The adaptability of bacteria to extreme cold environments has been demonstrated from time to time by various investigators. Metabolic activity of bacteria at subzero temperatures is also evidenced. Recent studies indicate that bacteria continue both catabolic and anabolic activities at subzero temperatures. Implications of these findings are discussed.
A population of cold-tolerant Antarctic bacteria was screened for their ability to tolerate other environmental stress factors. Besides low temperature, they were predominantly found to be tolerant to alkali. Attempt was also made to postulate a genetic basis of their multistress-tolerance. Transposon mutagenesis of an isolate Pseudomonas syringae Lz4W was performed, and mutants with delayed growth at low temperature were further screened for sensitivity to some other stress factors. A number of multistress-sensitive mutants were isolated. The mutated gene in one of the mutants sensitive to low temperature, acid and alkali was found to encode citrate synthase. Possible role of citrate synthase in conferring multistress-tolerance was postulated.
Arsenic (As), the metalloid, has been traditionally infamous for its toxicity. Because of its use by rulers to kill their rivals, it was once known as the poison of the kings or the king of poisons. It was also widely used as an agricultural insecticide. It kills humans quickly if consumed in large quantity. Also, regular small doses of arsenic cause various illnesses, such as patchy skin, digestive system disorders, peripheral neuritis, hepatic lesions and fatty degeneration of the heart, and other lifethreatening complications. Exposure to arsenic could be due to both natural and anthropogenic factors.In nature, arsenic occurs in four oxidation states: As(V), As(III), As(0) and As(−III). Occurrence of the highest oxidation states is more common and that of the two lowest oxidation states is rare. In aqueous aerobic environments, arsenate (AsO 4 −3 ), the pentavalent form, predominates. It tends to be strongly adsorbed onto common minerals (e.g. alumina). Arsenite (AsO 3 −3 ), the trivalent form, is more prevalent in anoxic environments and is substantially more toxic than arsenate. By virtue of its ability to bind sulphydryl groups, arsenic binds and inactivates enzymes. It inhibits the enzyme pyruvate dehydrogenase, essential for oxidation of pyruvate to acetyl-Co-A. In the process, it triggers cellular apoptosis. It also binds dithiols such as glutaredoxin and stimulates the production of hydrogen peroxide, thus leading to increase in oxidative stress. Inorganic arsenic trioxide, found in groundwater, affects voltage-gated potassium channels, thus creating neurological disturbances. This metabolic interference ultimately leads to death due to multiorgan failure. In the periodic table, arsenic is positioned just below phosphorus. Because of their similarity in electronegativity and radii, arsenate ion can replace phosphate in biological reactions. Thus, it can enter the early stages of metabolism, but fails to continue metabolism because of the rapid hydrolysis of the arsenate esters, compared with that of the phosphate esters. The hydrolysis of diesters is faster than that of triesters (Westheimer 1987). Arsenate esters are labile even if very low concentration of water is present. Presence of 0.5% water allows a half-life of less than 0.1 s at pH 9.0. Compared with phosphodiester-containing DNA, which has a half-life of 3×10 7 years, arsenodiestercontaining DNA spontaneously hydrolyses with an estimated half-life of 0.06 s at 25°C (Fekry et al. 2011).Analysis of arsenic-rich water and soil, sampled from different places, have so far revealed the presence of several types of bacteria belonging to the genera Acidithiobacillus, Bacillus, Deinococcus, Desulfitobacterium and Pseudomonas (Shivaji et al. 2005 and the references therein). These bacteria can tolerate arsenate in the presence of phosphate. The genes responsible for arsenic tolerance in these organisms are organized in an operon, called ars operon. Some bacteria can use arsenic as an electron donor; others can methylate inorganic arsenic or demethylat...
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