Microbicides in Coatings E-Book

Frank Sauer

• Herausgeber: Vincentz Network
• Kategorie: Fachliteratur
• Serie: European Coatings
• Sprache: Englisch
• Veröffentlichungsjahr: 2017

Seitenzahl: 282

Beschreibung

When it comes to protecting coatings, it is essential to strike the right balance between controlling germs in order to avoid economic damage on the one hand and tolerating microbial life where it is necessary and useful on the other. The new book from Frank Sauer provides a comprehensive overview of the working mechanisms and possible applications of microbiocides for coatings - invaluable for formulators and technicians as well as for business people with a basic knowledge of chemistry and biology.

Leseprobe

Frank Sauer

Microbicides in Coatings

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Frank Sauer

Microbicides in Coatings

Hanover: Vincentz Network, 2017

European Coatings Library

ISBN 978-3-86630-134-4

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European Coatings Library

Frank Sauer

Microbicides in Coatings

Preface

“Everyone is trying to accomplish something big, not realising that life is made up of little things.”

Frank Clark (1860 – 1936)

Microbicides are substances that represent two sides of the same coin. On one hand, they help to control microorganisms that are responsible for the deterioration of materials and for causing commercial damage worth billions of euros a year. On the other, they are regarded with suspicion because their action can have side-effects on humans or on the environment or both.

Microorganisms have been part of our biosphere for billions of years, during which time they have been extremely successful due to their ability to adapt to the most challenging of conditions. Human life as we know it would not have been possible without the tireless assistance of these tiny organisms. Our intention must therefore not be to combat germs wherever they are encountered – microbicidal measures should only be taken in situations where germs cause harm to humans, be it out of medical need or the need to protect a material.

This book seeks to provide an overview of the different aspects of material protection, covering the spectrum from basic information about the universe of microorganisms, to the innate properties of microbicides, to the state of the art and finally to legislative aspects. The biggest challenge in this regard has been deciding which of the key issues to select from the vast wealth of information available, without straying off course. It therefore goes without saying that it has not been possible to cover every detail in depth, as that would have substantially exceeded the scope of the book. Wherever appropriate, references are provided so that the reader can conduct further research.

The book also seeks to familiarise laboratory assistants, technicians, graduates, engineers and chemists with the principles of material protection in the field of coatings. However, it should also prove rewarding to business people with a basic knowledge of chemistry and biology.

I would like to thank all those colleagues who provided information on selected topics and proffered their advice and made various suggestions and recommendations.

My very special thanks go to my wife and my daughter for their endless patience during the preparation of this book and for their forbearance when I was so often unavailable for leisure pursuits, especially at the final stage of writing. Without their support, this book would not have been finalised in time. I am also greatly indebted to them for giving a reader’s perspective of the book.

