Effect of pH on microbial growth

pH is defined as the negative logarithm of the hydrogen ion concentration and it is the measurement of the hydrogen ion activity of a solution. A solution whose pH is less than 7 is acidic and the solution whose pH is more than 7 is basic; whereas pure water is neither acidic nor basic having pH 7. It is measured by pH scale which range from pH 0 to pH 14 where each pH unit represents a tenfold change in hydrogen ion concentration.

Majority of the microorganisms grow at the neutral pH value of 6.5 to 7.0, they are known as neutrophiles.  However, some can grow at acidic pH, between pH 0 to 5.5 and are called acidophiles. They includes Helicobacter pylori, Acetobacter aceti, Thiobacillus acidophilus etc. Alkalophiles also occurs that can grow at alkaline pH range of 8.5 to 11; for instance Natronomonas pharaonis, Thiohalospira alkaliphila etc. Fungi generally prefers acidic pH of 4 to 6 similar to algae.

Though microorganisms can grow over a wide range of pH value, there is a limit to their tolerance. Significant change in the pH value from optimum harms microorganisms by inhibiting enzyme activity and membrane transport proteins. It also disrupts the plasma membrane of cell. Ionization of the nutrient molecules gets altered due to the change in the external pH and this makes nutrient unavailable to cells.

Extreme alkalophiles like  Bacillus alcalophilus maintains their neutral cytoplasmic pH by exchanging internal sodium ions for external protons.  Other methods are also their for the maintenance of internal pH like neutrophiles uses antiport transport system for the exchange of  potassium for protons.

But with this antiport system, small variations in pH get corrected. Other mechanisms are there to correct drastic environmental pH changes. When pH becomes too acidic, some microorganisms begins to synthesize new proteins due to the activation of acidic tolerance response. Also, chaperones and heat shock proteins are also synthesized. This prevents the denaturation of the proteins and helps in the refolding of denatured proteins.

Some microorganism also has the ability to change the pH of their habitat by synthesizing basic or acidic metabolic waste products. They can make their environment alkaline by producing ammonia through amino acid degradation. Also, they can make their surrounding acidic by fermenting carbohydrates to form organic acids or by oxidizing reduced sulfur components to sulfuric acid as seen in Thiobacillus.

In laboratories, when media are prepared for the growth and study of microorganism, buffers are added to it. Buffer resists the change in pH by the addition of acidic or basic components. For example: phosphate buffer etc.