You find an unusual colony of E. coli floating on the surface of your Coke (pH 3). Describe three mechanisms by which those bacterial cells may withstand the low pH environment of soda by regulating their internal cellular pH.

Question

What will be an ideal response?

Answer 1

When cells are placed in pH conditions below their optimum, protons can enter the cell and lower internal pH to lethal levels. Therefore, many microbes have ways to regulate their internal pH levels.

a) Proton extrusion mechanisms: E. coli changes the pH of the medium using amino acid decarboxylases (removal of CO2 from amino acid, formation of amines). It consumes organic acids when growing at low pH, but produces these acids when growing in alkaline conditions. For example, E. coli has an antiporter protein on it's cell surface that brings glutamate into the cell in exchange for the decarboxylation product gamma-aminobutyric acid (GABA), which is transported out of the cell. The combined action of the decarboxylase and antiporter removes H+ from the cell.

Microbes can also prevent the unwanted influx of protons in other ways:
b) Acidophiles will transport other cations (K+ or Na+) into the cell to make it more basic.
c) Acidophiles often have high amounts of tetraether lipids. Altered cell membrane structures decrease proton permeability.
d) Alkaliphiles use antiport systems to scavenge protons. They will recruit protons to cell in exchange for expelling Na+.