All bacteria have characteristic minimum (Tmin) and maximum (Tmax) temperatures for growth. The determinants and regulators of these limits are not well understood, but extensive research efforts go into determining the mechanism of thermophily, particularly with regards to the nature of thermophile proteins and membranes (Lieph et al., 2006). The observations that heat sensitive mutations lead to a lowering of Tmax but not Tmin suggests that both are independent, single genome traits (Sundaram, 1986). Membrane lipids of thermophiles tend to have a greater abundance of high melting point (saturated long chain) fatty acids than their mesophilic counterparts (Suutari and Laakso, 1994). These can be further adjusted, depending on the growth temperature. Bacterial phospholipid bilayer membranes undergo a thermotrophic reversible transition between an ordered rigid gel (solid phase), and a fluid (liquid crystalline) phase. This transition involves the melting of the hydrocarbon chains in the interior of the bilayer. The temperature at which this takes place depends on the nature (melting point) of the membrane fatty acids (Cronan and Gelman, 1975).

The heterogeneity of membrane lipid composition implies that phase changes take place over a range of temperature, rather than at a discrete point, with liquid crystalline and solid gel phases existing simultaneously. Above the upper boundary, liquid crystals obtain while solid gels exist below the lower boundary. Depending on the type of organism, membrane bound proteins may, or may not, have any effect on the phase transition temperature (McElhany and Souza, 1976; Lipowsky, 1991). The quantity and quality (ratio and types) of membrane lipids and proteins of thermophiles have also been shown to vary with the growth temperature (Suutari and Laakso, 1994). In the eubacterium B. stearothermophilus this manifests as a decrease in total lipid, and an increase in membrane protein, whereas in the archae T. aquaticus the quantity of lipid increases with temperature. In B. stearothermophilus and E. coli, the existence of membranes in liquid crystalline phase is believed to be essential for performance of membrane functions. This implies a role for the membrane transition point in setting Tmin and Tmax. Changes in conformation of essential enzymes, leading to loss or reduction of catalytic activity, or alteration of important regulatory characteristics, which may lead to arrest of growth below certain temperature (Tmin), may also affect minimum temperature of thermophiles. Additionally, ribosomal assembly in thermophilic bacteria has been shown to be energy intensive and may thus contribute in setting a high minimum temperature for growth (Suutari and Laakso, 1994).

As for Tmin, determinants of Tmax are not well understood. Early suggestions were that the membrane lipid phase transition temperature may be important in setting the upper temperature. This position was prompted by observations in some thermopiles that increase in growth temperature led to an increase in the melting point of membrane lipids. It is contradicted however, by observations that the phase transition temperature may be varied by manipulating the lipid composition without affecting the Tmax, while (in B. stearothermophilus) mutations which lowered Tmax did not affect the lipid transition temperature (Kawada and Nosoh, 1981). In spite of these observations, membrane stability may be a major determinant of Tmax in so far as cell death may be accelerated by heat damage to the membrane above its optimum temperature (Kuhn et al., 1980). The fluidity of membrane lipid (transition phase) will therefore be an index of stability at a given temperature. Membranes of thermophiles have been shown to be more stable than that of mesophiles, and to increase in stability with temperature (Suutari and Laakso, 1994).

It is also believed that there is a limit to the gap that can exist between the transition temperature and Tmax, i.e., a limit to the level of fluidity of membranes in growing bacteria. Mutants of E. coli defective in lipid synthesis were unable to grow under conditions that led to the incorporation of very low melting point lipids. It is presumed that incorporation of such lipids in the membrane led to production of membranes that were too fluid and therefore unstable at the Topt. A similar observation was made in the growth of heat sensitive B. stearothermophius (Sundaram, 1986). This organism could not adjust its membrane phase transition point beyond 41 °C, and rapidly lost the ability to grow beyond 58°C, which became the Topt and Tmax, as well as the upper limit for membrane stability. The wild type organism could adjust its upper phase transition point to 65°C, and could grow at 72°C. The Tmax of microbes may also be affected by temperature stability of various macromolecules, particularly enzymes (Lindsay, 1995).

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