Microbiology Multiple Choice Questions on “Physical Agents”.
1. Among the following which one is the most effective method of killing microorganisms?
A. High temperature
B. Low temperature
C. High temperature, high moisture
D. Low temperature, high moisture
Clarification: High temperatures combined with high moisture are one of the most effective methods of killing microorganisms. It is important to distinguish between dry heat and moist heat in any procedure for microbial control.
2. How much time is required by spores of Clostridium botulinum to be killed by moist heat at 1200C?
A. 2 hours
B. 4 to 20 minutes
C. 24 hours
D. 6-7 hours
Clarification: Spores of Clostridium botulinum are killed in 4 to 20 minutes by moist heat at 1200C, whereas a 2 hours exposure to dry heat at the same temperature is required.
3. Vegetative cells are much more sensitive to heat than are spores.
Clarification: Vegetative cells are much more sensitive to heat than the spores because of the higher level of water activity in the vegetative cells.
4. Which of the following conditions are required for the determination of thermal death time?
A. temperature is varied
B. time is fixed
C. temperature is fixed and time is varied
D. temperature is fixed and time is not selected
Clarification: In thermal death time, the temperature is selected as the fixed point and the time is varied. It refers to the shortest period of time to kill a suspension of bacteria or spores at a prescribed temperature and under specific conditions.
5. Which of the following apparatus is used to provide steam under regulated pressure?
B. laminar air flow
D. hot oven
Clarification: The laboratory apparatus designed to use steam under regulated pressure is called an autoclave. It is essentially a double-jacketed steam chamber that can maintain steam under pressure and temperature.
6. The general pressure at which the autoclave is operated is ___________
A. 5 Psi
B. 20 Psi
C. 15 Psi
D. 40 Psi
Clarification: Generally the autoclave is operated at a pressure of approximately 15 Psi at 121.50C. It is not the pressure that kills the organisms but the temperature of the steam.
7. Some amount of air is also essential inside the chamber of the autoclave.
Clarification: In the operation of an autoclave it is absolutely essential that the air in the chamber be completely replaced by saturated steam. If air is present, it will reduce the temperature obtained within the chamber substantially below.
8. Which of the following methods is used for killing microorganisms of only certain types and not all microorganisms?
C. Boiling water
D. Fractional Sterilization
Clarification: In pasteurization, milk, cream, and certain alcoholic beverages are subjected to a controlled heat treatment that kills microorganisms of certain types but does not destroy all organisms.
9. Which of the following instruments is used to perform tyndallization?
B. Steam Arnold
C. Gas oven
Clarification: For tyndallization or fractional sterilization the apparatus used is the Steam Arnold however an autoclave can also be used if a free-flowing steam is used.
10. Which of the following actions occur due to low temperature?
A. coagulation of proteins
B. death of microorganisms
C. rate of metabolism is reduced
D. denatures proteins
Clarification: Temperatures below the optimum for growth depresses the rate of metabolism and if the temperature is sufficiently low, growth and metabolism ceases. Low temperature are also useful for the preservation of cultures.
11. Which of the following are resistant to desiccation?
Clarification: Streptococci are much more resistant to dessication, some survive weeks after being dried but species of Gram-negative cocci are very sensitive to dessication.
12. Passage of water from a low solute concentration into the cell is referred to as ______________
Clarification: The passage of water from a low solute concentration into the cell is termed plasmoptysis. The pressure built up within the cell as a result of this water intake is termed osmotic pressure.
13. Which of the following radiations have the energy to knock electrons away from molecules and ionize them?
A. Non-ionizing radiations
B. Acoustic radiations
C. Subatomic particles
D. Ionizing radiations
Clarification: Gamma rays and x-rays are called ionizing radiations because they have enough energy to knock electrons away from molecules and ionize them. When such radiations pass through cells, they create free hydrogen radicals, hydroxyl radicals and some peroxides which causes intracellular damage.
14. Wavelengths around ___________ have the highest bactericidal efficiency.
A. 150 Å
B. 3900 Å
C. 2650 Å
Clarification: The ultraviolet portion of the spectrum includes all radiations from 150 to 3900 Å. Wavelengths around 2650 Å have the highest bactericidal efficiency.
15. Which of the following inhibits DNA replication?
A. cathode rays
B. UV light
D. gamma rays
Clarification: Ultraviolet light is absorbed by many cellular materials but most significantly by the nucleic acids where it does the most damage. Pyrimidine dimers are formed due to which DNA replication is inhibited and mutations can result.
16. Which material is used for the Berkefeld filter?
A. diatomaceous earth
B. asbestos pad
D. sintered glass disks
Clarification: Filters can remove microorganisms from liquids or gases. The Berkefeld filter is made up of diatomaceous earth.