The pH value is a key factor in cell and tissue cultures

The term “pH” is derived from either “pondus hydrogenii” or “potentia hydrogenii”. Both are originally Latin phrases, with “pondus hydrogenii” meaning “weight of hydrogen” and “potentia hydrogenii” meaning “power of hydrogen”. The pH value is a non-dimensional number that measures how acidic or alkaline an aqueous solution is. It is defined as the negative decimal logarithm of the concentration of hydrogen ions. The strength of the acid or alkali is identified by color indicators. Every cellular process has an ideal pH at which it functions best. If the process deviates from this ideal pH value, it can lead to conformational enzyme changes that impair function, resulting in effects that are as severe as those caused by extreme temperature changes. If the process deviates drastically from the ideal pH value, it can even cause damage to the DNA’s secondary structure, α-helix, and β-pleated sheet, leading to irreversible denaturation. In order for cells to grow well, it is therefore vital that the culture medium has a stable pH value. Most mammalian cells feel right at home at a pH value of 7.4. Transformed cell lines prefer slightly lower pH values of 7.0 to 7.4 while normal fibroblast cell lines have a preference for slightly alkaline pH values of between 7.4 and 7.7. Many culture media have a phenol red pH indicator added to them. If the pH value is 7.4, the culture medium will be red. With 40 µM of phenol red in DMEM, the medium changes color when the pH value rises, turning pink at a pH value of 7.6 and purple at 7.8. This might mean that the medium is contaminated with yeast or a fungus. As pH values become increasingly acidic, the indicator turns from orange to yellow – potentially a sign of bacterial contamination. (See below for more information about hot-air sterilization.) A CO2 incubator stabilizes the set pH value in the culture medium by controlling the CO2 concentration. CO2 incubators play a key role in the activities of every cell and tissue culture laboratory – from growing adherent CHO cell cultures to growing cultures used in genome editing with the help of a CRISPR Cas9 system. Most of the time, the open hydrogen carbonate buffer system is used in the culture medium. This buffer system maintains a balance based on an exchange between the CO2 contained in the incubator atmosphere and the CO2 released in the culture medium.

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