Chapter 7: The Control of Microbial Growth
Below you will find answers to the "Review" study questions found at the end of this chapter in Microbiology: An Introduction, 7th edition.
  1. Lysis.
  2. Altered permeability and leakage of cell contents.
  3. Destruction of enzymes and structural proteins such as those in the plasma membrane.
  4. Interference with protein synthesis and cell division.
  5. Autoclave. Due to the high specific heat of water, moist heat is readily transferred to cells.
  6. Most organisms that cause disease or rapid spoilage of food are destroyed by pasteurization.
  7. Variables that affect determination of the thermal death point are:
  8. The innate heat resistance of the strain of bacteria.
  9. The past history of the culture, whether it was freeze-dried, wetted, etc.
  10. The clumping of the cells during the test.
  11. The amount of water present.
  12. The organic matter present.
  13. Media and incubation temperature used to determine viability of the culture after heating.
  14. Ionizing radiation can break DNA directly. However, due to the high water content of cells, the formation of free radicals (H• and OH•) that break DNA strands is likely to occur.
  15. Ultraviolet radiation damages DNA by the formation of thymine dimers.
  16. Microorganisms tend to die at a constant rate over a period of time. The constant rate is indicated by the straight line after exposure to the bactericidal compound.
Sterilization Method / Temp. / Time / Type / Preferred Use / Mechanism of Action
Autoclaving / 121°C / 15 min / Moist / Media, equipment / Protein denaturation
Hot air / 170°C / 2 hr / Dry / Glassware / Oxidation
Pasteurization / 72°C / 15 sec / Moist / Milk, alcoholic drinks / Protein denaturation
  1. All three processes kill microorganisms; however, as moisture and/or temperatures are increased, less time is required to achieve the same result.
  2. Salts and sugars create a hypertonic environment. Salts and sugars (as preservatives) do not directly affect cell structures or metabolism; instead, they alter the osmotic pressure. Jams and jellies are preserved with sugar; meats are usually preserved with salt. Molds are more capable of growth in high osmotic pressure than bacteria.
  3. Acts rapidly.
  4. Attacks all, or a wide range of, microbes.
  5. Is able to penetrate.
  6. Readily mixes with water.
  7. Is not hampered by organic matter.
  8. Stable.
  9. Does not stain or corrode.
  10. Nontoxic.
  11. Pleasant odor.
  12. Economical.
  13. Safe to transport.
Method of Action / Standard Use
a. Disrupts plasma membrane / Skin surfaces
b. Inhibits protein function / Antiseptic
c. Oxidation / Disinfect water
d. Denatures proteins, destroys lipid / Skin surfaces
e. Oligodynamic / AgNO3 to prevent gonococcal eye infections
f. Inactivation of proteins / Chemical sterilizer
g. Denatures proteins / Chemical sterilizer
h. Oxidation / Antiseptic
  1. Disinfectant B is preferable because it can be diluted more and still be effective.
  2. Quaternary ammonium compounds are most effective against gram-positive bacteria. Gram-negative bacteria that were stuck in cracks or around the drain of the tub would not have been washed away when the tub was cleaned. These gram-negative bacteria could survive the washing procedure. Some pseudomonads can grow on quats that have accumulated.

Below you will find answers to the "Multiple-Choice" study questions found at the end of this chapter in Microbiology: An Introduction, 7th edition.
  1. d
  2. b
  3. d
  4. d
  5. b
  6. b
  7. b
  8. a
  9. a
  10. b
Note: The answers to the Critical Thinking and Clinical Applications questions are available to instructors only, and are found in the Instructor's Manual.