The structure and adaptations of endotoxin in psychrophiles

Gram‐negative bacteria are characterized by a double‐membrane envelope. The outer leaflet of the outer membrane of this envelope is composed mainly of lipid A, the anchor and primary component of endotoxin. Psychrophilic bacteria thrive in cold marine environments, through adaptation to optimal growth at temperatures below 15°C and survival even below 0°C. It is the hypothesis of this work that adaptation to these conditions involves constitutive changes to lipid A structure, in particular the use of shorter acyl residues and/or more unsaturation than that found in closely related mesophiles, as predicted by the physical properties of lipids and the metabolic cold‐shock response of Escherichia coli and others. This hypothesis is tested by determination of the lipid A structures of two psychrophiles using chromatography, mass spectrometry, and chemical degradation analysis, and comparison to the structures of lipid A in related bacteria. MALDI‐TOF mass spectrometry of purified Psychrobacter cryohalolentis lipid A suggests predominant use of short (10–12 carbon) acyl residues and shows an unusual degree of variation, with a cluster of seven molecular species separated each from the next by 14 mass units. In addition, MALDI‐TOF mass spectrometry of Psychromonas marina lipid A shows a more homogenous structure and suggests the presence of unsaturated acyl residues. (this work funded by USNA NARC, DoD DTRA)