inactivation kinetics for beginners

Question 1

what is the kill curve?

Answer 1

• Part of a development phase which occurs in a lab
• Take samples at regular intervals and dilute culture so you get colony forming units that you can count on a agar plate.
• Treat sample to sterilise it before you dilute and identify colonies.
• Serial dilutions so when you plate out on to agar you want 30-300 colony forming units
• Anything under 30 is not statistically liable and anything above 300 is too hard to count.

Question 2

what is an asymptote curve?

Answer 2

regular intervals you get the same proportion of cells killed as per the unit of time. The blue line will never reach zero as It will lose the same number of cells each function of time.

Question 3

how can you make the kill curve a straight line?

Answer 3

If you want to make this a straight line you take a log of the survivors, this is a semi-athymic graph. You can then use the gradient to work out the rate of survival at a given temperature. We can then repeat this at different temperatures so you can build up a portfolio of survival at different temperatures (you can change what it is you are measuring).

Question 4

what is the d value?

Answer 4

The time taken, at a fixed temperature, to reduce the population by 90% (1-log). This gives a measure you can compare at different temperatures. It doesn’t matter which number you have on the y axis as long as it is a full log cycle- it’s a straight-line relationship.

Question 5

what is the thermal resistance curve?

Answer 5

The temperature change required to produce a 90% reduction (1-log cycle) in D-value. Can only be used for temperature changes not any other sterlisation agent

Question 6

what is the z-value?

Answer 6

• A measure of thermal resistance
• Indicator of efficiency
• Reference (indicator) organisms
• Bacillus stearothermophilus under steam sterilisation has a Z value of 10 degrees
• Bacillus subtilus under dry sterilisation has a z value of 20 degrees.

Question 7

when is a product deemed sterile?

Answer 7

• Inactivation on log scale, therefore no 0 on the log scale. Therefore, need a way to determine there is no organisms due to 0 not existing
• Measure via sterility assurance level (SAL)
• SAL= 10-6
• Minimum is the SAL value, even better if it is less then this value.

Question 8

what are d-values influenced by/

Answer 8

o Bacterial species
o Vegetative vs spore form
o Production method
o Nutrient environment (suspension media, carrier materials, culture media)
o Treatment dose (temperature, radiation, dose)