Steady state growth of \textit{E. Coli} in low ammonium environment

Ammonium is the preferred nitrogen source for many microorganisms. In medium with low ammonium concentrations, enteric bacteria turn on the nitrogen responsive (ntr) genes to assimilate ammonium. Two proteins in \textit{E. coli}, Glutamine synthetase (GS) and the Ammonium/methylammonium transporter AmtB play crucial roles in this regard. GS is the major ammonium assimilation enzyme below 1mM of NH$_{4}^{+}$. AmtB is an inner membrane protein that transports NH$_{4}^{+}$ across the cell membrane against a concentration gradient. In order to study ammonium uptake at low NH$_{4}^{+}$ concentration at neutral pH, we developed a microfluidic flow chamber that maintains a homogenous nutrient environment during the course of exponential cell growth, even at very low concentration of nutrients. Cell growth can be accurately monitored using time-lapse microscopy. We followed steady state growth down to micro-molar range of NH$_{4}^{+}$ for the wild type and $\Delta $amtB strains. The wild type strain is able to maintain the growth rate from 10mM down to a few uM of NH$_{4}^{+}$, while the mutant exhibited reduced growth below $\sim $20~uM of NH$_{4}^{+}$. Simultaneous characterization of the expression levels of GS and AmtB using fluorescence reporters reveals that AmtB is turned on already at 1mM, but contributes to function only below $\sim $30~uM in the wild-type. Down to $\sim $20~uM of NH$_{4}^{+}$, \textit{E.~coli} can compensate the loss of AmtB by GS alone.