Taking a Closer Look at Resistance

Organisms can also acquire a genetic resistance to a biocide via acquisition of a resistance gene from another microorganism. There are different modes of transfer, but the end result is that an organism that was previously sensitive to a biocide can suddenly acquire the genes that cause it to be resistant to the biocide or even multiple antimicrobial agents.

Oftentimes this kind of resistance is not stable. A mutation or resistance gene may only offer a survival advantage as long as the biocide is present. For instance, a mutation in a binding site that makes it less likely that a QAC can bind to a microorganism may also interfere with the binding of nutrients that are important for the cell to survive. While the QAC is present, the mutation acts as a survival advantage but once the QAC is removed, the mutant is still not able to absorb nutrients easily and may disappear from a population once the biocide is gone.

The Impact of Resistance

Many people get concerned about genetically acquired resistance to sanitizers, yet this is one of the least relevant forms. The confusion may be a result of the legitimate concern over genetically acquired resistance to antibiotics. However, antibiotics and the biocides that are used in sanitizers and disinfectants are different compounds used in different ways. Antibiotics often have a single binding site on a target microorganism and a single site at which they are active. They are also used at levels that are very close to the lowest possible level at which the antimicrobial is effective, referred to as the minimum inhibitory concentration (MIC). That means a mutation in a single binding site or active site in a microorganism can make that organism nearly immune to an antibiotic, particularly if the antibiotic is used at levels that are near its MIC.

Biocides have many ways that they can kill microorganisms. In some cases, there may be hundreds or even thousands of binding sites or places in a bacterial cell where the biocide is active. Even if a cell mutates so that a site on its surface no longer binds a biocide, it may have a very limited effect on the effectiveness of the biocide. Another factor to consider is use levels. A sanitizer or disinfectant is often used at many times the MIC for that antimicrobial. For example, the MIC for a typical QAC against many organisms is 0.5-2 ppm. An organism that acquires resistance to a QAC may be able to tolerate 2-5 times that much QAC to survive 1-10 ppm. However, QAC is used at 200-800 ppm in many applications, so this level of resistance really has little or no effect.

Intrinsic resistance is also not a particularly relevant resistance as long as due care is taken when selecting sanitizers and disinfectants. Because this characteristic is stable and is inherent to the nature of target microorganisms, the effectiveness of an antimicrobial can be tested against the microorganism and the antimicrobial’s label will indicate which microorganisms the treatment is effective against.

Often the most serious form of resistance is phenotypically acquired resistance, or organisms growing in biofilms or those protected by soil. The most important step for controlling these kinds of organisms is good cleaning practices. Yet when there is a microbial problem, many people change sanitizers on the assumption that organisms have acquired a genetic resistance or use the sanitizers at higher than recommend concentrations. Unfortunately, because most sanitizers and many disinfectants are poor cleaners and because the chemicals are often prevented from physically contacting microorganisms in biofilms or soil, such responses are ineffective. The correct response to this kind of resistance is to clean better.