Although the mechanisms by which organic acids inhibit growth of bacteria in mildly acidic foods are not fully understood, it is clear that intracellular accumulation of anions is a primary ...contributor to inhibition of bacterial growth. We hypothesize that intracellular accumulation of anions is driven by 2 factors, external anion concentration and external acidity. This hypothesis follows from basic chemistry principles that heretofore have not been fully applied to studies in the field, and it has led us to develop a novel approach for predicting internal anion concentration by controlling the external concentration of anions and pH. This approach overcomes critical flaws in contemporary experimental design that invariably target concentration of either protonated acid or total acid in the growth media thereby leaving anion concentration to vary depending on the pKa of the acids involved. Failure to control external concentration of anions has undoubtedly confounded results, and it has likely led to misleading conclusions regarding the antimicrobial action of organic acids. In summary, we advocate an approach for directing internal anion levels by controlling external concentration of anions and pH because it presents an additional opportunity to study the mechanisms by which organic acids inhibit bacterial growth. Knowledge gained from such studies would have important application in the control of important foodborne pathogens such as Listeria monocytogenes, and may also facilitate efforts to promote the survival in foods or beverages of desirable probiotic bacteria.
An erythromycin-resistant strain of probiotic Lactobacillus paracasei ssp. paracasei LBC-1 (LBC-1e) was added to part-skim Mozzarella cheese in alginate-microencapsulated or free form at a level of ...108 and 107cfu/g, respectively. Survival of LBC-1e and total lactic acid bacteria (LAB) was investigated through the pasta filata process of cheese making (in which the cheese curd was heated to 55°C and stretched in 70°C-hot brine), followed by storage at 4°C for 6wk and simulated gastric and intestinal digestion. This included incubation in 0.1 M and 0.01 M HCl, 0.9 M H3PO4, and a simulated intestinal juice consisting of pancreatin and bile salts in a pH 7.4 phosphate buffer. Some reductions were observed in both free and encapsulated LBC-1e during heating and stretching, with encapsulated LBC-1e surviving slightly better. Changes in total LAB losses during heating and stretching did not reach statistical significance. During storage, a decrease was observed in total LAB, but no statistically significant decrease was observed in LBC-1e. Survival during gastric digestion in HCl was dependent on the extent of neutralization of HCl by the cheese, with more survival in the weaker acid, in which pH increased to 4.4 after cheese addition. The alginate microcapsules did not provide any protection against the HCl. It is interesting that survival of the encapsulated LBC-1e was greater during incubation in H3PO4 than in the HCl gastric juices. Proper selection of simulated gastric digestion media is important for predicting the delivery of probiotic bacteria into the human intestinal tract. Neither free nor encapsulated LBC-1e was affected by incubation in the pancreatin-bile solution. Based on the level of probiotic bacteria in cheese needed to provide a health benefit and its survival during simulated gastric digestion, as determined in this study, it should theoretically be possible to lower the amount that needs to be ingested in cheese by up to a factor of 103 compared with other fermented dairy foods or when consumed as supplements.
Lactobacillus helveticus CNRZ 32 is recognized for its ability to decrease bitterness and accelerate flavor development in cheese, and has also been shown to release bioactive peptides in milk. ...Similar capabilities have been documented in other strains of Lb. helveticus, but the ability of different strains to affect these characteristics can vary widely. Because these attributes are associated with enzymes involved in proteolysis or AA catabolism, we performed comparative genome hybridizations to a CNRZ 32 microarray to explore the distribution of genes encoding such enzymes across a bank of 38 Lb. helveticus strains, including 2 archival samples of CNRZ 32. Genes for peptidases and AA metabolism were highly conserved across the species, whereas those for cell envelope-associated proteinases varied widely. Some of the genetic differences that were detected may help explain the variability that has been noted among Lb. helveticus strains in regard to their functionality in cheese and fermented milk.
We compared spray washing at 55.4
°C with 2% levulinic acid to that with lactic or acetic acid for decontamination of pathogenic bacteria inoculated onto meat surfaces, and their residual protection ...against later growth of pathogenic bacteria. The model systems included
Escherichia coli O157:H7 on beef plate,
Salmonella on chicken skin and pork belly, and
Listeria monocytogenes on turkey roll. In the decontamination studies, acid washes lowered recoverable numbers of pathogens by 0.6 to 1
log/cm
2 as compared to no-wash controls, and only lactic acid lowered the number of pathogens recovered as compared to the water wash. Washing with levulinic acid at 68.3 or 76.7
°C did not result in additional decontamination of
E. coli. Acetic acid prevented residual growth of
E. coli and
L. monocytogenes, and it reduced numbers of
Salmonella on chicken skin to below recoverable levels. Overall, levulinic acid did not provide as effective decontamination as lactic acid nor residual protection as acetic acid.
