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

A note on the alpha-quantile option

In this communication, we discuss some properties of a class of path dependent options based on the α-quantiles of Brownian motion. In particular we show that such options are well behaved in relation to standard options and comparatively cheaper than an equivalent class of lookback options

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

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

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

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

Microbial species and strain heterogeneity affect resistance to high pressure processing

Ten strains of each of Listeria monocytogenes (BHI & TSB-D), Escherichia coli (TSB-D), Lactiplantibacillus plantarum (MRS) and Saccharomyces cerevisiae (MEA) were exposed to High Hydrostatic Pressure (HHP; 200, 300 and 400 MPa, 10 min, 20 ◦C) to investigate the impact of species and strain variability in piezotolerance. L. monocytogenes was the most resistant, followed by E. coli, L. plantarum and S. cerevisiae. L. monocytogenes L6 was the most robust and NCTC 10357 the most sensitive strain, while overall survival was better in TSB-D (no glucose) than in BHI (naturally contains glucose) under similar pressures. Strains ranked differently according to their piezotolerance in the two media, while this was serotype-dependent in TSB-D. E. coli strain variability was detected under all conditions with O157 VT- and FAM 21843 as most robust and most sensitive respectively. L. plantarum FBR04 and ATCC14917 were the most resistant and sensitive (300 MPa), respectively, while for S. cerevisiae this corresponded to AD1890 and 028.0315 (200 MPa). Industrial relevance: This study confirms the importance of species and strain variability in HHP. The results are relevant for the improvement of decontamination efficiency predictions, the design of validation studies and the application of hurdle technology. Knowledge of microbial inactivation and strain variability under mild HHP conditions can allow fine-tuning of hurdle technology and lead to production of safer, more affordable HHP-treated food due to decrease of operating costs

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

The three lipocalins of egg-white: only Ex-FABP inhibits siderophore-dependent iron sequestration by Salmonella Enteritidis

Salmonella Enteritidis is the most prevalent food-borne pathogen associated with egg-related outbreaks in the European Union. During egg colonization, S. Enteritidis must resist the powerful anti-bacterial activities of egg white (EW) and overcome ovotransferrin-imposed iron-restriction (the most important anti-bacterial mechanism of EW). Many pathogens respond to iron restriction by secreting iron-chelating chemicals called siderophores but EW contains a siderophore-sequestering “lipocalin” protein (Ex-FABP) that is predicted to limit the usefulness of siderophores in EW. S. Enteritidis produces two siderophores: enterobactin, which is strongly bound by Ex-FABP; and the di-glucosylated enterobactin-derivative, salmochelin (a so-called “stealth” siderophore), which is not recognized by Ex-FABP. Thus, production of salmochelin may allow S. Enteritidis to escape Ex-FABP-mediated growth inhibition under iron restriction although it is unclear whether its EW concentration is sufficient to inhibit pathogens. Further, two other lipocalins (Cal-γ and α-1-ovoglycoprotein) are found in EW but their siderophore sequestration potential remains unexplored. In addition, the effect of EW lipocalins on the major EW pathogen, S. Enteritidis, has yet to be reported. We overexpressed and purified the three lipocalins of EW and investigated their ability to interact with the siderophores of S. Enteritidis, as well as their EW concentrations. The results show that Ex-FABP is present in EW at concentrations (5.1 μM) sufficient to inhibit growth of a salmochelin-deficient S. Enteritidis mutant under iron restriction but has little impact on the salmochelin-producing wildtype. Neither Cal-γ nor α-1-ovoglycoprotein bind salmochelin or enterobactin, nor do they inhibit iron-restricted growth of S. Enteritidis. However, both are present in EW at significant concentrations (5.6 and 233 μM, respectively) indicating that α-1-ovoglycoprotein is the 4th most abundant protein in EW, with Cal-γ and Ex-FABP at 11th and 12th most abundant. Further, we confirm the preference (16-fold) of Ex-FABP for the ferrated form (Kd of 5.3 nM) of enterobactin over the iron-free form (Kd of 86.2 nM), and its lack of affinity for salmochelin. In conclusion, our findings show that salmochelin production by S. Enteritidis enables this key egg-associated pathogen to overcome the enterobactin-sequestration activity of Ex-FABP when this lipocalin is provided at levels found in EW