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A graph with pH on the X axis and growth rate on the Y axis. One bell shaped curve peaks at about pH 3 and drops down, reaching a growth rate of 0 at pH 1 and 5.5. This line is labeled acidophile. Another bell curve peaks at pH 7 and drops to 0 at pH 5.5 and 8.5. This is labeled neutrophile. The final curve peaks at pH 9.5 and drops to 0 at pH of 7.5 and 11.5. This is labeled alkaliphile.
The curves show the approximate pH ranges for the growth of the different classes of pH-specific prokaryotes. Each curve has an optimal pH and extreme pH values at which growth is much reduced. Most bacteria are neutrophiles and grow best at near-neutral pH (center curve). Acidophiles have optimal growth at pH values near 3 and alkaliphiles have optimal growth at pH values above 9.

At the other end of the spectrum are alkaliphile s, microorganisms that grow best at pH between 8.0 and 10.5. Vibrio cholerae , the pathogenic agent of cholera , grows best at the slightly basic pH of 8.0; it can survive pH values of 11.0 but is inactivated by the acid of the stomach. When it comes to survival at high pH, the bright pink archaean Natronobacterium , found in the soda lakes of the African Rift Valley, may hold the record at a pH of 10.5 ( [link] ). Extreme alkaliphiles have adapted to their harsh environment through evolutionary modification of lipid and protein structure and compensatory mechanisms to maintain the proton motive force in an alkaline environment. For example, the alkaliphile Bacillus firmus derives the energy for transport reactions and motility from a Na + ion gradient rather than a proton motive force. Many enzymes from alkaliphiles have a higher isoelectric point, due to an increase in the number of basic amino acids, than homologous enzymes from neutrophiles.

A photo of a red lake
View from space of Lake Natron in Tanzania. The pink color is due to the pigmentation of the extreme alkaliphilic and halophilic microbes that colonize the lake. (credit: NASA)

Survival at the low ph of the stomach

Peptic ulcers (or stomach ulcers ) are painful sores on the stomach lining. Until the 1980s, they were believed to be caused by spicy foods, stress, or a combination of both. Patients were typically advised to eat bland foods, take anti-acid medications, and avoid stress. These remedies were not particularly effective, and the condition often recurred. This all changed dramatically when the real cause of most peptic ulcers was discovered to be a slim, corkscrew-shaped bacterium, Helicobacter pylori . This organism was identified and isolated by Barry Marshall and Robin Warren, whose discovery earned them the Nobel Prize in Medicine in 2005.

The ability of H. pylori to survive the low pH of the stomach would seem to suggest that it is an extreme acidophile. As it turns out, this is not the case. In fact, H. pylori is a neutrophile. So, how does it survive in the stomach? Remarkably, H. pylori creates a microenvironment in which the pH is nearly neutral. It achieves this by producing large amounts of the enzyme urease, which breaks down urea to form NH 4 + and CO 2 . The ammonium ion raises the pH of the immediate environment.

This metabolic capability of H. pylori is the basis of an accurate, noninvasive test for infection. The patient is given a solution of urea containing radioactively labeled carbon atoms. If H. pylori is present in the stomach, it will rapidly break down the urea, producing radioactive CO 2 that can be detected in the patient’s breath. Because peptic ulcers may lead to gastric cancer, patients who are determined to have H. pylori infections are treated with antibiotics.

  • What effect do extremes of pH have on proteins?
  • What pH-adaptive type of bacteria would most human pathogens be?

Key concepts and summary

  • Bacteria are generally neutrophiles . They grow best at neutral pH close to 7.0.
  • Acidophiles grow optimally at a pH near 3.0. Alkaliphiles are organisms that grow optimally between a pH of 8 and 10.5. Extreme acidophiles and alkaliphiles grow slowly or not at all near neutral pH.
  • Microorganisms grow best at their optimum growth pH . Growth occurs slowly or not at all below the minimum growth pH and above the maximum growth pH .

Fill in the blank

A bacterium that thrives in a soda lake where the average pH is 10.5 can be classified as a(n) ________.

alkaliphile

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Lactobacillus acidophilus grows best at pH 4.5. It is considered a(n) ________.

acidophile

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Short answer

Which macromolecule in the cell is most sensitive to changes in pH?

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Which metabolic process in the bacterial cell is particularly challenging at high pH?

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Questions & Answers

a solvent isopropyl myristate is use for the pre-treatment of?
Nasir Reply
what is microbiology
Ayeniyi Reply
microbiology is the study of all living organisms that are too small to be visible to the naked eyes, examples bacteria and fungi etc
Iheme
study of microorganism such as becteria etc
Muhammad
study of microorganisms and their effect on organisms life
Aqsa
characteristic of Gram negative bacteria
jane Reply
Characteristics of Gram Negative Bacteria As with Gram positive bacteria, Gram negative bacteria also contain the peptidoglycan polymer in their cell wall. While this polymer is thin (2 to 4 nanometers in thickness with just about 3 layers of peptidoglycan) in Gram negative bacteria, it's also com
Kaviya
it's also composed of long glycan strands that are cross-linked by peptide molecules. This composition serves a number of functions including protecting the bacterial cell from lysis
Kaviya
Good shot
Enoch
Thanks 😊
Kaviya
what was Hans Christian Gram's supported in the modern Microbiology?
Wilson Reply
what is microbial growth
Chisa Reply
The organism responsible for vulva ulcers
nyiter Reply
Why are vascular pathogen poorly communicable from person to person?
Aj Reply
Most vascular pathogens are poorly communicable from person to person because they need a medium to be communicated i,e a vector that would carry them from one person to other
Kaviya
what's the habit of protista
Afieahngwi Reply
They show both autotrophic and heterotrophic mechanisms...
Swetha
thanks
Afieahngwi
welcome...
Swetha
let me mention some water. Air .Food and so on
Gattiek Reply
causes of infectious diseases
Afieahngwi Reply
water.Air
Gattiek
infectious disease are caused by pathogenic micro organisms like bacteria ,fungi..
Swetha
What is pasteurization?
Wilson
are fungi prokaryote or eukaryotes?
Afieahngwi Reply
fungi are eukaryotes.
Swetha
All fungi are eukaryotes. Even micro fungi.
Lad
have..complex cellular organization and membrane bound nucleus ...and..also... having loops of DNA( like plasmids) as.bacteria
Swetha
what enzyme replaces rna nucleotides with dna nucleotides during replication?
Remi Reply
an enzyme called DNA ligase.
Jael
describe the acid fast staining procedure used in the diagnosis of tuberculosis
Salma Reply
bacterial morphology
lf_ Reply
what is the difference between biogenesis & abiogenesis
Mayuri Reply
biogenesis is when living comes out from other living things as a result of reproduction while a biogenesis is the process where living things comes out from non living things
Usman
living things come form other form living things is biogenesis. ....right?
Mayuri

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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