Langenfeld, May 2017Frank Sauer

For Marlis and Melina

Contents

1 Introduction to microbicides

1.1 Classification of microorganisms

1.1.1 Archaea

1.1.2 Bacteria

1.1.3 Eukaryotes

1.2 Microbicides

1.3 Mode of action of antimicrobial actives

2 Coatings preservation

2.1 In-can preservation

2.1.1 Formaldehyde and formaldehyde-releasing compounds (FA-R)

2.1.2 Isothiazolinone derivatives

2.1.3 Compounds with activated halogens

2.1.4 Summary of relevant properties for in-can preservation

2.2 Dry-film preservation

2.2.1 Fungicides for coatings protection

2.2.2 Algicides for coatings protection

2.2.3 Overview of fungicidal/algicidal product formulations

2.3 Plant hygiene

2.3.1 Prevention is better than cure

2.3.2 Where there is water, there is also life

2.3.3 Ten-point programme: disinfect operational facilities

3 Application aspects

3.1 Service life of microbicides

3.2 Optimisation of dosage

3.3 Formulation aspects

3.4 Remedial surface treatment

3.5 New developments in the field of material protection

3.5.1 Slow-release technology

3.5.2 New actives

3.6 Microbicides based on silver compounds

4 Microbiological and application test methods

4.1 Minimum inhibitory concentration

4.2 Determination of germ count

4.3 In-can challenge test

4.4 Agar diffusion test

4.5 Laboratory leaching tests

4.6 Semi-field leaching trials

4.7 International standards

5 Legislative aspects

5.1 Biocidal Product Legislation (BPR)

5.1.1 General aspects of the authorisation process

5.1.2 Article 95: List of active substances and suppliers

5.1.3 Treated articles

5.2 Interrelationship of the BPR and other legislation

6 Summary and outlook

7 References

Author

Index

1 Introduction to microbicides

Surfaces determine our daily lives in manifold ways. They define the borderline between an interior and exterior domain and they are essential for giving form to physical objects. Consequently, surfaces play a key role in our living environment.

Nature creates surfaces in a huge variety of ways, be it in the form of inorganic matter, such as rock, soil, sand, gas, and water, or in the form of organic matter, such as plants and living creatures. At all times, surfaces are subject to interactions, such as approximation, adhesion, transformation, penetration, diffusion, attack and – in the worst case – destruction.

In general, coatings are designed to build a specific, well-defined layer on top of surfaces. Such layers can confer tremendous functionality: they can have a signalling function (e.g. a traffic sign), a commercial purpose (e.g. an advertising hoarding), an infrastructural purpose (e.g. a pavement), a protective function (e.g. a thermal insulation system also known as ETICS/EIFS1) and finally a decorative function by means of which they create an attractive appearance or convey a philosophical message, as in the case of paintings and other artwork.

In the construction field, the protective function of coatings is very often combined with a decorative purpose, e.g. protection of walls against energy loss in conjunction with an external layer for creating an attractive façade that retains its appeal in the long term. The materials used for designing coatings also vary enormously and they are usually used in combination. An architectural paint can be a quite complex mixture of polymeric or pre-polymeric binder, organic/inorganic solid matter, such as pigments, fillers and diverse additives and solvents for keeping the particles in the liquid phase, which, in this specific case, is a prerequisite for applying the coating material by a simple technique, such as brushing, rolling or spraying. But coatings, as a generic term, are not limited to paints and plasters. They can also consist of completely different materials applied by numerous other techniques, such as metal coverings made of gold, copper, brass, chromium, lead, titanium, and platinum (e.g. for roofing churches or producing prostheses as well as implants in medicine), glass panels which are widely used for façades of skyscrapers, ceramic tiles, especially in wet areas, and a huge swathe of plastics – to mention just a few (see Figure 1.1).

Figure 1.1: Examples of different kinds of coating materials

Most coatings have one property in common. Sooner or later, they become susceptible to attack and destruction. This might come about as a result of natural climatic conditions or other environmental factors, man-made physical and/or chemical impact, or seemingly unremarkable species which are minute and work under-cover but which have been extremely successful and effective for billions of years: microorganisms.

Bacteria, yeasts, fungi, algae and lichens are remarkably adaptive to different environmental situations and can find their specific ecological niche even in the most inhospitable of conditions. Even materials such as concrete, plastics and the like which were long thought to withstand microbial degradation are not exempt from such attack (Figure 1.2)[1].

Even more diverse than the world of coating materials is the huge variety of microbial species found in nature. All these germs have a specific preference for certain living conditions, such as acidic/alkaline media, aerobic/anaerobic surroundings or shade/sunlight areas[2]. In addition, germs can interact with each other and particular species are even known to engage in a form of communication[3].

Whatever the specific living conditions are, microorganisms compete with each other for space which, depending on its location, provides water, food, heat, essential minerals or UV light for photosynthesis[4]. Even if we could create an almost totally sterile surface, e.g. by treatment with a strong disinfectant, that same surface is very likely to be quickly re-colonised once the microbicide has disappeared or lost its effectiveness[5]. The survival or elimination of any given species is determined by its toughness, a fundamental principle of nature that can also be readily observed in the animal kingdom[6].

