Antibiotic resistant bacteria : oak is the best in thwarting pathogenic growth

Drug resistant superbugs are expected to overwhelm the healthcare system, reverse a century’s worth of medical progress and claim more lives than cancer by 2050 unless efforts are accelerated to stop antibiotic resistant bacteria in their tracks.

While most studies to address the problem are focusing on the development of new drugs, a series of elegant studies in France has taken an unusual angle – analyzing the types of surfaces that can harbor drug resistant bacteria in healthcare settings.

Surfaces can serve as fomites, which refers to objects or materials likely to harbor infectious organisms, allowing them to promote the spread of pathogens when touched or used.

Scientists have long known that environmental surfaces are a potential reservoir for healthcare-associated infections. So the hunt has been underway in France to find a material with potent antimicrobial activity.

In the journal Antibiotics, Muhammad Tanveer Munir and colleagues at Ecole Supérieur du Bois in Nantes, located in northwestern France near the Atlantic coast, worked with collaborators from elsewhere in the region to take a fresh approach to potential materials in the healthcare environment.

They have discovered what may at first blush seem the most unlikely surface for healthcare settings – oak. Although his institution is aimed at studying wood, it is not designed to promote wood as a product for hospitals.

Wood is an old-school material and seems almost unthinkable for surfaces in a modern healthcare facility. It’s porous, and has nooks and crannies where bacterial colonies hypothetically might flourish.

Moreover, it just isn’t as sleek as stainless steel. But scientific research is proving otherwise.

“That is exactly the first perception – the porosity and organic nature of wood would somehow help the bacteria survive,” Munir told Medical Xpress.

Munir and his team compared oak to aluminum, polycarbonate and stainless steel surfaces and found that when four of the most notorious causes of drug-resistant bacterial infections were placed on these surfaces, oak fared best in thwarting pathogenic growth.

The finding was stunning – because oak would seem to be the least likely surface. Indeed, to most well-read armchair biologists the choice of oak as a material in hospitals would seem not only counterintuitive, but almost difficult to imagine as a surface for hospital bedrails, table tops, trays and other common items used in healthcare facilities.

But the research dramatically proved otherwise. Oak didn’t promote bacterial growth, it did the opposite – it inhibited it.

For example, the four bacterial species whose survival was studied in the research included Acinetobacter baumannii, Enterococcus faecalis, Klebsiella pneumoniae, and Staphylococcus aureus. Each species was analyzed on oak versus the three other materials.

“This investigation is a ‘One Health approach’ in collaboration with laboratories of multiple institutes including two public hospitals … and a veterinary school,” Munir said.

One-health approaches recognize the interconnection among people, animals, plants, and their shared environment.

“The multidisciplinary team of this project includes veterinarians, microbiologists, pharmacists, biologists, medical doctors, chemists, engineers and social scientists,” he added.

Millions of bacterial cells were deposited on each material, Munir explained, and their survival was measured on day 0, 1, 2, 6, 7 and 15 of the experiments.

Analyses were performed in triplicate for each material. When it came to oak surfaces, the team found the bacterial count decreased rapidly on transversal- and tangential-cut oak.

“Wood from a tree can be cut in various directions, the most commonly known are the tangential and transversal directions,” Munir explained.

“These two directions have a different cellular arrangement and their microscopic anatomy is different.

“For example, most of the fibers in wood run longitudinally in a tree, and when it is cut transversally, more cells would be exposed showing higher porosity, while the tangential – longitudinal – cut wood will have lower porosity.”

However, results of the research revealed no overall difference between transversal and tangential oak regarding a capacity to inhibit bacterial growth. Yet the question remains: Why would oak thwart antibiotic resistant bacteria?

The answer: Oak, like any plant, has natural antimicrobial capabilitites, Munir said.

“The tested bacteria are not the natural flora found on wood,” he explained.

“The wood material is hygroscopic, which means its fibers can absorb moisture and make it unavailable to these microbes, thus limiting their growth or killing them by a desiccation effect.

