What is resistant starch?

Initially, when the word "resistant starch" was coined, it was for the purpose of naming that fraction of starch which did not undergo hydrolyzation with alpha amylase and pullulanase even after 20 min of incubation in vitro. However, now it is more commonly used to refer to that fraction of starch which cannot be digested by the small intestine and is hence not available for absorption by the body.

There exist 4 different types of resistant starch (RS) fractions.

RS1- These starches have a physical coat or covering and are hence protected from the attack of enzymes. Eg: Whole grains, seeds, legumes. These can be made available by processes that interfere with their protection, like milling( in case of grains). Chemically, it can be measured as the difference between the glucose released due to enzyme digestion when the food sample is homogenized and when it is not homogenized. Due to physical protection, these foods are generally heat stable.

RS2- These structures have a relatively compact granular form thus allowing little water or enzymes to penetrate their insides and therefore resistant to breakdown. Eg: raw potatoes, green bananas. However, food processing can soften these starches. Chemically, their content is calculated as the difference between the glucose released upon enzymatic digestion from a boiled, homogenized sample to that of a non boiled non homogenized sample.

RS3- It is the most resistant starch fraction and is not at all susceptible to digestion by pancreatic amylase. It is measured as that fraction which is dispersed by Potassium hydroxide and dimethyl sulfoxide.

RS4- This starch contains novel chemical bonds that are formed because of interaction between the food and the chemical agents or additives used in its processing.

Certain activities like autoclaving of the food, parboiling, baking may increase the RS content of certain foods. On the other hand, microwave cooking, germination of legumes, fermentation may help reduce the RS content.

RS has a small particle size, bland in taste and low water holding capacity. They can be used as a dietary fiber or added to other foods to improve its fiber content or lower its calorific value. It is digested over a long period of time and is thus beneficial to be used by diabetics. It can also function as a laxative.




Reference:

http://onlinelibrary.wiley.com/doi/10.1111/j.1541-4337.2006.tb00076.x/pdf

Why can Deinococcus radiodurans survive high radiation dose?

Deinococcus radiodurans is one of the bacterium that is resistant to ionizing radiation. To say, that they evolved this capacity by natural selection or mutation is difficult, because the background radiation on earth is very low as compared to what this organism can endure for it to have evolved in this manner. However, it has been observed that dessication also causes double stranded DNA breaks similar to that which occur after radiation exposure. Hence,  it can be rationalized that Deinococcus actually developed a mechanism to combat dessication or dehydration which is now being proved useful to combat high doses of radiation.

This is not the only bacterium that shows radiation resistance . There are some other bacteria (Kineococcus radiotolerans, Rubrobacter xylanophilus) as well as archae (Thermococcus gammatolerans) that show radiation resistance. However this particular guy has received much attention. How come these apparently unrelated species show resistance to high doses of radiation? There have been 2 postulates about this. One states that radiation resistance was a widespread phenomenon in the beginning of the world and slowly has been lost by the species. The other says it is a result of convergent evolution or horizontal gene transfer.

It has been found that Deinococcus radiotolerans can survive a radiation dose of 5000 Gy without any adverse effect. What helps it withstand this high stress level? The factors responsible for this ability as well as the exact mechanisms have still not been confirmed but the possibilities have been enumerated.

The Deinococcus has a segmented genome with two chromosomes 2.64 Mb (Chromosome I) and 0.41 Mb (Chromosome II). Apart from this it also has a 0.18 Mb megaplasmid and a 0.045 Mb plasmid. It has between 4 to 10 genome copies per cell depending upon the bacterial growth phase which are stacked one over the other . The more the number of genome copies, more is the resistance to radiation stress. More the number of genome copies means there is more probability of survival of crucial genetic information. This information from the surviving copies may then be used to repair the other damaged strands. The condensed genome is also said to play a part in radiation resistance. Since, it is tightly packed together, even though the strands may be fragmented they cannot diffuse away from each other and hence repair may become somewhat easier.

It has been experimentally found that in presence of a lot of Mn (II) the bacterium can withstand radiation stress better than when it is depleted of them. However in both the instances, the impact is the same, its only that the survival is better in presence of Mn (II). It is not that the bacterium does not undergo damage but it has a very efficient  and error free repair mechanism to repair the damage that has taken place. In fact, the cell growth is halted till all the damaged DNA is repaired and then only the bacterium continues with its life cycle.

 Also, new loci have been found in the genome suggesting newer enzymes or at least novel mechanisms of DNA repair that need to be further looked into. However a lot of work still needs to be carried out so that the mysteries of this "strange" bacterium are revealed.

Reference:
http://www.biochem.wisc.edu/faculty/cox/lab/pdfs/38.pdf

More reading:

http://www.genomenewsnetwork.org/articles/07_02/deinococcus.shtml

What is Phage therapy?

Bacteriophages or simply phages, are viruses that infect bacteria. Phage therapy , as the name suggests strives to utilize these phages as antimicrobials. Frederick Twort (1915) and Felix d'Herelle (1917) independently discovered the bacteriophages. Seeing the potential of phages to be used as a treatment tool against bacterial infections, Herelle started his experiments. He got his first success when he used phages to cure dysentry in 1919. Thereafter,  different phage formulations against different diseases emerged in the market. But this hype was shortlived. With the advent of antibiotics, the phage cocktails were relegated to a backseat. Now, with the emergence of antibiotic resistant strains, phage therapy is again being looked into as a potential treatment tool against pathogenic bacteria.

Phages maybe effective against a single strain of bacteria or multiple strains. Hence, selection of the best possible match is essential. It has been found that phages generally attach themselves to those receptors on the bacterial surface which are responsible for the virulence of the bacteria. Hence, if the bacteria mutates itself so that the phage no longer attaches to it, there is an increased probability that the mutation would be in the receptors. However, even if the bacteria does acquire resistance to a phage, the phages themselves can mutate back effectively nullifying the acquired resistance of the bacteria. To prevent the rapid acquirement of resistance by the bacteria against phages, it has been proposed that a mixture of phages instead of one single type of phage be used.

The problem with phage therapy is that not much is known about the behavior of phages inside an eukaryotic organism. This requires a long and detailed experimentation. Although it is believed that the phages wouldnt directly affect the eukaryotic cells, there can be no surety of how they will behave with the beneficial bacterial flora already present within the human body. Another problem with phages is that they are rapidly eliminated from the body. So either longer circulating phage variants need to be selected or they must be shielded from the immune system of the body. Also, the bacterial flora of each individual may differ and thus it remains to be seen whether a standardized concoction of phages can be used for all people. Apart from this, there remain the usual problems of production and delivery of phages.

Phage therapy had been used extensively in some parts of the world like the former USSR. Pyophage and Intestinophage are two phage preparations widely available in the Republic of Georgia. Pyophage is a cocktail against pus producing bacteria while the other one is used against diarrhoea and gastrointestinal upsets.


References
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3586887/

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3400130/