Sterilisation 2 Flashcards
how do you produce a kill curve?
Heat then take regular samples
Put on an agar plate and see the culture that forms
Don’t exactly know how much is in each sample, so we do dilute the sample so we have 30-300 colony forming units- before we put on the plate
Put on an agar plate and see the culture that forms
how many colony forming units do we dilute to when creating a kill curve?
30-300
why do we want 30-300 colony forming units?
<30 isn’t significant
>300 would be too hard to see as the bacteria would clump
regarding a kill curve, what does an asymptote curve mean
what does it look like
number of survivors against time
if you measure at regular time intervals, you get the same proportion of cells killed as you did in the previous portion of time.
like a curved L
how can we give the kill curve straight lines
Log(survivors) against time
what would be the gradient of the Log(survivors) against time?
how quickly it dies at that particular temperature
the steeper the line in a log(survivors) against time graph means what?
more rapid death rate
the less steep means the bacteria are resisting
will organisms respond differently to different sterilisation methods
yes depending on their properties
what is the D value?
the time taken at a fixed temperature to reduce the population by 90%
when calculating the D value from log(survivors) against time, what values would. you use?
Can choose any value on the y axis- doesn’t matter AS LONG as its one full log cycle reduction (100-10, 10-1, 1-0.1)
Then see the interceptions on the x axis (time) and calculate the difference
what is a 90% reduction in terms of log cycles
one log cycle
what is the Z value?
measure of thermal resistance
the temperature change required to produce a 90% reduction in D value
is the Z value at a fixed temperature?
no just temperature in general
when calculating the Z value what graph must be used
Log D value against temperature
after you’ve calculated the Z value, you can conclude:
whenever you decrease the temperature by (Z value) degrees, you have a ….
1 reduction in log value
how could we deem a product sterile
To deem it sterile it must receive a minimum sterility assurance level (SAL)
why can there be no 0 as 0 would mean sterile
we are working on a log scale
what is the SAL value, what does this mean
10^-6
so we are looking to have a millionth of a bacterial cell present
as the SAL is less than the 30 colony forming units, how can we estimate the time needed to achieve the minimum SAL?
Can extrapolate the Log D value as it’s a logarithmic plot, can do this to the 10 -6 level as it’s the minimum SAL
then can see at what time this would occur
what are the D values influenced by?
• Bacterial species • Vegetative vs spore form • Production method • Nutrient environment o Suspension media o Carrier materials o Culture media • Treatment dose
