Most bacteria are neutrophiles, meaning they grow optimally at a pH within one or two pH units of the neutral pH of 7, between 5 and 8 (see Figure 9.35). Acidophiles have optimal growth at pH values near 3 and alkaliphiles have optimal growth at pH values above 9. Most bacteria are neutrophiles and grow best at near-neutral pH (centre curve). Each curve has an optimal pH and extreme pH values at which growth is much reduced. The curves show the approximate pH ranges for the growth of the different classes of pH-specific prokaryotes. is 7.0–7.5, but the minimum growth pH is closer to 4.2.
For example, the optimum growth pH of Salmonella spp.
These values can cover a wide range, which is important for the preservation of food and to microorganisms’ survival in the stomach. The lowest pH value that an organism can tolerate is called the minimum growth pH and the highest pH is the maximum growth pH (Figure 9.35). The optimum growth pH is the most favourable pH for the growth of an organism. Acidic foods have been a mainstay of the human diet for centuries, partly because most microbes that cause food spoilage grow best at a near neutral pH and do not tolerate acidity well. Sauerkraut and dishes such as pico de gallo owe their tangy flavour to their acidity. Lactic acid bacteria that ferment milk into yogurt or transform vegetables in pickles thrive at a pH close to 4.0. Moderate changes in pH modify the ionization of amino-acid functional groups and disrupt hydrogen bonding, which, in turn, promotes changes in the folding of the molecule, promoting denaturation and destroying activity. But the component most sensitive to pH in the cell is its workhorse, the protein. If H + ions are neutralized by hydroxide ions, the concentration gradient collapses and impairs energy production. The proton motive force responsible for production of ATP in cellular respiration depends on the concentration gradient of H + across the plasma membrane (see Cellular Respiration). Lipids are hydrolyzed by an extremely basic pH. The hydrogen bonds holding together strands of DNA break up at high pH. Extreme pH affects the structure of all macromolecules. Environments with pH values below 7.0 are acidic, with a high concentration of H+ ions those with pH values above 7.0 are considered basic. Recall that acidity is a function of the concentration of hydrogen ions and is measured as pH. Yogurt, pickles, sauerkraut, and lime-seasoned dishes all owe their tangy taste to a high acid content (Figure 9.34). Give examples of microorganisms for each category of pH requirement.Identify and describe the different categories of microbes with pH requirements for growth: acidophiles, neutrophiles, and alkaliphiles.Illustrate and briefly describe minimum, optimum, and maximum pH requirements for growth.
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