Bacterial growth and pH

ByJeevan ShresthaEmail: shresthajeevan3@gmail.com

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.

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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.

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.

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.

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.

Internal pH control in acidophiles

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.

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.

Internal pH control in Alkaliphile

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Source: Brock Biology of microorganisms

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