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Environmental Influences: pH

Background and Introduction:

The pH, which is a measure of the hydrogen ion concentration, directly influences the growth activities of microorganisms.  Each species has a definite pH growth range and pH growth optimum.  The pH range for a given microorganism is generally narrow; most do not grow in an environment in which the pH is more than 1 pH unit above or below their optimum.  Changes in pH can lead to denaturation of enzymes and other proteins and can interfere with membrane transport systems.  According to their tolerance for acidity or alkalinity, bacteria are classified as: 

  • acidophiles: acid-loving organisms, grow best at a pH of 0.1 to 5.5.  Most fungi and algae like a slightly acidic environment.  Some procaryotic members of Archaea can grow at a pH of 0. 

  • neutrophiles: organisms that grow optimally at pH 5.5 to 8.0.  Most bacteria and protozoa are neutrophiles.  

  • alkalophiles: base-loving organisms, grow best at a pH of 8.5 to 11.5. 

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