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The following points highlight the ten economically important materials that are being deteriorated by the activity of microorganisms. The economically important materials are: 1. Pulp Wood 2. Paper-Pulp 3. Finished Paper 4. Textile and Cordage 5. Painted Surfaces 6. Rubber 7. Leather 8. Metal Corrosion 9. Wood Deterioration 10. Food Spoilage.
Material # 1. Pulp-Wood:
Pulp-wood represents the wood which is used to manufacture paper. It has been estimated that almost about 10% of all the paper-wood cut is deteriorated by the action of microorganisms, particularly fungi.
Temperature and moisture together with an appropriate availability of oxygen play an important role in growing the fungi to deteriorate pulp-wood. Basidiomycetous fungi are responsible for “white rots” and “brown rots” of pulp-wood. This classification of rots is based mainly upon the constituent of the wood that is attacked.
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If one finds white rotten patches on the pulp-wood surface, it characterizes the degradation of brownish lignin leaving a white spongy cellulosic mass in the wood. Contrary to it, if there are brown rotten patches, they are the result of preferential microbial deterioration of the cellulose leaving behind a brown pinky mass predominantly of lignin.
When the moist pulp-wood is stored, its surface is attacked and degraded by some ascomycctous and deuteromycetous fungi. This degradation is characteristically called “soft rots”.
Material # 2. Paper-Pulp:
As we know, the raw material, e.g., wood, cotton, linen rags, etc. are treated physically or chemically for the purpose of separating and purifying cellulose fibrous in the form of fibrous pulp. This pulp is generally called “paper-pulp”.
Those paper-pulps which are prepared by chemical treatments generally possess less nutrients for microorganisms and hence are less susceptible to microbial attack than the physically (mechanically) prepared paper-pulps.
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However, microbial degradation of the paper-pulp may be encountered in the form of “paper-pulp slime” spots on the finished paper sheet. Paper-pulp slime is produced by the deposition of microorganisms and the subsequent enlargement of fibre, fines, and other debris from the water and compounds of the paper-making medium.
Bacteria, yeasts, moulds, algae, and protozoa have been isolated from pulp slimes. Bacteria, particularly capsulated bacilli such as Enterobacter aerogenes and Bacillus spp. represent the most important group of pulp slime producers. Sphaerotilus natans, the filamentous iron bacteria, can be found as part of the slime mass on those paper machines operating above pH 5.5.
The bacterium Alcaligenes viscosus var. dissimilis has been obtained from pink pulp slime. Species of Mucor, Penicillium, Trichoderma, Fusarium, and yeasts (Torula, Rhodotorula) are the fungi that have been isolated from pulp slimes in various paper-making industries.
Material # 3. Finished Paper:
Finished paper, i.e., the paper-sheet which is prepared by the refinement and fabrication of paper-pulp is also attacked by microorganisms. Various fungi (Penicillium spp., Aspergillus spp., Chaetomium, etc.) and bacteria are the main attackers as cellulose, the main constituent of the paper, is susceptible to them.
They may cause black, brown or yellow discoloration and spotting through “mildewing”. Glue or casein, the other constituents of the paper, also serve as substrate for certain microorganisms. This is the reason why some chemicals are generally added to the surface of the paper-sheet to avoid microbial attack.
However, the microorganisms produce certain chemicals during their metabolism and these chemicals cause staining or decolouration of the paper-sheet. Growth of cellulolytic microorganisms may result in either weakening of fibres, perforations and/or even complete destruction of the finished paper.
Material # 4. Textile and Cordage:
Textiles and cordages are susceptible to spoilage by certain microorganisms in raw, processing and finished stages. Loss of millions of rupees is estimated annually due to attack of microorganisms on these materials. The microorganisms involved in these deteriorations include both bacteria and fungi.
Moulds are the principal microorganisms responsible for the deterioration of cellulose fibres resulting in discolouration and weakening of fibre strength. The most important among bacteria are the aerobic Bacillus spp., Proteus vulgaris, and some actinomycctes, whereas the most important among fungi are Myrothecium verrucaria, Penicillium, Aspergillus, Alternaria, Hormodendrum, Cladosporium, Fusarium, etc.
