Kinetics of Antimicrobial action

Question 1

What are the sterilisation processes

Answer 1

1. elevated temperature- moist heat (autoclaving), dry heat
2. reactive gas (ethylene oxide)
3. irradiation (ionising radiation such as gamma rays)
4. filtration through a microorganism proof filter

Question 2

what are the kinetics of antimicrobial action

Answer 2

1. microorganisms and microbial structures show different sensitivity to sterilisation agents (eg. heat or radiation)
2. vegetative forms of bacteria, fungi and larger viruses show a greater sensitivity to sterilisation
3. reference organisms for testing sterilisation efficacy are usually bacterial spores

Question 3

what happens when a microbial population is exposed to a killing process

Answer 3

microbial populations generally lose viability in an exponential fashion, independently of the initial number of organisms
- plot of the logarithm of the fraction of survivors against the exposure time or dose of the killing process (eg. high temperature or radiation dose)

Question 4

what are survivor curves

Answer 4

have a linear portion which may be continuous (A), or modified by an initial shoulder (B), or by a reduced rate of kill at low survivor levels(C)

Question 5

what is the equation representing a linear survivor plot

Answer 5

Log10(N)= Log10(No) - (k x t/2.303)
where:
N- concentration of surviving cells after t mins
No- initial bioburden
K- inactivation rate constant

Question 6

what does the inactivation rate constant indicate

Answer 6

how quickly cells are being killed

Question 7

what is the D value

Answer 7

the resistance of an organism to a sterilising agent
- indicates the rate of kill, but doesn’t quantify the amount of microbial killing

Question 8

define the D value for heat treatments

Answer 8

the time taken at a fixed temperature to achieve a 90% reduction in viable cells

Question 9

define the D value for radiation treatments

Answer 9

the radiation dose required to achieve a 90% reduction in viable cells

Question 10

what does the calculation of D value assume

Answer 10

assumes a linear survivor curve

Question 11

what is the inactivation factor

Answer 11

a parameter that quantifies the amount of microbial killing

Question 12

What is the equation for calculating IF

Answer 12

IF= No/N
IF- inactivation factor
N- concentration of surviving cells after t mins
No- initial bioburden

Question 13

what is the equation that links the relationship between IF and D value

Answer 13

IF= No/N = 10^t/D
t- exposure time
D- d value

Question 14

how does bacterial killing rates change as the sterilisation temperature increases

Answer 14

eg. for Geobacillus spores
D121= 1.8 mins
D115= 9 mins
- higher temperature takes less time and increases bacterial killing rates

Question 15

what is the Z value

Answer 15

represents the increase in temperature needed to decrease the D value of an organism by 90%
- allows us to assess the influence of the temperature changes on thermal resistance

Question 16

what Z value is commonly used in calculations to design steam sterilisation processes

Answer 16

z values found by experiment are normally within range 10-14 degrees for different microbial species
- 10 degrees is often adapted in calculations

Question 17

what can be calculated if the Z value is known

Answer 17

the D value at one temperature can be calculated from the value of another

Question 18

What is the equation for calculating the D value using the Z value

Answer 18

Log D1- Log D2= T2-T1/Z
D1 is the D value at temperature T1
D2 is the D value at temperature T2

Question 19

What value does the F value use as a reference temperature

Answer 19

121 degrees

Question 20

what is the F value

Answer 20

expresses a heat treatment at any temperature in terms of an equivalent number of minutes exposure at 121 degrees Celsius
- eg. a steam sterilisation cycle with an F value of 8, would have a killing effect equivalent to that of 8 mins at 121 degrees

Question 21

what is needed to calculate the F value

Answer 21

need the Z value
- since Z values are often close to 10 degrees, a value of z=10 is assumed in F value calculations

Question 22

what is the F value designated as when a Z value of 10 is used

Answer 22

F0

Question 23

Write the equation for calculating the F0 value

Answer 23

F0=^- t x E10^(T-121/Z)

Question 24

explain how the F0 value is calculated

Answer 24

1. we split the entire autoclave cycle into many short exposure periods (^- t)
2. the lethal effects of all ^- t periods are added together to give the overall lethal effect of the cycle (F0)
3. the end part of the equation gives the killing effect at temperature T, relative to that at 121 degrees
4. killing effect is referred to as lethal rate