48 research outputs found
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Study on the effect of citric acid adaptation toward the subsequent survival of Lactobacillus plantarum NCIMB 8826 in low pH fruit juices during refrigerated storage
Pre-treatment of stationary phase cells of Lactobacillus plantarum NCMIB 8826 with citric acid (pH 3 to 6) for a short period of time significantly improved subsequent cell survival in several highly acidic fruit juices namely cranberry (pH 2.7), pomegranate (pH 3.5), and lemon & lime juices (pH 2.8). Although the mechanism for this adaptation is still unclear, the analysis of the cellular fatty acid content of acid adapted cells and the reverse transcription polymerase chain reaction (RT-PCR) showed a significant increase (by ~1.7 fold) of the cellular cyclopropane fatty acid, cis-11,12-methylene octadecanoic acid (C19:0cyclow7c) and a significant upregulation (~12 fold) of cyclopropane synthase (cfa) were observed, respectively, during acid adaptation. It is likely that these changes led to a decrease in membrane fluidity and to lower membrane permeability, which prevents the cells from proton influx during storage in these low pH fruit juices
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A novel approach in acidic disinfection through inhibition of acid resistance mechanisms; Maleic acid-mediated inhibition of glutamate decarboxylase activity enhances acid sensitivity of Listeria monocytogenes
Here it is demonstrated a novel approach in disinfection regimes where specific molecular acid resistance systems are inhibited aiming to eliminate microorganisms under acidic conditions. Despite the importance of the Glutamate Decarboxylase (GAD) system for survival of Listeria monocytogenes and other pathogens under acidic conditions its potential inhibition by specific compounds that could lead to its elimination from foods or food preparation premises has not been studied. The effects of maleic acid on the acid resistance of L. monocytogenes were investigated and found that it has a higher antimicrobial activity under acidic conditions than other organic acids, while this could not be explained by its pKa or Ki values. The effects were found to be more pronounced on strains with higher GAD activity. Maleic acid affected the extracellular GABA levels while it did not affect the intracellular. Maleic acid had a major impact mainly on GadD2 activity as also shown in cell lysates. Furthermore, it was demonstrated that maleic acid is able to partly remove biofilms of L. monocytogenes. Maleic acid is able to inhibit the GAD of L. monocytogenes significantly enhancing its sensitivity to acidic conditions and together with its ability to remove biofilms, make a good candidate for disinfection regimes
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Enzymatic hydrolysis of thermally pre‐treated chitin and antimicrobial activity of N,N'‐diacetylchitobiose
BACKGROUND: N,N’-diacetylchitobiose (GlcNAc2) is known to be highly functional and offers a wide range of applications, especially as an antimicrobial agent. In this study, a thermal pre-treatment process using steam under pressure in an autoclave, has been employed to facilitate subsequent enzymatic hydrolysis of chitin with chitinase from Streptomyces griseus.
RESULTS: Pre-treatment of chitin with 0.05 M sodium acetate buffer (pH=6.0) at 121ºC for 60 min, followed by enzymatic hydrolysis involving 24 h incubation, was found to be the best condition for producing the GlcNAc2. The GlcNAc2 obtained was tested regarding its antimicrobial activity against Gram-negative and Gram-positive strains and showed minimum inhibitory concentrations (MIC) at 5 and 10% w/v against Escherichia coli K-12 and Listeria monocytogenes 10403S, respectively.
