Question

Define the physical requirement for the following organisms: Psycrophile, mesophile, obligate anacrobe, thermophile, facultative anacrobe, extreme obligate halophile, acidophile, facultative halophile.

Answer 1

Microorganisms are sensitive to dramatic changes in their environments. Physical requirements include the physical growth conditions such as temperature, pH etc. Extreme conditions of temperature, pressure, drought, salinity, and pH affect the folding and functioning of critical biomolecules that are required for cellular integrity. However, some types of organisms can survive in these environmental conditions by adapting their cellular machinery.

Psychrophiles: These are microorganisms that grow at or below 0 °C, (optimum is 15-20 °C). They can live between -5 to +20°C . They live in cold environments, from the stratosphere to the deep-sea. When the seawater freezes, the salt gets concentrated in small pockets, and the freezing point of water may reach ?20 °C. The bacterium Psychrobacter cryopegella can grow at this temperature, between -10 to ?20 °C. Hence, most psychrophiles are also halophiles (microbes that grow in elevated salt concentrations). Psychrophiles have high levels of unsaturated fatty acids in order to modulate membrane fluidity. Their enzymes are cold-adapted with high specific activities at low temperatures and can carry out transcription and translation at low temperatures.

Mesophiles: Mesophiles are microorganism that grows best in moderate temperature, typically between 15 and 45 °C with an optimal temperature is 37 °C. They grow in habitats such as cheese and yogurt, wine. They can be aerobic, aerotolerant, facultative anaerobes and obligate anaerobes. Hence, they can live at different oxygen levels.

Thermophiles: Thermophiles are microorganisms that have adapted to live in extremely hot environments, for example: Thermus thermophilus that grows in Hot Springs at Yellowstone. Thermophiles can tolerate temperatures between 45-80⁰C. They have mostly chemoautotrophic mode of nutrition. Inorganic redox reactions act an energy source and utilize carbon dioxide as a carbon source (chemolithotophic). Thermophiles therefore are mostly anaerobes and can use hydrogen, sulphide, sulphur, and ferrous iron as electron donors. Their enzymes function at higher temperature while the lipids are rich in saturated fatty acids to maintain membrane stability at high temperature. There is increased number of ionic bonds between positive and negative charges of amino acids. Hence, densely packed hydrophobic region make them resist unfolding of proteins in aqueous environment. Increase number of disulphide bonds increases the stability of thermophillic enzymes, to help in oligomerization.

Acidophiles: Acidophiles are organisms that thrive under extremely acidic conditions (pH 0-6.5). Acidophiles are found in Lechuguilla Cave, Carlsbad, New Mexico, where the pH is 0.0. They are found in volcanic areas, mine drainades where there is extreme acidity. The low pH is due to metabolism of the organism. Acidophiles oxidize the elemental sulfur or sulfidic minerals to obtain energy. Acidophiles have proteins with more amino acids with neutral side-groups. They also actively pump protons out of the cell, thereby maintaining constant intracellular pH levels.

Extreme obligate Halophiles: Halophiles are bacteria that live only in high salinity upto 30% salt concentration. They can even live in salt crystals. They require high osmotic pressure for growth. Halobacterium salinarum can live in the Dead Sea. Their cytoplasm must therefore are isosmotic with the surrounding environment. Halophile proteins, can fold only in high salt conditions. This binding of protein to salt is dependent on presence of large number of acidic amino acids on the surface. Further, these proteins form more random-coil structures, rather than ?-helices. They have a membrane pump that pumps in potassium in while pumping sodium ion. This pump functions to maintain osmotic balance. Proteins such as cysteinyl-tRNA synthetase are active only in high salt concentration.

Facultative Halophiles: Facultative halophiles can grow high salt conditions but does not depend on high salt for growth. Hence, it can grow and reproduce both in high salt and non-salt conditions. They do not require high osmotic pressure for growth but can tolerate it. Staphylococcus aureus is a facultative halophile that can tolerate 7.5-20% salt. Hence, it is able to colonize the skin and nasal membrane, which are high salt areas.

Facultative Anaerobe: The facultative anaerobes can grow in the presence or absence of oxygen. They switch their metabolism according to their environment. However, they prefer aerobic environment as aerobic respiration generates the largest amount of energy for faster growth. They can also use fermentation mode or other terminal electron acceptors. E.coli, Corynebacterium are examples of facultative anaerobes.

Obligate anaerobes: Obligate anaerobes can only grow in the absence of oxygen. An oxygenated environment is toxic to them. They are sensitive to oxygen because they lack superoxide dismutase enzyme and catalase that protects cell from reactive oxygen species. They usually grow at bottom of a media tube as oxygen poisons them. Example is Clostridium botulinum, Clostridium perfringens.