Figure 1.2: Microbial infestation on plastic, concrete, stone and roof tiles

Just as humans have conquered the planet on a macroscopic level, microbial species have conquered the microcosm. Their success is due to an ability to rapidly adapt to changing external conditions that is significantly supported by their very high reproduction rates[7]. Only those germs which are capable of quickly developing a biological response to environmental stress and challenges will survive. The new, successful genetic material is then readily duplicated and passed down to subsequent generations.

The conquest of surfaces by these germs reflects two sides of the same coin. On one side, they are absolutely essential for human life (e.g. digestion, metabolism, acid mantle)[8]. Without the help of microbial species, human life as we know it would be inconceivable.

On the other, germs are responsible for the deterioration of substrates (Figure 1.3) and consequently for causing economic damage or even harm to plants, animals and humans. Recent decades have seen a remarkable increase in the microbial infestation of façades, for instance. The environmental conditions which microorganisms need to flourish in have improved considerably due to continuing eutrophication of the atmosphere with organic pollutants and to dramatic changes in the global climate[9]. In Germany, alone, the annual economic damage arising from microbially induced defacement and bio-corrosive deterioration is valued at EUR 8 to 16 billion[10, 11].

When it comes to providing protection for coatings, it is essential to strike the right balance between controlling germs so as to avoid disease and economic damage on one hand and tolerating microbial life where it is necessary and useful on the other. Thus, whole-scale eradication of microorganisms per se is not the right objective at all. Germs should only be combated in areas where they adversely impinge on the human sphere. This calls for an intelligent strategy for developing antimicrobial products which act not only where they are truly needed but also in dosage levels which minimise the adverse impact on the environment and humans and which simultaneously control the target germs with high efficacy, thereby avoiding the development of resistance. This point is discussed further in Section 3.

Figure 1.3: Examples of deteriorating substrates

1.1 Classification of microorganisms

The term ‘microorganism’ derives from the Greek μικρός (mikros) “small” and όργανισμός (organismόs) “organism” and describes a microscopic living species which consists of either unicellular or multicellular structures[12, 13]. Microorganisms were first discovered in 1674 by van Leeuwenhoek who observed bacteria through a single-lens microscope of his own design[13].

The classification of microorganisms is quite a complex area and a full treatment would be beyond the scope of this book. Consequently, only an overview is provided in this section. For comprehensive details, the reader should refer to specialist literature, such as[14–24]. The currently accepted classification of life forms recognises three domains[25]:

– Archaea

– Bacteria

– Eukaryota

Archaea and bacteria are subsumed under the term ‘prokaryotes’ meaning a unicellular organism with relatively simple cell compartment structures[26]. In contrast, ‘eukaryotes’ have much more sophisticated cell structures enclosed within membranes; in particular, they possess a membrane-bound cell nucleus. The names ‘prokaryote’ and ‘eukaryote’ are derived from the Greek πρό (pro) “before” and ευ (eu) “well”/“true” and κάρυον (karyon) “nut”/“kernel”[27–29]

1.1.1 Archaea

Archaea were initially dedicated to the domain bacteria (‘archaebacteria’). However, this classification is outdated because these microorganisms possess unique properties which distinguish them from both bacteria and eukaryotes[30]. They were originally regarded as extremophiles that live in harsh environments, such as volcanic hot springs, but have since been found in a wide range of other natural habitats, such as fens, soil and sea water[31]. Archaea in plankton are believed to be one of the most abundant groups of organisms on earth[32]but they have also been found in the human colon and the navel[31]. Finally, archaea play an important part in global environmental processes, such as the carbon and nitrogen cycles[32].

