Biocides and Disenfection

Question 1

Define: Antimicrobial agent

Answer 1

Chemicals agents used to kill or inhibit growth of microbes

Question 2

Define: Sterile

Answer 2

No microbes present. An absolute term

Question 3

Define: Disinfectant / antiseptic

Answer 3

A substance that kills only vegetative cells. Disinfectants are applied to non-living objects. Antiseptics are applied to living tissues

Question 4

What do sanitisers do?

Answer 4

Kill only 99.9% of vegetative cells, thereby reducing the microbial load

Question 5

What are the conditions for autoclaving

Answer 5

15 lbs/in2 at 121°C for 15-20 min

Question 6

Why is wet heat used in autoclaving

Answer 6

Wet heat can penetrate deeper into the cell causing the denaturation of macromolecules and cell death at lower temperatures

Question 7

What methods can be used to assess biocide activity

Answer 7

Disk diffusion. (efficacy is proportional to zone size)

Quantitative suspension tests. (D Value determines efficacy)

Question 8

What is the correct used of biocides

Answer 8

• Appropriate concentration
• Sufficient time

*Think about how many microbes would be on that surface to begin with. If 106 on surface, 99.9999% biocide would kill all but 1 cell.

Question 9

What are the modes of action of Biocides

Answer 9

Upon close contact with the cell wall and membrane, the physical structures are damages and can no longer carry out processes. At sub-lethal concentrations damage may be tolerated/ repaired

Question 10

How do phenols act as biocides?

Answer 10

Damage to proteins affects membrane permeability & general cellular function
Inappropriate in food production as humans are sensitive to taste

Question 11

How do alcohols act as biocides?

Answer 11

Denatures proteins and dissolves lipids. Effective against viruses.

Question 12

How do halogens act as biocides?

Answer 12

Iodine: combines with tyrosine residues often found in enzyme active sites.
Chlorine: releases nascent oxygen in water (oxidising affect) Cl2 inactivates proteins

Question 13

How do oxidising agents act as biocides?

Answer 13

• H2O2 forms superoxide which rapidly degenrates O2 and H2O.
• Disrupts disulphde bonds in membrane and cell wall

Question 14

How do dyes act as biocides?

Answer 14

• Acridine Orance inhibits DNA replication and causes mutations
• Crystal violet inhibits cell wall synthesis of gram positive bacteria

Question 15

Explain why choice of disinfectant is important.

Answer 15

No single multipurpose chemical exists - choice depends on:

1) surface to be cleaned
2) concentration of organic matter present
3) importance of toxic residues

Question 16

How do capsules prevent cell death?

Answer 16

They prevent the biocide reaching the active cite by offering an extra polysaccharides layer. Biofilms can form to cover community .

Question 17

Identify the term that can describe a disinfectant that can inhibit the growth of fungi

Answer 17

Fungistatic