Distinct Effects of Sorbic Acid and Acetic Acid on the Electrophysiology and Metabolism of Bacillus subtilis

Abstract

b Sorbic acid and acetic acid are among the weak organic acid preservatives most commonly used to improve the microbiological stability of foods. They have similar pK a values, but sorbic acid is a far more potent preservative. Weak organic acids are most effective at low pH. Under these circumstances, they are assumed to diffuse across the membrane as neutral undissociated acids. We show here that the level of initial intracellular acidification depends on the concentration of undissociated acid and less on the nature of the acid. Recovery of the internal pH depends on the presence of an energy source, but acidification of the cytosol causes a decrease in glucose flux. Furthermore, sorbic acid is a more potent uncoupler of the membrane potential than acetic acid. Together these effects may also slow the rate of ATP synthesis significantly and may thus (partially) explain sorbic acid's effectiveness.

Intracellular pH Response to Weak Acid Stress in Individual Vegetative Bacillus subtilis Cells

“…The data of the study by van Beilen et al show that sorbic acid has an effect on the membrane potential, while acetic acid carries only bulk volume protons across the membrane until steady state is reached, leaving ⌬ relatively unaffected. In line with this, and in accordance with data previously obtained in yeast (reviewed by Orij et al [17]), van Beilen et al (4) showed that at the population level, growth rate and pH i behavior in B. subtilis can be correlated. We now also demonstrate a similar correlation at the single-cell level, demonstrating that pH i can be assessed as a good indicator of individual bacterial health.…”

“…van Beilen et al showed that sorbic acid is unable to recover pH i during acid stress. These observations reflect the notion that sorbate acts as a classical uncoupler, which shuttles protons over the membrane, whereas acetate is believed to do this to a much lesser extent (4). The data of the study by van Beilen et al show that sorbic acid has an effect on the membrane potential, while acetic acid carries only bulk volume protons across the membrane until steady state is reached, leaving ⌬ relatively unaffected.…”

“…Growth rate and average colony pH i calculations with Multichannel-SporeTracker were always performed from the time point at which a clearly detectable surface increase, i.e., growth, had resumed. pH i was then generally ϳ7.5, values seen previously by van Beilen et al in liquid populations (4,20).…”

“…The acidification of the cytoplasm, in turn, inhibits metabolism. A recent study by van Beilen et al (4) shows that sorbic acid has an ability to act as a classical uncoupler, transporting protons over the membrane, whereas acetic acid, which is less lipophilic, does so to a much lesser extent. This is corroborated by the fact that sorbic acid has a greater effect on the membrane potential, while acetic acid only carries bulk volume protons across the membrane until steady state is reached.…”

Sodium Acetate Coated Tenofovir-Loaded Chitosan Nanoparticles for Improved Physico-Chemical Properties

“…The second step requires dropwise addition of sodium hydroxide aqueous solution to raise the pH of the chitosan aqueous solution to 4.76 which is the pKa value of acetic acid (24). This enables a selective deprotonation of half glacial acetic acid and produces half acetate ion according to Henderson Hasselbach equation while (25) concomitantly increasing sufficiently the amount of sodium cation in the medium.…”