Non-invasive assessment of myocardial ischaemia is a cornerstone of the diagnosis of coronary artery disease. Measurement of myocardial blood flow (MBF) using positron emission tomography (PET) is ...the current reference standard for non-invasive quantification of myocardial ischaemia. Dynamic myocardial perfusion cardiovascular magnetic resonance (CMR) offers an alternative to PET and a recently developed method with automated inline perfusion mapping has shown good correlation of MBF values between CMR and PET. This study assessed the repeatability of myocardial perfusion mapping by CMR in healthy subjects.
Forty-two healthy subjects were recruited and underwent adenosine stress and rest perfusion CMR on two visits. Scans were repeated with a minimum interval of 7 days. Intrastudy rest and stress MBF repeatability were assessed with a 15-min interval between acquisitions. Interstudy rest and stress MBF and myocardial perfusion reserve (MPR) were measured for global myocardium and regionally for coronary territories and slices.
There was no significant difference in intrastudy repeated global rest MBF (0.65 ± 0.13 ml/g/min vs 0.62 ± 0.12 ml/g/min, p = 0.24, repeatability coefficient (RC) =24%) or stress (2.89 ± 0.56 ml/g/min vs 2.83 ± 0.64 ml/g/min, p = 0.41, RC = 29%) MBF. No significant difference was seen in interstudy repeatability for global rest MBF (0.64 ± 0.13 ml/g/min vs 0.64 ± 0.15 ml/g/min, p = 0.80, RC = 32%), stress MBF (2.71 ± 0.61 ml/g/min vs 2.55 ± 0.57 ml/g/min, p = 0.12, RC = 33%) or MPR (4.24 ± 0.69 vs 3.73 ± 0.76, p = 0.25, RC = 36%). Regional repeatability was good for stress (RC = 30-37%) and rest MBF (RC = 32-36%) but poorer for MPR (RC = 35-43%). Within subject coefficient of variation was 8% for rest and 11% for stress within the same study, and 11% for rest and 12% for stress between studies.
Fully automated, inline, myocardial perfusion mapping by CMR shows good repeatability that is similar to the published PET literature. Both rest and stress MBF show better repeatability than MPR, particularly in regional analysis.
Expansion of the myocardial extracellular volume (ECV) is a surrogate measure of focal/diffuse fibrosis and is an independent marker of prognosis in chronic heart disease. Changes in ECV may also ...occur after myocardial infarction, acutely because of oedema and in convalescence as part of ventricular remodelling. The objective of this study was to investigate changes in the pattern of distribution of regional (normal, infarcted and oedematous segments) and global left ventricular (LV) ECV using semi-automated methods early and late after reperfused ST-elevation myocardial infarction (STEMI).
Fifty patients underwent cardiovascular magnetic resonance (CMR) imaging acutely (24 h-72 h) and at convalescence (3 months). The CMR protocol included: cines, T2-weighted (T2 W) imaging, pre-/post-contrast T1-maps and LGE-imaging. Using T2 W and LGE imaging on acute scans, 16-segments of the LV were categorised as normal, oedema and infarct. 800 segments (16 per-patient) were analysed for changes in ECV and wall thickening (WT).
From the acute studies, 325 (40.6%) segments were classified as normal, 246 (30.8%) segments as oedema and 229 (28.6%) segments as infarct. Segmental change in ECV between acute and follow-up studies (Δ ECV) was significantly different for normal, oedema and infarct segments (0.8 ± 6.5%, -1.78 ± 9%, -2.9 ± 10.9%, respectively; P < 0.001). Normal segments which demonstrated deterioration in wall thickening at follow-up showed significantly increased Δ ECV compared with normal segments with preserved wall thickening at follow up (1.82 ± 6.05% versus -0.10 ± 6.88%, P < 0.05).
Following reperfused STEMI, normal myocardium demonstrates subtle expansion of the extracellular volume at 3-month follow up. Segmental ECV expansion of normal myocardium is associated with worsening of contractile function.
Many strains of Streptococcus thermophilus synthesize extracellular polysaccharides. These molecules may be produced as capsules that are tightly associated with the cell, or they may be liberated ...into the medium as a loose slime (i.e., "ropy" polysaccharide). Although the presence of exopolysaccharide does not confer any obvious advantage to growth or survival of S. thermophilus in milk, in situ production by this species or other dairy lactic acid bacteria typically imparts a desirable "ropy" or viscous texture to fermented milk products. Recent work has also shown that exopolysaccharide-producing S. thermophilus can enhance the functional properties of Mozzarella cheese, but they are not phage-proof. As our understanding of the genetics, physiology, and functionality of bacterial exopolysaccharides continues to improve, novel applications for polysaccharides and polysaccharide-producing cultures are likely to emerge inside and outside the dairy industry. This article provides an overview of biochemistry, genetics, and applications of exopolysaccharide production in S. thermophilus.