CONCLUSION: The extent of swelling and crystallite size of chitin increased with the pre-treatment residence time, and enhanced the rate of subsequent hydrolysis using chitinase
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Investigation into the antimicrobial activity of fumarate against Listeria monocytogenes and its mode of action under acidic conditions
Organic acids such as fumarate are commonly used as antimicrobials in foods. Apart from the classical mechanism of intracellular dissociation, weak acids are active through important additional mechanisms which are not well-defined. Fumarate, based on its low dissociation constants is expected to have a low antimicrobial activity which is not the case, suggesting additional antimicrobial effects. Previously, fumarate has been shown to inhibit the GAD system of E. coli and therefore, we investigated for first time how it affects this system in Listeria monocytogenes. We found that fumarate is highly antimicrobial towards L. monocytogenes under acidic conditions. We also show that in cell lysates and similarly to E. coli, fumarate inhibits the GAD system of L. monocytogenes. However, despite the inhibition and in contrast to E. coli, L. monocytogenes is able to counteract this and achieve a higher extracellular GAD output (measured by GABA export) in the presence of fumarate compared to its absence. The latter is achieved by a dramatic 9.44-fold increase in the transcription of gadD2 which is the main component of the extracellular GAD system. Interestingly, although maleate, the cis-isomer of fumarate results in a more dramatic 48.5-fold gadD2 upregulation than that of fumarate, the final GADe output is lower suggesting that maleate might be a stronger inhibitor of the GAD system. In contrast, the GADe removes more protons in the presence of fumarate than in the presence of HCl at the same pH. All the above suggest that there are additional effects by fumarate which might be associated with the intracellular GAD system (GADi) or other acid resistance systems. We assessed the GADi output by looking at the intracellular GABA pools which were not affected by fumarate. However, there are multiple pathways (e.g. GABA shunt) that can affect GABAi pools and we cannot conclusively suggest that GADi is affected. Furthermore, similarly to maleate, fumarate is able to eliminate L. monocytogenes in biofilms under acidic conditions. Overall, fumarate is a good candidate for L. monocytogenes decontamination and biofilm removal which is not toxic compared to the toxic maleate
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Determination of the relative effects of temperature, pH and water activity in food systems: a meta-analysis study
The aim of this study is to use ComBase to determine the relative effects of temperature, pH, and water activity in the
inactivation rates of Salmonella enterica in a range of foods. This is performed to determine whether any of the above factors
have a dominant effect on survival. The inactivation rates of Salmonella were obtained from original raw data in the ComBase
browser and from complete ComBase data for Salmonella. A total of 972 data of different types of food systems and data of
individual types of food from ComBase were analysed. Over the range of 0–90°C, the z values calculated for the food data is
14°C. At 0–46°C relevant to intermediate moisture foods (IMF), the z values for the food data was 22°C, indicating a
moderate effect of temperature. The z value for inactivation at 47–90°C was 11°C, indicating that temperature has an important
effect on survival. This study shows that the effect of temperature is clearer at high temperatures than in the low temperature
region. It suggests that the inactivation of Salmonella in food systems is slightly dominated by temperature and that the pH
and aw levels appear to be less influential
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Biofilm formation of Salmonella enterica and the central role of RpoS sigma factor in stress resistance
Non-typhoidal Salmonella is considered as the leading cause of foodborne illness and it has been associated with highprofile outbreaks in many groups of foods. This work examines the contribution of phenotypic properties related to survival (biofilm formation) and how these are linked with the genetic and functional variability of rpoS gene and RpoS status respectively in Salmonella enterica. The test strains were Salmonella serovars Anatum, Enteritidis (466), Enteritidis (496), Hadar, Heidelberg, Montevideo, Newport and Virchow and two Typhimurium strains previously characterised as either RpoSpositive or RpoS-negative. RpoS status was found to affect biofilm formation. The capability of Salmonella to resist stress and survive under unfavourable conditions can vary between strains. We confirmed that the two strains that were previously sensitive to various stresses harboured significant mutations in the rpoS gene. The rpoS sequencing not only confirmed a link between RpoS and biofilm formation, but it also revealed a link with differences in the utilisation of carbon sources. The RpoS-negative phenotype was linked with an increased growth under different carbon sources suggesting that a functional RpoS is a burden for growth which is in agreement with the SPANC hypothesi
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High phenotypic variability among representative strains of common Salmonella enterica serovars with possible implications for food safety
Salmonella is an important foodborne pathogen whose ability to resist stress and survive can vary between strains. This variability is normally not taken into account when predictions are made about survival in foods with negative consequences. Therefore, we examined the contribution of variable phenotypic properties to survival under stress in 10 Salmonella serovars. One strain (S. Typhimurium 10) was intentionally an RpoS-negative however, another one (S. Heidelberg) showed an rpoS mutation rendering it inactive. We assessed an array of characteristics (motility, biofilm formation, bile resistance, acid resistance and colony morphology) showing a major variability between strains associated with a 10- to 19-fold difference between the highest and the lowest strain for most characteristics.
