Bacterial growth and pH

This slide explains how pH affects microbial growth and how bacteria are classified on the basis of optimum pH

1. By
JeevanShrestha
Email:
[email protected]

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 cytoplasmicpH can harm
bacteria by disrupting the plasma membrane or
inhibiting the activity of enzymes and membrane
transport proteins.
Most procaryotesdie if the internal pH drops much
below 5.0 to 5.5.

6. Acidophiles(Helicobacter pylori)
optimum in pH range 0-5.5
Acidophilesare 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.
Acidophileshave 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 neutrophilesare 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 neutrophilesare 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, argininedecarboxylasesand 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.
Alkaliphilehelps in membrane transport.
Alkaliphilesecretes various enzymes such as cellulase,
pectinase, chitinaseetc.

10. Internal pH control in Alkaliphile

12. 12

13. Source: Brock Biology of microorganisms

14. 1. Madigan MT, MartinkoJM & 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