Mechanism of antimicrobial action of sodium metasilicate against Salmonella enterica serovar Typhimurium.

Sodium metasilicate (SMS) is an alkaline antimicrobial approved by the U.S. Department of Agriculture for use in poultry processing and ready-to-eat poultry products. The objectives of this study were to determine the effectiveness of SMS against Salmonella enterica serovar Typhimurium in suspension and to elucidate the antimicrobial mechanism of action of SMS. Salmonella Typhimurium (ATCC 14028) was exposed to 0 (positive control), 0.5%, 1%, 2% (wt/vol) SMS and 0.1 N NaOH (high pH) solutions for 1, 10, and 30 min. The viability of Salmonella Typhimurium cells treated with different SMS concentrations and high pH was determined on selective and nonselective media and by staining with fluorescent propidium iodide (PI) and SYTO9 nucleic acid stains in combination with flow cytometry. Transmission electron microscopy of Salmonella Typhimurium cells was performed to observe the changes at the cellular level following exposure to SMS and high pH treatments. Treating Salmonella Typhimurium cells with SMS (as low as 0.5%) resulted in immediate inactivation of Salmonella with no detectable survivors. The breakage in membrane integrity and loss of cell viability was observed by PI uptake by cells treated with SMS with subsequent flow cytometry. Salmonella Typhimurium cells exposed to SMS and high pH appeared wrinkled, vacuolated, and lysed with their cytoplasmic material leaking into extracellular matrix on transmission electron micrographs. The findings from this study indicate that SMS acts on the cytoplasmic membrane and causes lysis of the cells and leakage of intracellular contents.

Antimicrobial effects of sodium metasilicate against Listeria monocytogenes.

Sodium metasilicate (SMS) is a U.S. Department of Agriculture-approved antimicrobial for use in meat and poultry processing and has been known to be effective against various foodborne pathogens. However, its antimicrobial mechanism has not yet been revealed. In this study, we attempted to elucidate the mechanism by which SMS inactivates Listeria monocytogenes, a Gram-positive bacterial pathogen encountered commonly in ready-to-eat meat and poultry products. L. monocytogenes (Scott A) cells were treated with different concentrations of SMS (1.0, 2.0, 3.0, 4.0, 5.0, and 6.0% [wt/vol]) and compared with high pH treatment (0.1, 0.2, and 0.3N NaOH solutions) for 1, 10, and 30 min. SMS exhibited concentration and time effects on inactivation of L. monocytogenes. The effect of SMS on the membrane integrity and viability of L. monocytogenes was determined by use of propidium iodide (PI) and SYTO9 nucleic acid stains with subsequent flow cytometry. The breakage in membrane integrity was observed by uptake of PI by cells treated with SMS with subsequent flow cytometry. Ultrastructural changes from corresponding transmission electron micrographs further revealed the disruption in the cytoplasmic membrane and changes in the morphology of the cells treated with SMS and high pH. The results from flow cytometry experiment and transmission electron microscopy study indicated that following SMS treatment, the membrane integrity of L. monocytogenes was compromised leading to leakage of intracellular contents and subsequent cell death.

The effect of aggregation on the electrical conductivity of spin-coated polymer/carbon nanotube composite films.

This paper considers the feasibility of replacing indium tin oxide (ITO) with spin-coated, polymer-based composite films that are filled with multiwalled carbon nanotubes (MWNTs). The coating mixture consists of a solvent with low volatility, a dissolved thermoplastic polymer, and MWNTs. The high aspect ratio of MWNTs and their good electrical conductivity enable electrical percolation at very low concentrations, so that films can be prepared that conduct electricity while retaining good optical transparency. Although the MWNTs are driven to aggregate by Van der Waals interactions, the high viscosity of the polymer/solvent solution enables the preparation of metastable, homogeneous dispersions. However, exposing the mixtures to shear leads to aggregation, the magnitude of which depends on the duration of the shear. This effect could be observed directly in spin-coated films using both optical microscopy and conductivity measurements, with aggregation causing a drop in conductivity at high nanotube loading, and more complex non-monotonic behavior at concentrations approaching the percolation threshold.

Growth of Listeria monocytogenes at 10°C in Milk Preincubated with Selected Pseudomonads 1.

Preliminary studies involving co-inoculation of Listeria monocytogenes with Pseudomonas fragi into whole or skim milk demonstrated that neither inhibition nor stimulation of growth occurred for either organism. Additional investigations involved preincubation of whole milk, skim milk, and 10% reconstituted nonfat dry milk (NDM) for 3 d at 10°C with P. fragi , Pseudomonas fluorescens P26, P. fluorescens T25, or P. fluorescens B52, followed by inoculation with L. monocytogenes and further incubation at 10°C. Growth curves of L. monocytogenes were constructed for each treatment combination and generation times were statistically compared for differences. Results indicated that L. monocytogenes did not affect growth or survival of the preincubated Pseudomonas spp. However, growth rates of L. monocytogenes were significantly (P<0.05) enhanced in milks preincubated with pseudomonads. Doubling times of L. monocytogenes were reduced by up to 3 h when grown in milk preincubated with Pseudomonas spp. Three strains of P. fluorescens showed more stimulation of the growth rate of L. monocytogenes compared to P. fragi in preincubated whole or skim milk but not in preincubated NDM. Milk composition had little effect on growth of either genus when incubated alone. Results of this study indicate that L. monocytogenes can grow in the presence of Pseudomonas spp. either as a co-inoculant or following preincubation in milk at 10°C. Furthermore, data suggest that the presence of the pseudomonads may enhance growth of L. monocytogenes in milk.

Compositional and Selected Functional Properties of Whey Protein Concentrates and Lactose-Hydrolyzed Whey Protein Concentrates 1.

Commercial whey protein concentrate (WPC) products, manufactured by ultrafiltration with and without lactose hydrolysis, were compared for proximate composition, mineral and trace mineral composition and for protein solubility and viscosity parameters. Protein concentration ranged from 30.5 to 52.7%, while ash content ranged from 5.9 to 12.0%. Extent of lactose hydrolysis in lactose-hydrolyzed WPCs was estimated at 60 to 75% of the initial lactose level. Protein solubility of 10% protein dispersions of the WPC samples ranged from 90 to 100% and was not affected by heating WPC dispersions at 65°C for 30 min or by increased centrifugation force in solubility determination from 40,000 × g to 100,000 × g. All WPC dispersions exhibited pseudoplastic flow behavior as indicated by flow behavior indices (n) of less than 1.0. WPC dispersions possessed a low viscosity as indicated by consistency index (k) data, and k values decreased slightly after heating. Lactose hydrolysis had no apparent effect on solubility or viscosity properties of the WPC dispersions. Alteration of electrophoretic mobility of polyacrylamide gel electrophoresis was observed for α-lactalbumin in lactose-hydrolyzed WPC samples.