87-1 |
Influence of heat adaptation on changes in membrane lipid composition and quantities of verotoxin in E. coli O157:H7 |
H. G. YUK, Food Science & Technology, Mississippi State University, Box 9805, Mississippi State, MS 39762 and D. L. Marshall.
Understanding the ability of E. coli O157:H7 to adapt to temperature fluctuations is an important area of investigation because these fluctuations occur commonly in nature and during food storage, processing, and transportation. Although many studies have determined that heat resistance is due to the presence of heat shock proteins, one should not ignore the function of the bacterial membrane as a protective barrier. Because membranes are the first structures involved in protecting a cell from thermal stress, there is a need to study response of membrane components to thermal stresses. The objective of this study was to measure changes in membrane lipid composition and changes in verotoxin production of heat adapted E. coli O157:H7. Heat adapted cells were made by incubating cultures at 37°C for 18 hours then shifted 42°C for 18 hours then followed by an additional 15 hours at 45°C. After incubation at each temperature, each population was measured for heat tolerance (D57°C), membrane lipid composition, and the quantity of released verotoxin. Membrane lipids were measured by gas chromatography and verotoxin was measured by optical density (450 nm) of ELISA reactions. D values of control cells at 37°C and heat adapted cells at 42 and 45°C were 7.8, 8.8, and 10.4 minutes, respectively. The increase of growth temperature increased the amount of hexadecanoic (palmitic) acid (C 16:0) and decreased the amount of octadecenoic (oleic) acid (C 18:1) in membrane lipids. The quantities of released verotoxin of control cells and heat adapted cells at 42 and 45°C were O.D. 1.4336, 1.8173, and 1.8054, respectively. D values, amount of saturated fatty acid, and the amount of released verotoxin increased significantly with elevated growth temperatures. These changes may be due to a protective response against elevated incubation temperature and may increase the risk of foodborne disease with E. coli O157:H7.
Session 87, Food Microbiology: General
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