In food processing the question often arises regarding the need to rotate sanitizers on a regular basis to ensure continued efficacy. The concern is that extended use of a particular chemical sanitizer will select a resistant population thereby rendering the sanitization regime ineffective. Before addressing this question, there are some aspects of bacterial resistance to antimicrobials that need to be clarified. There are numerous antimicrobial compounds available for use both in vitro and in vivo. Antibiotics, disinfectants, preservatives and sanitizers are all considered antimicrobials, but their modes of action and likelihood of developing a resistant population are very different and sometimes confused.
Resistance is a term that can be defined differently depending on the scientific forum in which it is used. It is important to note that use of the term resistance in many instances must be interpreted with some caution. Where antibiotics are used in the treatment of infection, resistance can be defined as a significant reduction in susceptibility resulting in a high likelihood of treatment failure. In terms of chemical sanitizers, resistance can be defined as the microorganism’s ability to survive exposure to an antimicrobial compound regardless of concentration and contact time.
Unlike antibiotics, an increase in minimum inhibitory concentration (MIC) of a sanitizing chemical does not necessarily correlate with treatment failure. Should the MIC for an antibiotic used against a particular microorganism increase, the consequences can be significant indicating possible therapeutic failure or the need for a different course of treatment. Where sanitizers are concerned, increased MICs are often misinterpreted as indicating development of resistance. In cases where a microorganism has adapted to survive at reduced antimicrobial concentrations or lesser contact times, the term tolerance is more appropriate.
Microbial Resistance. Resistance can be innate, acquired or achieved by adaptation:
Innate or intrinsic resistance is related to the general physiology of the microorganism and stems from properties or mechanisms already present. For example, Bacillus sp. are intrinsically resistant to benzoates due to their ability to metabolize this compound.
Acquired resistance results from genetic changes that occur due to mutation or through acquisition of extra-chromosomal genetic material such as plasmids. Acquired resistance has been studied extensively in relation to antibiotic use, but as yet, limited studies have been done with regard to chemical sanitizers.
Resistance through adaptation occurs following a successive increase in antimicrobial exposure to sub-lethal concentrations. This type of resistance is often unstable and the microorganism can revert to a sensitive state if the antimicrobial is removed from the growth environment.
Chemical sanitizers are effective against planktonic cells but cells within a biofilm have been shown to survive sanitizer exposure. Biofilms are aggregates of microorganisms that adhere to each other and produce a protective matrix consisting of polymeric substances such as exopolysaccharides. The ability of the biofilm matrix to exhibit increased tolerance to environmental stresses, such as sanitizer treatment, may be due to several factors. These include chemical interaction of sanitizer with the biofilm matrix, the production of degradative enzymes that breakdown the sanitizing chemical and physical separation of sanitizer from cell surfaces due to the protective effect of the polymeric matrix. In this instance, by definition, microbial cells have not developed resistance, but have created surroundings that enable cells to tolerate the hostile environmental conditions.
Antibiotics are used to combat infections in humans and animals, whereas chemical sanitizers are used to control microbial contamination on hard surfaces. The mode of action of chemical sanitizers differs markedly from antibiotics. Antibiotics have specific target sites such as DNA or protein synthesis and can be species specific. Chemical sanitizers, however, have multiple target sites and affect multiple cellular components through physicochemical interaction and chemical reactions. When used at maximum permissible concentrations they act indiscriminately in a relatively short time frame, physically damaging the cell by destroying the cell wall and denaturing components such as proteins and nucleic acids essential for cell survival. Because chemical sanitizers attack a cell on multiple fronts, development of resistance to sanitizers through genetic mutation or modification is unlikely.
Chemical sanitizer use in the U.S. food industry is regulated by the Federal Insecticide, Fungicide, and Rodenticide Act, and, by law, users must adhere to label instructions. Approved-use concentrations of hard-surface sanitizers are greater than the tolerable range for microbes normally encountered in food processing. With this in mind, no evidence exists that proper use of sanitizers in food processing will result in the development of a resistant microorganism population.
All chemical sanitizers have advantages and disadvantages. Some may have better efficacy against bacteria than against yeasts and mold, whereas others may have greater tolerance to soil load. In instances where the microflora of a food processing line has shifted due to changing production conditions, sanitizer rotation may be advantageous. Therefore, rotating sanitizers in food production facilities is a worthwhile action when spectrum of microbial activity rather than development of resistant microorganism population is the primary concern.