The RpoS status of isolates did not affect variability in the characteristics with the exception of resistance to NaCl, acetic acid, lactic acid, and the combination of acetic acid and salt where the variability between the highest and the lowest strain was reduced to 3.1-fold, 1.7-fold, 2-fold and 1.7-fold respectively, showing that variability was significant among RpoS-positive strains. Furthermore, we also found a good correlation between acid resistance and lysine decarboxylase activity showing its importance for acid resistance and demonstrated a possible role of RpoS in the lysine decarboxylase activity in Salmonella
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Amino acids other than glutamate affect the expression of the GAD system in Listeria monocytogenes enhancing acid resistance
The Glutamate Decarboxylase (GAD) system is important for survival of L. monocytogenes and other microorganisms under acidic conditions. Environmental conditions influence the function of the GAD system. Until now, the only conditions known to lead to increased transcription of the GAD system are the stationary phase in rich media and anoxic conditions. Previously, we showed that transcription of the GAD system requires unidentified compounds other than glutamate present in rich media. Following a test looking at various compounds we identified for first time that peptone, tryptone and casamino acids activate the GAD system under oxic conditions suggesting that amino acid(s) other than glutamate and/or peptides are important for the above process. The defined medium, where the GAD system is inactive, once it is supplemented with the above compounds results in an active intracellular and extracellular GAD system and increased acid resistance. Through functional genomics we show that these compounds are required for GadD2 activity and although we previously showed that GadD3 is active part of the intracellular GAD system, the supplementation did not activate this gene. The above is explained by the fact that only gadD2 transcription was upregulated by these compounds while the transcription of gadD1 and gadD3 remained unaffected. Together our results show that the L. monocytogenes GadD2 decarboxylase is activated in the presence of amino acids or peptides other than glutamate, a finding that has important implications for acid tolerance and food safety
A systematic quantitative determination of the antimicrobial efficacy of grape seed extract against foodborne bacterial pathogens
Concerns regarding the role of antimicrobial resistance (AMR) in disease outbreaks are growing due to the excessive use of antibiotics. Moreover, consumers are demanding food products that are minimally processed and produced in a sustainable way, without the use of chemical preservatives or antibiotics. Grape seed extract (GSE) is isolated from wine industry waste and is an interesting source of natural antimicrobials, especially when aiming to increase sustainable processing. The aim of this study was to obtain a systematic understanding of the microbial inactivation efficacy/potential of GSE against Listeria monocytogenes (Gram-positive), Escherichia coli and Salmonella Typhimurium (Gram-negative) in an in vitro model system. More specifically, for L. monocytogenes, the effects of the initial inoculum concentration, bacterial growth phase and absence of the environmental stress response regulon (SigB) on the GSE microbial inactivation potential were investigated. In general, GSE was found to be highly effective at inactivating L. monocytogenes, with higher inactivation achieved for higher GSE concentrations and lower initial inoculum levels. Generally, stationary phase cells were more resistant/tolerant to GSE as compared to exponential phase cells (for the same inoculum level). Additionally, SigB appears to play an important role in the resistance of L. monocytogenes to GSE. The Gram-negative bacteria under study (E. coli and S. Typhimurium) were less susceptible to GSE as compared to L. monocytogenes. Our findings provide a quantitative and mechanistic understanding of the impact of GSE on the microbial dynamics of foodborne pathogens, assisting in the more systematic design of natural antimicrobial-based strategies for sustainable food safety
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Ιn Vitro screening of γ-Aminobutyric Acid and Autoinducer-2 signalling in lactic acid bacteria exhibiting probiotic potential isolated from natural black conservolea olives
In the present study, 33 strains of lactic acid bacteria (LAB) previously isolated from natural black Conservolea olives were assessed for their probiotic potential in vitro, as well as for their autoinducer-2 (AI-2) activity under standard growth conditions and the production of γ-aminobutyric acid (GABA). The probiotic tests included the in vitro resistance to low pH and resistance to bile salts, the evaluation of bile salt hydrolase activity, as well as safety tests regarding their possible haemolytic activity and their antimicrobial activity against pathogens. The results indicated that 17 strains were able to survive in low pH and in the presence of bile, with 15 of them also exhibiting partial bile salt hydrolase activity. None of the strains exhibited haemolytic activity or inhibited the growth of any of the examined pathogens. Moreover, the strains displayed generally low AI-2 activity under the growth conditions tested, regardless of the species. Interestingly, in contrast to what has been found in most foods, none of the isolates were found to produce GABA after 48 h of growth. The results from the AI-2 activity and extracellular GABA detection were considered as unexpected for LAB with probiotic attributes