Thesis

Studies on the growth, survival, interaction and detection of potentially pathogenic Listeria and Bacillus species in infant milk formulae

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Awarding institution
  • University of Strathclyde
Date of award
  • 1996
Thesis identifier
  • T8594
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Abstract
  • The heat resistance of L. monocytogenes may be significantly improved by modification of the cell’s cultivation and heat treatment environments. The recovery of heat treated Listeria cells was influenced by the cultivating, heating, diluting, enriching and enumerating conditions. Rough cell and colony forms of L. monocytogenes emerged during these thermal studies, which were successfully isolated and identified using current enrichment/enumeration media and diagnostic kits. Both morphological forms exhibited thermal death rate curves, although the rough form was revealed to be more heat resistant. Determination of the cell’s thermotolerance was achieved by linearisation of the survivor curves via the formula (logNo-logN)a = kt+c. Serotype 4b strains of L. monocytogenes were shown to be more thermotolerant and the efficacy of the recommended FDA and USDA enrichment methods was improved by the use of a primary non-selective enrichment stage. A practical method for the identification of Bacillus spp. was developed. A detailed microbiological analysis of 125 infant milk formulae was performed which identified various preparation and storage conditions that influenced the type and number of organisms present. Some Bacillus cereus II isolates produced diarrhoeal enterotoxin in reconstituted infant formulae (supplemented with maltodextrin) at ambient and near refrigeration temperatures. Bacillus spp. were recovered from these products which were previously implicated in either clinical infections and/or food related illnesses, while L. monocytogenes was not detected. The efficacy of current cleaning and sterilisation procedures to eliminate B. cereus from contaminated infant feeding bottles was evaluated. Antagonistic studies between 178 Bacillus spp. (isolated from reconstituted infant formulae) and L. monocytogenes revealed that 22.5% exhibited various levels of antagonism against both morphological forms of Listeria This antagonistic activity was demonstrated in solid, semi-solid and liquid media. These antagonistic Bacillus exhibited various levels of antagonism against other members of the genus Listeria. The study was concluded with an investigation into the nature of this antagonism.
Advisor / supervisor
  • Anderson, John G.
  • Anderton, Annette
Resource Type
DOI
EThOS ID
  • uk.bl.ethos.487780
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