“The other mechanism is the chemical effect. The trees have a chemical defense mechanism against microbes in form of extractives – chemical compounds. These compounds are present even in dead wood material and have antimicrobial activities.

“We used untreated oak wood in this study because we have previously observed antimicrobial activities of this wood, and our objective was to study the natural antimicrobial potential of wood material.”

The French research arrives in the midst of a global coronavirus pandemic, which is overshadowing all other infectious diseases.

But in late November, Dr. Tedros Adhanom Ghebreyesus, director general of the World Health Organization, declared antimicrobial resistance as worrisome as the coronavirus pandemic.

“Antimicrobial resistance may not seem as urgent as a pandemic but it is just as dangerous,” he said.

While most of the world is riveted on the growing number of infections caused by SARS-CoV-2, which have exceeded 63 million worldwide, the drug resistance crisis threatens to reverse a century of medical progress, Ghebreyesus told a WHO news conference.

An estimated 700,000 people die annually of infections directly linked antimicrobial resistant organisms, according to WHO’s estimates, which also includes deaths attributed to a range of resistant microbes, such as fungi.

The agency additionally predicts drug resistance will likely soar uncontrollably to become the globe’s leading cause of death by 2050 unless measures are undertaken now to rein in drug resistant infections.

Munir, meanwhile, concludes that considering oak surfaces in hospitals might help lower the burden of drug resistant infections. “Bacteria responsible for healthcare-associated infections may survive for days to months on the commonly used hospital surface materials.”

Although wood unfairly is perceived as an unhygienic material “our research showed that the four most common bacteria responsible for healthcare-associated infections survive least on this material compared to other inanimate surfaces.”

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Wood is an organic material and a renewable resource of nature. It is an eco-friendly material as compared to glass, plastic, and metals that cause environmental disorders i.e., pollution or health hazards [1]. It is also an important constituent of nature-based themes aimed to improve the psychological well-being of inhabitants [2].

Untreated wood surfaces are traditionally used for food preparation, cutting, fermentation, and packaging [3]. Wood and wood products are also used as flooring and beddings in animal husbandry practices where they contribute to improvement in the health and welfare of animals [4,5].

Meanwhile, the safety of wood material in hygienically significant places is questioned, owing to its porosity and hygroscopic nature. However, studies have shown that some commonly used wood speices have antimicrobial activities [6,7,8] and can be looked on as a safe material for indoor uses in hygienically significant places [2,9] and as food contact surfaces [3,10,11].

Therefore, the antimicrobial properties of this material are investigated either to validate its safety as a hygienic surface or for the discovery and identification of the active antimicrobial compounds present in it [9,12,13,14,15,16].

Various diagnostic methods are used to determine the antimicrobial properties of wood to evaluate the safety of this material via screening tests and/or quantify the presence of any active compounds [6,16]. Moreover, such tests can help identify the factors affecting the antimicrobial behavior of wood such as the nature of the microbes (type and resistance), the wood characteristics and variability (age, location, part, and treatment) and the environment (humidity, moisture, and temperature) [7,8,10].

In addition, such methods can also be used to evaluate the efficacy of disinfectants and treatments used to increase the antimicrobial effectiveness of surfaces [17,18,19,20].

In general, antimicrobial properties of wood are studied via extractive-based methods, where compounds are extracted using solvents (S1 in Supplementary Materials) and then subjected to conventional antimicrobial testing methods such as agar diffusion and broth dilution [8,12,21,22].

Meanwhile, the direct methods such as surface contact test, microbe recovery protocols (S2 in Supplementary Materials), and bioluminescence assay can assess the surface contamination of wood [23,24,25]. However, to our knowledge, there are no specific standard methods available for wood material to directly determine its antimicrobial potential or surface contamination.

Further, mass spectrometry and chromatography help in the identification and characterization of active compounds [16,26,27]. Each method has its own benefits and disadvantages regarding its suitability for the handler.