Figure 1.4: Halophiles in water bodies of very high salt concentration

Thanks to state-of-the art techniques employed in molecular biology, especially the polymerase chain reaction (PCR), archaea are now known to be widely distributed in nature and common in all habitats on earth[33]. The ribosomal2 genes from species found in diverse environmental surroundings have been analysed and numerous organisms which had not been cultured in the laboratory thus far have now been classified as belonging to the archaea domain[34, 35]. Interesting examples of archaea species living under extreme conditions are hyperthermophiles[36] and halophiles[37]. Hyperthermophiles can thrive in very hot environments, sometimes even at temperatures exceeding 100 °C and under high pressure, e.g. on the walls of hydrothermal vents in the vast depths of the sea. One of the toughest species here is Strain 121 which can double its population in 24 hours at temperatures of 121 °C under pressure in an autoclave[38]. Hyperthermophiles were first discovered by Brock[39, 40] in hot springs in Yellowstone National Park in 1965. Since then, more than 70 hyperthermophiles species have been discovered[41].

Halophiles are found particularly in water bodies that have very high salt concentrations, such as the Great Salt Lake in Utah and the Dead Sea[42]. A subset of this species has a red appearance due to the presence of carotenoid compounds (Figure 1.4).

There is no clear evidence that archaea are pathogenic or parasitic, but they are known to act as ‘commensals’, which are organisms that cohabit with other organisms by using the same nutrient base. One example is methanogens, which profit from a supply of food in the guts of humans and ruminants and which in turn support digestion due to their vast numbers[32].

1.1.2 Bacteria

Bacteria and archaea evolved from an ancient common ancestor[25]. The first forms of life on earth appeared approximately 4 billion years ago in the form of unicellular microorganisms. For about 3 billion years, bacteria and archaea dominated the terrestrial habitats on our planet, such as soils, fens, water bodies and other natural compartments[43, 44], but nowadays they can even be detected on radioactive waste[45] and in manned spacecraft[46].

The total number of bacteria on earth is unimaginably high. The following examples illustrate how, in terms of ‘head count’, bacteria cells are by far the predominant life form on our planet, vastly outnumbering the world’s human population:

– Scientists estimate the totality of bacteria on earth to be roughly 5 x 1030 [47], i.e. a five followed by 30 zeros. The respective biomass is greater than that of all the terrestrial plants and animals taken together[48].

– One gramme of soil contains approximately 40 million bacterial cells, and even natural freshwater has been found to host a million bacterial cells per millilitre[49].

– Approximately 80 million bacteria are exchanged during every intense kiss. However, this should not discourage couples from keeping this tradition, as bacterial exchange stimulates the human immune system and boosts the body’s defences[50].

Even in the most extreme habitats on earth, such as the Mariana Trench at a depth of 11 kilometres researchers hypothesised the existence of bacteria there[51, 52].

Also, human body parts can be extensively colonised by bacteria, especially the gut flora and the skin. There are roughly 10 times as many bacterial colonists in human flora as there are cells in the human body[53]. But most of them are harmless and some are beneficial. Only a minority are pathogenic and capable of causing various diseases[49].

Research on bacterial species is by no means exhausted, as only a small proportion of them have been fully characterised. In addition, many bacteria species could not yet been cultured in the laboratory[54].

Bacteria come in all shapes and sizes. Their cell dimensions are in the single-digit micrometre range, and so the cells are normally invisible to the unaided eye[49].

Spherical (‘coccus’, pl. ‘cocci’) or rod-shaped (‘bacillus’, pl. ‘bacilli’) bacteria are the most common species, but there are also spiral-shaped (‘spirella’), comma-shaped (‘vibrio’) or tightly coiled-shaped (‘spirochaetes’) representatives, beside others[49]. Cell shape is important because it can help a given species to find food or to escape predators[55, 56].

Bacterial cells often aggregate in a specific way, e.g. in pairs, chains, clusters, and filaments. The last of these is often enclosed by a sheath containing diverse individual cells[49].

One important property in the context of cell aggregation is the formation of biofilms, which often occur at boundary layers. Biofilms can be detected particularly in aqueous systems, either on the water surface or at the interface with a solid phase (Figure 1.5). Biofilms are typically slimy layers containing embedded microorganisms, such as bacteria, algae, and fungi.

