap1p promoter access or activity. The five Sko1p target genes described here are suitable models for studying the interplay between osmotic and oxidative responses at the molecular and physiological levels.

Engineering of the pyruvate dehydrogenase bypass in Saccharomyces cerevisiae: role of the cytosolic Mg(2+) and mitochondrial K(+) acetaldehyde dehydrogenases Ald6p and Ald4p in acetate formation during alcoholic fermentation

Acetic acid plays a crucial role in the organoleptic balance of many fermented products. We have investigated the factors controlling the production of acetate by Saccharomyces cerevisiae during alcoholic fermentation by metabolic engineering of the enzymatic steps involved in its formation and its utilization. The impact of reduced pyruvate decarboxylase (PDC), limited acetaldehyde dehydrogenase (ACDH), or increased acetoacetyl coenzyme A synthetase (ACS) levels in a strain derived from a wine yeast strain was studied during alcoholic fermentation. In the strain with the PDC1 gene deleted exhibiting 25% of the PDC activity of the wild type, no significant differences were observed in the acetate yield or in the amounts of secondary metabolites formed. A strain overexpressing ACS2 and displaying a four- to sevenfold increase in ACS activity did not produce reduced acetate levels. In contrast, strains with one or two disrupted copies of ALD6, encoding the cytosolic Mg(2+)-activated NADP-dependent ACDH and exhibiting 60 and 30% of wild-type ACDH activity, showed a substantial decrease in acetate yield (the acetate production was 75 and 40% of wild-type production, respectively). This decrease was associated with a rerouting of carbon flux towards the formation of glycerol, succinate, and butanediol. The deletion of ALD4, encoding the mitochondrial K(+)-activated NAD(P)-linked ACDH, had no effect on the amount of acetate formed. In contrast, a strain lacking both Ald6p and Ald4p exhibited a long delay in growth and acetate production, suggesting that Ald4p can partially replace the Ald6p isoform. Moreover, the ald6 ald4 double mutant was still able to ferment large amounts of sugar and to produce acetate, suggesting the contribution of another member(s) of the ALD family.

Re-assessment of the influence of yeast strain and environmental factors on glycerol production in wine

Increasing glycerol production is of concern for wine-makers in improving the quality of certain wines. We have compared the impact of strain and relevant environmental factors influencing glycerol production under the same conditions, i.e. standardized conditions simulating enological fermentation. The glycerol production of 19 industrial wine strains ranged from 6.4 to 8.9 g l-1 and varied significantly between strains. The production of acetate and succinate was also found to differ substantially depending on the strain but no significant strain-dependent variation was observed for acetaldehyde. Interestingly, high glycerol production was not correlated to high production of acetate or acetaldehyde, which are undesirable in wine. A detailed study with two low or two high glycerol-producing strains showed that temperature and the initial concentration of nitrogen had little effect on the amount of glycerol formed, although agitation or a nitrogen source composed mainly of ammoniacal nitrogen slightly enhanced glycerol production. The influence of environmental factors remained minor while the predominant factor for glycerol variability in wine was attributed to the strain. Taking into account wine-making constraints, the results indicate that achieving a high glycerol content in wine requires the selection or improvement of yeast strains rather than the control of growth and cultivation conditions.

Modulation of glycerol and ethanol yields during alcoholic fermentation in Saccharomyces cerevisiae strains overexpressed or disrupted for GPD1 encoding glycerol 3-phosphate dehydrogenase

The possibility of the diversion of carbon flux from ethanol towards glycerol in Saccharomyces cerevisiae during alcoholic fermentation was investigated. Variations in the glycerol 3-phosphate dehydrogenase (GPDH) level and similar trends for alcohol dehydrogenase (ADH), pyruvate decarboxylase and glycerol-3-phosphatase were found when low and high glycerol-forming wine yeast strains were compared. GPDH is thus a limiting enzyme for glycerol production. Wine yeast strains with modulated GPD1 (encoding one of the two GPDH isoenzymes) expression were constructed and characterized during fermentation on glucose-rich medium. Engineered strains fermented glucose with a strongly modified [glycerol] : [ethanol] ratio. gpd1delta mutants exhibited a 50% decrease in glycerol production and increased ethanol yield. Overexpression of GPD1 on synthetic must (200 g/l glucose) resulted in a substantial increase in glycerol production ( x 4) at the expense of ethanol. Acetaldehyde accumulated through the competitive regeneration of NADH via GPDH. Accumulation of by-products such as pyruvate, acetate, acetoin, 2,3 butane-diol and succinate was observed, with a marked increase in acetoin production.