Developing a novel test method to determine the efficacy of antimicrobial materials

Cunliffe, Alexander James (2024) Developing a novel test method to determine the efficacy of antimicrobial materials. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Antimicrobial materials are becoming increasingly popular as a method of controlling microbial growth due to the raised awareness of infection control and hygiene. Standardised test methods help to demonstrate the efficacy of these materials before they are implemented in end-use scenarios. However, some methods do not reflect the in-use conditions of the materials, such as the use of high relative humidity that prevents an inoculum from evaporating on a surface. This in turn allows an antimicrobial material to remain active if it requires moisture, which is often the case. It is of utmost importance to ensure that these standardised test methods accurately simulate the end-use environment to provide confidence that their efficacy will remain once applied to the point-of-use. To achieve this, a series of experimental decisions were tested including environmental conditions (e.g., temperature, relative humidity, and airflow) as well as other factors (e.g., inoculum volume and concentration, incubation period). To reproducibly assess these parameters, an environmental control chamber was designed, and 3D printed. Additionally, the impact of droplet evaporation on bacterial deposition and the prolonged efficacy of antimicrobial materials was assessed. The environmental conditions significantly (p < 0.05) affected the survival of bacteria on an inert (stainless steel) surface by causing the evaporation of the inoculum on the surface, promoting cell death via desiccation. Furthermore, a prototype environmental control chamber was developed that was capable of maintaining temperature (room temperature to 37 oC), relative humidity (15 to 100 %) and airflow in a stable manner. The environmental conditions (temperature, relative humidity) as well as methodological decisions (surface composition, droplet volume, droplet contamination with microorganisms) significantly affected the evaporation rate of droplets on a surface in some instances. All of the factors described affect the ability of an antimicrobial material to exhibit or retain its efficacy, and novel standardised test methods need to accommodate these factors if they are to accurately simulate a variety of end-use scenarios. Future work should aim to continue to develop both the novel chamber and realistic conditions in standardised test methods to determine antimicrobial material efficacy more accurately when in use.