They can be regarded as a primary life form because the oldest fossils found to date originated from microorganisms in biofilms that lived 3.2 billion years ago[57]. The biofilm is a proven life form in view of the fact that it is so widespread in nature. The vast majority of microorganisms live in biofilms[58].

Biofilms contain water by way of main component, plus microorganisms and their secretion products, so-called extracellular polymeric substances (EPS) such as polysaccharides, proteins, lipids and nucleic acids. In combination with water, these biopolymers can form hydrogels, giving rise to a slimy matrix of more or less stable shape[59, 60].

Figure 1.5: Biofilm in a toilet flush

Different microbial strains usually coexist within a biofilm, so that, for example, in the space of just a few hundred micrometres aerobic and anaerobic bacteria can occur. The biofilm matrix is often interspersed with channels, pores and voids which allow for mass transfer and water supply. Inside a biofilm, mass transport mainly takes place by diffusion[59, 60].

Furthermore, microorganisms possess an intercellular communication system known as ‘quorum sensing’[161, 62]. The bacteria use this system to coordinate a set of processes in biofilms by activating genetic programmes in the various cells. Surface contact with corresponding microbial cells causes some genes to be switched on and others to be switched off. Specific signalling molecules enable microbial species to transmit information, and gene transfer with neighbouring cells is also known. Thus, a biofilm constitutes a flexible and capable life form that is essentially comparable to a more sophisticated, multicellular organism[57].

A biofilm provides effective shelter even in areas where maximum cleanliness is assumed (Figure 1.6). It protects against environmental stress, such as major fluctuations in pH and temperature, lack of food, and UV light. Finally, it confers a certain amount of protection against those chemical molecules which are designed to control germs: microbicides. This last property is possible because the chemical molecules either are unable to readily penetrate the biofilm or because unfavourable conditions in the biofilm hamper the efficacy of the microbicide.

It is known, for instance, that bacteria in a biofilm can retard their metabolism to such an extent that they enter a state of dormancy or stand-by mode. In this condition, they ingest virtually no antibiotic substances, thereby essentially protecting themselves against external stress[63]. Although this strategy of inactivity is successful at this micro level, it may not necessarily serve as a role model for the macrocosm.

Biofilms are ubiquitous. To mention only a few areas, they can be detected in soil, sediment, glacial ice, on rocks, plants and animals (here especially on mucous membranes), in technical equipment, such as pipes and tanks, and even in spacecraft[57, 64].

Human oral, dermal and intestinal florae are prominent examples of substantial bacterial communities which exist, as it were, right under our noses. A specific interaction occurs between the host and the bacteria involved. Germs which benefit from the host without impairing it are called ‘commensals’. When both the host and the bacteria benefit from each other, this interaction is called ‘mutualism’. Bacteria perform numerous tasks in such relationships. They are crucial for the maturation of our immune systems in the first years of our lives. They also prevent our body from being colonised by pathogenic germs and they support digestion processes. Any imbalance in this relationship can cause diseases[57].

Compared with this positive role in the aforementioned examples, biofilm formation in in- dustrial production can have a severe negative impact on technical equipment. Parts of a given biofilm might be released from a boundary layer to the water phase (Figure 1.7) and translocated by the water flow to other macroscopic areas, thereby disseminating microbial life to other production areas, e.g. piping, filter equipment, valves, and tanks. Appropriate plant hygiene measures are essential for microbial control in plant components. In the worst-case scenario, production has to be halted completely in order that massive product contamination or a health threat posed to production staff (e.g. by Legionella spp.) may be dealt with.

Figure 1.6: Biofilm in a washing machine

A consequence of biofilm formation and unhindered proliferation is that the products of microbial degradation, such as acids, can trigger bio-corrosion that will gradually destroy the colonised substrates. Even alloyed steel is not immune to such attack. Just as dental enamel is attacked by plaque, with the formation of caries, contaminated materials in industrial production are likely to experience bio-corrosive attack as well. Microbial induced corrosion (MIC) can reduce the service life of a production site. In conjunction with potential shutdown time and increased energy consumption, this phenomenon is estimated to be responsible for financial losses in the double-digit billion euros range[65].

