In the battle to slow or prevent the transmission of viruses, such as the novel coronavirus, continuously active disinfectants could provide a new line of defense, according to a recent University of Arizona study released on the health sciences preprint server MedRxiv.
While disinfecting high-contact surfaces is an important practice to prevent the spread of pathogens, these surfaces can be easily re-contaminated after the use of conventional surface disinfectants.
Alternatively, continuously active disinfectants work to actively kill microorganisms and provide continued protection over an extended period of time.
“During the course of respiratory illnesses such as COVID-19, aerosols released during sneezing and coughing contain infectious viruses that will eventually settle onto various surfaces,” said Luisa Ikner, associate research professor in the Department of Environmental Science and lead author of the study.
“Factors including temperature, humidity and surface type can affect how long viruses such as SARS-CoV-2 will remain infectious after surface deposition.”
“The only tools we have currently in reducing the environmental spread of viruses via surfaces are hand sanitizer, hand washing and the disinfection of surfaces,” said Charles Gerba, a microbiologist and professor of environmental science in the College of Agriculture and Life Sciences.
“This technology creates a new barrier in controlling the spread of viruses in indoor environments.”
Gerba and his research team designed and conducted the study – which was funded by Allied BioScience, a company that manufactures antimicrobial surface coatings – to evaluate continuously active antimicrobial technology and its potential use against the transmission of viruses.
“We evaluated this technology by testing a modified antimicrobial coating against the human coronavirus 229E, which is one of the viruses that causes the common cold,” Gerba said.
“Even two weeks after the coating was applied, it was capable of killing more than 99.9% of the coronaviruses within two hours.”
Human coronavirus 229E is similar in structure and genetics to SARS-CoV-2 but causes only mild respiratory symptoms. It can therefore be safely used as a model for SARS-CoV-2 to evaluate antiviral chemistries.
The results from these experiments may provide new opportunities for controlling the environmental transmission of COVID-19.
“The standard practice of surface disinfection using liquid-based chemistries according to product label instructions can render many viruses – including the coronaviruses – noninfectious,” Ikner said.
“In contrast, high-touch surfaces treated with continuously active disinfectants are hostile environments to infectious viruses upon contact and demonstrate increasing effectiveness over time.”
Continuously active disinfectant technology has been around for almost a decade but has been focused primarily on controlling hospital-acquired bacterial infections, such as invasive methicillin-resistant Staphylococcus aureus, or MRSA.
UArizona researchers from the Mel and Enid Zuckerman College of Public Health investigated the impact of antimicrobial surface coatings in reducing health care-associated infections in two urban hospitals.
The results of that study were published in October and found a 36% reduction in hospital-acquired infections with the use of a continually active antimicrobial.
“As communities are reopening after weeks of stay-at-home restrictions, there is significant interest in minimizing surface contamination and the indirect spread of viruses,” Gerba said.
Previous research on the environmental spread of viruses through contaminated surfaces modeled the spread of germs and the risk of infection in an office workplace.
In that study, a contaminated push-plate door at the entrance of an office building led to the contamination of 51% of commonly touched surfaces and 38% of office workers’ hands within just four hours. With the use of disinfecting wipes, environmental contamination was reduced to 5% of surfaces and 11% of workers’ hands.
“Antimicrobial coatings could provide an additional means of protection, reducing the spread of coronaviruses in indoor environments and public places where there is continuous contamination,” Gerba said.
“We’re evaluating a number of products right now and believe it may be the next major breakthrough in environmental infection control.”
The ability of titanium dioxide (titania, TiO2) to act as a photocatalyst has been known for 90 years (Renz 1921), and its role in the “chalking” of paint (formation of powder on the surface) is well known (Jacobsen 1949).
Interest in the application of the photocatalytic properties of TiO2 was revived when the photoelectrolysis of water was reported by Fujishima and Honda (1972), and this activity was soon exploited both for the ability to catalyse the oxidation of pollutants (Carey et al. 1976; Frank and Bard 1977) and the ability to kill microorganisms (Matusunga 1985; Matsunaga et al. 1985).
Photocatalytic surfaces can be superhydrophilic, which means that water spreads on the surface, allowing dirt to be washed off, and commercial uses include self-cleaning windows (e.g. San Gobain Bioclean™, Pilkington Active™ and Sunclean™; Chen and Poon 2009) and self-cleaning glass covers for highway tunnel lamps (Honda et al. 1998).
There are currently over 11,000 publications on photocatalysis. Although an early study showed no improved antimicro-bial activity of TiO2 for disinfection of primary wastewater effluent (Carey and Oliver 1980), many subsequent studies have shown the usefulness of photocatalysis on TiO2 for disinfection of water (Chong et al. 2010).
These include killing of bacteria (Rincón and Pulgarin 2004a) and viruses from water supplies (Sjogren and Sierka 1994),
tertiary treatment of wastewater (Araña et al. 2002), purifying drinking water (Wei et al. 1994; Makowski and Wardas 2001), treatment of wash waters from vegetable preparation (Selma et al. 2008) and in bioreactor design to prevent biofilm formation (Shiraishi et al. 1999).
TiO2- coated filters have been used for the disinfection of air (Jacoby et al. 1998; Goswami et al. 1997, 1999; Lin and Li 2003a, b; Chan et al. 2005). The advantage of using photocatalysis along with conventional air filtration is that the filters are also self-cleaning. TiO2 has also been used on a variety of other materials and applications (Table 1).
The potential for killing cancer cells has also been evaluated (reviewed by Blake et al. 1999; Fujishima et al. 2000).
In recent years, there has been an almost exponential increase in the number of publications referring to photocatalytic disinfection (PCD), and the total number of publications now exceeds 800 (Fig. 1).
Some of the early work was reviewed by Blake et al. (1999) and sections on photocatalytic disinfection have been includ- ed in several reviews (Mills and Le Hunte 1997; Fujishima et al. 2000, 2008; Carp et al. 2004); reviews of the use in disinfection of water (McCullagh et al. 2007; Chong et al. 2010) and modelling of TiO2 action have been published (Dalrymple et al. 2010). In this review, we explore the effects of photoactivated TiO2 on microorganisms.
For a more detailed discussion of the photochemistry, the reader is directed to the excellent reviews by Mills and Le Hunte (1997) and Hashimoto et al. (2005). TiO2 is a semiconductor. The adsorption of a photon with sufficient energy by TiO2 promotes electrons from the valence band (evb−) to the conduction band (e −), leaving a positively charged hole in the valence band (hvb+; Eq. 1).
The band gap energy (energy required to promote an electron) of anatase is approx. 3.2 eV, which effectively means that photocatalysis can be activated by photons with a wave- length of below approximately 385 nm (i.e. UVA).
The electrons are then free to migrate within the conduction band. The holes may be filled by migration of an electron from an adjacent molecule, leaving that with a hole, and the process may be repeated.
The electrons are then free to migrate within the conduction band and the holes may be filled by an electron from an adjacent molecule.
This process can be repeated. Thus, holes are also mobile. Electrons and holes may recombine (bulk recombination) a non-productive reaction, or, when they reach the surface, react to give reactive oxygen species (ROS) such as O2−⋅ (2) and ⋅OH (3). These in solution can react to give H2O2 (4), further hydroxyl (5) and hydroperoxyl (6) radicals.
Reaction of the radicals with organic compounds results in mineralisation (7). Bulk recombination reduces the efficiency of the process, and indeed some workers have applied an electric field to enhance charge separation, properly termed photoelectrocatalysis (Harper et al. 2000).
There are three main polymorphs of TiO2: anatase, rutile and brookite. The majority of studies show that anatase was the most effective photocatalyst and that rutile was less active; the differences are probably due to differences in the extent of recombination of electron and hole between the two forms (Miyagi et al. 2004).
However, studies have shown that mixtures of anatase and rutile were more effective photocatalysts than 100% anatase (Miyagi et al. 2004) and were more efficient for killing coliphage MS2 (Sato and Taya 2006a).
One active commercially available preparations of TiO2 is Degussa P25 (Degussa Ltd., Germany) which contains approx. 80% anatase and 20% rutile. The increased activity is generally ascribed to interactions between the two forms, reducing bulk recombination.
Brookite has been relatively little studied, but a recent paper showed that a brookite–anatase mixture was more active than anatase alone (Shah et al. 2008).
A silver-doped multiphase catalyst was shown to have increased photocatalytic activity, but its antimicrobial activity was not reported (Yu et al. 2005a). Indoor use of photocatalytic disinfection is limited by the requirement for UVA irradiation.
Modified catalysts can reduce the band gap so that visible light activates the photocatalysis. This has been shown for TiO2 combined with C, N and S, metals such as Sn, Pd, and Cu, and dyes (Fujishima and Zhang 2006), but activity is generally lower than when activated with UVA. This area is currently the subject of much research.
The antimicrobial activity of UVA-activated TiO2 was first demonstrated by Matsunaga and coworkers (Matusunga 1985; Matsunaga et al. 1985). Since then, there have been reports on the use of photocatalysis for the destruction of bacteria, fungi, algae, protozoa and viruses as well as microbial toxins.
TiO2 can be used in suspension in liquids or immobilised on surfaces (Kikuchi et al. 1997; Sunada et al. 1998; Kühn et al. 2003; Yu et al. 2003a; Brook et al. 2007; Yates et al. 2008a, b; Ditta et al. 2008).
The ability to eliminate microorganisms on photocatalytic self-cleaning/ self-disinfecting surfaces may provide a useful additional mechanism in the control of transmission of diseases along with conventional disinfection methods.
Copper and silver ions are well characterised for their antimicrobial activities and can also enhance the photocatalytic activity. Combinations of Cu2+ and Ag+ with TiO2 therefore provide dual function surfaces (see below).
Photocatalytic action on microorganisms
Photocatalysis has been shown to be capable of killing a wide range of organisms including Gram-negative and Gram-positive bacteria, including endospores, fungi, algae, protozoa and viruses, and has also been shown to be capable of inactivating prions (Paspaltsis et al. 2006). Photocatalysis has also been shown to destroy microbial toxins.
As far as the authors are aware, only Acanthamoeba cysts and Trichoderma asperellum coniodiospores have been reported to be resistant (see below), but these have not been extensively studied.
The ability to kill all other groups of microorganisms suggests that the surfaces have the potential to be self-sterilising, particularly when combined
with Cu or Ag. However, for the present, it is correct to refer to photocatalytic surfaces or suspensions as being self- disinfecting rather than self-sterilising. Many studies have used pure cultures, although there are reports of photo- catalytic activity against mixed cultures (van Grieken et al. 2010) and of natural communities (Armon et al. 1998; Araña et al. 2002; Cho et al. 2007a).
The great majority of studies have been performed with Escherichia coli, and there are far too many to give a complete list in this review. Some examples of different strains used and applications are shown in Table 2. Examples of other Gram-negative bacteria that are suscep- tible to PCD are shown in Table 3. They include cocci, straight and curved rods, and filamentous forms from 19 different genera.
Most studies showed that Gram-positive bacteria were more resistant to photocatalytic disinfection than Gram-negative bacteria (Kim et al. 2003; Liu and Yang 2003; Erkan et al. 2006; Pal et al. 2005, 2007; Muszkat et al. 2005; Hu et al. 2007; Sheel et al. 2008; Skorb et al. 2008).
The difference is usually ascribed to the difference in cell wall structure between Gram-positive and Gram-negative bacteria. Gram-negative bacteria have a triple-layer cell wall with an inner membrane (IM), a thin peptidoglycan layer (PG) and an outer membrane (OM), whereas Gram-positive bacteria have a thicker PG and no OM. However, a few studies show that Gram-positive bacteria were more sensitive.
Lactobacillus was more sensitive than E. coli on a Pt- doped TiO2 catalyst (Matsunaga et al. 1985). methicillin- resistant Staphylococcus aureus (MRSA) and E. coli were more resistant than Micrococcus luteus (Kangwansupamon- kon et al. 2009).
Dunlop et al. (2010) showed that MRSA were more sensitive than an extended spectrum β- lactamase (ESBL)-producing E. coli strain, but less sensi- tive than E. coli K12. Enterococcus faecalis was more resistant than E. coli, but more sensitive than Pseudomonas aeruginosa (Luo et al. 2008).
Conversely, Kubacka et al. (2008a) showed no difference in sensitivity between clinical isolates of P. aeruginosa and E. faecalis. Van Grieken et al. (2010) saw no difference in disinfection time for E. coli and E. faecalis in natural waters, but E. faecalis was more resistant in distilled water.
These differences may relate to different affinities for TiO2 (close contact between the cells and the TiO2 is required for optimal activity – see below) as well as cell wall structure.
Gram-positive bacteria that have been shown to be killed by PCD are shown in Table 4 and include species of 17 different genera, including aerobic and anaerobic endospore formers. The endospores were uniformly more resistant than the vegetative cells to PCD.
Fungi, algae and protozoa
Fungi, algae and protozoa that have been shown to be susceptible to PCD are shown in Tables 5 and 6. These include 11 genera of filamentous fungi, 3 yeasts, 2
amoebae, 1 Apicomplexan, 1 diplomonad, 1 ciliate and 7 algae, including 1 diatom. Fungal spores were generally more resistant than vegetative forms, and Trichoderma harzianum spores in particular were resistant to killing under the conditions tested (Giannantonio et al. 2009). Cysts of Acanthamoeba showed only a 50% reduction during the treatment time and may have been killed if the treatment time had been extended (Sökmen et al. 2008).
Viruses that have been shown to be killed by PCD are shown in Table 7.
Most studies were on E. coli bacteriophages in suspen- sion, which have been demonstrated for icosahedral ssRNA viruses (MS2 and Qβ), filamentous ssRNA virus (fr), ssDNA (phi-X174) and dsDNA viruses (λ and T4).
Other bacteriophages include Salmonella typhimurium phage PRD-1, Lactobacillus phage PL1 and an unspecified Bacteroides fragilis phage. Mammalian viruses include poliovirus 1, avian and human influenza viruses, and SARS coronavirus (Table 7).
Photocatalytic activity has been shown to be capable of inactivating bacterial toxins including Gram-negative en- dotoxin and algal and cyanobacterial toxins (Table 8).
Mechanism of killing of bacteria
The mode of action of photoactivated TiO2 against bacteria has been studied with both Gram-positive and Gram-negative bacteria. The killing action was originally pro- posed to be via depletion of coenzyme A by dimerization and subsequent inhibition of respiration (Matsunaga et al. 1985, 1988).
However, there is overwhelming evidence that the lethal action is due to membrane and cell wall damage. These studies include microscopy, detection of lipid peroxidation products, leakage of intercellular components, e.g. cations, RNA and protein, permeability to low- molecular-weight labels, e.g. o-nitrophenyl-galactoside (ONPG), and spectroscopic studies.
Changes in cell permeability
Indirect evidence for membrane damage comes from studies of leakage of cellular components. Saito et al. (1992) showed that there was a rapid leakage of K+ from treated cells of Streptococcus sobrinus AHT which occurred within 1 min of exposure and paralleled the loss of viability.
This was followed by a slower release of RNA and protein. Leakage of K+ was also shown to parallel cell death of E. coli (Hu et al. 2007; Kambala and Naidu 2009). Huang et al. (2000) showed an initial increase in permeability to small molecules such as ONPG which was followed by leakage of large molecules such as β-D-galactosidase from treated cells of E. coli, suggesting a progressive increase in membrane permeability.
Membrane damage has been shown with cells labelled with the LIVE-DEAD® BacLight™ Bacterial Viability Kit which uses the fluorescent dyes Cyto 9, which stains all cells green, and propidium iodide, which only penetrates cells with damaged membranes and stains cells red. Gogniat et al. (2006) showed that permeability changes occurred in the membrane soon after attachment of E. coli to the TiO2, and we have seen similar changes (Ditta and Foster, unpublished).
However, no damage was detected on a visible light active PdO/TiON catalyst until the catalyst had been irradiated (Wu et al. 2010b). SEM clearly showed membrane damage after irradiation on this catalyst (Wu et al. 2008, 2009a, b, 2010b; see Fig. 2).
More information: Katherine D Ellingson et al. Impact of a Novel Antimicrobial Surface Coating on Healthcare-Associated Infections and Environmental Bioburden at Two Urban Hospitals, Clinical Infectious Diseases (2019). DOI: 10.1093/cid/ciz1077