Doctoral Dissertations

Date of Award

5-1992

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Food Science and Technology

Major Professor

P.M. Davidson

Committee Members

H.O. Jaynes, F.A. Draughon, G.L Christen, D.A. Bemis

Abstract

Lipid profile of nine strains of Listeria monocytogenes and its variation in the presence of antimicrobials (parabens and salt) and varying temperatures was determined. In addition, an attempt was made to develop L. monocytogenes cells with a defined fatty acid composition. The resistance of these cells and parent strains of L. monocytogenes to antimicrobials was compared. The effect of the antimicrobials on lipid synthesizing enzymes was carried out. Correlations between cytoplasmic membrane lipid composition, or enzymes, or both, and susceptibility of L. monocytogenes to antimicrobials were determined.

The average total lipid content was 6.65% (dry weight basis). The two major phospholipids found were phosphatidyl glycerol and cardiolipin. Phosphatidyl serine and phosphatidyl ethanolamine were detected only in strain Scott A and LM4C, respectively. Large percentages of branched chain fatty acids (C15:0br and C17:0br) were characteristic of all strains. The major differences among strains was the relative percentages of these fatty acids as well as variable presence of certain long chain fatty acids.

Phospholipid composition and fatty acid profiles of two strains of L. monocytogenes varied greatly in the presence of antimicrobials and at different temperature. The percentage of phosphatidyl glycerol increased and the cells secreted saturated fatty acids. Also, decreases in relative percentages of branched chain fatty acids and variable changes in long chain fatty acids were found with increasing concentration of antimicrobials in the growth medium. Increasing temperature indicated similar results with the exception that most of the medium chain fatty acids decreased with temperature increased.

Cells adapted to saturated fatty acids (C14:0 and 018:0) were more resistant to antimicrobials than the parent strains. An opposite trend was observed for the cells adapted to unsaturated fatty acid (C18:1). The effect of the antimicrobials on fatty acid synthesizing enzymes indicated that antimicrobials altered enzyme activity.

The resistance of L. monocytogenes may be mediated by lipid composition. Results support a correlation between lipid composition and susceptibility of L. monocytogenes. Therefore, food preservation processes alter lipid composition of L. monocytogenes could result in changes in susceptibility to antimicrobials.

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