pH scale is used to express the acidity or alkalinity of a solution on which neutrality is pH 7. The pH scale extends from pH 0.0 to pH 14.0, and each pH unit represents a tenfold change in hydrogen ion concentration. Those pH values that are less than 7 are said to be acidic, and those greater than 7 are alkaline (or basic).
pH is a measure of the hydrogen ion activity of a solution and is defined as the negative logarithm of the hydrogen ion concentration.
(Source: Brock Biology of Microorganisms)
Though some microorganisms are able to grow under extreme pH conditions (pH<2 or >10), most of the microorganisms have pH optima between 5 and 9. Each species has a well-defined pH growth range and pH growth optimum. On the basis of pH requirements, microorganisms can be classified as:
- Acidophiles
- Neutrophiles
- Alkaliphiles
Acidophiles “Acid loving”
Acidophiles are organisms that can withstand and even thrive in acidic environments where pH value ranges from 0.5 to 5. Fungi as a group tend to be more acid-tolerant than bacteria and prefer acidic surroundings of pH 4 to 6. Several bacteria are also acidophilic. For example, several species of Thiobacillus and several genera of Archaea, including Sulfolobus and Thermoplasma are obligate acidophiles. Archaeon Sulfolobus acidocaldarius is a common inhabitant of acidic hot springs; it grows well around pH 1 to 3 and at high temperatures.
Obligate acidophiles can not grow at all at neutral pH.
Acidophiles can be found in volcanic areas, hydrothermal sources, deep-sea vents, or in the stomachs of animals.
Acidity inhibits most microbial growth and is used frequently for food preservation (e.g.: pickling).
Neutrophiles “Neutral Loving“
Most bacteria prefer neutral pH, their pH optimum for growth is between pH 6.5 to 7.5. Majority of microorganisms including human pathogens are neutrophiles.
Alkaliphiles “Alkali loving’
Some microorganisms have high pH optima for growth (pH 8.0 to 11.5) and are called alkaliphiles. Alkaliphiles include prokaryotes, eukaryotes, and archaea. For example, Vibrio cholerae and Alkaligens faecalis have optimal pH of 9 and are inactivated by the acid of the stomach.
Extreme alkalophiles have growth optima at pH 10 or higher. For example, soil bacterium Agrobacterium grows at pH 12. Alkaliphilic microorganisms are usually found in soda lakes and high carbonate soils and sometimes even in garden soils.
Some alkaliphiles are used in commercial industries. Biological detergents contain alkaline enzymes, such as alkaline cellulases and/or alkaline proteases produced from alkaliphiles.
Summary
| Class | Definition | Example |
| Acidophile | Growth optimum between pH 0 and 5.5 | Sulfolobus, Thiobacillus ferroxidans, Lactobacillus |
| Neutrophile | Growth optimum between pH 5.5 and 8.0 | Escherichia coli, Salmonella, Staphylococci |
| Alkalophile | Growth optimum between pH 8.0 and 11.5 | Vibrio cholerae, Bacillus alcalophilus, Natronobacterium |
Coping with pH changes
Though the optimal pH for growth (i.e., pH of the external environment) varies among microorganisms, their intracellular pH always remain near neutral. Most prokaryotes die if the internal pH drops much below 5.0 to 5.5. Drastic variations in cytoplasmic pH can harm microorganisms by disrupting the plasma membrane or inhibiting the activity of enzymes and membrane transport proteins.
Microorganisms respond to external pH changes using mechanisms that maintain a neutral cytoplasmic pH. There are several proposed mechanisms by which microorganisms adjust to changes in external pH such as :
- Internal buffering system of microorganisms contributes to pH homeostasis.
- The plasma membrane is impermeable to protons. Extreme alkaliphiles like Bacillus alcalophilus maintain their internal pH closer to neutrality by exchanging internal sodium ions for external protons (H+) using Na+/H+ antiport.
- Acid tolerance response: This response is activated, when the external pH becomes too acidic. For example, when the pH drops below 5.5 to 6.0, Salmonella enterica serovar Typhimurium and E. coli synthesize an array of new proteins. A proton-translocating ATPase contributes to this protective response, either by making more ATP or by pumping protons out of the cell.
- Synthesis of chaperone proteins: Chaperone proteins such as acid shock proteins and heat shock proteins prevent the acid denaturation of proteins and aid in the refolding of denatured proteins.
- Microorganisms frequently change the pH of their own habitat by producing acidic or basic metabolic waste products. Fermentative microorganisms form organic acids from carbohydrates, whereas chemolithotrophs like Thiobacillus oxidize reduced sulfur components to sulfuric acid. Other microorganisms make their environment more alkaline by generating ammonia through amino acid degradation.
References and further reading
- Brock Biology of Microorganisms (15th Edition) by Michael T. Madigan
- Prescott’s Microbiology (10th Edition) by Joanne Willey, Linda Sherwood, Christopher J. Woolverton
