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The ISO 27447 method, titled "Test Method for Antibacterial Activity of Semiconducting Photocatalytic Materials", determines the ability of photocatalytic materials to inhibit the growth of microorganisms or kill them. Both hard surfaces (non-porous) and textiles could contain a photocatalyst or have photocatalytic films and can be tested using this method by measuring the number of viable microorganisms on the surface after being exposed to ultraviolet light.
There are two methods that can be used depending on the surface being tested. The "film adhesion method" is used to test the antimicrobial efficacy of hard surfaces while the "glass adhesion method" is used to test textiles. Both methods involve inoculating the test sample with a standardized microbial suspension, placing either film or glass on the suspension, and then exposing it to ultraviolet light. After the contact time, microbial reductions are calculated.
Summary of the ISO 27447 Test
- The test microorganism is prepared, usually by growth in a liquid culture medium.
- The suspension of test microorganism is standardized by dilution in a nutritive broth (this affords microorganisms the potential to grow during the test).
- Sterilized filter paper is placed into the bottom of a sterile glass petri dish. Sterile water is then added, and a sterile glass tube or rod is placed on top of that. Control and test surfaces are placed on top of the glass rod. This prevents the carriers and the filter paper from touching and helps maintain moisture during the test.
- Control and test surfaces are inoculated with microorganisms, in triplicate, and then the microbial inoculum is covered with a thin, sterile film (or glass). Covering the inoculum spreads it, prevents it from evaporating, and ensures close contact with the antimicrobial surface.
- The petri dishes containing inoculated samples are covered with moisture conservation glass.
- Microbial concentrations are determined at "time zero" by elution followed by dilution and plating.
- A control is run to verify that the neutralization/elution method effectively neutralizes the antimicrobial agent in the antimicrobial surface being tested.
- Inoculated, covered control and test carriers are allowed to incubate undisturbed under ultraviolet light (at a known intensity) for the duration of the contact time, usually 8 hours.
- Another set of inoculated, covered control and test carriers are allowed to incubate undisturbed in the dark for the duration of the contact time.
- After incubation, microbial concentrations are determined. Reduction of microorganisms relative to initial concentrations and the control surface is calculated.
Strengths of the ISO 27447 Test Method
- The method is quantitative and results tend to be reproducible, provided the inoculum does not spill off of the target area after being covered with the thin film or glass.
- The method tests for both bacteriostatic (growth-inhibiting) and bactericidal (bacteria-killing) properties.
- Microbial concentrations are standardized, and bacteria are provided with nutrients during the incubation period, which provides them with ample opportunity to grow if surfaces aren't sufficiently antimicrobial. This is in contrast to certain other antimicrobial tests, where microbes are "incubated" in non-nutritive suspensions, which itself may be stressful over long periods.
- The method stipulates triplicate experimentation, which helps researchers estimate the precision of the individual tests and increases overall experimental accuracy.
Weaknesses of the ISO 27447 Test Method
- The ISO 27447 method is not necessarily representative of actual surface contamination events, since a relatively dilute liquid microbial inoculum is spread over a considerable surface area, and then is kept wet (usually for a period of 8 hours). Most of the time, microbial contaminants dry quickly onto surfaces. This limits the time that an aqueous medium is available to facilitate interaction between the antimicrobial surface and microorganisms. This means that ISO 27447 is a "best-case" sort of test for many photocatalytic surfaces.
- It is not uncommon for "control" surfaces to demonstrate low-level antimicrobial activity, so identification of an ideal control surface may be challenging.
- The method does not include a "pass/fail" criterion for the calculated levels of antimicrobial activity observed in test samples, making determinations of antimicrobial activity difficult.
Though the ISO 27447 test is somewhat "best-case," it is an excellent way to quantify the antimicrobial activity level of a photocatalytic antimicrobial surface.