Few reviews exist on the subject of testing the antimicrobial potential of different materials [28,29,30,31]. It is believed that this is the first review of the suitability of these methods for wood and hygienically important microbes, particularly those that can be responsible for infections in the healthcare setting, and among them those being multiresistant to antibiotics.

Therefore, this article aims to describe the available antimicrobial assays, their suitability to wood material in different forms, along with their advantages and disadvantages regarding utilization.

This information is intended to serve as a guideline for researchers and field experts regarding the application of suitable methods in wood science, microbiology, hygiene, and the discovery of novel antimicrobial agents.

Results and discussion

A total of 57 articles were obtained to identify the methods of antimicrobial testing of wood material (Table 1). Further studies were added to describe the prospective methods (i.e., autobiography) of studying the antimicrobial properties of different compounds in the form of extractives.

Table 1

Summary of publications selected for full-text review.

Material	Microorganism	Objective of the Study	Methods	Main Findings	Reference
Oak and pine	Staphylococcus aureus, Salmonella enteritidis	Survival of pathogens on wooden surfaces in healthcare facilities	Swabbing, planning, and plate count	Wood surfaces showed antimicrobial properties	[2]
Oak wood	Isolates of S. aureus	Oak in hospitals, the worst enemy of Staphylococcus aureus	Direct disc diffusion method	The method was efficient to show the antimicrobial properties of wood	[6]
Pine and spruce wood-associated polyphenols	Salmonella, Listeria monocytogenes, S. epidermidis, S. aureus, Candida tropicalis, Saccharomyces cerevisiae	The antimicrobial effects of wood-associated polyphenols on food pathogens and spoilage organisms	Microbial cell wall permeability and membrane damage	Several stilbenes showed antimicrobial activities against food pathogens and spoilage organisms	[13]
Populus lasiocarpa, P. tomentosa	N/A	Characteristics of antibacterial molecular activities in poplar wood extractives	GC/MS	The molecules were identified that are known to have antimicrobial properties	[16]
Abies alba, Q. rubra, European oak, Fagus sylvatica	S.aureus, E. coli, P. aeruginosa, E. faecalis	Direct screening method to assess antimicrobial behavior of untreated wood	Direct disc diffusion method	The method was efficient to show the antimicrobial properties of wood	[7]
Larch (Larix decidua Mill.) and Pine (Pinus sylvestris L.)	Bacillus subtilis, S. aureus, Enterococccus faecium, Pseudomonas aeruginosa	Testing the antimicrobial activities of different wood and their parts against different bacteria	Direct disc diffusion, paper disc diffusion	Antimicrobial activities depended upon the type of wood, part of tree, and type of bacteria	[8]
Spruce wood (P. abies), glass, polypropylene	L. monocytogenes	An assessment of bacterial transfer from wooden ripening shelves to cheeses	Food contact with surface	Wooden shelves had the lowest transfer rate of bacteria compared to other surfaces	[10]
Wood and other cutting boards	S. Enteritidis	Transfer of bacteria to food after cleaning the surfaces	Swabbing and contact press	Efficacy of cleaning methods was tested	[17]
Spruce wood shelves	L. monocytogenes	Survival of bacteria after the cleaning and sanitation of cheese preparation boards	Surface contact/blot planning and blending	Bacteria could not be cleaned by brushing and rubbing	[18]
Wood and other archeological objects	Variety of microbes	Isolation, characterization, and treatment of microbial agents responsible for the deterioration of archaeological objects	Swabbing	All samples were contaminated with various types of surface degrading microbes	[20]
P. sylvestris, Picea abies	E.coli	Effect of extractives and thermal modification on antibacterial properties	Plate count method	Thermal treatments and extraction influence on the antimicrobial properties of wood	[21]