Biofouling is a preliminary stage of bio-corrosion. As the biofilm proliferates, macroscopic effects on substrates, such as discolouration, clogging, slime formation, gas evolution and release of odours, can be detected. At this stage, although the biofouling impairs performance, it does not damage the material[65].

Biofilms on underwater bodies, e.g. large container vessels and on maritime sensor systems, fall into this category. Biofouling on container ships is responsible for greatly increasing drag, leading to a substantial loss of vessel speed and higher fuel consumption. To solve this problem, industry has developed special antifouling paints for marine application that defer biofilm formation by releasing biocidal substances.

The surface of any given bacteria species is usually composed of a cell wall and a cell membrane, which together are called the ‘cell envelope’. The cell wall commonly comprises polysaccharide chains that are crosslinked by peptide molecules to form a large three-dimensional, net-like macromolecule known as a ‘peptidoglycan’[66]. This boundary layer confers stability and shape and is essential for keeping the bacterial cell alive. The cell walls of bacteria differ from those of archaea, which do not contain peptidoglycan. They are also distinct from the cell walls of fungi, which contain chitin instead of peptidoglycan[67].

As far back as 1884, Gram[68] developed a stain test that allows for a coarse classification of bacteria. This test is based upon the differential staining of cell walls as a function of structure (Figure 1.8). The first step consists is staining the bacteria under test with a solution of crystal violet (also known as pyoctanine blue) and phenol, followed by treatment with an iodine/iodide complex (Lugol’s solution), which produces a deep blue appearance on all bacteria. In the second step, the coloured bacteria are treated (washed) with 96 percent alcohol. Bacterial cell walls of higher thickness remain deep blue (‘gram-positive’) whereas thin cell wall structures are decolourised as the gram stain is washed off (‘gramnegative’). When counterstained with a second dye (fuchsin or safranin), gram-negative bacteria appear red or red orange[68].

Figure 1.7: Parts of a biofilm disseminating microbial life to other production areas

Although this principle was established more than 130 years ago, it still represents state-of-the-art microbiological testing, because Gram staining is an important distinguishing feature and serves as a taxonomic criterion. Together with bacterial morphology, i.e. the study of bacterial shape, it enables many bacterial species to be allocated to one of the following groups: – Gram-positive bacilli

– Gram-negative bacilli

– Gram-positive cocci

– Gram-negative cocci

Gram-positive bacteria include the genera Streptococcus, Enterococcus, Staphylococcus, Listeria, Clostridium, while Escherichia, Salmonella, Klebsiella, Proteus, Enterobacter, PseudomonasandLegionella are gram-negative[69].

Not all bacteria can be classified by this technique, however, as there are also species that react in a variable or indefinite way to the Gram stain. For instance, tuberculosis-inducing Mycobacteria, such as Mycobacterium tuberculosis, contain within their cell walls mycolic acids, which are lipid substances that resist Gram staining[70]. The remedy in this case is to employ the ‘Ziehl-Neelsen stain’, also known as the ‘acid-fast stain’[71].

Many crucial biochemical processes take place across the interface between the cell interior and the exterior domain. As a consequence, the cell wall is a major point of attack for microbicides. One way to control germs, for instance, is to deploy antibiotics to inhibit peptidoglycan synthesis inside the bacterial cell[67].

The structural differences in cell walls, as described above, are also crucial to germ control, as some antibiotics are effective only against gram-positive bacteria, and not against gram-negative species[72].

A subset of gram-positive bacteria has the ability to form dormant structures without a detectable metabolism. Such ‘endospores’[73] can survive extreme conditions, e.g. in the space vacuum[74], and remain viable for millions of years[75, 76].