Moulds are essentially more important deteriorants of cotton textiles and their growth is favoured by high humidity, moderate temperature and diminished light. The bacteria caused damage by their proteolytic enzymes in woollen material which represents a protein, namely, keratin.
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The nature of spoilage of textiles and cordages can be categorized as follows:
(i) Discolouration of fabric strain caused by pigment-producing (chromogenic bacteria) or coloured spore-forming (dematiaceous fungi) microorganisms.
(ii) Loss of strength due to attack by microbial enzymes (Moulds on cotton fabrics and bacteria on wool).
(iii) Change in the pH of the fibre resulting in change in shade of the dye.
Material # 5. Painted Surfaces:
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Painted surfaces of the material are also subject to attack by microorganisms unless the paints contain effective fungicidal ingredients. Painted surfaces exhibit evidence of mould-spotting or discolouration under certain environmental conditions. This discolouration is due to products of microbial metabolism of organic constituents of the paint.
Many moulds such as Aspergillus, Penicillium, Pullularia, Phomu glomerata, Alternaria, and Cladosporium and a bacterium called Flavobacterium marinum have been isolated from “mildewed” or “mouldy” painted surfaces. Pullularia spp. are considered to be the most common cause of mould-spots on painted surfaces.
Material # 6. Rubber:
Rubber is subject to microbial deterioration, particularly natural rubbers rather than the synthetic ones like neoprene. The deterioration is serious in electrical insulation of buried cables and in the sealing rings of underground sewage pipes where the seals can decay long before the concrete pipes themselves need replacing.
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The organisms responsible are various fungi and actinomycetes. Some of the accelerators used in the polymerization of rubber, such as dehydroabietyl ammonium pentachlorophenate, can help to prevent decay because they have biocidal properties. To prevent this degradation some biocides may be added during manufacture.
Material # 7. Leather:
We all know that several microorganisms harbour the living animals. When the animals die and their skin is removed, the microorganisms continue to be present on the hides. When the hides are taken for processing, several changes take place in the micro-flora.
If the leather or hide is preserved by drying and salting, most microorganisms are killed. Contrary to it, if they are soaked in water to keep them soft, the microorganisms multiply rapidly. Sometimes, undesirable microorganisms multiply and spoil the leather.
Besides, bacteria, some species of Aspergillus, Penicillium, Cladosporium, etc. are known to attack the leather and cause hardening of it. The spoilage of leather goods is very common under warm humid condition. On account of microbial attack, various types of leather goods are deformed and spoiled.
Material # 8. Metal Corrosion:
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Growth of several microbial species plays an important role in corrosion of metal pipes and result in serious problem particularly in oil and gas delivery systems. Bacteria such as Gallionella, Crenothrix, and Leptothrix species cause metal corrosion in aerobic conditions by oxidizing metal and forming metallic oxides as corrosion products.
Thiobacillus species, the sulphur-oxidizing bacteria, produce high concentrations of sulphuric acid in acrobic condition that causes corrosion. But, aerobic corrosion is not as serious as anaerobic corrosion. Desulfovibrio desulfuricans, the sulphur-reducing bacterium, is especially important in the corrosion of metals in anaerobic conditions by causing graphitization.
Graphitization is a process in which a metal-pipe losses much of its metal, becomes soft and brittle, and easily broken. Anaerobic microbial corrosion of steel results in more localized pitting which, sometimes, cause perforation of the pipe.
Material # 9. Wood Deterioration:
Forests are among the most valuable of all our resources as they provide us wood which is used for various purposes.
The microorganisms cause decay of wood and there are two types of wood decay:
(i) destruction of lignin (or infrequently cellulose) resulting in white or spongy rotten wood. This type of instruction is mainly caused by Tramets pini and Ganoderma applanatum, and
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(ii) destruction of cellulose resulting in brown, soft and easily powdered wood. This destruction is caused by Pliaeolus sp., Letinus lepideus, Serpula lacrymans, and Poria incrassata.
Material # 10. Food Spoilage:
Air-borne microorganisms are often troublesome in our home and in industries where food and food-products are manufactured.
