7th May
Disinfectants have a broad spectrum of activity that inactivates or kills microorganisms on living tissue and inanimate surfaces in specific or non-specific ways. The application of disinfectants is essential for maintaining contamination control within pharmaceutical and healthcare areas.
There are several different approaches for evaluating disinfectant suitability - this article considers the Minimum Inhibitory Concentration (MIC). This is the lowest concentration of a disinfectant (or other antimicrobial agent) that prevents visible growth of a microorganism after a given contact period. Importantly, the operative word is ‘inhibitory’ - the disinfectant might kill the microorganisms, or it might inhibit them (or a combination thereof). The criterion for assessment, in this case, is the inhibition of growth. This is assessed by way of an in vitro susceptibility test.
The MIC test sets out to define the ‘breakpoint’ - the concentration of disinfectant which defines whether a species of bacteria is susceptible or resistant to the antibiotic (or, in other words, the dilution where bacteria begin to show resistance). Operationally, the in use concentration should be at least two dilutions away from the breakpoint to guard against dilution error.
A step beyond determining the MIC is the Minimum Bactericidal Concentration (MBC) - the minimum concentration of an antibacterial agent that results in bacterial death 1. The MBC has some limitations around culturability, where the aggregation of bacterial cells occurs and the ‘colony forming unit’ provides an underestimate of cell numbers 2.
The origin of the MIC test relates to the studies performed by Sir Alexander Fleming during the 1940s 3. The MIC can be a useful measure of disinfectant suitability and assessing it can help determine the effectiveness of a given disinfectant, at a selected concentration, against a particular strain of bacteria 4.
The MIC is essentially the "threshold" of disinfectant concentration needed to stop bacterial growth. It is expressed as a quantitative measure, in micrograms per millilitre (μg/mL) or milligrams per litre (mg/L). This is determined through laboratory tests where bacteria and fungi are individually exposed to a range of disinfectant concentrations. Its application in the clinical field helps doctors to select the most appropriate antibiotic for treating a bacterial infection, helping to prevent resistance development 5.
Across all applications, understanding the MIC helps to track the emergence and spread of resistance in bacterial strains. This can occur when the concentration or contact time needs to increase to achieve the same level of inhibition (or a combination of both variables).
The MIC is typically determined using either broth or agar dilution methods. With broth dilution, bacteria or fungi are grown in a liquid medium containing varying concentrations of the antimicrobial agent. The antimicrobial concentration is adjusted into the correct concentration by mixing stock antimicrobial with media. The adjusted antimicrobial is serially diluted into multiple tubes (or wells) to obtain a gradient. With agar dilution, antibiotics are incorporated into a solid growth medium (agar plates) 6. The most commonly cited source for the test method is the Clinical and Laboratory Standards Institute of the United States of America 7.
After incubation, the lowest concentration that shows no visible bacterial growth is the MIC.
If the MIC is below a predetermined breakpoint, the microorganisms are considered susceptible to the antimicrobial, meaning it is likely to be effective under laboratory conditions. If the MIC is above a breakpoint, the microorganisms are considered resistant and the antibiotic is unlikely to be effective.
Some organisms may fall into an intermediate category, where the antimicrobial might be effective in certain circumstances, but the risk of failure is greater. It should also be noted that ‘effective under laboratory conditions’ only means that where a suspension test is conducted, this should pass. However, variables relating to different surface types and conditions, as well as understanding how a disinfectant behaves ‘in the field’ (cleanroom), can demonstrate that a disinfectant may not be as effective as the initial laboratory tests might suggest.
Thus, to understand disinfectant efficacy we need knowledge of:
These are in addition to several other factors.
In general, lower MIC scores are more effective antimicrobial agents. However, this needs to be treated with caution. The breakpoint and range of dilutions differ by disinfectant and bacterial species. Therefore, comparing MICs of different disinfectants is not only based on the numerical value but on how far the MIC is from the breakpoint. For example, a strain of Pseudomonas aeruginosa has an MIC of 2 μg/mL for Disinfectant A and Disinfectant B. Looking at the dilutions for Disinfectant A, at 2 μg/mL, this strain of P. aeruginosa is four dilutions away from the breakpoint. Whereas, for Disinfectant B, the same strain of P. aeruginosa at an MIC of 2 μg/mL is two dilutions away from the breakpoint. Hence, based on MICs, this strain of P. aeruginosa is more susceptible to Disinfectant A than Disinfectant B.
Furthermore, disinfectants of the same family will differ from each other in relation to their formulation. On this basis, we can see that concentrations range considerably for different agents 8:
In summary, the MIC is an important measure in microbiology (as well as for infectious disease management). This is in terms of assessing the minimum level of antimicrobial activity required to suspend the growth of microorganisms and with helping to monitor resistance. However, making sense of how a disinfectant will behave in practice requires an assessment of other factors, in addition to the MIC.
1. French GL (2006) Bactericidal agents in the treatment of MRSA infections--the potential role of daptomycin. Journal of Antimicrobial Chemotherapy. 58 (6): 1107–17
2. Robert É, Lefèvre T, Fillion M et al. (2015). Mimicking and understanding the agglutination effect of the antimicrobial peptide thanatin using model phospholipid vesicles. Biochemistry. 54 (25): 3932–41
3. Fleming A (1944). On the antibacterial action of cultures of a penicillium, with special reference to their use in the isolation of B. influenzae. H. K. Lewis & Co. Ltd. OCLC 25424051
4. Alajlan AA, Mukhtar LE, Almussallam AS et al. (2022) Assessment of disinfectant efficacy in reducing microbial growth. PLoS ONE 17(6): e0269850
5. Magréault S, Jauréguy F, Carbonnelle E, Zahar JR () When and How to Use MIC in Clinical Practice?. Antibiotics. 11 (12): Article 1748
6. Andrews JM (July 2001). Determination of minimum inhibitory concentrations. The Journal of Antimicrobial Chemotherapy. 48 Suppl 1 (suppl 1): 5–16
7. CLSI. M100 Performance Standards for Antimicrobial Susceptibility Testing, 29th ed.; Clinical and Laboratory Standards Institute: Wayne, PA, USA, 2019
8. Andrews JM. Determination of minimum inhibitory concentrations. J Antimicrob Chemother. 2001;48 Suppl 1:5-16. doi: 10.1093/jac/48.suppl_1.5
