Microbially induced deterioration of architectural heritages: routes and mechanisms involved

Biodeterioration can be defined as a geophysical and geochemical process that causes undesirable physical, chemical, mechanical and aesthetic alterations and damages to historic monuments and artworks. It is a complex process that illustrates the interaction of micro-organisms with its substratum and environment [1-3]. These stone structures are highly susceptible to damage by weathering and atmospheric conditions (such as light, temperature, humidity, pollutants and acid rain) [1,4-6] because of their chemical nature and petrologic properties (texture, high porosity etc). The high porosity allows penetration of water along with corrosive ions, acids and salts inside the porosity of the stone and cause severe damage to them. Besides this, stone surface supports the growth of some characteristic group of microorganisms which includes alkaliphiles, halophiles, epiliths and endoliths [7-10] which cause deterioration in many ways (Table 1). These micro-organisms through different known mechanisms of deterioration cause harm to the stone surfaces of monuments and artworks resulting in an irreversible and irreparable loss of their physical strength, aesthetic appearance, value and information [1,6,10-16] (Table 2).Since the time, industrial revolution began the deterioration and weathering of heritage monuments and artworks became noticeable. Environmental conditions like relative humidity, temperature, wind, light and rainfall plays a crucial role in colonization and establishment of microbial communities on the stone surfaces of monuments and artworks [1,4,5]. The problem is more pronounced in tropical areas where the high temperature, high relative humidity and high annual rainfall favor the growth of diverse group of microorganisms. Microbial growth and activity is a function of the environment that surrounds them. For instance, seepage of the rain water and subsequent dampening and moistening of the vertical walls of the monuments favor the colonization of diverse gr