Desiccation Tolerance in Prokaryotes Term Paper

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Desiccation Tolerance in Prokaryotes

Water is very important for life. Indeed, the processes of life, both external and internal even, at the cellular and the molecular level, are governed by water. Without water, most living organisms suffer from what is known as water stress.

This water stress can be due to the loss of water or dehydration. Desiccation is a special case of dehydration where drying takes place in air. Alternatively, another form of water stress is due to the excessive accumulation of salts. This is relatedly called osmotic stress. Osmosis seeks to reduce this accumulation by moving fluids across a concentration gradient. While most living beings cannot survive without water, lesser species belonging to the eukarya group -- that includes both bacteria and a more primitive organism archaea show remarkable tolerance to water stress.

Responses to water stress takes place at a supracellular level as well as a cellular level. This work will be concerned with the latter. Thirst response or the secretion of anti-diuretic hormones in animals indicates that there is water stress. This stress is remedied by either increasing the intake of water or reducing the loss of water. Plants also have a supracellular response. Closing of stomata on leaves indicate the need for decrease in transpiration rates. Another defense mechanism has origins in evolution. Cactus leaves have evolved into thorns to preserve water-loss. The green fleshy stems store water and conduct photosynthesis. This work will be dedicated to exploring the current research being done in explicating the idea of desiccation tolerance in prokaryotes -- namely bacteria and archaea (though not a lot of research is being done in the latter, these organisms having been only recently identified).

Desiccation tolerance is also called anhydrobiosis -- or a form of suspended animation. The concept was identified a few centuries ago. (Schierbeek, 1959). This suspended animation lasts until water is available to resume proper metabolic function. At its basic, mechanistic level, this process engenders the use of sugars (sucrose, etc.) to replace the hydrogen bonds that sustain the viability of protein and other molecules in cellular metabolism. Or, as will be shown in this work, other methods are employed to preserve the structure function of proteins and the integrity of DNA.

Bacteria are critical to most molecular biology research. If a DNA sample is to be studied or created in large quantities for expression purposes for (for instance) protein synthesis, researchers make use of Escherichia coli. The DNA is inserted into the E. coli chromosomal plasmid. The bacteria are then spotted on a plate that contains nutrients and moisture that allows the bacteria to grow and reproduce. Each spot is then allowed to multiply (almost exponentially) in a medium rich for bacterial growth. This medium is aqueous. It is kept at a temperature of 37 "C -- the ideal temperature for bacterial growth. E. coli then multiplies by the typical bacterial reproductive process of geometric cell division, thus creating large quantities of the DNA to be studied. The point for using, what is called as recombinant DNA techniques, is not only to illustrate that bacteria drive research, but that bacteria thrive under moist conditions at the right (and mild) temperature. The operative terms being -- moist and mild. As such, E. coli, despite its abundance is not a good candidate for desiccation tolerance and research has shown that this is indeed true.

On the other hand however, bacteria such as the cyanobacterium Nostoc commune and Deinococcus radiodurans thrive under extremely dry conditions of very high or very low temperature where water as a metabolite is not easily available. In fact, both these bacteria, especially D. radiodurans can withstand radioactivity that would easily kill all other living things. This correlation between tolerance to radioactivity and desiccation is important. Researchers believe that the mechanisms of both are intrinsically linked. That since radioactivity is not a natural phenomenon on earth, the tolerance to radioactivity is a manifestation of the tolerance to conditions of extreme dryness. In turn, therefore, to understand one form of tolerance, mechanistically, is to understand the other.

Prokaryotes have developed a mechanism for survival under extreme and harsh conditions that has earned them the moniker of extremophiles. These include theromophiles (able to exist at high temperatures) and acidophiles (able to survive under conditions of low pH) and halophiles (in conditions of high salinity). Thermophiles were first discovered in the depths of Yellowstone National Park's hot springs, where the temperature of the water exceeds that of its boiling point.

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Halophilic archaea have been discovered in salt crystals from millions of years ago. (Rothschild & Mancinelli, 2001).

