pH Requirements of Microorganisms

Swallowing Vibrio cholerae is harmless for most healthy people. Stomach acid has pH of roughly 1.5 to 3.5, and V. cholerae is acid-sensitive.

A person typically need to swallow about 100 million of them for enough to survive at the acidic stomach to establish infection in the small intestine. That's part of why cholera outbreaks track so closely with contaminated water sources capable of delivering that kind of dose, and why taking cholera-contaminated food with something that neutralizes stomach acid measurably raises the risk of infection.

Shigella tells the opposite story. It's remarkably acid-tolerant, and as few as 10 to 100 organisms can cause disease. There's no large safety margin from stomach acid to rely on; a small, incidental exposure is enough.

Same barrier, same organ, wildly different outcomes, because the two organisms sit at opposite ends of the pH tolerance spectrum this article is actually about. Understanding where a microorganism falls on that spectrum isn't just a classification exercise; it's a big part of why some infections require a contaminated reservoir and a large dose, and others require almost nothing at all.

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.

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.

These extreme tolerance limits describe the outer edge of what a microorganism can survive, not its optimum. The acidophile/neutrophile/alkaliphile classification below is based on optimum growth pH specifically, the pH at which an organism grows fastest, which is usually a narrower range than what it can merely tolerate. On the basis of pH requirements, microorganisms can be classified as:

1. Acidophiles
2. Neutrophiles
3. Alkaliphiles

Acidophiles “Acid loving”

Acidophiles are organisms that can withstand and even thrive in acidic environments, generally with an optimum growth pH below 5.5. 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, with an optimum for growth generally between pH 5.5 and 8.0. 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 Alcaligenes 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

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 :

1. Internal buffering system of microorganisms contributes to pH homeostasis.
2. 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.
3. 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.
4. 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.
5. 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.

How to Remember

• The three-zone scale, as one line: acid below 5.5, neutral from 5.5 to 8, alkaline above 8. The boundaries are shared points on a single continuous scale, not three separate ranges to memorize independently.
• Why infectious dose varies so much between pathogens: the more acid-sensitive an organism is, the more of it has to survive the stomach to cause disease, so more needs to be swallowed in the first place. Vibrio cholerae (acid-sensitive) needs a huge dose; Shigella (acid-tolerant) needs almost none.
• The one rule that applies no matter which category an organism falls into: whatever the external pH is, the inside of the cell insists on staying near neutral. An acidophile living at pH 2 does not have an acidic cytoplasm; its entire biology is built around defending a neutral interior against that environment.
• The alkaliphile's trick, in one phrase: trade what's outside for what's needed inside. Extreme alkaliphiles pump sodium out and pull protons in through a Na+/H+ antiporter, converting an overabundance of external sodium into the protons needed to keep the cytoplasm from drifting too alkaline.

Key exam facts in one table

Where Students Get Confused

• Confusing an organism's absolute pH tolerance limits with its optimum growth pH. The article notes some microorganisms survive below pH 2 or above pH 10, but the acidophile/neutrophile/alkaliphile classification is based specifically on where growth is fastest, a narrower range than mere survival.
• Assuming an acidophile's internal cytoplasm is acidic. It isn't. Every category of organism here maintains a near-neutral internal pH; what differs is the external environment they're adapted to tolerate and the mechanisms they use to defend that internal neutrality.
• Treating the acid tolerance response as the same thing as being an acidophile. The acid tolerance response in organisms like Salmonella and E. coli is an inducible stress response, triggered when external pH drops, involving new protein synthesis. It's a defensive reaction in a neutrophile, not evidence that the organism is adapted to grow optimally at low pH.
• Missing the connection between acid tolerance and infectious dose. This is a frequently tested clinical application: acid-sensitive pathogens like V. cholerae require a large infecting dose because most organisms die in the stomach, while acid-tolerant pathogens like Shigella require very few organisms to cause disease.

References and further reading

• Madigan, M. T., Bender, K. S., Buckley, D. H., Sattley, W. M., & Stahl, D. A. (2018). Brock Biology of Microorganisms (15th ed.). Pearson.
• Willey, J. M., Sherwood, L. M., & Woolverton, C. J. (2016). Prescott's Microbiology (10th ed.). McGraw-Hill Education.