A Review of Standardized Antibacterial Material Testing Methods
Microorganism causes disease after reaching out to a potential host. They move around the environment in various but typically passive ways such as aerosols, direct contact between two animated objects and fomites. Certain microbes can retain their pathogenic potential whilst outside their host for a longer period of time. According to studies, they can survive from days to few weeks on inanimate surfaces like metals and plastics which are usually considered hygienic surfaces. Most common examples of such materials are door handles, light switches, lift buttons, digital locks, etc.
Microorganism causes disease after reaching out to a potential host. They move around the environment in various but typically passive ways such as aerosols, direct contact between two animated objects and fomites. Certain microbes can retain their pathogenic potential whilst outside their host for a longer period of time. According to studies, they can survive from days to few weeks on inanimate surfaces like metals and plastics which are usually considered hygienic surfaces. Most common examples of such materials are door handles, light switches, lift buttons, digital locks, etc.
Both clinical and non-clinical settings are prone to growth of microorganisms. Various methods are used to control microorganisms on surfaces in those settings. In clinical environments, methods to chemically disinfect surfaces are often used, but may be performed inadequately, (through poor adherence to cleaning protocols), allowing pathogens to be spread more rapidly throughout wards, following recontamination of disinfected surfaces via contact with fomites. Antimicrobial testing laboratory is need of the hour to take on microorganisms.
Testing the Efficacy of Antimicrobial Materials (AMM)
Standardized test methods are required and considered inevitable in the development of a unique antimicrobial material. In order to define a material as antimicrobial, efficacy should be evaluated under reproducible conditions that mimic later in-use environments. In case the set threshold is not satisfied, the surface then cannot be taken as antimicrobial.
The method needs to enable them who have special interest in AMM to establish a confidence in their material, offering some positive data that bolsters further exploration for testing the material either under conditions more precise to the intended point of use or even in practice. Let's say, bacterial inocula utilized in standardized testing (~105–108 CFU/mL) are considerably higher than that found in the most potential end-use settings (e.g., ~102–104).
In addition, renowned microbial testing laboratory performs the relevant standardised test method to validate the reproducibility of an antimicrobial material and achieve results that are all within the natural error range for such test. The validity of data resulted from individual test method should be acknowledged. On the other hand a growing need exists for accurate and reproducible methods, as many different AMM fall short when being tested by independent reviewers.
Standardised Tests for AMMs in Vitro
Here are the five general categories of test for an antimicrobial material in vitro:
1. High surface area to volume ratio tests - Such methods focus on maximising the contact between the surface and the microorganism, so the cells and the surface are essentially always touching and interacting. Here the bacteria are kept between the test sample and the sterilised non-antimicrobial material such as glass or plastic. The most used test method for antimicrobial materials in this category is ISO 22196:2011 (and similar methods such as JIS Z 2801).
2. Agar zone of inhibition tests - Methods that utilise zones of inhibition, for which there are two existing standardized methods, are relatively quick and simple but may provide only an indicator of whether any antimicrobial effect might be present under permanently wet conditions. The first, ISO 20645:2004 is based on a disk diffusion method, where the AMM is placed on top of an inoculated nutrient agar plate, incubated at the required temperature for a set time depending on the requirements for the bacteria being tested (such as at 37 °C for 24 h for E. coli).
3. Suspension tests - Suspension methods focus on inoculating and incubating bacteria in a liquid medium containing the antimicrobial surface, and then determining the remaining viable CFUs by taking an aliquot of this liquid and performing a dilution plate count using it. The process evaluates the materials that exhibit antimicrobial-release properties. However, due to the inoculum not being placed directly on the material, only surfaces that release antimicrobials can get tested.
4. Adhesion tests - Such tests intend to find the bacteria count that can adhere to an antimicrobial material. Generally the testing is performed by two ways. The first requires that the test surface is inoculated with the bacteria and incubated for 1 to 4 h. Non-adhering bacteria are detached and either the surface with attached cells is added to a liquid medium or an agar slab is kept on the top of the surface and incubated (for counting CFUs), or the microorganisms can be stained (e.g., live-dead staining) to know cells per unit area.
5. Biofilm tests - A biofilm is an accumulation of microorganisms that are connected with a surface and often become encased in a matrix of polysaccharide material. They are one of the most common forms that microorganisms take on Earth and can cause many problems when they form in specific artificial environments. Owing to their impact on health, industry and the environment, the ability to either destroy a biofilm or to stop it from forming in the first instance using antimicrobial materials is a focus of rising importance.