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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.
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.
A bacterium that thrives in a soda lake where the average pH is 10.5 can be classified as a(n) ________.
alkaliphile
Lactobacillus acidophilus grows best at pH 4.5. It is considered a(n) ________.
acidophile
Which macromolecule in the cell is most sensitive to changes in pH?
Which metabolic process in the bacterial cell is particularly challenging at high pH?
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