This study used Lactobacillus casei 334e, an erythromycin-resistant derivative of ATCC 334, as a model to evaluate viability and acid resistance of probiotic L. casei in low-fat Cheddar cheese and ...yogurt. Cheese and yogurt were made by standard methods and the probiotic L. casei adjunct was added at approximately 10⁷ CFU/g with the starter cultures. Low-fat cheese and yogurt samples were stored at 8 and 2 °C, respectively, and numbers of the L. casei adjunct were periodically determined by plating on MRS agar that contained 5 μg/mL of erythromycin. L. casei 334e counts in cheese and yogurt remained at 10⁷ CFU/g over 3 mo and 3 wk, respectively, indicating good survival in both products. Acid challenge studies in 8.7 mM phosphoric acid (pH 2) at 37 °C showed numbers of L. casei 334e in yogurt dropped from 10⁷ CFU/g to less than 10¹ CFU/g after 30 min, while counts in cheese samples dropped from 10⁷ CFU/g to about 10⁵ after 30 min, and remained near 10⁴ CFU/g after 120 min. As a whole, these data showed that low-fat Cheddar cheese is a viable delivery food for probiotic L. casei because it allowed for good survival during storage and helped protect cells against the very low pH that will be encountered during stomach transit.
Sodium reduction in cheese can assist in reducing overall dietary Na intake, yet saltiness is an important aspect of cheese flavor. Our objective was to evaluate the effect of partial substitution of ...Na with K on survival of lactic acid bacteria (LAB) and nonstarter LAB (NSLAB), pH, organic acid production, and extent of proteolysis as water-soluble nitrogen (WSN) and protein profiles using urea-PAGE, in Cheddar cheese during 9mo of storage. Seven Cheddar cheeses with molar salt contents equivalent to 1.7% salt but with different ratios of Na, K, Ca, and Mg cations were manufactured as well as a low-salt cheese with 0.7% salt. The 1.7% salt cheeses had a mean composition of 352g of moisture/kg, 259g of protein/kg and 50% fat-on-dry-basis, and 17.5g of salt/kg (measured as Cl(-)). After salting, a faster initial decrease in cheese pH occurred with low salt or K substitution and it remained lower throughout storage. No difference in intact casein levels or percentage WSN levels between the various cheeses was observed, with the percentage WSN increasing from 5% at d 1 to 25% at 9mo. A greater decrease in intact αs1-casein than β-casein was detected, and the ratio of αs1-casein (f121-199) to αs1-casein could be used as an index of ripening. Typical changes in bacteria microflora occurred during storage, with lactococci decreasing gradually and NSLAB increasing. Lowering the Na content, even with K replacement, extended the crossover time when NSLAB became dominant. The crossover time was 4.5mo for the control cheese and was delayed to 5.2, 6.0, 6.1, and 6.2mo for cheeses with 10, 25, 50, and 75% K substitution. Including 10% Mg or Ca, along with 40% K, further increased crossover time, whereas the longest crossover time (7.3mo) was for low-salt cheese. By 9mo, NSLAB levels in all cheeses had increased from initial levels of ≤10(2) to approximately 10(6)cfu/g. Lactococci remained at 10(6) cfu/g in the low-salt cheese even after 9mo of storage. The propionic acid concentration in the cheese increased when NSLAB numbers were high. Few other trends in organic acid concentration were observed as a function of Na content.
The structure and dynamics of microbial populations play a significant role during cheese manufacture and ripening. Therefore, fast and accurate methods for identification and characterization of the ...microbial populations are of fundamental importance to the cheese industry. In this study, we investigate the application of the automated ribosomal intergenic spacer analysis (ARISA) for the assessment of the microbial dynamics in cheeses differing in salt cation level and type. We developed a database of the observed and theoretical length of the 16S‐23S intergenic spacer of common lactic acid bacteria (LAB) found in cheese and used the database to describe the structure and dynamics of microbial populations during ripening. Salt content and cation concentration did not significantly influence the overall bacteria structure, except that lower salt levels resulted in enhanced starter survival. Presence of nonstarter LAB was detected by ARISA and denaturing gradient gel electrophoresis (DGGE) after 3 months for all the cheeses analysed. ARISA used as fingerprinting method, proved to be a rapid and inexpensive technique for the discrimination of LAB in cheese and demonstrated higher resolution and performance in comparison with DGGE. SIGNIFICANCE AND IMPACT OF THE STUDY: Microbial communities play important roles during cheese making and ripening, hence rapid inexpensive methods to characterize this microbiota are of great interest to both academic and industrial scientists. The application of automated ribosomal intergenic spacer analysis (ARISA) was used to examine the microbial ecology of Cheddar cheese differing in salt level and type. ARISA is well suited to the analysis of the microbial ecology of cheese during ripening. Additionally, the results confirm that salt concentration influences starter culture survival in the cheese matrix, while significant differences were not observed in the nonstarter lactic acid bacteria.
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