Adaptation Rates of E. Coli in Monovalent Salt Solutions

Abstract

A complete description of bacterial populations requires a large-scale model of how populations adapt to their environment. In this study we start with a bacterial pre-culture that has been grown in LB (Lennox Formula) broth and use 500 μL aliquots of preculture to grow E. coli colonies in a shaker at 37°C and ∼300rpm. These experiments are conducted in 500 mL Erlenmeyer flasks with a working volume of 125 mL and cell density is measured by optical density at 600 nm. In concentrated monovalent salt solutions, E. coli growth rate is affected by both the positive and the negative ions. For both chloride and bromide salts, we obtain the toxicity sequence K > Na >Li. Comparing the effect of anions, we find that bromide salts are more toxic than chloride salts. For adaptation studies to environmental stress, we use either 500 mM NaCl or a temperature shift from 37 to 41.5°C. Adaptation at the population level is marked by a sudden increase in population growth. We discuss the observed timescale of the adaptation process and its functional form which are useful for constructing mathematical models for bacterial populations.