Bacteria regulate their internal pH through various mechanisms to survive in different external pH environments. The internal pH of cells is maintained near neutral through the use of buffers and ion pumps, even when the external pH is highly acidic or alkaline. Acidophilic bacteria can tolerate very low external pH levels down to 0 through specialized internal pH regulation. Neutrophilic bacteria have mechanisms to reduce effects of low external pH on their internal pH. Alkaliphilic bacteria maintain a slightly acidic internal pH despite living in high pH environments and also regulate internal pH through homeostasis.

1. By
Jeevan Shrestha
Email:
shresthajeevan3@gmail.com

2. pH refers to the hydrogen ion conc.
i.e. acidity or alkalinity of a solution.
Neutral – pH 7, Acidic < pH 7,
Basic > pH 7
Most organisms shows a growth range of 2-3 pH units.

3. 3
5
Organism through their metabolic activity, produce
acid or alkaline metabolites which are elaborated out of
cell. The presence of these metabolite changes pH of
medium.
The internal pH of a cell must stay relatively close to
neutral even though the external pH is highly acidic or
alkaline.
Internal pH regulated by BUFFERS and near neutral
adjusted with ion pumps.

4. Buffers-salt of weak acid or base that keeps hydrogen
ions conc. constant by maintaining an equilibrium with
hydrogen ions of the solution.
Neutral pH- phosphate buffers, alkaline pH- borate
buffers, acidic pH – citrate buffers
 Bacterial growth rates greatly influence by pH & is
largely based on nature of proteins.
 Changes in the external pH also might alter the
ionization of nutrient molecules and thus reduce their
availability to the organism.

5. Bacteria have limitation to their acidity tolerance.
When the external pH is low, the concentration of H+ is
greater outside than inside and H + will move into the
cytoplasm which lower internal pH.
Drastic variations in cytoplasmic pH can harm
bacteria by disrupting the plasma membrane or
inhibiting the activity of enzymes and membrane
transport proteins.
Most procaryotes die if the internal pH drops much
below 5.0 to 5.5.

6. Acidophiles (Helicobacter pylori)
optimum in pH range 0-5.5
Acidophiles are found in sulfuric pools and geysers, areas
polluted by acid mine drainage and even our own stomachs.
The low pH of volcanic areas and environments associated with
metal ore mining is caused by the activities of acidophilic
microorganisms that oxidize reduced sulfur compounds to
sulfuric acid.
Acidophiles have great role in acid mine drainage. Ferroplasma,
which was found growing at pH 0 in acid mine drainage in Iron
Mountain in California.

7. Internal pH control in acidophiles

8. Neutrophiles(E. coli)
Optimum in pH range 5.5 – 8
Majority of neutrophiles are found in soil and water.
They are well-suited to survive inside a human body as
human blood and tissues has pH 7.2+0.2 So, most of the
pathogenic bacteria are neutrophiles.
When neutrophiles are exposed to low pH environment,
there occur a mechanism to reduce the effect of decreased
internal pH. There will be an induction of glutamate
decarboxylases, arginine decarboxylases and RpoS-
dependent oxidative systems etc.

9. Alkaliphiles (Bacillus alcalophilus)
optimum in pH range 8-14
Internal pH is slightly acidic in comparison to the pH of
an environment.
Internal pH is maintained by homeostasis.
Alkaliphile helps in membrane transport.
Alkaliphile secretes various enzymes such as cellulase,
pectinase, chitinase etc.

10. Internal pH control in Alkaliphile

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13. Source: Brock Biology of microorganisms

14. 1. Madigan MT, Martinko JM & Parker J (2000) Brock's
Biology of Microorganisms, 9th edn. Englewood
Cliffs, NJ: Prentice Hall.
2. http://www.ehow.com/list_6774091_list-
neutrophilic-acidophilic-heterotrophic-
bacteria.html
3. www.journals.asm.org