Sterility Maths Flashcards
What is the kinetics for the rate of death of microorganisms in sterilisation?
First order
How would you work out experimentally the rate at which microorganisms are dying due to sterility?
Suspension of microorganisms that are defined temp for a defined time
Plate out a defined volume of microorganisms suspension on suitable agar plates and incubate
Count number of colonies and calculate the number of viable microorganisms present each time point
Plot some graphs
Give and annotate the equation to calculate the log of microorganisms at a specific time point:
log10 (Nt) = log10 (No) - k x t
Nt= microorganisms present at time t
No= microorganisms present at time 0
k= constant value at each temp
t= time
Give the equation to calculate half life of a killing setting:
t1/2= 0.301/k
What is the correlation between half life and the initial number of organisms?
t1/2 is independent of the number of microorganisms initially
What is decimal reduction time (D)?
Defined as the time to reduce the bioburden to 1/10 of its initial value (min)
Give the equation to calculate the decimal reduction time of a microorganism:
D=1/k
What is the correlation between the decimal reduction time and the initial number of organisms?
D is independent of the number of organisms initially
What are D values specific for and how can they be used?
Relate to a particular microorganism and temperature
D values can be used to predict how long it takes to reduce the number of microorganisms to a specific level or by a specific amount
What is the correlation between log D and temperature?
As temperature increases, the logD decreases linear
What is a Z value in sterility?
The change in temp required to cause a 10-fold change in the D value (ºC)
Give and annotate the equation to calculate the Z value:
Z= T1-T2/ log10(D of T2)- log10 (D of T1)
T= temperature
D= D value
Give the equation to calculate the the D value of a second temperature when a Z value is present:
D of T2= D of T1 x 10^(T1-T2/Z)
What is the traditional time and temp for heat sterilisation?
121ºC for 15 mins
What is the F value and give an example?
The time required at a constant 121°C to have the same killing affect as the actual temp/time used
e.g if a F value is 10, then the process at whatever time/temp is equivalent to 10 minutes at 121ºC for killing bacillus