P. sylvestris, P. abies	S. aureus, E. faecalis, E. coli, Streptococcus pneumoniae	Antibacterial properties of wooden extracts	Direct (extractive) agar diffusion method	Extractive showed antimicrobial properties	[22]
Oak and Douglas fir wood	Wood degrading microbes	Interaction of bacteria and fungi on wooden surfaces	Scanning electron microscopy and plate contact test	Environmental factors’ influence on the microbial interaction on wooden surfaces	[23]
Melamine, vinyl chloride, stainless steel, wood, and acrylonitrilebutadiene styrene	Total microbial count	ATP bioluminescence values are significantly different depending upon the material surface properties of the sampling location in hospitals	ATP bioluminescence, SEM, agar stamp/blotting	ATP and colony-forming unit (CFU) were different for wooden surfaces	[25]
Wood and plastic	Foodborne bacteria	Analysis of microbial community and food-borne bacteria on restaurant cutting boards	Pyrosequencing	Distribution of 32 genera was identified	[32,33]
Wood, plastic, vinyl, quarry clay tile	L. monocytogenes	Efficacy of sonicating swabs to recover microbes from surfaces	Sonicating swab compared to cotton, sponge, and foam swab	Sonicating swabs recovered significantly higher number of microbes	[34]
Contact surfaces including wood	Erwinia herbicola	Evaluation of two surface sampling methods for microbial detection on materials by culture and qPCR	Sponge and swabbing used for sample collection and tested by qPCR and plate count	qPCR is more sensitive than culturing, and swabbing was more efficient than sponge	[35]
Pterocarpus spp. and poplar wood	White and brown rot fungus	Evaluation of antimicrobial activity of ethanol and aqueous extracts	Wood mass loss calculation and gas chromatography-mass spectrometry	The wood extracts provided protection against degradation owing to antimicrobial properties	[36]
Wood and bamboo cutting boards	Vibrio parahaemolyticus	Efficacy of disinfectant to clean the cutting boards	Stirring method for microbial recovery	More microbes were recovered from plastic as compared to wood and bamboo	[37]
Wood cutting board and other surfaces	Methicillin-resistant Staphylococcus aureus (MRSA)	Microbial survival on five environmental surfaces	Swabbing	Survival and recovery of microbes depends upon the type of surfaces and moisture conditions	[38]
Calabrian and Sicilian chestnut, cedar, cherry, ash, walnut, black pine, poplar	Salmonella, Listeria, E.cli, S. aureus, Lactic acid bacteria (LAB)	Formation and characterization of early bacterial biofilms on different wood typologies	SEM for biofilm observation and paper disc method to determine antimicrobial activities	LAB represent efficient barriers to the adhesion of the main dairy, pathogens, probably due to their acidity and bacteriocin generation	[39]
Rubber wood cutting boards, plastic, glass	E. coli, S. aureus	Effectiveness of domestic antibacterial products in decontaminating food contact surfaces	Agar overlay method for microbial recovery	This method gave good results for testing the cleanability of surfaces	[40]
Pine and plastic	E. coli, P. aeruginosa, S. aureus, L. monocytogenes	Efficacy of electrolyzed water to inactivate different bacteria on cutting boards	Swabbing	Treatment was efficient for reducing microbial contamination	[41]
Poplar wood	E.coli	Confocal spectral microscopy—An innovative tool for the tracking of pathogen agents on contaminated wooden surfaces	Confocal spectral laser microscopy	The microbes could be located for their distribution by this method	[42]
Melia azedarach wood	Agrobacterium tumefaciens, Dickeya solani, Erwinia amylovora, P. cichorii, Serratia pylumthica, Fusarium culmorum, Rhizoctonia solani	Wood preservation potential of extracts	Direct diffusion method	Antimicrobial properties were observed using the disc diffusion method	[43]
Wooden toothpicks	Variety of microbes	Determination of microbial contamination of wood	Wet preparation techniques, concentration techniques, culture, biochemical tests	Wooden samples were found contaminated with a wide range of microorganisms	[44]
