BIODETERIORATION

BIODETERIORATION

• Deterioration is defined as a loss of structural capacity with time as a result of the action of the external agents or material leaching.
• Biodeterioration is the chemical or physical alteration of a product that decreases its usefulness for its intended purpose, caused by micro or macroorganisms or their enzymes.
• In 1965, Hueck defined Biodeterioration as “Any undesirable change in the properties of a material caused by the vital activities of organisms”. This is the widely accepted definition for Biodeterioration.
• In 1981, Rose defined Biodeterioration as “The process by which Biological agents are the cause of the lowering of quality or value”.
• Biological deterioration caused by Insect attack and Fungal growth and the other form of Deterioration is caused by adverse environmental conditions such as extremes of dampness or wide fluctuations in relative humidity associated with large variations in day and night temperatures, light and atmospheric pollutants.
• Both Biodegradation and Biodeterioration degrades the materials but Biodegradation results in Positive aspects of microbial activity and Biodeterioration results in Negative aspects of microbial activity.
• Like disease, Biodegradation has three different phases

a)    Phase I – Infection/Contamination                            

b)    Phase II - Incubation

c)     Phase III – Manifestation (Symptoms start appearing)

Microbial Biodeterioration of Materials

• Many cultural heritage materials are at risk of Biodeterioration by microorganisms including Metals, Glass, Ceramics, Paper, Paintings, Wood, Coatings, Synthetic polymers, Cellulose, Food, Fabrics, Teeth and Mummified bodies.
• Biodeterioration plays an important role in the degradation of Stone in Historic buildings, Monuments and Archaeological sites.
• Microorganisms that have been demonstrated as the causative agents in Deterioration of Stone include Bacteria, Archaea, Cyanobacteria, Algae, Fungi and Lichens.
• Additionally, Stone objects may support novel communities of microorganisms (e.g., alkaliphiles, halophiles, and endoliths) that function in the Biodeterioration process.

Effect of Microbial metabolites in Biodeterioration

1) Effect of Acids

• Microbial Biodeterioration of Stone is widely thought to occur through the action of Organic and Inorganic acids produced as metabolic by-products.
• Bacteria isolated from Mexico, are capable of producing Calcium carbonate - dissolving exudates. However, not all Organic acids produced by microorganisms cause immediate dissolution of Stone. For example, Oxalic acid may have a protective role by the formation of Calcium oxalate on Stone surfaces.

2) Effect of Exopolymers

• Biofilm exopolymers play an important role in Deterioration of Stone because of their proximity to the Stone substratum.
• Bacteria produce the polymers as Biofilm growth is initiated, and adhere directly to the stone.
• Bacterial exopolymers are large macromolecules consisting of varied sugar molecules exhibiting several kinds of functional groups, including acidic carbonyls. These functional groups are often capable of binding cations in solution. For example, negatively charged carboxylic and hydroxyl groups of exopolymeric materials, such as alginic acid, form complexes with the mineral surface and may leach calcium from limestone matrices.

 3) Discoloration

• Microbial pigments frequently cause discoloration of stone. While these metabolites may not cause physical damage, they can cause aesthetic problems.
• Monuments are particularly susceptible to this form of discoloration.
• Two pigment-producing fungi were isolated from the monument: Epicoccum nigrum and Drechslera sp.
• Interestingly, in a growth medium of low salt concentration, Epicoccum nigrum appeared milky white in color, while in a high calcium concentration medium, the fungus produced a red pigment. The red stain may have been an exudate produced to protect the fungus from the stresses of its habitat on the stone, such as ultraviolet radiation or ionic strength.

Strategies for the Control of Microbial Biodeterioration

1) Environmental Control

• Microorganisms can persist in dry environments. Active metabolism, however, requires appropriate levels of relative humidity and temperature.
• A combination of low humidity and low temperature is the simplest way to control microbial growth, but this treatment may be less effective for control of fungi and is impractical in outdoor situations.
• Regular cleaning may be the most effective treatment for preventing Biofilm formation and subsequent Biodeterioration of materials in Historic buildings and monuments.

2) Biocides

• The application of Biocides has become a routine practice in the conservation of Cultural heritage materials. However, environmental issues have severely limited the number of available effective biocidal chemicals for use in conservation.
• Biofilm bacteria respond differently to Biocides and are generally more resistant than unattached cells. Because microorganisms are capable of rapidly acquiring chemical resistance, no one chemical can be relied on for long-term use; frequently several chemicals need to be combined in order to achieve effective eradication of Biofilm populations.
• Biocides are a difficult tool for preservation, because

a)     Many are too caustic for environmental use.

b)    They are not strong enough to discourage microbial growth.

c)     The microorganisms ultimately develop resistance.

3) Consolidants

• Consolidants have been used for some time to conserve Archaeological stone from Biological and Chemical weathering.
• Consolidation is a means of generating structural strength in disintegrating material and is an artificial means of repairing the damage caused by natural processes.
• The efficacy of Consolidants on outdoor stone is controversial, because they can upset the natural saturation and evaporation of moisture from within the stone, often resulting in exfoliation and cracking of stone surfaces.
• Application of Consolidants is not easily reversible, which is a serious drawback when dealing with Ancient monuments. Some Consolidants may also discolor as they degrade because of Aging, Photochemical processes and Oxidation.
• Two of the most common types of Consolidants used for Monuments and Archaeological stone are Ethoxysilanes and Acrylic resins.
• While problems associated with the use of Consolidants for the protection of Archaeological stone are numerous, they are only one of the means of preventing the Disintegration of stone grains due to exposure and weathering.
• The addition of Biocides to Consolidants would help to prevent Microbial degradation, increasing the longevity of the treatment.
• Commercial Consolidants are susceptible to Biodegradation. Commercially available and environmentally acceptable Biocides could be used as additives in Consolidants.