Biomedical subjects

Linda V Thomas

Publications and source records attributed to Linda V Thomas.

4 recordsLinked to original sources

PubMed2006

Interactions of nisin with glutathione in a model protein system and meat.

Loss of nisin activity in meat has been ascribed, in part, to the formation of a nisin-glutathione adduct. Activity is lost more quickly in raw meat than in cooked meat, and this has been taken as evidence that the reaction is enzyme mediated. Formation of the nisin-glutathione adduct has been confirmed but is shown not to be enzyme mediated. Retention of activity in cooked meat is shown to be due to the loss of free sulfhydryl groups during cooking as a result of the reaction of glutathione with proteins and not a result of the inactivation of endogenous enzymes. Microbial enzymes do not appear to play a role, as similar losses are seen in raw and cooked meat extracts, both of which contained undetectable levels of microorganisms.

Animals↗

PubMed2004

Investigation of the effectiveness of Ascopyrone P as a food preservative.

Ascopyrone P (APP), a novel antibacterial from fungi, was evaluated as a food preservative. Efficacy was generally assessed by comparing the time taken for test strains to grow to 10(6) CFU/g in food +/- APP. In chilled chicken soup, 2000 mg kg-1 APP prevented Bacillus cereus, Listeria monocytogenes, Pseudomonas fluorescens, Salmonella and Escherichia coli reaching this threshold for >60 days. Good activity was also observed at 500-1000 mg kg-1 but not against L. monocytogenes. No activity was observed against Saccharomyces cerevisiae. Activity was reduced at 20 degrees C, although 2000 mg kg-1 was still effective against B. cereus and P. fluorescens. APP was less effective in chilled cooked meat systems and ineffective in raw meat. In a cooked meat system at 8 degrees C, bacteriostatic effect was generally observed at 2000 mg kg-1 against Salmonella typhimurium, E. coli and P. fluorescens but not against L. monocytogenes or Lactobacillus sake. Activity against Gram-negative enteric bacteria was enhanced by low temperature. In milk, 2000 mg l-1 was effective against P. fluorescens at chilled but not ambient temperature. APP was ineffective against yeasts and the mould Byssochlamys in apple juice. A minimum of 2000 mg kg-1 APP would appear to be necessary for antibacterial efficacy in food, although low-temperature storage may help. Observed variations in sensitivity may be related to APP stability, which decreases >pH 5.5. Toxicology testing is needed before consideration of APP for food use.

Anti-Bacterial Agents↗

PubMed2002

Effective use of nisin to control Bacillus and Clostridium spoilage of a pasteurized mashed potato product.

Heat-resistant spore-forming bacteria such as Bacillus and Clostridium can survive and grow in cooked potato products. This situation represents both a public health problem and an economic problem. The natural food preservative nisin is used in heat-treated foods to prevent the growth of such bacteria. A cocktail of Clostridium sporogenes and Clostridium tyrobutyricum spores was inoculated into cooked mashed potatoes, which were vacuum packed, pasteurized, and incubated at 8 and 25 degrees C. The shelf life of the mashed potatoes at 25 degrees C was extended by at least 58 days with the addition 6.25 microg of nisin per g. At 8 degrees C, in control samples not containing nisin, the natural contaminant Bacillus grew, but the inoculated Clostridium strains did not until the temperature was raised to 20 degrees C after 39 days. No bacterial growth occurred in nisin-containing samples. The shelf life of the mashed potatoes was extended by at least 30 days with 6.25 microg of nisin per g. In trials involving a cocktail of Bacillus cereus and Bacillus subtilis strains, 6.25 microg of nisin per g extended the shelf life of mashed potato samples that were not vacuum packed by at least 27 days at 8 degrees C. At 25 degrees C, 25 microg of nisin per g extended shelf life by a similar period. Shelf life extension was also observed at lower nisin levels. Microbiological analysis of the mashed potato ingredients showed that a high spore level was associated with the onion powder. It is emphasized that the preservative and the ingredients must be well mixed to ensure good nisin efficacy. Nisin remained at effective levels after pasteurization, and good retention was observed throughout the shelf life of the mashed potatoes.

Anti-Bacterial Agents↗

PubMed1997

Spatial interactions between subsurface bacterial colonies in a model system: a territory model describing the inhibition of Listeria monocytogenes by a nisin-producing lactic acid bacterium.

The effect of spatial separation on interactions between subsurface bacterial colonies was tested using a model system: the inhibition of Listeria monocytogenes by nisin-producing and nisin-non-producing Lactococcus lactis subsp. lactis. Separation distance was controlled by altering the number of inoculum organisms within the agar. Mean separation distance was calculated by determining the mean volume available to each cell at the start of the experiment. Inhibition was assessed by comparing the growth of L. monocytogenes in pure culture with its growth in the presence of Lac. lactis subsp. lactis. Increasing the distance between colonies resulted in an exponential decrease in inhibition. When L. monocytogenes and Lac. lactis subsp. lactis colonies were within 100 microns of each other, the increase in cell numbers per L. monocytogenes colony was only 0.6 c.f.u. (which indicated some cells had become non-viable). This was a log reduction of 3.5 compared to the pure culture control. A separation distance of 1000 microns resulted in a L. monocytogenes colony growth increment of 2.5 x 10(2) c.f.u. per colony, a log reduction of 3.0 compared to the control. Increasing the separation distance to 3000 microns resulted in a L. monocytogenes colony growth increment of 1.3 x 10(6) c.f.u. per colony, a log reduction of 0.9 compared to the control. The effects of nisin and acidity were investigated by using a nisin-non-producing strain of Lac. lactis subsp. lactis and by buffering the medium. Data were obtained for the effect of separation on inhibition, as well as competition between colonies of the same species. The inhibition was mathematically described in terms of a simplified 'territory' model of immobilized bacterial growth. There was a strong qualitative agreement between the mathematical model and the experimental data. It was concluded that the phenomenon of propinquity is of important consideration when modeling and predicting microbial growth within solid food systems.

Antibiosis↗