Evaluating disinfectants (IC3)

Question 1

4 stages of binary fission

Answer 1

1) Lag phase
starting processes eg. rna and enzyme synthesis

2) Log / Exponential phase
Increasing at constant rate

3) Stationary phase
growth rate = death rate

4) Death phase

Question 2

Definition of MIC

Answer 2

MIC: Minimum Inhibitory Concentration
Inhibit growth (static, not killing), determined visually after incubation

Question 3

Definition of MEC

Answer 3

MEC: Minimum Effective Concentration
Lowest concentration where disinfectant is bactericidal
Risky, as a slight dilution of disinfectant will cause disinfectant to not be effective already

Question 4

Definition of MRC

Answer 4

MRC: Minimum recommended concentration
Best for determining bactericidal concentration

Question 5

Definition of MBC / MLC

Answer 5

MBC: Minimum Bactericidal Concentration
Lowest conc to cause a 3 log reduction

MLC: Minimum Lethal Concentration
Any microbe, not just bacteria

Question 6

Definition of MSC

Answer 6

MSC: Minimum Selective Concentration
Concentration where resistant strains have a competitive advantage

Avoid use if there are different strains of bacteria

Question 7

Rank the different minimum concentrations from best to worst (acronym)

Answer 7

RBESI

MRC
Recommended, takes into account external factors eg. organic matter, hardness

MBC / MLC
Conc to cause a 3 log reduction
May not be possible due to toxicity

MEC
Conc of disinfectant to be bactericidal

MSC
Will kill some bacteria but have some strains resistant to it

MIC
Inhibit only

Question 8

What is the most important consideration when conducting a culture

Answer 8

Must know the initial amount of microbes!

Question 9

How to calculate number of bacteria (CFU) per ml or gram? What must be considered?

Answer 9

Must consider dilution factor

CFU = (Colonies x dilution factor) / volume of plate

Question 10

What are some considerations when testing disinfectant efficacy? (5 points)

Answer 10

1. Using the appropriate minimum concentration (MRC, MEC, MBC)
2. Mechanism of activity of disinfectant
   Important to determine test duration, test is run sufficiently long to see a positive result / test is not stopped prematurely and result in false negative
3. Presence of post-exposure effect
   Some disinfectants that target metabolism of microbe can continue to be effective even when disinfectant is removed
4. Environment that microbe is in
   pH, temperature, time, concentration
5. Amount of microbes (before disinfectant use) must be large enough
   Too little microbes -> May think disinfectant is too effective

Question 11

What is used to dilute the disinfectant in an experiment?

Answer 11

Hard water

Question 12

Test procedure for disinfectant (rough steps of what to prepare)

Answer 12

Prepare carrier, test organism and inoculum
Inoculate, dry and transfer carriers
Expose dried inoculum to test substance or control substance
Neutralise test substance and Isolate test substance
Recovery of test substance: Incubation
Count surviving test organism

Question 13

What are 3 examples of neutralisation of disinfectant?

Answer 13

1) Chemical neutralisation
2) Dilution
3) Filtration

Question 14

What is a carrier

Answer 14

Surface or object to be inoculated with the bacteria (test organism)

Question 15

What is CFU

Answer 15

Colony forming unit

Question 16

What is inoculum

Answer 16

Bacteria in soil load (organic contaminant)

Question 17

What is neutralisation

Answer 17

a process to quench microbicidal or microbistatic activity of a test substance (disinfectant or preservative)