Resistance is the ability of a microorganism to exhibit reduced sensitivity to an antimicrobial treatment that would be effective against other organisms. There are several kinds of resistance, including intrinsic resistance, phenotypically acquired resistance, and genetically acquired resistance.
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Intrinsic resistance is the ability of an organism to be insensitive to an antimicrobial condition due to the nature of the microorganism. For instance, some microorganisms can form bacterial spores that enable them to survive conditions like extreme temperatures and drying, as well as exposure to many disinfectants and sanitizers. In particular, non-oxidizing antimicrobials, such as phenolics, alcohol, and quaternary ammonium chloride (QAC), are unable to penetrate a spore coat. And with oxidizing biocides, it generally takes far higher levels and exposure times to inactivate a spore compared to a normal microorganism. For example, it may take 5,000 parts per million (ppm) and several minutes or more to inactivate a spore compared to only 50 ppm of chlorine and 30 seconds.
Another form of intrinsic resistance is displayed by mycobacteria, which have a cell wall that is very hydrophobic and contains a lot of natural wax. This can prevent many biocides, especially non-oxidizing biocides, from penetrating the cell wall. This barrier can be overcome but it requires a higher level of biocide, longer exposure time or the use of other ingredients.
Intrinsic resistance is generally a very stable trait and is closely linked to the basic structure of various microorganisms. In general, the intrinsic resistance of microorganism to biocides is, from most resistant to least resistant: spores>mycobacteria>non-enveloped viruses>gram negative bacteria>gram positive bacteria>enveloped viruses.