There is much hype around residual antimicrobials, and you might be wondering what these are and what the difference is between different residual antimicrobials. Residual antimicrobials are surface treatments that provide long-term disinfection or sanitation protection to surfaces. This is different from how one typically thinks about episodic disinfection: the disinfecting chemical is put onto a surface and works at that moment in time, but as soon as a pathogen lands on that surface, the cycle starts again. Let’s dive into the “what” and “how” behind different types of residual antimicrobials! Residual antimicrobials fall into two main categories by the US EPA:
- Disinfectants with Residual Efficacy: These disinfectants provide ongoing antimicrobial effects beyond the initial application. They are effective for a specified duration (e.g., 24 hours) and can reduce re-contamination by germs on high-touch surfaces. They meet EPA standards for disinfection efficacy.
- Supplemental Residual Antimicrobial Products: These include coatings, paints, and solid surfaces. While they don’t meet EPA disinfection standards, they serve as supplements to standard disinfection practices. Their efficacy duration varies (weeks to years), and they undergo durability assessments with other chemicals.
A better term to describe residual antimicrobials is to consider them persistent antimicrobials. They provide ongoing antimicrobial protection in a way that normal episodic disinfection does not.
Ever wonder how something applied to a surface could have residual efficacy? Let’s look at the different enabling chemistries for residual antimicrobials, how they work, and their pros and cons.
1. Quaternary Ammonium Compounds (Quats): These are commonly used in healthcare settings. Quats work as disinfectants by breaking open the membranes of bacteria and outer coatings of viruses. They can have residual efficacy (depending on the formulation) and are effective against a wide range of bacteria and viruses. Because quats work by direct contact with a pathogen, they can be less efficacious when a soil load is present or when used on textured or soft surfaces. This means that a surface needs to be physically cleaned of soil for the underlying technology to work.
An emerging concern around quats is their safety. Certain reproductive and developmental problems have been linked to quats, as well as respiratory issues like COPD. Additionally, in recent years, certain MDROs (multi drug-resistant organisms) have started to become resistant to disinfection by chemical quats. Quat binding is another issue that plagues this chemistry. Quat binding occurs when the active ingredient in disinfectants, quaternary ammonium chloride (quat), becomes attracted to and absorbed into fabrics, such as cotton. This can reduce the overall efficacy (some estimates as high as 50%) of the disinfectant delivered to surfaces, creating scenarios where the surface may not be fully disinfected and lead to quat-resistant organisms.
2. Silver-Based Disinfectants and Residual Antimicrobials (Ag): Silver ions and silver nanoparticles have antimicrobial properties and can provide residual protection. The antimicrobial properties of silver have been known for centuries, but the exact mechanism of how it kills bacteria is still not fully understood. However, it is known that silver kills by releasing a silver ion (Ag+). These ions can damage cell membranes, interfere with cell metabolism, and DNA damage. Like quats, silver needs to be close enough to a pathogen to get a silver ion to destroy it. This reduces efficacy with soil load and textured surfaces unless the surface is physically cleaned. Also similar to quats, pathogens are beginning to become resistant to silver as an antimicrobial.
3. Copper-Infused Surfaces (Cu): Copper has natural antimicrobial properties that have been known to man for over 6,000 years. Surfaces coated with copper or copper alloys can continuously reduce microbial contamination. Like silver, copper works by releasing a copper ion (Cu+). When microbes land on a copper surface, these ions puncture the cell membrane, damaging DNA and RNA and disrupting the whole cell. Like silver, copper needs to be close enough to a pathogen to get a copper ion to destroy it. This reduces efficacy with soil load and textured surfaces, again requiring the surface to be physically cleaned to work. Additionally, high levels of copper in water or food can lead to copper toxicity, which can damage the liver, kidneys, heart, and brain, limiting where and how much this antimicrobial can be used.
4. Titanium Dioxide nanoparticles (TiO2): While it has photocatalytic properties and is effective in removing organic contaminants, its primary applications are in self-cleaning glass and other surfaces. Titanium dioxide needs ultraviolet (UV) light to produce reactive oxygen species (ROS). They generate ROS in the form of hydroxyl radicals and superoxide anion radicals. These radicals damage bacterial cell membranes, proteins, and DNA. However, the downside of using titanium dioxide as an antimicrobial is the need for UV light, where, without UV, the antimicrobial activity disappears. This is largely why titanium dioxide is not used in disinfecting applications. Titanium oxide does not release any potentially concerning chemicals or metal ions, making it generally safe for use.
5. Cerium Oxide nanoparticles (CeO2): Cerium oxide has similar ROS activity to titanium dioxide with one major exception: cerium oxide can make ROS without UV light and can be engineered to turn this mechanism on when a pathogen is near cerium oxide. This makes the production of the ROS more specific than what titanium dioxide can do. Cerium oxide can be modified (or mediated) with silver to boost its ROS production and ability to make hydrogen peroxide the primary form of ROS from relative humidity and other available sources of water. This makes silver-mediated cerium oxide antimicrobials more tolerant to surface dirt and does not require a clean surface to work. This is the major performance difference that cerium oxide has over other residual antimicrobials.
Like titanium dioxide, cerium oxide is considered safe and is used in many consumer products. Unlike titanium dioxide, silver-mediated cerium oxide can be manufactured to preferentially make hydrogen peroxide, which is potent in reducing MDROs and biofilms. Cerium oxide also does not require UV or other light for it to work as an antimicrobial. In the case of silver-mediated cerium oxide, the trigger to produce hydrogen peroxide is the reason we disinfect in the first place: harmful pathogens.
How can these antimicrobials be used to improve human health and well-being? Think of all the high-contact surfaces one encounters within public spaces, such as public transit, schools, hospitals, and gyms. It is often impossible to clean and disinfect these surfaces between each person. This can be due to manpower, the cost of disinfectants, and the dwell time needed for disinfectants to work. Residual antimicrobials reduce disease-causing germs between episodic cleaning to reduce disease transmission from these surfaces. The drawback of many residual antimicrobials is the need for a pristine (or freshly cleaned) surface to expose the antimicrobial to a germ deposited on the surface. Chemistries like cerium oxide that produce hydrogen peroxide can work even with interference from moderate amounts of dirt and buildup.
Another exciting application is the ability to disinfect soft and highly textured surfaces (think hospital linens and vinyl patient tables). Hospital curtains are often overlooked and expensive to maintain hygienically, from the manpower to remove and replace them to aggressive laundering with appropriate chemicals to disinfect. Many hospital curtains are replaced when visibly soiled. One study estimated that 92% of curtains were contaminated with bacteria within a week of use. Residual antimicrobials can be used in soft surface applications to reduce bacteria proliferation, reducing the need for constant washing and chemicals on these critical surfaces. When used appropriately, residual antimicrobials can minimize the use of water and chemicals while improving the overall cleanliness of both hard and soft surfaces. For those entrusted with patient safety and healthcare outcomes, these can be a powerful tool in reducing harmful disease-causing pathogens on surfaces. Choosing the right residual antimicrobial technology for your facility can help in the fight against HAIs. Need help in selecting the right residual antimicrobial for your facility? Find a reputable consulting company that specializes in this area that can help identify the best solutions to your toughest HAI environmental surface challenges.