Eucalyptus globulus wood	B. subtilis, S. aureus, S. epidermis, E. coli, C. krusei, P. aeruginosa C. parapsilosis, C. glabrata, C. albicans, Saccharomyces cerevisiae	Extraction of bioactive compounds from biomass of forest management and wood processing	Well diffusion method	Antimicrobial compounds were identified	[45]
Spruce wood	L. monocytogenes, L. innocua	Comparison of methods for the detection of listeria on porous surfaces	Sponge swabbing	Porosity influences the recovery of microbes	[46]
Rubber wood and plastic	L. monocytogenes	Transmission of bacteria from raw chicken meat to cooked chicken meat through cutting boards	Rinsing with normal saline to remove bacteria and meat contact to study transmission	Surfaces play role in transmission of bacteria	[47]
Cork wood	S. aureus and E. coli	Evaluation of antimicrobial properties of cork	Agar dilution method	Cork has antimicrobial properties	[48]
Wood of P. heldreichii Christ. var. leucodermis	S. aureus, S.epidermidis, E. coli, Enterobacter cloacae, Klebsiella pneumoniae, P. aeruginosa, C. albicans, C. tropicalis, C. glabrata	Chemical composition and biological activity of the essential oil from pine wood	GC and GC/MS and Agar dilution method	Antimicrobial activities of pine wood were identified and characterized	[49]
Hardwood, carpets, vinyl and porcelain tiles	S. aureus, Aspergillus niger	Microbial survival on floor materials	Bulk rinsate, agar plate contact, vacuum suction	Microbial survival depends on the recovery method and surface type in hospitals (vet and human) and office buildings	[50]
Spruce fir boards (P. abies)	L. monocytogenes, L. innocua	Sanitizing wooden boards used for cheese maturation by means of a steam-mediated heating process	Planning and cotton swabbing and then stomacher	Both recovery methods showed identical results	[51]
Pine, poplar, spruce	E. coli, L. monocytogenes, P. expansum	Comparative study of 3 methods for recovering microorganisms from wooden surfaces in the food industry	Planning, grinding and brushing	Humidity, type of wood and microbe, and recovery method influenced the recovery rates	[52]
Sapwood and heartwood of the larch	K. pneumoniae,MRSA	Antimicrobial properties of wood against hygienic microbes	Blotting and vibration	Microbial quantities decreased after contact with wood	[53]
Quercus baloot	C. albicans	Evaluation of anticandidal potential of wood	Thin-layer chromatography, contact bioautography, disc diffusion method, broth microdilution	Chemical constituents were identified and antimicrobial activities were reported	[54]
Maple and Beech	Aerobic mesophilic microorganisms Enterobacteriaceae, Pseudomonas spp.	Hygienic aspects of using wooden and plastic cutting boards	Swabbing	Survival of microbes on different cutting boards before and after cleaning	[55]
Pine, larch, spruce, beech, maple, poplar, oak, polyethylene	E.coli, E. faecium	Studying the survival of pathogenic organisms in contact with wood material	PCR and culture-based recovery methods	Wood material has antimicrobial properties	[56,57]
Maple wood, steel, ceramic and carpet	Enterobacter aerogenes	Longer contact times increase cross-contamination of Enterobacter aerogenes from surfaces to food	Vortex for microbial recovery plate count method for enumeration	Contact time, food, and surface type all had highly significant effects on the log percent transfer of bacteria	[58]
Poplar	E. coli, P. expansum	Assessment of Penicillium expansum and Escherichia coli transfer from poplar crates to apples	Grinding/blending	There is a low transmission of microbes from wood to food (apple) as compared to glass and plastic	[59]
Wood, stainless steel, Formica, polypropylene	Salmonella Typhimurium	Recovery and transfer of Salmonella Typhimurium from four different domestic food contact surfaces	Swabbing (vortexting), contact pressing (635 g) and food contact	Number of microbes recovered and their transfer from wood to food was lowest compared to other surfaces	[60]
Poplar	B. cereus spores, E. coli cells	Behavior of bacteria on poplar wood crates by impedance measurements	Direct contact (wood in broth)	Microbes in contact with wood present in broth showed decrease in CFU	[61]
