## 6 Conclusions and Future Perspectives  
Bacteria have evolved sophisticated mechanisms to survive and grow under acidic conditions. The particular solutions used by a given species to the problems created by high proton concentrations clearly depends on its natural environment and on its unique physiology and metabolic traits. _E. coli_ and _L. monocytogenes_ are unrelated phylogenetically yet acidity presents common challenges to both; protecting the internal pH, protecting the genetic material, and critical enzyme function. The different mechanisms deployed by these two bacteria under acidic conditions are summarized in Table 1. They both have very finely tuned regulatory circuits to activate the expression of protective and homeostatic functions. Alternative sigma factors play a critical role in reprogramming the transcriptional landscape in both species, RpoS in _E. coli_ and \(\sigma^{\text{B}}\) in _L. monocytogenes_. In both cases, a key research question remains to be answered; how is the initial acid-specific signal detected by the cell? Answering this question has proven difficult partly because the signal could be either direct (e.g., proton concentration) or indirect (e.g., protein unfolding, altered concentration of some ion or metabolite, etc.). As we have seen, both species use amino acid decarboxylation as means of buffering the cytoplasmic pH against a potentially lethal drop in pH. The regulation of these systems in _E. coli_ is very complex, which likely highlights the importance to fitness of getting this decision right. Given the diverse roles that glutamate plays in the cell this should probably not come as a surprise. In _L. monocytogenes_ the presence of three glutamate decarboxylase systems highlights its importance to acid tolerance but leaves us with much to learn about how they are collectively or independently regulated. It is also clear that strain to strain differences exist within this species in the extent to which they rely on each of the three glutamate decarboxylases systems [24; 175]. Given the critical role of these systems in acid tolerance, they represent a potentially useful target for the development of next-generation antimicrobials. Preventing their activity could potentially reduce the growth and survival of pathogenic strains of either _E. coli_ or _L. monocytogenes_ in acidic food products and furthermore could serve to reduce the likelihood of these pathogens surviving in the acidic conditions of the stomach after ingestion. Indeed, one recent study has shown that the GAD system of _L. monocytogenes_ can be inhibited by maleic acid [176], suggesting that small molecule inhibitors might be a possibility. Other common themes that emerge from this comparative review of the protective systems used by these bacteria include mechanisms to limit proton ingress, protection of proteins, and nucleic acids from damage. Both species have specific mechanisms to alter the composition of their cell envelope in response to acidic environments. The role of cyclopropane fatty acids in reducing the permeability of the membrane of _E. coli_ to protons is clear. The changes that occur in the lipid bilayer of _L. monocytogenes_ are known, but further research is needed to determine how these changes protect the cell and to understand the regulatory processes involved. Preserving the function of critical enzymes in the cell is likely to be the problem faced by all microbes exposed to extreme acid stress. The role of the extra-cytoplasmic protein chaperones HdeA and HdeB in _E. coli_ in preserving the integrity of periplasmic proteins at low pH is now well established, but the role of intracellular chaperones and proteases is less clear at this stage. DNA protection and repair appear critical in both species, although the precise mechanisms of how acidic pH leads to DNA damage still needs further investigation. Perhaps the greatest difference regarding the acid stress responses found in these two bacteria lies