Term Paper: Desiccation Tolerance in Prokaryotes Water

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[. . .] It is very possible that genomic and phenotypic factors (discussed later in this work) combine along with protectants such as trehalose. Different media concentrations induced E. coli to generate different compounds. Researchers induced the E. coli production of glycine betaine by varying the NaCl concentrations and the consequent osmotic stress. Glycine betaine has been implicated in desiccation protection for several higher plans such as grams and tobacco. Glycine betaine has also been suggested as a protectant against desiccation for certain lactobacilli. (Teunissen et al., 1992). from short-term viability experiments.

In this sense, both trehalose and glycine betaine can be considered compatible solutes because their presence (in response to the osmotic stress that draws water away from the cell) is compatible with the processes that occur within the cell. This compatibility arises no matter how high the concentrations of these solutes. One must remember however, that there is an artificial construct to attempting to render a desiccation-protection mechanism from osmotic stress. This is because osmotic stress occurs rarely in nature and the water loss from air-drying (or desiccation) is higher and relentless). On the other hand, one might consider osmotic stressors are necessary to understand desiccation protection under laboratory condition

In Welsh and Robert's experiments, glycine betaine had no discernible effect on desiccation tolerance. The action of glycine betaine as yet another mechanism of desiccation tolerance has been shown through the process of solute exclusion. (Lows, 1985). This concept proposed that, under water stress, solutes such as glycine betaine are excluded from the protein sphere and surrounding water, around which, this solute creates a protective barrier. It is possible that this would explain the short-term (but not long-term) desiccation tolerance. In addition to replacement of water, hydrogen bonds to prevent the denaturation of protein. It is also possible that the "glassy" trehalose also acts as a membrane barrier to the outside drying elements. E. coli does not produce trehalose in response to air-drying. But externally applied trehalose either diffuses into the cell or enters during structural phase changes in the outer membrane. (Leslie et al., 1995). It is also possible therefore, that bacteria that can produce trehalose in response to drying also have developed a mechanism by which trehalose produced by the bacterium can make its way to the surface of the cells.

It has been mentioned at least twice here that E. coli does not have a natural water-stress response. This does not make it particularly viable in dry conditions. Gowrishankar and co-workers have identified a third mechanism of this water stress. And it is referred to (in addition to solute inclusion or solute exclusion) by a term (coined by the authors) anhydrotic stress. This stress is related directly to the loss of water in the cell, and the consequent inhibition of bacterial growth due to permeable liquids such as glycols. This stress is alleged to be directly correlated with the increase of L-ornithine in the bacterial cell. It has also been mentioned above that genomic factors probably have a lot to do with resistance to desiccation. Gowrishankar and his co-workers have shown that in addition to L-ornithine correlation, certain transposons in the E. coli plasmid may also be associated with anhydrotic stress. (UmaPrasad & Gowrishankar, 1998). This has been observed in Gram-positive as well as Gram-negative bacteria. The authors believe that this additional information would be important in identifying why E. coli is not naturally tolerant but several other extremophile bacteria maintain their viability on drying.

The identifiable qualitative difference between Gram-positive and Gram-negative bacteria is that the former responds to the Gram stain which is a violet colored stain that interacts with iodine to produce a violet color. The Gram staining process also involves rinsing the bacterial sample with alcohol and counterstaining with saffranine dye. While positive bacteria turn violet, the negative bacteria which do not for complexes with the Gram stain are stained red to pink with saffranine. The quantitative difference between Gram-positive and Gram-negative bacteria is the amount of peptidoglycan in the bacterial cell wall. Positive strains of bacteria have almost five times as much as Gram-negative bacteria. The peptidoglycan layer complex with the Gram stain giving positive bacteria the distinct violet colors. This might have consequences to desiccation protection.

