UKPSC Prelims 2024 General Studies Paper-I · Question 78 of 147

Correct — B, Psychrophiles.

Psychrophiles are the cold-loving organisms. In the standard classification of microbial growth by temperature, a psychrophile grows best at about 15 °C or below, can still grow at 0 °C, and typically stops growing at around 20 °C.

The band in the stem, 0-15 °C, is the textbook psychrophile band.

The classes are named by Greek prefixes for what the organism loves: psychro- (cold), meso- (middle), thermo- (heat). Read the prefix and the temperature band follows.

The idea to carry away: the class is fixed by the temperature at which growth is fastest, not by the temperatures the organism can merely survive.

That one distinction also separates true psychrophiles from psychrotrophs, which tolerate a refrigerator but grow best at 20-30 °C.

• (a)
  Thermophiles — Thermo- means heat. Thermophiles grow best at roughly 45-80 °C, the hot end of the scale and the opposite of the stem's 0-15 °C band.

  It is the right label for heat-loving organisms such as Thermus aquaticus of Yellowstone's hot springs, which grows best near 70 °C and gave PCR its Taq polymerase.

• (c)
  Hydrophiles — Hydro- means water, not cold. Hydrophile is not a temperature class; the figure 0-15 °C may bring cold water to mind, but the word itself carries no temperature meaning.

  It is the right term for water-attracting substances in chemistry, and hydrophily is pollination by water, as in Vallisneria and Hydrilla in fresh water and the seagrass Zostera in the sea.

• (d)
  Mesophiles — Meso- means middle. Mesophiles grow best in the moderate band of roughly 20-45 °C. Many survive being chilled, but surviving at 0 °C is not the same as growing best there, which is what the stem asks.

  It is the right label for organisms that grow best near body temperature, including Escherichia coli and most human pathogens at about 37 °C.

Every organism has a minimum, an optimum and a maximum growth temperature, its cardinal temperatures. Growth rate climbs slowly from the minimum, peaks at the optimum and collapses quickly above it, because enzymes and membranes fail when overheated.

Microbiologists group organisms by where the optimum lies: psychrophiles (about 15 °C or below), mesophiles (roughly 20-45 °C), thermophiles (roughly 45-80 °C) and hyperthermophiles (80 °C and above). Textbooks draw the boundaries slightly differently, but the order of the ladder does not change.

Psychrotrophs, also called psychrotolerant organisms, sit between the first two rungs: they grow at refrigerator temperatures of 0-7 °C but grow fastest at 20-30 °C. They are common spoilers of chilled food.

• Psychrophiles grow best at about 15 °C or below, can grow at 0 °C, and typically cannot grow above roughly 20 °C.
• The prefixes are Greek: psychro- means cold, meso- means middle, thermo- means heat, and hydro- means water; -phile means loving.
• Mesophiles grow best at roughly 20-45 °C; Escherichia coli and most human pathogens grow best near 37 °C.
• Thermophiles grow best at roughly 45-80 °C; Thermus aquaticus of Yellowstone's hot springs, the source of Taq polymerase, grows best near 70 °C.
• Hyperthermophiles grow best above 80 °C; Methanopyrus kandleri strain 116 grew at 116 °C under 0.4 MPa gas pressure, optimum 100 °C, and at 122 °C under 20 MPa.
• Psychrotrophs grow at refrigerator temperatures of 0-7 °C but grow fastest at 20-30 °C; Pseudomonas species that spoil chilled milk are typical.
• Psychrophile membranes are rich in unsaturated fatty acids, which keep the membrane fluid at low temperature.
• Enzymes of psychrophiles are more flexible than their mesophile counterparts and work in the cold, which is why they are used in cold-wash detergents.
• Hydrophily is pollination by water, as in Vallisneria and Hydrilla in fresh water and Zostera in the sea; the term refers to water, not temperature.

• Treating 0-15 °C as a survival range instead of a growth optimum; a mesophile may survive chilling, but the stem asks where growth is best.
• Confusing psychrophiles with psychrotrophs: both grow in a refrigerator, but psychrotrophs grow best at 20-30 °C, so they do not fit 'grow best' in 0-15 °C.
• Reading hydro- as cold because cold water comes to mind; the prefix means water and says nothing about temperature.
• Swapping the two ends of the scale under time pressure: thermo- is heat, psychro- is cold.

• practice — not a real PYQ

  Thermus aquaticus, the bacterium from which the Taq polymerase used in PCR was isolated, grows best at about 70 °C. It is best classified as a:

  • (a)Psychrophile
  • (b)Mesophile
  • (c)Thermophile
  • (d)Hyperthermophile
  Answerc — An optimum near 70 °C falls inside the thermophile band of roughly 45-80 °C. (a) Psychrophiles grow best at 15 °C or below. (b) Mesophiles grow best at roughly 20-45 °C. (d) Hyperthermophiles have an optimum of 80 °C or above, which 70 °C does not reach.
• practice — not a real PYQ

  A bacterium multiplies slowly in milk stored at 4 °C but multiplies fastest at about 25 °C. It is best described as a:

  • (a)Psychrophile
  • (b)Psychrotroph
  • (c)Thermophile
  • (d)Hyperthermophile
  Answerb — Growth at refrigerator temperature combined with an optimum of 20-30 °C is the definition of a psychrotroph (psychrotolerant organism).

  (a) A psychrophile would grow best at 15 °C or below, not at 25 °C. (c) and (d) Thermophiles and hyperthermophiles grow best above 45 °C and 80 °C respectively, with minimum growth temperatures far above 4 °C.

• practice — not a real PYQ

  Most bacteria that cause disease in humans grow best at about 37 °C. Such organisms are called:

  • (a)Psychrophiles
  • (b)Mesophiles
  • (c)Thermophiles
  • (d)Halophiles
  Answerb — 37 °C sits inside the mesophile band of roughly 20-45 °C, which is why human body temperature suits most pathogens. (a) Psychrophiles grow best at 15 °C or below. (c) Thermophiles grow best at roughly 45-80 °C. (d) Halophiles are named for salt tolerance, not temperature.
• practice — not a real PYQ

  Consider the following statements: 1. The prefix psychro- in psychrophile refers to cold. 2. Hyperthermophiles grow best above 80 °C. 3. Hydrophiles are organisms that grow best between 0 °C and 15 °C. Which of the statements given above is/are correct?

  • (a)1 only
  • (b)1 and 2 only
  • (c)2 and 3 only
  • (d)1, 2 and 3
  Answerb — Statement 1 is correct: psychro- is Greek for cold. Statement 2 is correct: hyperthermophiles have an optimum of 80 °C or above. Statement 3 is wrong: hydro- means water and hydrophile is not a temperature class; the 0-15 °C organisms are psychrophiles.

  So (a) misses statement 2, while (c) and (d) accept statement 3.