pH and it's impact on Microbial growth

 The self-ionization of water is a continuous process when it is in pure form. In this process, when there is collision of two H₂O molecules they dissociate into H+ and OH-  ions. There is no free hydrogen ions in the water. Because the free hydrogen ions have tendency to attach to the H₂O molecule to form hydronium ion. This means, you will always find  H₃O⁺ or hydronium ions in water, instead of free H+ ions.

What is pH ?

pH stands for potential of hydrogen, or power of hydrogen. pH can be defined as measurement of hydrogen ion concentration in a solution, or mathematically it can be defined as negative logarithm of hydrogen ion concentration.

Formula of pH : —log [H+]

pH is applied only to aqueous solutions. That means where there is water, there is pH. pH was first described by Sorensen in 1909. pH reveals the acidity or basicity of water.
  The pH scale ranges from 0 to 14 where the value 7 indicates the neutral pH. pH less than 7 indicate acidic conditions, and greater than seven indicate basic or alkaline. In other word pH is about calculating the free hydronium ions and free hydroxyl ions in a given solution.

A solution with more H+ ions is acidic and gives a pH value less than 7 Similarly, a solution with more OH- ions, is basic and gives a value greater than seven.

Importance of pH

Certain reactions in our body require certain value of pH. Anything higher or lower value may cause damage. For example, if too much of hydrochloric acid is produced in the stomach, the patient will be advised to take antacids like magnesium hydroxide in order to neutralize the excess pH.
  In the same way plants cannot grow if there is continuous rising and falling of pH of soil. Animals in the river cannot survive when acid rains fall on the river.
  Similarly, pH plays an important role on microbial growth.

There were cases where microbes did not show growth in the absence of right pH conditions, even they were supplied with all the required nutrients.
  The pH value where a microbe can grow its best, is called the optimum pH.
Most bacteria grow best at a pH value near to 7, which means, most bacteria are neutrophils.
Some bacteria can grow at a pH range between 3 and 4. These are called acedophiles.
Alkaliphiles are the bacteria that can tolerate pH between 8 and 11.

Impact of pH on Microbial growth

The bacterial cell consists of several protein molecules, lipids, and nucleic acids. And the cell hosts several biochemical reactions. All these activities are regulated when there is optimum pH.
In lower pH conditions, the increased hydrogen ions break the weak hydrogen bonds of protein side chains and finally change the shape of the protein.
  When a protein is not in its original shape, it cannot perform its routine function, and ultimately the bacteria cannot survive.

There are two methods to measure pH in a microbiology lab. The first one is using pH paper. The pH paper changes its color when it is dipped into a solution. This color change is based on acidity and basicity of the sample solution.
Later, the color of the paper will be compared with the color chart provided along with the paper. These papers are coated with a pigment called Flavin, which is extracted from red cabbage. Flavin has ability to change color when it comes in contact with an acid or base. This method will not provide the exact pH value. However, this will provide a value which is closer to the actual pH value.
The other method to measure pH that gives an accurate value is using a pH meter.