## 3 SRB Activities in Acidic Environments
### Response of SRB to Acidic Environments
#### 3.1.1 The Maintenance of pH in Cytoplasm  
When exposed to acidic conditions, most neutrophic bacteria need to sustain a compatible cytoplasmic pH that lies within pH of 7.4-7.8 [28; 29] to maintain pH homeostasis with optimum function and structural integrity of the cytoplasmic proteins to support their growth [30]. Then, they are able to neutralize the pH of the cytoplasm relative to the external pH condition. The maintenance of cytoplasmic pH can be done by restricting proton permeation, increasing pumping proton out of cytoplasm and increase proton consumption. Restriction of Proton Permeation  
Proton from environment go into bacteria cytoplasm via plasma membrane. The driven force of proton flow or proton motive force (pmf) is generated by the proton electrochemical potential difference (\(\Delta\mu_{H^{+}}\)) which can be calculated by membrane potential difference (\(\Delta\psi\)) and pH gradient (\(\Delta pH\)) across a membrane [28]. \[\Delta\mu_{H^{+}}=F\Delta\psi-2.3 RT\Delta pH \tag{1}\]  
where \(F\) is Faraday constant (96,480 JV\({}^{-1}\)mol\({}^{-1}\)), \(R\) is the gas constant (8.315 JK\({}^{-1}\)mol\({}^{-1}\)) and \(T\) is temperature (K). Membrane potential difference (\(\Delta\psi\)) is generated as a result of the transfer of protons. When the potential exists across the membrane, a permeable ion travels through membrane with respect to this potential until electrochemical equilibrium is established (\(\Delta\mu_{H^{+}}=0\)). pH gradient has been reported to have a more dominant effect than the difference in membrane potential [28; 31]. Biologically, most cytoplasm membrane is built basically as a bilayer of lipids which has low permeability of protons and most nutrients. The membrane contains channel protein which is a special arrangement of amino acids embedded in the cell membrane to allow the influx of protons and nutrients and extrusion of waste products [32]. Hence, the permeability of proton through membrane and membrane channels may limit the flow of proton through bacterial membrane. Previous literature reported and summarized the change of the membrane composition of bacteria when they encounter low pH condition in _E. coli_ and _Streptococcus mutans_[33; 34]. Recently, [PERSON] et al. [35] reported _D. vulgaris_ when exposed to acidic conditions, had the largest differently expressed gene for acid-related in the Gene Ontology (GO) term \"GO0016021- integral component of membrane\". The study revealed the adjustment in the cell membrane composition under acid stress and this change was the main factor that strengthen the adaptation of bacteria to low extracellular pH. Modifying the membrane channel size also contributes to the response of bacteria to acidic conditions. [PERSON] et al. [36] reported that _Acidithiobacillus ferrooxidans_ showed the gene expression to attempt to modify the channel by forming a large L3 loop as their response to acidic condition. This resulted in decreasing the influx of proton through membrane. However, there was no report discussing modification of the channel size of the membrane of SRB regarding to acid stress. Increase in Pumping Proton Out of the Cytoplasm  
The proton pumps are directed outwardly all over the external pH range; accordingly, the potential is always negative inside [37]. The enhancement of increasing outflux of proton from cytoplasm is essential for maintaining pH homeostasis with low extracellular pH. Excess proton in cytoplasm may inhibit ATP hydrolysis and photochemical reactions [38]. F\({}_{1}\)F\({}_{0}\)-ATPase have ability to hydrolyse ATP to promote outflux of protons outside cells, hence, this enzyme can be considered as effective maintenance of pH homeostasis to prevent internal acidification. _Enterococci_ was shown to have cytoplasmic pH regulated by F\({}_{1}\)F\({}_{0}\)-ATPase under acidic conditions [39]. The same finding was found in _E. coli_[40]. When the pmf decreases below a threshold level as the extracellular pH decrease, F\({}_{1}\)F\({}_{0}\)-ATPase may act as a proton exporter [41].