Effect of pH on microbial growth and distribution

pH is a measure of the hydrogen ion activity of a solution. pH is defined as the negative logarithm of the hydrogen ion concentration (expressed in terms of molarity).

pH = ^-log [H⁺] = log(1/[H⁺])

The pH scale extends from pH 0.0 to pH 14.0.

The habitats in which microorganisms grow vary widely-from pH 0 to 2 at the acidic end to alkaline lakes and soil that may have pH values between 9 and 10.

pH strongly affects microbial growth. Each species has a definite pH growth range and pH growth optimum.

• Acidophiles have their growth optimum between pH 0 and 5.5; 
• Neutrophiles, between pH 5.5 and 8.0; 
• Alkalophiles prefer the pH range of 8.0 to 11.5. 
• Extreme alkalophiles have growth optima at pH 10 or higher.

In general, different microbial groups have characteristic pH preferences. Most bacteria and protists (protozoa and algae) are neutrophiles. Most fungi prefer more acidic surroundings, about pH 4 to 6; photosynthetic protists also seem to favour slight acidity.

Many archaea are acidophiles. For example, the archaeon Sulfolobus acidocaldarius is a common inhabitant of acidic hot springs; it grows well around pH 1 to 3 and at high temperatures. The archaea Ferroplasma acidarmanus and Picrophilus oshimae can grow at pH 0, or very close to it.

Although microorganisms will often grow over wide ranges of pH and far from their optima, there are limits to their tolerance

Maintenance of pH

Drastic variations in cytoplasmic pH can harm microorganisms 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. Changes in the external pH also might alter the ionization of nutrient molecules and thus reduce their availability to the organism.

Microorganisms respond to external pH changes using mechanisms that maintain a neutral cytoplasmic pH. Several mechanisms for adjusting to small changes in external pH have been proposed.

Ø  The plasma membrane is impermeable to protons.

Ø  Neutrophiles have an antiport transport system to exchange potassium for protons.

Ø  Extreme alkalophiles like Bacillus alcalophilus maintain their internal pH closer to neutrality by exchanging internal sodium ions for external protons.

Ø  Internal buffering also may contribute to pH homeostasis.

•      If the external pH becomes too acidic, other mechanisms come into play.

ü  When the pH drops below about 5.5 to 6.0, Salmonella enterica serovar Typhimurium and E. coli synthesize an array of new proteins as part of their acidic tolerance response (ATR).

ü  A proton-translocating ATPase is activated and makes more ATP or  pump protons out of the cell and thus contributes to this protective response, 

ü  If the external pH decreases to 4.5 or lower, chaperone proteins such as acid shock proteins and heat shock proteins are synthesized. These 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. Because microorganisms change the pH of their surroundings, buffers often are included in media to prevent growth inhibition by large pH changes.

Phosphate is a commonly used buffer and is a good example of buffering by a weak acid (dihydrogen phosphate, H₂PO₄^-) and its conjugate base (monohydrogen phosphate, HPO₄ ^2-).

H⁺ + HPO4 ^2-⎯⎯→ H₂PO₄^- 

OH^- + H₂PO₄^-   ⎯⎯→ HPO₄ ^2- + HOH

If protons are added to the mixture, they combine with the salt form to yield a weak acid. An increase in alkalinity is resisted because the weak acid will neutralize hydroxyl ions through proton donation to give water.

Peptides and amino acids in complex media also have a strong buffering effect.

 

pH	[H+] Molarity		Environmental examples	Microbial examples
0	10–0	Increasing acidity	Concentrated nitric acid	Ferroplasma, Picrophilus oshimae
1	10–1		Gastric contents, acid thermal springs	Dunaliella acidophila
2	10–2		Lemon juice Acid mine drainage	Cyanidium caldarium, Thiobacillus thiooxidans, Sulfolobus acidocaldarius
3	10–3		Vinegar, ginger ale Pineapple
4	10–4		Tomatoes, orange juice Very acid soil
5	10–5		Cheese, cabbage Bread	Physarum polycephalum, Acanthamoeba castellanii
6	10–6		Beef, chicken Rain water Milk, Saliva	Lactobacillus acidophilus,        E.coli, Pseudomonas aeruginosa, Euglena gracilis, Paramecium bursaria
7	10–7	Neutrality	Pure water, Blood	Staphyloccus aureus
8	10–8	Increasing alkalinity	Seawater	Nitrosomonas spp.
9	10–9		Strongly alkaline soil,  Alkaline lakes
10	10–10		Soap	Microcystis aeruginosa, Bacillus alcalophilus
11	10–11		Household ammonia
12	10–12		Saturated calcium hydroxide solution
13	10–13		Bleach, Drain opener
14	10–14

                         The pH scale and microorganisms with their growth optima.