The osmolyte tetrahydropyramidine hydroxyectoine can also be used as a desiccation protectant. It is shown to work irrespective of the Gram-nature of the bacteria. Researchers Manzanera et al. aver that it produces better protection and desiccation tolerance than the ubiquitous trehalose, especially in Gram-negative bacteria. (Manzanera, Vilchez, & Tunnacliffe, 2004). They base their findings by comparing the protection strengths of hydroxyectoine vs. trehalose in the Gram-negative Pseudomonas putida. Their results show that hydroxyectoine produced better protection than trehalose in P. putida. When these experiments were conducted for (also Gram-negative) E. coli, the final results were varied. In the case of external applications, both compatible solutes performed equally well in providing dessication tolerance. Bacteria do not metabolize of synthesize hydroxyectoine, unlike they do trehalose. But there is osmotic uptake of hydroxyectoine into the cell body. Such an uptake naturally reduces the induced production of trehalose and it reduces the ability of E. coli to protect itself.

The authors believe that mechanistically, hydroxyectoine does not possess enough hydroxyl groups that will hydrogen bond with cellular proteins replacing the water molecules and preventing denaturation. Indeed, a molecule of hydroxyectoine only possesses one -OH group, unlike trehalose. Manzanera and co-workers experimented with trehalose induction under high saline conditions vs. saline mixed with hydroxyectoine and glycine betaine. In the latter cases, perhaps due to osmolyte (compatible solute) uptake, the synthesis of adequate amounts of trehalose to tolerate drying adequately was not produced. This was seen in the results. These studies provide added substantiation for the water replacement model for mechanism of desiccation tolerance. One of the upshots of this study was also the development of a methodology for protecting E. coli to be used in laboratory settings. This includes inducing trehalose growth and then placing E. coli under desiccant conditions in the presence of a veneer of trehalose (in its glassy state).

Much like the acinetobacter family of prokaryotes, the adaptation to drying conditions (or conditions of osmotic stress) can give rise to pathogens. This means that the ability to protect against drying will become a health hazard. Enterobacter sakazakii can be found in baby formula powder. (Breeuwer et al., 2003). Its mechanism for dessication tolerance can also be traced to the internal production of trehalose in response to water stressful conditions.

As has been mentioned previously (first page), that non-reducing disaccharides substantiate the water replacement model. It is fairly obvious however, that sucrose and trehalose are only one part of the overall mechanism. This mechanism however, also cannot be universally applied to every species prokaryotes or otherwise. There is a lack of comprehensive research especially when it comes to Gram-positive bacteria. A study of the eukaryotic minute animal bdelloid (leech-like) rotifers (Phylum rotifera) indicates that other mechanisms must be at play. The results of such studies (on multicellular organisms) can do no better than further the cause of identifying mechanisms in prokaryotes.

Bdelloid rotifers are small animals that are found on films of water, especially in birdbaths. Though they live under moist conditions, they are remarkably well adapted to dry conditions and are ubiquitous in dry or very cold environments. When researchers fed, dehydrated and rehydrated bdelloid rotifers, the viability was greater than seventy-five percent. (Lapinski & Tunnacliffe, 2003). Under dehydrated conditions, the researchers discovered that there was no evidence of sucrose, trehalose, or any other mono- or polysaccharide. Lapinski and Tunnacliffe indicate that pursuing alternative mechanistic models might be necessary. The researchers also found (much like in the trehalose studies) that for full tolerance to take effect it took time. This meant that the organism (eukaryote or otherwise) undergoes physiological changes to counter the effects of drying. This is different from Deinococcus radiodurans, which is restored to full function just a few hours after stress. Indeed, they found that the viability of the rotifers decreased to fifty percent or less as soon if the drying process was rapid. This meant that the changes necessary for tolerance were not in place.

Lapinski and Tunnacliffe used the results of their research to posit questions of other models: namely, the protein restructuring to counter desiccation. They point to the identification of late embryogenic abundant (LEA) proteins as a potential protecting agent. An LEA like protein has also been isolated in nematodes. Indeed, it was relatively late that anhydrobiosis was identified in extremophilic bacteria. Nematodes and certain desert-viable plants and seeds were the first species on which anhydrobiosis-identifying research was conducted (Bartels & Salamini, 2001).

The research by Bartels and Salamini took a different approach. Realizing that organic sugar glasses and consequent water-replacement theory did not explain anyhydrobiosis in all species that exhibited it, they began seeking genomic information, a "protein-set" or a "gene-set." LEA proteins that are distributed throughout different species are perhaps one of these.… [END OF PREVIEW]

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