Exam 2: Topic 13

Question 1

General chemical agents usually damage microbes by one of two mechanisms. Describe these two mechanisms of action.

Answer 1

• Denatures proteins by breaking disulfide bridges.
• Positive charge of heavy metals bonds with negative charge of ionic amino acids therefore denaturing proteins.
• Disrupts membrane integrity by losing hydrophobic affect.

Question 2

Describe the mechanism of action for Iodine

Answer 2

Antiseptic; target proteins; iodine denatures proteins by breaking disulfide bridges

Question 3

Describe the mechanism of action for H2O2

Answer 3

• H2O2 covalent bond breaks into free radicals and is attracted to covalent bonds in a disulfide bridge and bonds with an atom in each sulfur atom, breaking the bond.
• 3% antiseptic
• 40% sterilizer

Question 4

H2O2 is incredibly effective against microbes (even spores if the concentration is
high enough). Explain why, in low concentrations, H2O2 is relatively safe as a
topical agent for human skin.

Answer 4

• The lower the concentration, the less aggressive the solution is. Human skin can tolerate low levels of oxidative agents.
• At low concentrations (typically around 3%), it releases oxygen when it decomposes.

Question 5

Describe the mechanism of action for heavy metals. Describe a medical
application of AgNO3 (silver nitrate) in a neonatal unit. Why aren’t metals used
more often as antimicrobial chemicals?

Answer 5

• Heavy metals are usually ions with a positive charge.
• Silver (Ag+): Silver bonds with the negative charges of an ionic amino acids:
○ Ionic bonds are disrupted
○ silver nitrate = antiseptic
• A medical application would be to put silver nitrate drops into the eyes of newborns to prevent the infection from gonorrhea and chlamydia, which would lead to blindness if left untreated.

Question 6

Describe the mechanism of action for detergents (e.g. soap). Are detergents
sanitizers, disinfectants, or sterilizers?

Answer 6

• Considered sanitizer
Scrubbing action allows soap particles to enter membrane, which creates weak spots by pulling off phospholipids, and breaks apart membrane

Question 7

Describe the mechanism of action for alcohol. What concentration of alcohol is
needed to be effective?

Answer 7

• Antiseptic
• Ethanol (effective at 60-65%): Disrupts membrane integrity by losing hydrophobic effect.
• Isopropyl alcohol (35%)

Question 8

Two alcohols commonly used as antiseptics are ethanol and isopropanol. Why is
isopropyl alcohol effective at a lower concentration (compared to ethanol)?

Answer 8

Alcohols work by disrupting membrane integrity by losing the hydrophobic effect within the membrane. IPA is more effective at lower concentrations, because the molecule has more nonpolar bonds and is more effective at disrupting membrane integrity.

Question 9

QACs are the most widely used disinfectants in the USA. Define a QAC and be
able to recognize the basic structure of a QAC.

Answer 9

• Quaternary Ammonium Compounds
• Disinfectants; creates leaky membranes.
• N+ in middle surrounds by alternating R groups.

Question 10

Describe the mechanism of action for QACs.

Answer 10

• Membrane disruption and thinning.
• Leaking of cytosolic contents.
• Precipitation of cellular materials.
Disruption of essential gradients

Question 11

Briefly describe why the following physical techniques have antimicrobial
properties: heat, freezing, desiccation, too much salt or sugar, UV rays, ionizing
rays

Answer 11

• Heat: denatures proteins
• Freezing: Slow down metabolism and lyse cells
• Desiccation: removes water and stops metabolism
• Excess salt/sugar: creates hypertonic environment and stops metabolism
• UV Rays: potent mutagen, mutate DNA to create nonfunctional proteins
• Ionizing Rays: potent mutagen, mutate DNA to create nonfunctional proteins