[Functional and structural changes of E. coli membranes induced by low temperature freezing]

Area of Science:

• Microbiology
• Cell Biology
• Biochemistry

Background:

• Low temperature freezing impacts cellular functions in bacteria like E. coli.
• Understanding membrane integrity post-freezing is crucial for cell survival and function.

Purpose of the Study:

• To investigate the effects of freezing on E. coli cell respiration, ATP synthesis, and membrane permeability.
• To identify the specific mechanisms of cellular damage caused by freezing.

Main Methods:

• Measurement of endogenous respiration and NADH-stimulated respiration.
• Assay of electron transport chain activities (dehydrogenase, oxidase) and cytochrome content.
• Assessment of ATP synthesis capacity and membrane ATPase activity.
• Analysis of proton permeability using pH changes.
• Spin labeling studies to probe membrane lipid dynamics.

Main Results:

• Freezing decreased endogenous respiration but stimulated NADH respiration in E. coli.
• Electron transport chain and ATPase activities remained largely unchanged.
• Frozen cells lost the ability to synthesize ATP via artificial proton motive force.
• Proton permeability barrier was disturbed, correlating with cell survival.
• Spin labeling indicated changes in the membrane surface and hydrophobic lipid zone, without inducing lipid peroxidation.

Conclusions:

• Freezing damages the E. coli cell membrane's penetrability barrier, affecting respiration substrates and potentially uncoupling oxidative phosphorylation.
• Membrane alterations, not electron transport chain dysfunction, are key to freezing-induced damage.
• The integrity of the membrane penetrability barrier is directly linked to cell survival after freezing.