Dear Aakash,

Yes, high or low pH can kill bacteria by disrupting their cellular processes and structures. Bacteria have an optimal pH range for growth and survival, and when the pH of their environment falls outside this range, it can harm their survival.

When the pH of the environment becomes too high or too low, it can cause changes in the bacterial cell membrane and walls. This can lead to membrane permeability changes, disrupting essential cellular processes and structures such as ion exchange, metabolism, and protein synthesis.

At a high pH, the hydroxide ions (OH-) concentration increases, making the environment more basic or alkaline. In such an environment, the cell membrane of bacteria can become damaged and lose its selective permeability, leading to loss of cellular contents and, ultimately, cell death.

At a low pH, hydrogen ions (H+) concentration increases, making the environment more acidic. This can cause the bacterial cell membrane to become damaged and more porous, also resulting in cell death.

In the case of skin microflora bacteria, which are adapted to the slightly acidic pH of the skin, exposure to high or low-pH environments can lead to their death or reduced growth. This can be beneficial in some instances, such as when trying to control the growth of pathogenic bacteria. Still, it can also have unintended consequences, such as disrupting the skin microbiome's balance and promoting opportunistic pathogens' development.

Yours sincerely,

Edgar M Cambaza

Hi Sir Aakash! it’s surely will depend on the organism as different species prefer different pH values. Certain bacterial genera may survive in pH as low as 4 and others in pH as high as 7. Thus, different bacterial genera have different pH needs, but will die if subjected to pH that is detrimental for them. Our skin's pH is normally mildly acidic (4.0-6.0). It is demonstrated that skin with pH values below 5.0 is in a better condition than skin with pH values above 5.0 (Lambers, Piessens, Bloem, Pronk & Finkel, 2006). This is because the acidity, can fight off pathogenic microorganisms and free radicals that can hasten the effects of aging. Most of our skin microbiomes are harmless or even beneficial and they mostly thrive in a neutral pH. In terms of disinfection, a high pH can kill most of them as according to (Mcbride, 2018) Disease cannot survive in an alkaline state. The mechanism of action according to (Schneegurt, 2021) is that the component most sensitive to pH in the cell is its workhorse, the protein. 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. However, if skin’s pH rises into the alkaline range, its natural balance is disturbed. Essential epidermal lipids cannot be synthesised and skin loses water and dries out. In this condition, the outer layer of skin (or epidermis) is no longer able to work as a protective barrier (Eucerin, 2023). We can destroy most of the skin’s normal flora but note that it has a detrimental effect on our skin. Therefore, It is essential to maintain our skin's pH balance in order to hinder bacterial development and to restore and maintain the ideal acid environment.

References:

1. Lambers, H., Piessens, S., Bloem, A., Pronk, H., & Finkel, P. (2006, October 1). Natural skin surface pH is on average below 5, which is beneficial for its resident flora. International Journal of Cosmetic Science; Wiley-Blackwell. https://doi.org/10.1111/j.1467-2494.2006.00344.x

2. L. (2021b, February 11). 8.3: The Effects of pH and Temperature on Microbial Growth. Biology LibreTexts. https://bio.libretexts.org/Courses/Manchester_Community_College_(MCC)/Remix_of_Openstax%3AMicrobiology_by_Parker_Schneegurt_et_al/08%3A_Microbial_Growth/8.03%3A_The_Effects_of_pH_on_Microbial_Growth

3. Eucerin. (n.d.). Eucerin: About Skin | Skin’s pH Eucerin. https://int.eucerin.com/about-skin/basic-skin-knowledge/skins-ph

At extreme pHs you start to get increased non-enzymatic acid/base chemistry occurring which can deplete essential metabolites and generate toxic byproducts (a good example are the formation of furans from sugars under acidic conditions as can be replicated in the lab by adding sulphuric acid to some sugar). Some amino acids in proteins are acidic/basic so large pH changes can change their protonation which in turn impacts on their ability to form ionic bonds needed to prevent proteins from denaturing. There is also the need for proton gradients used for ATP generation and to transport metabolites across the membranes which becomes less energetically favorable outside of the normal physiological range.

As n ote above, it depends on the microorganism - some only grow at extremes of pH and others produce resistant elements that preserve viability at extremes.

As a rule of thumb, industrial practices consider the range of >2 to