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J. Bacteriol. doi:10.1128/JB.01753-06
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

The influence of amino acids on low density Escherichia coli responses to nutrient downshifts

Department of Environmental Biotechnologies, ISRIM Scarl, Località Pentima Bassa, 21 - 05100 Terni, Italy

^* To whom correspondence should be addressed. Email: s.bodini{at}isrim.it.

	Abstract

A vast bibliography exists on nutrient effects on high-density cultures, while it has been overlooked that low densities of starving cells are often the rule in natural environments. By means of a novel sensitive -galactosidase assay, we examined E. coli transitions to minimal media when the cell concentration was 100 - 10,000 cells per ml. As in high density cultures, the enzyme activity depended on amino acid availability and was subject to catabolite repression and stringent control. In all conditions tested, despite the presence of other nutrient sources, the relation between -galactosidase activity and the l-amino acid pool was hyperbolic. The affinity constant K_AA, when the amino acid pool was the only nutrient source, averaged 14 µM after 90 minutes and increased up to 222 µM after 4.5 hours. While investigating the transition from lag to exponential phase, we observed that the cells did not enter into starvation mode in the presence of amino acids, even when the nutrient amount was insufficient to support full survival. Based on these premises, the switch from starvation to hunger was investigated in relation to the amino acid pools. A critical range of concentrations was then recognized at which E. coli linearly synthesised -galactosidase despite, at the same time, suffering a large decrease in cell viability. Since both -galactosidase production and dilution rate were reduced by more than half in the absence of leucine, we discussed the contribution of leucine to cell recovery capabilities.