Poplar and pine	Total microbial counts, S. aureus	Hygienic properties exhibited by single-use wood and plastic packaging on the microbial stability for fish	Vortexing to recover microbes and enumerated by the TEMPO® system	Microbes decreased fastest on wood	[62]
Leucaena leucocephala	Trichoderma viride, Fusarium subglutinans, A. niger	Antimicrobial properties of wood treated with natural extracts	GC-MS, direct diffusion method	Antifungal properties were observed	[63]
P. abies, Larix decidua	P. funiculosum, P. ochrochloron, A. niger, C.albicans, A. flavus, A. ochraceus, E.coli, S. aureus, Micrococcus flavus, B. cereus, L. monocytogenes, P. aeruginosa, Pectobacterium atrosepticum, Pec. carotovorum, Dickeya solani	Antimicrobial properties of bark and wood extracts	GC-MS, microdilution method	The extracts showed antimicrobial properties, minimum inhibitory concentration (MIC) was determined	[64]
Quercus incana	S.aureus, Micrococcus luteus, B. subtilis, E. coli, Ps. pickettii, Shigella flexneri, A. niger, A flavus	Identification, isolation, and characterization of novel antimicrobial compounds	Disc diffusion method, well diffusion method	Two new compounds were identified with their antimicrobial properties	[65]
Q. suber, Q. macrocarpa, Q. montana, Q. griffithii, Q. serrata	B. subtilis, S. pneumonia, E. coli, S. aureus, A. niger, Penicillium spp., Fusarium oxysporum	Antimicrobial characterization combining spectrophotometric analysis of different oak species	Paper disc diffusion method and UV spectrophotometric analysis	Antimicrobial properties and active compounds were identified	[66]
Rubber wood	Campylobacter jejuni	Transfer of Campylobacter jejuni from raw to cooked chicken via wood and plastic cutting boards	Rinsing with normal saline and then counting CFU by combined most-probable-number (MPN)-PCR	Transfer during uncooked/cooked meat chopping on unscored and scored cutting boards	[67]
Heartwood of Scots pine (P. sylvestris)	L. monocytogenes, E. coli	Pine heartwood and glass surfaces: easy method to test the fate of bacterial contamination	Plate count and broth turbidity test	Wood does not allow the survival of microbes	[68]
P. sylvestris and P. abies	MRSA, E.coli O157:H7	Microbial survival on extractive-treated glass cylinders was studied	Vortexting and plate count method	Extractive showed antimicrobial properties	[69]
P. sylvestris and P. abies	S. aureus, E. coli, S. pneumoniae, S. enterica Typhimurium	Antimicrobial properties of volatile organic compounds (VOCs) of wood	Glass chamber and plate count method	VOCs reduced the microbial survival	[70]
30 species of trees	B. cereus, S. aureus, L. monocytogenes, Lactobacillus plantarum, E. coli, Salmonella infantis, P. fluorescens, C albicans, Saccharomyces cerevisiae, A. fumigatus, Penicillium brevicompactum	Antimicrobial and cytotoxic knotwood extracts and related pure compounds and their effects on food-associated microorganisms	Broth dilution and agar well dilution methods	Antimicrobial properties were observed	[71]
Beech wood (F. sylvatica L.)	Gloeophyllum trabeum, Trametes versicolor	Phenolic extractives of wound-associated wood of beech and their fungicidal effect	Spectrophotometrically analyzed and a paper disc screening test	Wood wounds have defensive chemicals to counter fungal invasion	[72]
Hard maple and plastic cutting boards	E. coli	Bacterial retention and cleanability of cutting boards with commercial food-service maintenance practices	Wet sponge swabbing	Microbial recovery was 0.25% and 0.1% from plastic and wood respectively in dry conditions and was similar in wet conditions

According to the literature findings, it was possible to categorize the methods into two broad groups based on form of test material used e.g., solid wood or extractives. Furthermore, they were subclassified into different groups according to the methodology, as shown in Figure 2.

reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7277147/

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More information: Ju-Chi Chen, et al. Survival of Bacterial Strains on Wood (Quercus petraea) Compared to Polycarbonate, Aluminum and Stainless Steel, Antibiotics (2020) DOI: 10.3390/antibiotics9110804