1.1.3 Eukaryotes

The characteristic feature of eukaryotes (Figure 1.9) is the cell nucleus: this contains the genetic information (DNA) and is enclosed by a so-called nuclear envelope, a membrane consisting of two lipid bi-layers. The multiple pores in the membranes regulate the transport processes of diverse molecules, such as proteins, ribonucleic acids (RNA) which play an important role in biological processes, adenosine triphosphate (ATP) which is the universal and directly available energy carrier in cells, water, ions and other small molecules[77–79]. All multicellular organisms, including humans, animals, plants and fungi belong to the domain Eukarya. Moreover, eukaryotes contain further membrane-bound structures, called organelles. Examples here are the mitochondrion, which is the ‘power plant of the cell,’ and the Golgi apparatus which plays an important role in cell metabolism[80, 81]. Plants and algae additionally contain chloroplasts (Greek: χλωρός (chlōrós) “green” and πλáστης (plastes) “the one who forms”)[82], organelles which can carry out photosynthesis[83].

Figure 1.8: Gram test on bacteria; left: Staphylococcus aureus (positive, violet), E. coli (negative, red); right: Pseudomonas aeruginosa (negative, red)      Source: [69] picture modified

Table 1.1: Contrasting properties of eukaryotes and prokaryotes

Eukaryotes

Prokaryotes

Domain

Eukaryota

Bacteria

Archaea

Cell nucleus

yes

no

no

Membrane-bound organelles

yes

no

no

Cell type

usually multicellular

usually unicellular

usually unicellular

Location of the genetic material

cell nucleus

cytoplasm

cytoplasm

Cell compartment structures

intense

poor

poor

Cell dimension [μm]

10 – 100

1 – 10

1 – 10

Examples

humans, animals, fungi, algae, plants & diverse unicellular species

E. coli

Methanogenium sp.

Fungi

Fungi are eukaryotic microorganisms which have cells containing a mitochondrion and a cytoskeleton (Greek: kýtos “cell”), a network made of proteins featuring thin and filamentary cell sub-structures which give the cell shape and mechanical stability and which are responsible for movement and transports within the cell[84].

In the past, fungi were classified as belonging to the plant kingdom because of their immobile lifestyle. Nowadays, they are considered an independent kingdom by virtue of their physiological and genetic characteristics and would appear to be more related to the animal kingdom. One commonality between fungi and animals, for example, is the fact that both use glycogen for storing carbohydrates – plants employ starch for this purpose[85]. Most fungal cell walls contain chitin[86], unlike plant cell walls, which are usually made of cellulose[87].

What particularly distinguishes fungi from plants is a lack eukaryotes of chlorophyll, which is essential for photosynthesis. Together with animals and the majority of bacteria and archaea, fungi are classified as being ‘heterotrophic’, i.e. they feed on organic matter in their surroundings by chemically degrading the organic material to use as anenergy source and recycling its nutrients[88].

Figure 1.9: Illustration of important structural elements of

Fungi are important decomposing organisms (‘destruents’) which, besides bacteria, are essential for the degradation of organic matter (Figure 1.10), such as cadavers, excrement, necrotic tissues and the like. By this method, large amounts of organic waste are continually returned to the eternal cycle of life[89]. Without, such an extremely effective recycling system, the human race would probably have ‘choked’ in debris for ages.

Fungi also play a very important role in the degradation and utilisation of biomolecules, such as lignin (an integral component wooden structures), cellulose, hemicellulose (an umbrella term for mixtures of polysaccharides of variable composition) and keratin (a major constituent of nails, claws, hooves, horn, barbs, feathers etc.). Lignin makes up from 20 % to 30 % of the dry weight of a wooden structure. Besides cellulose and chitin, this class of chemical ranks among the most common organic matter on earth[90]. By dint of teamwork among fungi, bacteria and animalcules, these biomolecules are eventually transformed into humus, which serves as an essential nutrient basis for the plant kingdom. In other words, mi- crobial degradation causes organic matter to rise from the dead to generate new life, in the manner of the phoenix.

