## 2 Background and Knowledge Gap
### Physiological Relevance Under Low-Anmonium Conditions  
In many natural and host environments, such as the urinary tract or certain soil microenvironments, external ammonium concentrations can drop to the low micromolar  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Substrate** & \begin{tabular}{c} **Gradient** \\ **(Cs vs. Trans)** \\ \end{tabular} & \begin{tabular}{c} **Vrev (mV)** \\ **(Experiment)** \\ \end{tabular} & \(P_{anion}\)–\({}^{1}\)\(P_{cation}\)\({}^{*}\) \\ \hline \multirow{4}{*}{**Omp-Pst2**} & \begin{tabular}{c} Ammonium \\ chloride \\ \end{tabular} & 200/50 mM & 22.46 \(\pm\) 7 & 1:05 \\ \cline{2-6}  & \begin{tabular}{c} Ammonium \\ sulfate \\ \end{tabular} & 200/50 mM & 20.06 \(\pm\) 5 & 1:06 \\ \hline \multirow{4}{*}{**OmpF**} & \
\begin{tabular}{c} Ammonium \\ chloride \\ \end{tabular} & 200/50 mM & 18.5 \(\pm\) 5 & 1:09 \\ \cline{2-6}  &
\begin{tabular}{c} Ammonium \\ sulfate \\ \end{tabular} & 200/50 mM & 18\(\pm\)4 & 1:09 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Bi-ionic permeability ratios. Calculated cation flux rate of the different substrates (\(P_{anion}\)–\(P_{cation}\)\({}^{*}\)) for the current flux through Omp-Pst2 and OmpF in ammonium sulfate and ammonium chloride, respectively. Bi-ionic reversal potential \(V_{rev}\) needed to obtain zero current for a concentration gradient for the respective substrate only. Using the Goldmann–Hodgkin–Katz equation gives the permeability ratio \(P_{anion-}/P_{cation+}\) for the ion current flux through Omp-Pst2 and OmpF [23].  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Porin**} & **Salt** & \begin{tabular}{c} **G (Trimer)** \\ **[nS]** \\ \end{tabular} & \begin{tabular}{c} **G (Mono)** \\ **[nS]** \\ \end{tabular} & **Vrev [mV]** & \
\begin{tabular}{c} **Bulk Cond. [mS/cm]** \\ **50 mM Salt** \\ \end{tabular} &
\begin{tabular}{c} **Calculated Flux** \\ **@ 1 \(\mu\)M (ions/s)** \\ \end{tabular} \\ \hline \multirow{2}{*}{**Omp-Pst2**} & **NH4 Cl** & 0.53 \(\pm\) **0.29** & 0.177 & 22.46 & 6.2 & \(\approx\)**6680** \\ \hline \multirow{2}{*}{**Omp-Pst2**} & **(NH4)2 SO4** & 1.17 \(\pm\) **0.6** & 0.39 & 20.06 & 11.78 & \(\approx\)**5520** \\ \hline \multirow{2}{*}{**OmpF**} & **NH4 Cl** & 0.4 \(\pm\) **0.25** & 0.133 & 18.5 & 6.2 & \(\approx\)**4300** \\ \hline \multirow{2}{*}{**OmpF**} & **(NH4)2 SO4** & 1.1 \(\pm\) **0.6** & 0.367 & 18 & 11.78 & \(\approx\)**5150** \\ \hline \hline \end{tabular}
\end{table}
Table 2: Estimated ammonium ion flux through Omp-Pst2 and OmpF porins under defined bi-ionic gradients.  
range (10 \(\upmu\)M or less) [58; 59; 60]. Under such conditions, passive diffusion through porins may become the main route for ammonium to reach the periplasm [16; 18; 21; 61]. When active transporters are either saturated or regulated down, the permeability of the porins can determine how quickly the cell acquires nitrogen. By modeling a realistic gradient of approximately 1 \(\upmu\)M ammonium across the outer membrane, the measured flux rates demonstrate how quickly a bacterium can gather ammonium passively. The finding that Omp-Pst2 can move approximately 6,000 ammonium ions per second under such a small gradient suggests that porin-mediated uptake could account for a significant portion of Providencia stuarti's total ammonium acquisition when nitrogen is limited [1; 62; 63].  
From a therapeutic perspective, these findings point toward two possible antimicrobial strategies: