RESUMEN
Listeria monocytogenes is a food-borne, pathogenic, psychrotolerant bacterium that grows at refrigeration temperatures. Long-range membrane order of the parent (10403S) and of a cold-sensitive mutant ( cld-1) deficient in odd-numbered, branched-chain fatty acids was measured using the width of the central line of spectra of an electron paramagnetic resonance probe, 4,4-dimethyl-2-heptyl-2-hexyloxazolidine- N-oxyl (7N14), that locates deep in the hydrocarbon region of the membranes. The line width decreased from 0.9 to 0.5 milliTesla (mT) over the temperature range of 0-10 degrees for strain 10403S and -5 to 32 degrees C for strain cld-1 independent of protein state (heat denatured or intact). This provided new evidence for phase transitions in the membranes. When strain cld-1 was grown in medium supplemented with 2-methylbutyric acid, which restores anteiso fatty acids and the ability to grow at low temperature, the change in central line width as a function of temperature resembled that of strain 10403S. The temperatures at which the central line width became 0.8 mT corresponded to those at which growth became very slow in both strains (3-5 degrees C for 10403S, 15 degrees C for cld-1) as determined by Arrhenius plots. These data underscore the critical role of odd-numbered anteiso fatty acids in influencing the lower temperature limits of growth through their effects on long-range membrane fluidity.