Sterilisation 2

Question 1

how do you produce a kill curve?

Answer 1

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

Question 2

how many colony forming units do we dilute to when creating a kill curve?

Answer 2

30-300

Question 3

why do we want 30-300 colony forming units?

Answer 3

<30 isn’t significant

>300 would be too hard to see as the bacteria would clump

Question 4

regarding a kill curve, what does an asymptote curve mean

what does it look like

Answer 4

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

Question 5

how can we give the kill curve straight lines

Answer 5

Log(survivors) against time

Question 6

what would be the gradient of the Log(survivors) against time?

Answer 6

how quickly it dies at that particular temperature

Question 7

the steeper the line in a log(survivors) against time graph means what?

Answer 7

more rapid death rate

the less steep means the bacteria are resisting

Question 8

will organisms respond differently to different sterilisation methods

Answer 8

yes depending on their properties

Question 9

what is the D value?

Answer 9

the time taken at a fixed temperature to reduce the population by 90%

Question 10

when calculating the D value from log(survivors) against time, what values would. you use?

Answer 10

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

Question 11

what is a 90% reduction in terms of log cycles

Answer 11

one log cycle

Question 12

what is the Z value?

Answer 12

measure of thermal resistance

the temperature change required to produce a 90% reduction in D value

Question 13

is the Z value at a fixed temperature?

Answer 13

no just temperature in general

Question 14

when calculating the Z value what graph must be used

Answer 14

Log D value against temperature

Question 15

after you’ve calculated the Z value, you can conclude:

whenever you decrease the temperature by (Z value) degrees, you have a ….

Answer 15

1 reduction in log value

Question 16

how could we deem a product sterile

Answer 16

To deem it sterile it must receive a minimum sterility assurance level (SAL)

Question 17

why can there be no 0 as 0 would mean sterile

Answer 17

we are working on a log scale

Question 18

what is the SAL value, what does this mean

Answer 18

10^-6

so we are looking to have a millionth of a bacterial cell present

Question 19

as the SAL is less than the 30 colony forming units, how can we estimate the time needed to achieve the minimum SAL?

Answer 19

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

Question 20

what are the D values influenced by?

Answer 20

•	Bacterial species  
•	Vegetative vs spore form  
•	Production method  
•	Nutrient environment  
o	Suspension media  
o	Carrier materials  
o	Culture media  
•	Treatment dose