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Industry expert Tony O’Lenick explains the difference between a bacteriostatic and bacteriocidal.
Bacteriostatic and bacteriocidal are both terms that relate to microbial growth. They are antimicrobials that include anti-bacterial, anti-viral, anti-fungal and anti-parasitic compounds. Bacteriostatic and bacteriocidal differ from each other by how they affect the bacteria.
A bacteriostat is a compound that keeps bacteria from reproducing. It does not kill bacteria if they are already present, but it keeps their number from increasing. A bactericide, on the other hand, is a compound that kills bacteria. If a product already is contaminated with bacteria, the addition of a bacteriostat may slow the growth but will not kill the existing bacteria. To kill existing bacteria, a bactericide must be added.
The concept of selective toxicity is important here; in cosmetic formulations, one generally wants a compound that kills bacteria and fungi without causing harm to the human using it. Cyanide, for example, is a very effective bacteriocidal compound but it does not make a good additive to cosmetic products since it also is quite toxic to the person using it.