In contrast, many species of fungi benefit not just from dead matter but also from living plants; this poses a substantial threat – in the form of plant fungal diseases – to commercially important agricultural crops[91]. Without appropriate preventive or counter-measures, these diseases may lead to crop failure or even to a total loss of harvest. Many tree fungi also belong to this group of plant pathogens[92].

Fungal diseases do not just attack plants. Humans can be attacked by certain fungi species as well, especially the skin, hair, nails and mucous membranes. A multiplicity of bacteria and fungi typically colonise the human skin but do not cause any harm in normal circumstances. They settle in the upper dermal layers and feed on dead skin cells and sweat. However, factors such as stress, a compromised immune system and hormonal changes can trigger human diseases by fungal species that are usually harmless. That said, the benefits of fungal activities to humankind compensate for the disadvantages. For instance, fungi are employed in the production of alcohol, citric acid, and vitamin C and are even used for medical purposes. Production of the antibiotic penicillin is a prominent example of this. Certain species of fungi are important for the maturation of dairy products, such as cheese. Brewer’s yeast, wine lees and baker’s yeast are wellknown examples of beneficial microorganisms drawn from the group of unicellular fungi[88].

Figure 1.10: Degradation of organic matter by fungi

Fungi can be roughly classified by their propagation mode as either unicellular (e.g. yeasts) or multicellular organisms, so-called ‘hyphae’-forming fungi. Hyphae are cylindrical, threadlike structures several micrometres in diameter and up to several centimetres in length (Figure 1.11). Most fungi use hyphae for vegetative growth[93]. Propagation usually occurs at the hyphal tips where branching occasionally occurs, leading to a complex interconnected network that is collectively called a mycelium[94].

Hyphae are usually subdivided into cells which are separated from each other by a dividing wall called the septum. The latter contains pores that support an exchange of cytoplasm. In addition to chitin, the hyphal cell walls contain hemicellulose, lipids, proteins and other chemical compounds. The shape of hyphae may differ substantially from one fungal species to another. For instance, parasitic fungi often develop suckers that spread over vegetable cells in order to collect plant nutrients[88].

Most people have preconceptions of what fungi look like. But this biological structure with its characteristic shape, the so-called fruiting body (‘sporocarp’), does not constitute the entire fungus. It is only a minor part of the entire organism which is responsible for reproduction, survival and dissemination via formation of spores. In fact, it is the capillary and in most cases the invisible network of hyphae (e.g. in soil or on wood) that constitutes the major part of a fungus[88].

In terms of size, fungi range from microscopic small species to readily identifiable, large mushrooms. The largest known fungus in the world belong to the genus Armillaria (honey fungus) and was found in a National Forest in Oregon, USA[95]. With a surface area of more than 8.8 km2, an approximate age of 2400 years and an estimated weight of 600 tonnes, it is regarded as being one of the largest and oldest living organisms on earth. Fungi are thought to have been in existence for nearly 1 billion years[88]. Today, approximately 100,000 species of fungus are known, with some experts estimating their total number in the range of several millions[96].

Fungi usually thrive when the – humidity is higher than 70 %

– environment is slightly acidic (pH: 4.6 to 6.5)

– temperature is in the range 10 to 35 °C

Figure 1.11: Hyphae structures of fungi

Species of black mould can often be detected on building façades. These fungi contain dark pigments (melanin) in the hyphae and in the spores that protect them from UV radiation[97]. As a consequence, the appearance of a façade can turn – in the worst case – from a primarily pale, pleasant colour to a black-spotted, ugly surface in the course of just a few years. In a way, this process bears comparison with human skin, which also contains melanin compounds. Sunbathing turns the skin colour from beige to brown (or red, given the wrong attitude). Excessive exposure to sunlight can eventually compromise the appearance of human skin, even to the point of causing pathological changes[98].

Some examples of the black mould (‘Dermatiaceae’) mentioned above are species such as Ulocladium, Cladosporium, Alternaria, Aureobasidium, Stachybotrys, and Phoma. Alternaria