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.
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.
Phenotypically Acquired Resistance
The ability of microorganisms to become insensitive to an antimicrobial treatment as a result of how and where the organism grows is considered phenotypically acquired resistance. An example is biofilms, complex communities of microorganisms like bacteria, yeast, molds, protozoa, and viruses. Biofilms attach to surfaces and secrete a material that strengthens and protects the biofilm. Organisms in a biofilm are far more resistant to antimicrobial agents than organisms that are freely in suspension. This increased resistance occurs because antimicrobial agents can’t physically reach the microorganisms through the secreted material or they are inactivated by the material.
Organisms that are on a soiled surface or even in solution with a heavy soil load are also often very resistant to biocides. As with biofilms, this is a result of the biocide being inactivated by the soil or physically prevented from reaching the organism.
Unlike intrinsic resistance, phenotypically acquired resistance is not a stable trait of microorganisms. If the organisms in a biofilm are suspended in solution so that they are no longer protected by the secreted material, the organisms are as sensitive to a biocide as an organism that was not in the biofilm. Or, if the soil is removed, the organisms will become sensitive to biocides. This is one reason why it is important to clean a surface before sanitizers are used.
Acquired Genotypic Resistance
Genetically acquired resistance is insensitivity to a biocide that a microorganism gains either via a mutation or through a transfer of resistance genes from one organism to another. A mutation is a change in an organism’s DNA, and on rare occasions, can make a microorganism resistant to biocides. Exposure to antimicrobials at sub-lethal levels over time can encourage this kind of mutation. Thus, it is critical that all sanitizers and disinfectants are used at the recommended concentrations and in the recommended way. It is also important that biocides drain properly. Pooling or standing solutions of antimicrobials that get diluted to below lethal levels increase the chance that a mutant resistant to that biocide can develop.