Microorganismos sicrofilos: adaptaciones y aplicaciones biotecnologicas

RUBERTO L.

Rosario

Conferencia; 2do Taller de Criobiologia en Ciencias Medicas; 2007

UNESCO CHAIR IN CRIOBIOLOGY - Laboratorio de investigaciones en criobiologia - UNR

Bacteria are ubiquitous and they can proliferate all around the world under a broad range of environmental conditions. Some of these conditions are considered as “extreme”, such as high and low temperatures, high pressure, high and low environmental pH, high osmolarity and several others. Understanding of the adaptation mechanisms that allow bacteria to growth and proliferate under such conditions are very attractive from both physiological and biotechnological point of view. Among extremophiles, psychrophilic bacteria are those able to colonize cold environments. Morita (1975) made a distinction between psychrophiles and psychrotrophs (psychrotolerants) bacteria considering the optimum and maximum temperatures at which they can survive. Psychrophiles can not survive at temperatures above 20ºC while psychrotolerant bacteria can growth at temperatures around 0°C but their optimum temperature is 20°C or higher. Both of them, psychrophiles and psychrotolerant are interesting for biotechnological procedures. Cold environments challenge psychrophilic bacteria in several different ways, being the most important the low thermal energy and the high viscosity of aqueous environment at low temperatures (D´Amico et al 2006). Cytoplasmic membrane and enzymes of psychrophiles show the most studied adaptation mechanism. In a biological membrane, the physical properties are determined by their lipidic composition. Variation in temperature results in modification of the type and proportion of fatty acids present in a bacterial membrane. This adaptation, called “homeoviscous adaptation” was first reported for E.coli by Sinensky (1969). In this way, the presence of a high fraction of monounsaturated, polyunsaturated and methyl branched fatty acids appear as the main mechanisms used by psychrophilic bacteria in order to adapt membranes at low temperatures. Cold-adapted enzymes are able of catalysis at low temperatures by the destabilization of the active site, the entire protein structure or both of them (Hoyoux et al. 2004). This adaptation that allows these enzymes to work at low temperature turns it in highly thermo-labile. Most of psychrophiles organisms, including bacteria, use antifreeze proteins (AFPs) and other cryoprotectans as a shield against the damage produced by large-size ice crystals. AFPs act binding to ice crystals through a large complementary surface and creating thermal hysteresis, allowing psychrophiles to decrease the temperature at which an organisms can grow (Jia and Davies 2002). Cryoprotection activity is also related with trehalose and other called “compatible solutes”, which acts by a colligative effect and preventing protein denaturation and aggregation (Phadtare 2004). Exopolysaccharides in a high concentration might play an important role in psychrophilic adaptation modifying the physico-chemical surroundings of bacteria, creating an environment that allows cell adhesion, retention of water and sequestration and concentration of nutrients. Extracelullar enzymes are retained inside this matrix which protects them against cold denaturation (Mancuso-Nichols et al 2005). Genome sequencing and proteomics of psychrophilic microorganisms show over-expression of cold-shock response proteins, mainly proteins involved in unsaturated fatty acid synthesis and degradation of steroids or hopanoids. It is interesting to remark the enhanced antioxidant capacity generated by the presence of catalases and superoxidedismutases found in C.psychroerythraea and D.psychrophila in response to the increased production of reactive oxygen species at low temperatures. Most of the cold adaptations mentioned above have potential applications in the biotechnological field. Some of them are shown in table 1. Enzymatic catalysis at low or moderate temperatures, avoiding the need of heating (that represents one of the most determining cost at industrial scale), is one of the main advantage obtained from psychrophiles. The study of psychrophiles is under permanent evolution and the broad diversity of microorganisms in cold region, including Antarctic and Arctic ecosystems, promises new developments in the biotechnological field.