Is there one correct microbiological test method for every product?
Antimicrobial test methods are generally used as screening tools to assist us in predicting the end-use performance characteristics of our products. Choosing the correct test method for both the active antimicrobial agent used and for the proposed end-use is critical in allowing us to predict functionality in the real-world. In most cases, for non-porous, hard surfaces, test methods that allow for direct inoculation of a test surface would be required. The most suitable Industrial Standard test method for ceramic tiles, for example, would be the ISO 22196 (or JIS Z2801). In this test method, a standard amount of bacteria are applied to a test surface. After a specific time, usually 18-24 hours, the surviving bacteria are retrieved from the surface and counted.
The amount of bacteria surviving on the treated surface is directly compared to the amount of bacteria surviving on an untreated control. A reduction of over 99% of bacteria, under these test conditions is generally viewed as superior. However, these standard test methods are not applicable to all end-use situations. Modification to these standard test methods are often employed to prove effectiveness under more specific end-use conditions. For example, quick antimicrobial kill can be demonstrated by changing the contact time of this test method from 18 hours to 2 hours. Moreover, the bacteria may be changed to demonstrate effectiveness to more clinically relevant organisms such as Methicillin Resistance Staphylococcus aureus (MRSA) or spore forming organisms such as Clostridium difficile (C-diff). In many cases, the relative humidity of the use environment will greatly affect the survivability of the microorganisms on the surface. This needs to be a factor when determining the correct parameters for any test method. Untreated controls are essential so that the performance of the added antimicrobial agent can be differentiated from the natural resistant properties of any given surface (absorption, drying, hydrophobicity, etc).