Cognating the extremophilic behavior of prokaryotes has two-edged consequences. The first is that understanding the mechanism of desiccation tolerance allows researchers to understand the role of water in living things. On the other hand, pathogenic bacteria which sustain under harsh conditions can cause disease. In order to disinfect a laboratory bench of bacterial contaminants, an alcohol wipe is sufficient, since bacteria cannot grow in the presence of ethyl alcohol. Also, heating is supposed to kill bacteria and restrict its growth. Bacteria of the Acitenobacter genus: Acinetobacter baumannii (Jawad, Snelling et al., 1998). have shown remarkable desiccation tolerance. These bacteria have been found on the fingertips of medical staff and other dry surfaces in hospitals in the United Kingdom. These bacteria are responsible for cross-contamination outbreaks several times in these hospitals in the last twenty years.

From a mechanistic standpoint, the formation of trehalose has been implicated in the protection of the bacterium against desiccation. Trehalose is a reducing sugar. It is a disaccharide similar to sucrose and maltose. During the process of digestion, trehalose is broken down into two molecules of sucrose. Trehalose has approximately half the sweetness of sucrose when ten percent solutions were compared. From the standpoint of bacterial desiccation tolerance, two properties of trehalose stand out. It is not as hygroscopic as sucrose or maltose. Its hygroscopicity is less than one percent as opposed to seventy-five percent for sucrose. It has a relatively low melting point (97 "C, when compared to that of sucrose between 160 to 180 "C). Perhaps more relevant is its relative high glass transition temperature. Its Tg is 79 degrees as opposed to 52 degrees C. For sucrose. This high temperature allows trehalose to remain stable for wider temperature ranges. The high glass transition temperature also accounts for its significantly low hygroscopicity. Glass transition temperature is defined as temperature below which polymers achieve glass like status. This glass phase renders a compound inert to outside influences. It also slows down reaction rates. This inertness induces a "hibernation" of sorts in the bacterium thus allowing it to survive dry conditions. This means it also does not allow the exchange of material on either side of this "glass partition." Thus it preserves the remaining moisture on the inside of the bacterial cell.

Another theory suggests that the sugar domains of trehalose prevent internal desiccation by providing hydrogen bonding with the proteins inside the cell and with the phospholipid membrane bilayers in the cell membrane. Protein denaturation would have resulted if the water molecules that stabilize the tertiary structures of proteins (and hence the function) would have been removed due to desiccation. That however, does not happen; or, the process of drying is retarded. This theory however, has come under considerable criticism because it does not explain the advantage of trehalose over other sugars which would also replace water hydrogen bonding. In this case however, the chemical inertness of trehalose glass at a higher temperature prevents desiccation. By the same reasoning, the significantly lower Tg of sucrose becomes susceptible to the Maillard reaction between sugars and amino acids in proteins that result in the browning of food.

Researchers Welsh and Herbert showed the use of trehalose in protecting against desiccation by introducing trehalose externally into the medium surrounding E. coli and then subjecting it to short- and long-term drying. (Welsh & Herbert, 1999). E. coli showed resistance to drying in the presence of trehalose. Welsh and Herbert demonstrated that the desiccation-resistant efficacy of trehalose only occurs when it has reached a glasslike state. They measured the bacterial viability with respect to time and found that it decreased until a specific level of dryness. After that the trehalose began exerting its protecting mechanism.

Internal creation of trehalose within primitive systems is also not without precedence. Trehalose is naturally created in Saccharomyces cerevisiae when it is freeze-dried. While freeze-drying is not the same of air-drying or desiccation, it allowed researchers to test the notion that trehalose can also be manufactured by a species in response to water stress. In addition to external application of trehalose, Welsh and Herbert also sought to force E. coli to create trehalose in the presence of stress. When subjected to a medium of high salt content, E. coli -- to protect against this osmotic stress produced trehalose. The induced trehalose possessing bacteria then also resisted drying. The mere presence of a.....

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