Influence of Amino Acids on Low-Density Escherichia coli Responses to Nutrient Downshifts

Abstract

ABSTRACT A vast bibliography on nutrient effects on high-density cultures exists, while it has been overlooked that low densities of starving cells are often the rule in natural environments. By means of a novel sensitive β-galactosidase assay, we examined Escherichia coli transitions to minimal media when the cell concentration was 100 to 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 relationship between β-galactosidase activity and the l -amino acid pool was hyperbolic. The affinity constant when the amino acid pool was the only nutrient source averaged 14 μM after 90 min and increased up to 222 μM after 4.5 h. While investigating the transition from lag phase 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 at which E. coli linearly synthesized β-galactosidase despite, at the same time, suffering a large decrease in cell viability was then recognized. Since both β-galactosidase production and the dilution rate were reduced by more than half in the absence of leucine, we examined the contribution of leucine to cell recovery capabilities.