## 7 Conclusions  
This study employed a simple and dependable electrophysiological technique [35; 36] to quantify the transport of ammonium through two bacterial outer membrane porins: Omp-Pst2 from _Providencia stuartti_ and OmpF from _Escherichia coli_[24; 28; 36]. Forming an artificial lipid bilayer and applying controlled salt gradients [36] allowed us to record the electrical currents generated by individual porin channels, revealing several key insights [36]. (1) Higher ammonium flux through Omp-Pst2  
Under identical ammonium chloride gradients, Omp-Pst2 conducted approximately \(\approx\) 5000-6000 ammonium ions per second, whereas flux from OmpF conducted around \(\approx\) 4000-5000 ions per second. Even when ammonium sulfate was used, Omp-Pst2 maintained a higher overall flux than OmpF. These results suggest that in environments with limited ammonium [1; 62; 85], Omp-Pst2 can deliver nitrogen to the periplasm faster than OmpF. (2) Greater selectivity in OmpF  
Although OmpF transported fewer ammonium ions overall, it did so with a stronger preference for ammonium over its counterion (chloride or sulfate). In contrast, Omp-Pst2 allowed a higher proportion of counterions to pass alongside ammonium. This suggests that OmpF may play a more pronounced role in maintaining ionic balance within the periplasm [86] while Omp-Pst2 focuses on maximizing throughput. (3) Influence of counterions  
Switching the accompanying anion from chloride to sulfate altered the transport speed and selectivity of each porin. Specifically, sulfate reduced ammonium flux slightly compared to chloride. This observation highlights the fact that the mixture of available salts in real bacterial cells can directly influence the efficiency with which porins import ammonium. Taken together, these results shed light on how Gram-negative bacteria adjust their outer-membrane channels to optimize nitrogen uptake under different environmental conditions [1; 58; 62]. Omp-Pst2 is especially well-suited for rapid ammonium delivery, which could be advantageous for _Providencia stuartti_ growing in nutrient-poor environments, such as a catheterized urinary tract [86; 87]. Conversely, OmpF's stronger selectivity may help _Escherichia coli_ maintain periplasmic ionic balance in more variable habitats [15; 88; 89; 90; 91; 92]. Understanding these complementary rolesis important for both basic microbiology and identifying potential weak points in bacterial nutrient acquisition [15; 16; 22; 88; 89; 90; 91; 92]. For instance, blocking or misdirecting a portion that typically supplies ammonium may hinder a pathogen's ability to thrive in nitrogen-limited environments [93; 85; 94]. Limitations  
While these findings are encouraging, several factors must be considered when interpreting the results:  
**Simplified Bilayer Model**  
All experiments were conducted in a purified, laboratory-made lipid bilayer. Real bacterial outer membranes are much more complex--they contain lipopolysaccharidesaccharides, various lipids, and other proteins that can influence how porins behave. As a result, the transport rates measured here may differ from those in an actual bacterial cell [13; 15; 95; 96; 97; 98; 92]. **Absence of Cellular Context**  
In living bacteria, porin levels are regulated in response to environmental signals, and other proteins (such as periplasmic binding proteins) may work together with porins to optimize ammonium uptake. None of these regulatory or cooperative factors were present in the bilayer system, so the \"pure\" porin behavior described here may not fully capture what happens in vivo [17; 99; 99; 100; 101]. **Limited Porin Selection**  
This study focuses on just two general diffusion porins, Omp-Pst2 and OmpF. Many other porins exist in different bacterial species, each with its own transport characteristics. Therefore, one should be cautious about generalizing these findings to all Gram-negative bacteria [13; 15; 96; 97; 98; 99; 102; 103; 22; 104; 91; 105].