pH is Primary Control in Microbiology
The Effects of pH on Microbial Growth
High pH affects bacterial membranes by disrupting their structural and functional integrity, leading to a loss of selective permeability, the leakage of cellular contents, and potentially cell death. High pH also collapses the crucial proton motive force, impairing energy production and cellular processes. While some specialized alkaliphile bacteria can adapt to high-pH environments, most bacteria lack the necessary pH homeostasis systems and are negatively impacted by alkaline conditions.
The optimum growth pH is the most favorable pH for the growth of an organism. The lowest pH value that an organism can tolerate is called the minimum growth pH and the highest pH is the maximum growth pH. These values can cover a wide range, which is important for the preservation of food and to microorganisms’ survival.
For example, the optimum growth pH of Salmonella spp. is 7.0–7.5, but the minimum growth pH is closer to 4.2
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
How does pH affect the cell surface membrane?
Changes in intracellular pH can potentially affect virtually all cellular processes, including metabolism, membrane potential, cell growth, movement of substances across the surface membrane, state of polymerization of the cytoskeleton and ability to contract in muscle cells.
More importantly, pH changes will also affect cell function and cause tissue damage, by altering the structure and activity of proteins, ionic conductance of the neural membrane, and neuronal excitability
An extremely alkaline pH of
HYDROXY pH 12.5 to 13.8
is highly destructive to bacterial membranes, leading to cell death through several mechanisms because their membranes are not designed to withstand such extreme conditions.
Deleterious effects of pH 12.5 - 13.5 on bacterial membranes
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Destruction of membrane lipids:
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A pH of 12.5 to 13.5 is strong enough to trigger the saponification of membrane lipids, breaking them down into their constituent fatty acid and glycerol components. This process completely dissolves the membrane structure, causing the cell's contents to leak out.
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Protein denaturation:
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Most enzymes and proteins, including the integral membrane proteins responsible for transport and signaling, are adapted to function within a very specific and narrow pH range (around 7.4 to 7.8 for non-extremophiles). The extremely high concentration of hydroxide ions at pH 13.5 causes irreversible denaturation by disrupting the hydrogen bonds and modifying the ionization of amino acid functional groups. This destroys the proteins' three-dimensional structure and eliminates all cellular functions.
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Collapse of the proton motive force (PMF):
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The PMF is a critical energy source created by an electrochemical gradient of protons across the membrane, which is used to generate ATP and drive transport systems. At pH 13.5, the massive imbalance of protons between the cell interior and exterior overwhelms the membrane's ability to maintain the PMF, leading to a complete shutdown of energy production.
- Permeability disruption: