January 2021

CONTRACT/PROJECT NUMBER: 301014052

A REVIEW ON WOOD’S ANTIMICROBIAL CHARACTERISTICS AND TECHNOLOGIES TO IMPROVE ANTIMICROBIAL PROPERTIES OF WOOD PRODUCTS

info@fpinnovations.ca www.fpinnovations.ca

Zeen Huang, Scientist, New Construction Materials

FPlnnovation~

Project Number: 301014052

ACKNOWLEDGEMENTS

This project was financially supported by the Canadian Forest Service under the Contribution Agreement existing between the Government of Canada and FPInnovations.

APPROVER CONTACT INFORMATION Rod Stirling Manager, New Construction Materials

REVIEWER Simon Gandrieau, Scientist, Environment and Sustainability

Fabrice Roussiere, Scientist, New Construction Materials

AUTHOR CONTACT INFORMATION Zeen Huang, Scientist, New Construction Materials

Disclaimer to any person or entity as to the accuracy, correctness, or

completeness of the information, data, or of any analysis thereof

contained in this report, or any other recommendation,

representation, or warranty whatsoever concerning this report.

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EXECUTIVE SUMMARY

In this report, a state-of-the-art review on wood properties, technologies, and wood modification to improve the

antibacterial and antiviral performance of wood products has been made as a part of the emerging technologies

project. This literature review contains three sections: Testing Methods for Antimicrobial Characteristics of Wood

Materials; Antimicrobial Characteristics of Untreated Wood; and Modification and Surface Coating Technologies

to Improve Antimicrobial Properties. Wood products may inhibit microbial transfer through chemical and physical

means. Surface modification and the use of antimicrobial coatings may further enhance antimicrobial activity. To

justify investment in developing wood products with antimicrobial surfaces, more information is needed on the

size of the potential market, and technical and regulatory risks.

ii

TABLES OF CONTENTS

EXECUTIVE SUMMARY ................................................................................................................................................. I

1 OBJECTIVES ........................................................................................................................................................ 1

2 INTRODUCTION ................................................................................................................................................. 1

3 TESTING METHODS FOR ANTIMICROBIAL CHARACTERISTICS OF WOOD MATERIALS...................................... 1

4 ANTIMICROBIAL CHARACTERISTICS OF UNTREATED WOOD ............................................................................ 4

4.1 Effects of Wood’s Physical Characteristics on Its Antimicrobial Behaviors ............................................. 4

4.1.1 Porous Structure ......................................................................................................................... 4

4.1.2 Hygroscopicity and Capillary Action ............................................................................................ 5

4.2 Effects of Wood Species and Chemical Characteristics on Wood’s Antimicrobial Behaviors .................. 6

4.2.1 Wood Species with Antibacterial Behaviours ............................................................................. 6

4.2.2 Wood Extractives with Antibacterial and Antivirus Properties ................................................... 7

4.3 Canadian Wood Species with Antimicrobial Activity ............................................................................... 9

5 MODIFICATION AND SURFACE COATING TECHNOLOGIES TO IMPROVE ANTIMICROBIAL PROPERTIES ........10

6 CONCLUSIONS AND RECOMMENDATIONS .....................................................................................................12

7 REFERENCES ....................................................................................................................................................13

iii

LIST OF TABLES

Table 1. Pros and cons of the methods used to study the antimicrobial behavior of wood material (from

Munir et al 2020a) ................................................................................................................................ 3

Table 2. Ranking of wood species depending upon the hygienic suitability (Adapted according to Munir et al

2019)..................................................................................................................................................... 6

Table 3. Antimicrobial actions of wood chemicals against microbes (Munir et al 2019) .................................. 7

Table 4. Flavonoid compounds and their antibacterial activity (Ahmad et al 2015) ......................................... 8

Table 5. Antiviral activities of flavonoid compounds (Ahmad et al 2015) ......................................................... 9

Table 6. The mechanisms of action and characteristics of commonly used metallic nanoparticles (Dizaj et al

2014)................................................................................................................................................... 11

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1 OBJECTIVES

To conduct a state-of-the-art review on wood’s antimicrobial characteristics and technologies that improve the

antimicrobial properties of wood products.

2 INTRODUCTION

The antimicrobial properties of wood have attracted a lot of research interest due to the widespread application

of wood and wood-based materials in our daily life and in healthcare facilities (Schettler 2016, Munir et al 2019,

Munir et al 2020a, Ahmad et al 2015, Teng et al 2018). The antimicrobial properties of wood refer to the

antibacterial, antiviral and antifungal behaviors. As a part of the emerging technologies project, this work is to

perform a state-of-the-art review on wood properties, technologies, and wood modification to improve the

antibacterial and antiviral performance of wood products. The aim is to identify new market opportunities for

Canadian wood products. The technologies that enhance the antibacterial or antiviral surface properties of wood

products are likely to be in demand given the current COVID-19 pandemic, and increased consumer awareness

and institutional awareness of surface transmission hazards.

3 TESTING METHODS FOR ANTIMICROBIAL CHARACTERISTICS OF

WOOD MATERIALS

A comprehensive review on the testing methods of antimicrobial properties of wood materials has been made by

a French team based on a search on the research in the past 20 years from 2000 to March 2020 and thereby 57

selected published original research articles (Munir et al 2020a). According to the literature findings, they

categorized the testing methods into two broad groups, “Direct methods” and “Extractive-based methods”,

according to the form of test material used (e.g., solid wood or extractives).

In direct methods, the microbial survival is studied after direct contact with wood samples. These methods provide

a better understanding of the role of the physical structure of solid wood as a microorganism inhibitor and are

easy to implement because usually no chemical handling or complicated preparation steps are required. On the

other hand, the direct methods may require an extra step of sterilization of the test material, e.g., by autoclaving,

ultraviolet irradiation, gamma radiation, fumigation, or by disinfection with alcohols, some of which may change

the chemical composition of the wood surfaces and interfere with the antimicrobial properties of wood. The direct

methods can be achieved with different methodologies such as Agar Diffusion Method (Direct Wood Disc Agar

Diffusion Method, Sawdust-Filled Well Diffusion Method) and Evaluation of Microbial Survival on Wood Surfaces

(Microbial Recovery, Molecular Biology Methods, Adenosine Triphosphate (ATP) Bioluminescence Assay, and

Microscopy of Microbes on Wood).

In the methods to study the antimicrobial properties of wood extractives, experiments are conducted to evaluate

the bio-active chemical compounds, the non-structural components contained in wood which enhance wood’s

resistance to microorganisms. The extractive-based methods can give precise information of antimicrobial activity

of specific extractive compounds but need an extraction step and requires chemical handling. Methodologies of

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agar diffusion and dilution methods, broth dilution methods, measurement of wood mass loss, bioautography and

bio-active antimicrobial ingredient identification (e.g., mass spectrometry) can be grouped into this type of test

methods.

The above-mentioned test methods and procedures have been described in this comprehensive review and their

advantages and disadvantages were discussed and summarized (Table 1) (Munir et al 2020a).

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Table 1. Pros and cons of the methods used to study the antimicrobial behavior of wood material (from Munir et al 2020a)

Method Name Procedure Advantage Disadvantage D

irec

t m

eth

od

s

Direct diffusion method (Well and disc)

The wood material is directly placed on

microbe-inoculated agar or in a well and

incubated for recommended time

Presence of the zone of inhibition is

considered a positive result

1. Rapid and time saving 2. Applicable for low amount of material 3. Adapted for screening

1. Disc preparation time 2. High variability for quantitative applications 3. Studies only the effect of agar-diffused chemicals 4. May require the sterilization of wood

samples

Culture-based microbial survival test

Initial microbial quantity is inoculated on wood samples and after the incubation time, the microbes are recovered, cultured,

and viable cells are counted

1. Can study the structural and chemical role of wood components 2. Qualitative and quantitative results 3. Applicable for low amount of material

1. Difficulty in recovering all microbes present in pores 2. Microbial quantification is an extra step needed 3. Only viable cells are identified, while there

can be still non-viable infectious cells present

Microscopy The behavior and distribution of

inoculated microbes on wooden structures

is observed via microscopy

1. Rapid and time saving 2. Applicable for low amount of material 3. Adapted for screening

1. May require the fixation of samples 2. Difficult to differentiate microbial structures from wooden structures 3. May require competencies of image analysis

ATP luminescence The ATP of microbes on wood is measured 1. Rapid and easy 2. Applicable for low amount of material 3. Adapted for screening

1. Difficult to differentiate the microbial

ATP from other organic debris

2. Adapted only for solid surfaces

Molecular biology methods The quantity and viability of microbes is

tested via nucleic acid amplification Accurately measures the microbial survival

1. Expensive 2. Require sophisticated handling

Extr

acti

ve-b

ased

met

ho

ds

Extractive-based diffusion and dilution method Extractives are placed on agar or in agar

wells, or in broth, after loading on filter

paper discs or directly

1. Adapted for qualitative and quantitative antimicrobial studies 2. Specific chemicals can be extracted

depending upon the solvent used

1. Involves chemical handling Extra step of extraction 2. One solvent cannot extract all active components 3. Does not study the role of structure of wood

Bioautography

Extractives are loaded on a

chromatographic layer, and then the

diffusion of active chemicals is studied for

their antimicrobial properties

1. Adapted for qualitative antimicrobial studies 2. Specific chemicals can be extracted

depending upon the solvent used and

identified on the basis of their diffusion on the

chromatographic layer

1. Involves chemical handling and extraction 2. One solvent cannot extract all active components 3. Does not study the role of structure of wood 4. Not a quantitative method

Mass spectrometry The total profile of microbes is measured 1. Applicable for a low amount of material

2. Accurately measure the content of the

active ingredient

For more specific results, the identified

compounds are supposed to be tested by other

culture-based methods

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4 ANTIMICROBIAL CHARACTERISTICS OF UNTREATED WOOD

4.1 Effects of Wood’s Physical Characteristics on Its Antimicrobial Behaviors

Everyday, hospitals, healthcare facilities and food industries are dealing with the problem of transfer of

contamination from the solid surfaces of infra-structures, tools and equipment. Thus, surface hygiene is an

important factor for preventing environmental contamination and infection. The surfaces have different

properties depending on the material used (Aviat et al 2016). Wood is a commonly used renewable resource in

manufacturing contact surfaces, but traditionally its reputation is not good as a non-hygienic and non-cleanable

material. On the other hand, in the last decades, several studies proved that wood is a better surface to control

microbial growth and minimize microbial transmission (Ismail et al 2017, Laireiter et al 2013, Montibus et al 2016,

Pailhoriès et al 2017, Tiwari et al 2006).

Hygienic characteristics of wood are often misunderstood because of its organic, porous and moisture absorbing

surface. In fact, these properties are beneficial because the organic nature of wood makes it environment-friendly,

the absorption potential of wood can cause desiccation conditions for microbes, and the wood extractives can kill

or inhibit harmful microorganisms (Alpert 2006, Milling et al 2005a). The characteristics of wood allow to decrease

the use of chemical agents for cleaning operations, which are a big concern regarding chemical hazard and

antimicrobial resistance (Fahimipour et al 2018, Obe et al 2018).

In this section, the effects of some key physical characteristics of untreated wood on its microbe inhibiting

potential and the antimicrobial mechanism are reviewed.

4.1.1 Porous Structure

Wood is a complex porous material leaving open spaces on the surface in the form of pores. The size of pores

varies in different species of wood, ranging from less than 2 nm to more than 50 nm, which can be divided into

three categories according to the IUPAC classification: micropores (pore size < 2 nm), mesopores (2 nm < pore

size < 50 nm) and macropores (pore size > 50 nm) (IUPAC 1994, Zdravkov et al 2007). These tiny holes could retain

microbes and make the cleaning difficult. From this viewpoint, the wood surfaces are easily considered to be more

contaminated than other non-porous surfaces (Abrishami et al 1994, Carpentier 1997). However, the behavior of

wood surface retaining bacteria does not strictly mean that bacteria are then, necessarily, transferred to

something which is in contact with wood (Ismail et al 2015). For instance, a study has indicated that the wood

pine absorbs inoculum more rapidly as compared to other smooth materials (Soares et al 2012) and the difficulty

to recover microbes from wood surfaces means that these organisms are stuck inside wood structures (Cliver

2006). Thus, it can be assumed that these bacteria do not contaminate the contact objects like food or hands

(Ismail et al 2017, Montibus et al 2016, Carpentier 1997). Vainio-Kaila et al also observed that Colony Forming

Units (CFUs) of Escherichia coli and Listeria monocytogenes decreased faster on pine heartwood as compared to

glass surface and did not increase the next day (Vainio-Kaila et al 2011).

Furthermore, the wood’s porosity helps to dry the surface more rapidly than alternative non-porous materials. In

drier environments, contact plates removed significantly less bacteria from wood samples such as European

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maple, beech and oak than from polyethylene (Gehrig et al 2000). Wood and bamboo are considered as rough

and porous materials, whereas plastic, stainless steel, and glazed ceramic tile are considered as smooth and less

porous materials. Chiu et al found that the recovery of Vibrio parahaemolyticus from plastic cutting boards and

stainless steel surfaces was significantly greater than from bamboo and wooden boards (Chiu et al 2006). In

addition, it is also observed by other researchers that Vibrio parahaemolyticus survived better on smooth surfaces

as compared to porous material probably because smooth surfaces could maintain higher surface moisture

conditions for longer time (Shi et al 2017).

The presence of more pores in wood means more exposure of extractives from cut cells and deeper retention of

bacteria inside the wood. It was noted that the penetration depth of E. coli and spores of B. subtilis in wooden

cutting boards in transversal direction was different from in longitudinal direction and the bacteria and spores

could enter deeper (around 3 mm) in transversal cuttings than in longitudinal cut woods (Prechter et al 2002). A

later study on the antimicrobial activity of oak wood samples collected from three different locations in France

using a direct diffusion method against Staphylococcus aureus and Acinetobacter baumannii showed that the

transverse face discs exhibited higher antimicrobial activity (Munir et al 2020b).

On the other hand, the porosity not only offers difficulties in microbial recovery but may also provide shelter to

some of them. A study reported that when stressed by aeration of the liquid culture medium, the Campylobacter

jejuni cells were protected from death when a block of beech wood was present in the broth, indicating that the

wooden pieces kept in broth eliminated the possibility of desiccation, which may have resulted in the survival of

the bacteria on the wood (Boucher et al 1998).

4.1.2 Hygroscopicity and Capillary Action

Wood is a hygroscopic material that can adsorb or desorb water in response to temperature and relative humidity

of the atmosphere surrounding it (Hartley and Hamza 2016). The ability of wood to take moisture from the

environment can lead to desiccation of bacteria. As most bacteria are desiccation-sensitive and require a water

potential of -2.8 MPa or less for growth in wood, which is significantly above the moisture content of air-dried

wood stored indoors, the dried wood does not offer enough water for microbial growth and multiplication (Milling

et al 2005b, Stienen et al 2014).

The hygroscopicity of wood also makes it to absorb moisture faster than other non-porous contact surfaces, which

explains why the microbes survive longer on smooth and non-absorptive surfaces such as metal and plastics. For

example, Methicillin-Resistant Staphylococcus aureus (MRSA) was observed to survive longer on plastic, vinyl,

flannel cloth and glass as compared to wood surface (Coughenour 2009), and the turkey coryza agent was

observed to survive for shortest period on wood as compared to aluminum, glass and dust (Cimiotti et al 1982).

The property of wood absorbing moisture and liquid microbial inoculum rapidly compared to non-absorptive

materials also lead to lower recovery concentration or the number of recovered bacteria on contact from wood

surface (Moore et al 2007) especially with prolonged contact time (Miller et al 1996).

However, once the fiber saturation point is reached the wood does not absorb more moisture anymore and the

hygroscopic antimicrobial potential will decrease. By comparing the survival of E. coli on wood and polyethylene

under different moisture content, both wood and polyethylene showed very high numbers of bacteria in high

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moisture conditions, but the bacterial number was lower on wood in drier environments (Gehrig et al 2000).

Therefore, if wood surfaces are exposed to external weathering conditions, especially, abundant rain and high

humidity environment, the passive effect of wood against microbes may decrease. Moisture content above the

fibre saturation point would not be found on interior wood products under normal end use conditions.

4.2 Effects of Wood Species and Chemical Characteristics on Wood’s Antimicrobial

Behaviors

4.2.1 Wood Species with Antibacterial Behaviours

Every wood species has specific anatomy (physical characteristics) and chemistry (chemical characteristics relating

to wood components and extractives), both of which impact the ability of microorganisms to survive on wood.

Table 2 (Munir et al 2019) lists the wood species which have been studied by different researchers and their

ranking depending on the hygienic suitability against various bacteria including Staphylococcus aureus (S. aureus),

Pseudomonas aeruginosa (P. aeruginosa) and Escherichia coli (E. coli). S. aureus and P. aeruginosa are two

common hospital acquired bacteria (Bankier et al 2019), and E. coli are bacteria found in the environment, foods,

and intestines of human and animals (CDC 2020). These three kinds of bacteria have been chosen for testing

antibacterial properties of wood and extractives in many research reports (Table 2 and Table 4). Among the wood

species which have been investigated, pine, oak and larch showed the highest capacity in inhibiting the growth of

different bacteria. Maple, spruce, beech, poplar and Douglas fir have also been widely studied for their

antibacterial properties (Munir et al 2019).

Table 2. Ranking of wood species depending upon the hygienic suitability (Adapted according to Munir

et al 2019)

Reference* Bacteria Ranking

[1] Staphylococcus aureus, Pseudomonas aeruginosa, Enterobacter faecium, and B. subtilis

Pine > larch

[2] E. coli and E. faecium Pine = oak = larch > maple > spruce > beech > poplar Pine = oak > larch = maple = spruce = beech = poplar

[3] Bacillus subtilis and Pseudomonas fluorescens

Oak > spruce

[4] Poultry manure flora Pine > larch = maple

[5] E. coli 0157:H7 White ash > red oak > black cherry > maple

[6] Escherichia coli and E. faecium Pine > poplar = beech

[7] S. aureus, P. aeruginosa, and Acinetobacter baumannii

Oak > Douglas fir = pine > poplar

[8] [9] [10] S. aureus, E. coli, Enterococcus faecalis, Streptococcus pneumoniae

Pine > spruce

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* [1] Laireiter et al 2013; [2] Milling et al 2005a; [3] Koch et al 2002; [4] Milling et al 2005b; [5] Miller et al 1996; [6]

Schonwalder et al 2002; [7] Munir et al 2017; [8] Vainio-Kaila et al 2013; [9] Vainio-Kaila et al 2015; [10] Vainio-Kaila et al

2017.

4.2.2 Wood Extractives with Antibacterial and Antivirus Properties

Wood consists of cellulose, hemicellulose, lignin, and extractives. The extractives are a group of extraordinarily

diverse compounds that are soluble in neutral solvents or water. Usually, the content of the extractives is less

than 10 per cent in the wood of trees from temperate zones, but in some tropical and subtropical wood species

the amounts can be much larger (Fengel and Wegener 1984, Hillis 1987). The extractives are regarded as non-

structural wood constituents. Their primary function is to protect the tree from biodegradation. For example, a

study on the colonisation by the bacterium Escherichia coli on Scots pine sapwood and heartwood, and Norway

spruce showed that all the tested wood samples caused more rapid decrease of bacterial numbers compared to

glass, which was used as reference material. On the other hand, both thermal modification and removal of

extractives through extraction resulted in an increase of the bacterial count on all the samples compared to

untreated wood samples (Vainio-Kaila et al 2013).

The components in wood extractives with antimicrobial activities include tannins, phenolic acids, flavonoids and

terpenoids. The mode of action and targets of these different wood chemicals were grouped and listed in Table 3

(Munir et al 2019) according to some published review papers (Ahmad et al 2015, Silva et al 2010, Teodoro et al

2015).

Specifically, in a review on therapeutic potential of flavonoids and their mechanism of action against microbial

and viral infections (Ahmad et al 2015), the flavonoid compounds with antibacterial and antiviral activity against

different bacteria are identified in Table 4 and the flavonoid compounds with antiviral activity against different

types of virus and their sources are listed in Table 5 according to a series of published references.

Table 3. Antimicrobial actions of wood chemicals against microbes (Munir et al 2019)

Target Wood chemicals

Cell wall and cell membrane Flavonoids, tannins, aldehydes, phenolic acids, terpenoids, alkaloids, terpenes

Nucleic acid Flavonoids, aldehydes, alkaloids

Metals metabolism Tannins

Protein synthesis Aldehydes, tannins

Energy metabolism Flavonoids, phenolic acids

Adhesion and Biofilm formation Phenolic acids, quinones

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Table 4. Flavonoid compounds and their antibacterial activity (Ahmad et al 2015)

Flavonoid Antibacterial and antiviral activity against bacteria

Quercetin Staphylococcus aureus, Bacillus subtilis, Bacillus cereus, Escherichia

coli, Helicobacter pylori, Pseudomonas aeruginosa, P. fluorescens,

Enterobacter aerogenes

Apigenin Streptococcus pyogens, Streptococcus viridans,

Streptococcus jaccalis, Enterobacter cloacae, Vibrio cholera,

Enterococcus faecalis, Escherichia coli, Staphylococcus aureus,

Pseudomonas aeruginosa, Bacillus cereus, Bacillus subtilis,

Klebsiella pneumoniae, Bacillus subtilis Salmonella typhimurium

Rutin Bacillus anthracis, Pseudomonas aeruginosa, E. coli, Klebsiella

pneumoniae, Salmonella typhimurium, Bacillus cereus

Baicalin Staphylococcus aureus, Pseudomonas aeruginosa

Chrysin Streptococcus jaccalis, Streptococcus baris, Streptococcus

pneumonia, E. coli

Hydroxyethylrutoside Clostridium perfringens

Datisetin Proteus vulgaris

Hydroxyethylrutoside Pseudomonas aeruginosa

iso-liquiritigenin Staphylococcus aureus

Saponarine Enterobacter cloacae, E. aerogenes, Pseudomonas aeruginosa

Lucenin Enterobacter cloacae, E. aerogenes, Pseudomonas aeruginosa

Bartramia flavone Enterobacter cloacae, E. aerogenes, Pseudomonas aeruginosa

5,7-dimethoxyflavanone-4′-O-β-D-glucopyranoside Klebsiella pneumonia

5,7-dimethoxyflavanone-4′-O-[2″-O-(5‴-O-trans-cinnamoyl)-

β-D-apiofuranosyl]-β-D-glucopyranoside

Klebsiella pneumonia

5,7,3′-trihydroxy-flavanone- 4′-O-β-D-glucopyranoside Klebsiella pneumonia

Naringenin-7-O-β-D-glucopyranoside Klebsiella pneumonia

Quercetin-3-Orutinoside (Rutin) Klebsiella pneumonia

Kaempferol-3-Orutinoside (nicotiflorin) Klebsiella pneumonia

Kaempfero-3-o-glucoside Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa,

Bacillus cereus, Bacillus subtilis

Morin-3-O-arabinoside P. fluorescens, V. cholera, L. monocytogenes

Genkwanin Vibrio cholera, Enterococcus faecalis, E. coli

Rhamnocitrin Vibrio cholera, Enterococcus faecalis, Micrococcus luteus,

Shigella sonnei

Quercetin-5,3′dimethylether Vibrio cholera, Enterococcus faecalis, Micrococcus luteus,

Shigella sonnei

Rhamnazin Vibrio cholera, Enterococcus faecalis

Kaempferol Vibrio cholera, Enterococcus faecalis

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Table 5. Antiviral activities of flavonoid compounds (Ahmad et al 2015)

Flavonoid Source Activity against virus

Orientin Trollius chinensis Para influenza type3 virus

Vitexin Trollius chinensis Para influenza type3 virus

Quercetin Kalanchoe pinnata, Genus crataegus HCV, polio, herpes simplex

Glabranine

7-O-methyl-glabranine

Tephrosia madrensis,

Tephrosia viridiflora and Tephrosia crassifolia

Dengue virus

Ladanein Marrubium peregrinum HCV

Naringenin Citrus paradisi HCV

Chrysosplenol C Pterocaulon sphacelatum

Tanacetum parthenium

Polio virus

5-hydroxy-7,8-Dimethoxyflavone Mosla scabra Anti-influenza viruses

Apigenin Mosla scabra Anti-influenza viruses,

HCV,

Enteovirus-71

Moralbanone Morus alba Herpes simplex type 1

virus

Acacetin Mosla scabra Anti-influenza viruses

Eudraflavone B

hydroperoxide

Morus alba Herpes simplex type 1

virus

Leachianone G Morus alba Herpes simplex type 1

Liquiritigenin Glycyrrhiza uralensis

G. glabra

HCV

Quercetin and 3-O-glycosides Bauhinia longifolia Mayaro virus

7-hydroxyisoflavone Hedysarum theinum Enterovirus71

4.3 Canadian Wood Species with Antimicrobial Activity

As described in Section 4.2.1, the most common wood species which have been studied for antibacterial activity

are pine, oak, larch, maple, spruce, beech, poplar and Douglas fir. Pine, oak and larch showed the highest potential

for inhibiting the growth of different bacteria according to many investigations listed in Table 2. Canada is the

third most forested country in the world (Natural Resources Canada 2019) and has plenty of lumber products. In

Canada, the principal softwood lumber species are spruce, pine, hemlock, Douglas fir, larch and western red cedar,

while the predominant hardwood species are birch, maple and oak (Historica Canada 2020). Therefore, Canada

has almost all the wood species with antimicrobial properties especially antibacterial behaviors discussed above

from its domestic forest and timber market. However, it is noted that the studied wood species listed in Table 2

were from European forest tree species, and there is limited information available specifically on Canadian

species. Therefore, it is necessary to investigate the suitability of Canadian wood species’ antimicrobial activity

and compare with the information available on European species from the same genera.

In a study conducted at University of Ottawa and University of University of British Columbia (Omar et al 2000),

the wood of 14 samples of hardwood trees obtained in eastern Ontario which were used traditionally as medicine

by First Nation’s people were screened for antimicrobial activities with eight strains of bacteria. It was found that

the ethanol extractives from red maple, sugar maple, white birch, beech, hybrid popular and red oak were active

against the bacteria S. aureus and/or other bacteria such as B. subtilus and M. phlei, etc. The wood extractives

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from red oak showed the highest inhibitory efficacy. The screening result in this study was obtained through an

extractive-based method. It would be also useful to evaluate the antibacterial activity of these hardwood species

through a direct testing method.

Through this literature survey, it seems that neither direct method nor extractive-based method has been

reported to evaluate the Canadian softwood species such as pine, spruce and Douglas fir for their antimicrobial

activity against bacteria and virus.

5 MODIFICATION AND SURFACE COATING TECHNOLOGIES TO

IMPROVE ANTIMICROBIAL PROPERTIES

Some untreated wood exhibits antimicrobial activity against bacteria and other microbes owing to its physical

microstructure and specific chemical profile of extractives. A study has showed that the high inhibitory activity

against bacteria of Scots pine sapwood and heartwood and Norway spruce could not be explained alone by either

the amount of extractives or the faster drying of the wood surface, but is possibly a combination of both factors

(Vainio-Kaila et al 2013). Therefore, care must be taken in selecting modification methods to improve wood’s

antimicrobial property to ensure that the modification procedure does not result in change of the physical and

chemical characteristics of the untreated wood species which exhibit good antimicrobial activities.

Only a few research works have been found on surface modification technologies to improve antimicrobial

properties. A novel process was developed by chemically attaching quaternary ammonium compounds with

strong antibacterial activity to hemlock wood using supercritical carbon dioxide (Xu et al 2016). The quaternary

ammonium compound was chemically attached to hemlock by using hexamethylene diisocyanate (HDI) as a linker

via a carbamate/urethane linkage in supercritical carbon dioxide. As a result, the chemically modified hemlock

wood demonstrated exceptional antibacterial activity and improved dimensional stability as well.

There is a current trend to use nanotechnology for wood treatment especially for wood preservation. Wood

treatments based on nanoparticles is considered to play an important role in the next generation of wood

protection systems in order to achieve self-cleaning surfaces, scratch and weathering resistance, and biocides

properties, and promising findings have been reported in evaluation of the performance of nanoparticles from

silver (Ag), boron (B), copper (Cu), zinc (Zn), zinc oxide (ZnO), zinc borate (B2O6Zn3), and titanium dioxide (TiO2) on

wood protection (Borges et al 2018). For this purpose, nanotechnology can be applied through impregnation of

wood with a suspension of metallic nanoparticles (Teng et al 2018). On the other hand, researchers have reported

that some metallic nanoparticles (especially metal oxide nanoparticles) show great antibacterial effects. The

representative metallic nanoparticles are Ag and Ag2O, ZnO, Cu and CuO, and TiO2, among which ZnO and TiO2

also present encouraging potential as antiviral agents (Dizaj et al 2014, Borges et al 2018, Akhtar et al 2019). A

combination of different metallic nanoparticles could target a broad-spectrum of bacteria and increase the

antibacterial efficiency (Bankier et al 2019, Dizaj et al 2014). The common metallic nanoparticles used as

antimicrobial agents and their mechanisms of action are summarized in Table 6 (Dizaj et al 2014).

However, through this literature survey, it is also noted that the current studies on impregnated wood with the

suspension of metallic nanoparticles mainly concentrated on the improvement of wood’s natural durability and

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resistance to wood rot fungi, and wood’s dimensional stability. It would be meaningful to evaluate the

antibacterial and antiviral behaviors of the metallic nanoparticle-treated wood towards a hygienic environment.

Furthermore, the potential risk relating to introduction of metallic nanoparticles into wood is also crucial, but

there have not been sufficient risk assessments for human health and the environment impacts of some metallic

nanoparticles yet (Teng et al 2018, Borges et al 2018).

Table 6. The mechanisms of action and characteristics of commonly used metallic nanoparticles (Dizaj

et al 2014)

Type of the

nanoparticles

Proposed mechanism of

antimicrobial action

Main characteristics as

antimicrobial agent

The main factors that

influence antimicrobial

activity Ag nanoparticles Ion release; induction of pits and gaps in the bacterial

membrane; interact with disulfide or sulfhydryl groups

of enzymes that lead to disruption of metabolic

processes. DNA loses its replication ability and the cell cycle halts

at the G2/M phase owing to the DNA damage (in the

case of Ag2O).

High antimicrobial activity against both bacteria and

drug-resistant bacteria, antifungal activity on spore-

producing fungal plant pathogens, high stability,

nontoxicity.

Particle size and shape of particles.

ZnO nanoparticles ROS generation on the surface of the particles; zinc

ion release, membrane dysfunction; and nanoparticles

internalization into cell.

Photocatalytic activity; high stability; bactericidal effects

on both Gram-positive and Gram-negative bacteria;

antibacterial activity against spores which are resistant

to high temperature and high pressure.

Particle size and concentration.

TiO2 nanoparticles Oxidative stress via the generation of ROS; lipid

peroxidation that cause to enhance membrane fluidity

and disrupt the cell integrity.

Suitable photocatalytic properties; high stability;

effective antifungal for fluconazole resistant strains. Crystal structure, shape and size.

Au nanoparticles Attachment of these nanoparticles to membrane

which change the membrane potential and then

cause the decrease the ATP level; and inhibition of

tRNA binding to the ribosome

Nontoxicity, not inducing any ROS-related process; high

ability to functionalization, polyvalent effects; ease of

detection; photothermal activity.

Roughness and particle size.

Si nanoparticles Influencing the cell functions such as cell

differentiation, adhesion and spreading. Non-toxicity; stability. Particle size and shape.

CuO nanoparticles Crossing of nanoparticles from the bacteria cell

membrane and then damaging the vital enzymes of

bacteria.

Effective against Gram-positive and Gram-negative

bacteria; high stability; antifungal activity. Particle size and concentration.

MgO and CaO nanoparticles

Damaging the cell membrane and then causing the

leakage of intracellular contents and death of the

bacterial cells.

Effective against both Gram-positive and Gram-negative

bacteria; high stability; low cost; availability. Particle size, pH and concentration.

In addition to treating wood by impregnation, it is also possible to incorporate different metallic nanoparticles

into melamine coatings on the surface of wood materials, wood-based furniture and flooring to improve

antibacterial properties (Kandelbauer and Widsten 2009). Jia et al have studied the antibacterial ratio (AR) of Ag

nanoparticle-loaded hydroxyl zirconium sodium phosphate (Ag-HZDP), Ag nanoparticle-loaded zeolite (Ag-Z),

Nano-TiO2 and Nano-ZnO which were dispersed in melamine resin solution and coated on wood floor and found

that the two types of Ag nanoparticles exhibited highest antibacterial efficiency (the AR > 90%), and the Nano-

TiO2 and Nano-ZnO also showed considerable antibacterial property (AR > 80%) (Jia et al 2019). A water-based

wood coating containing 1.5% ultrasonic-treated nano-TiO2 slurry modified with 0.5% polyoxyethylene

octylphenol ether and 3% sodium polyacrylate exhibited excellent drying performance and antibacterial rate

against E. coli and S. aureus reached 95% (Qian et al 2014).

Many wood products used in high touch areas have a coating (varnish, paint, melamine paper, etc.) on their

surface. In these cases, it is the antimicrobial properties of the coating that are most important, not the underlying

wood. In these cases, the wood is only a substrate and the wood physical or chemical properties will not play a

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major role. In the past several years, a couple of literature reviews published have discussed systematically the

strategies of chemical modifications and the chemicals applied for antimicrobial surface coatings. Adlhart et al

have discussed different chemistry-based approaches, e.g. anti-adhesive surfaces, contact-killing surfaces and

biocide-releasing surfaces to achieve functional antimicrobial coating (Adlhart et al 2018). In another report,

Swartjes et al have overviewed the main chemicals or bio-active materials used for antimicrobial surface coatings

especially with bacterial killing function, including antibiotics, antimicrobial peptides, antibacterial enzymes,

nanoparticles, quaternary ammonium compounds, polymers in passive coatings, super-hydrophobic compounds,

and chitosan (Swartjes et al 2015).

A commercial antimicrobial surface coating called CuVerro Shield™ produced by Aereus Technologies can be used

to solid surface of wood composites and nearly any form or type of other material such as metals, tiles, polymers,

and plastics to form a strong, durable and permanent antibacterial copper coating. The coated surface is reported

to kill 99.9% of harmful bacteria. This proprietary product is registered to make public health claims with both the

Environmental Protection Agency (EPA) in the United States in 2008 and Pest Management Regulatory Agency

(PMRA) in Canada subsequently (Cuverro Bactericidal Copper Surfaces, 2021).

ARAUCO Inc. has produced a type of laminate panel using InCopper™ antimicrobial technology, a formulation that

actively protects the laminate surface against fungi, mold and mildew, and bacteria that can cause stains and odor.

The proprietary InCopper™ agent is registered by EPA in 2018 for its antimicrobial properties but currently

available only in the United States, consisting of a mixture of copper salts that act as an antimicrobial agent,

continuously protecting the laminate surface. It is introduced to the decorative sheet when the sheet is saturated

with resins before lamination to the wood substrate. However, as described by the company, this product does

not protect the users or others against bacteria, viruses, germs or other disease-causing organisms, and should

not be considered as a means to disinfect or sanitize for the purpose of providing health benefits or protecting

human health (ARAUCO, 2021).

6 CONCLUSIONS AND RECOMMENDATIONS

Different testing methods have been used to evaluate wood’s antimicrobial properties, including Direct Methods

and Extractive-Based Methods. In Direct Methods, wood blocks or wood dusts are used directly to examinate their

antimicrobial effectiveness against different microbes, whereas in Extractive-Based Methods, extractives are

extracted from wood and evaluated their antimicrobial efficiency. Extractive-based methods can help to

understand the antimicrobial activity of specific compounds. However, wood’s antimicrobial properties rely on

both chemical and physical features. As a result, only direct methods can give an accurate assessment of

antimicrobial performance of a wood surface. In the physical aspect, wood’s porous structure, hygroscopicity and

capillary action impact microbial survival and the transfer of microbes through contact surface. In the chemical

aspect, the different types of chemicals of wood extractives including tannins, phenolic acids, flavonoids and

terpenoids have antibacterial and antiviral activities. In the research of antibacterial activity, both direct methods

and extractives-based methods (especially direct methods) have been used for studying many untreated wood

species such as pine, oak, larch, maple, spruce, beech, poplar and Douglas fir. Three types of bacteria,

Staphylococcus aureus, Pseudomonas aeruginosa and Escherichia coli, which are closely related to human health

and environment, are the most common bacterial species chosen in assessing antibacterial activity of wood and

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extractives. In the study of antiviral activity, the research so far has largely concentrated on wood extractives from

medicinal plants. Different research studies have indicated that among the most common wood species which

have been studied for antibacterial behaviors, the untreated wood species showing most potential of antibacterial

property are pine, oak and larch. So far these wood species reported with high antibacterial activity are mainly

from European forests. There is a lack of appropriate investigations on Canadian wood species from the same

genera in order to compare with the information available on European species.

Many studies reported that some metallic nanoparticles (especially metal oxide nanoparticles) exhibit strong

bacterial property and antiviral efficiency. The examples of these metallic nanoparticles include Ag, AgO, CuO, ZnO

and TiO2. A combination of different metallic nanoparticles could target a broad-spectrum of bacteria. While there

is a trend to treat wood with suspensions of metallic nanoparticles, the objectives of studies in this area were

mainly investigating methods of improving resistance to biodegradation and dimensional stability. More work is

needed to evaluate the nanoparticle-treated wood’s antimicrobial behaviors, and to assess potential impacts on

human health and the environment. Such information would be needed to obtain registration from Health

Canada’s Pest Management Regulatory Agency (PMRA).

Some metallic nanoparticles such as AgO, Zno and TiO2 have also been incorporated into melamine resin coatings

for improving the antibacterial and antiviral performance of the surface of wood-based materials, furniture and

flooring. Different chemistry-based approaches such as anti-adhesive surfaces, contact-killing surfaces and

biocide-releasing surfaces could be developed by introducing various functional types of chemicals or bio-active

materials to achieve functional antimicrobial surface coating.

In commercial production, an antimicrobial surface coating called CuVerro Shield™ containing strong antibacterial

copper component has been produced and used to produce solid surface of wood composites, registered by both

EPA in the United States and PMRA in Canada. In addition, a commercial panel with laminated decorative surface

sheet containing a mixture of copper salts using InCopper™ antimicrobial technology has introduced and

registered by EPA for its antimicrobial properties against fungi, mold, and odor-causing bacteria.

Based on this literature review, the following recommendations are suggested:

- Assess the market potential for wood products with antimicrobial properties for use in health care settings

and high touch areas. A substantial market would be needed to justify the high cost of meeting regulatory

requirements for antimicrobial products.

- Determine the technical, market and regulatory barriers to using existing commercial antimicrobial

surface coatings on Canadian wood species.

- Evaluate the degree to which Canadian wood products used in high touch areas could inhibit the transfer

of human pathogens.

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