For autoclave validation, the D-value of Geobacillus stearothermophilus is considered to be 2.5 minutes.
First, let us understand the mathematics behind it.
What is a D-value?
What is the D-value formula?
How is it calculated?
The D-value (Decimal Reduction Time) is the time required at a specific temperature to reduce a microbial population by 90%, or 1 log (i.e., to kill 90% of the microorganisms present). For example, if the D-value is 2.5 minutes at 121°C, it means tht 90% of microbes will be killed in 2.5 minutes at that temperature.
D = t/(log N₀ – log N)
Where:
D = D-value (Decimal Reduction Time)
t = time taken for sterilization
N₀ = initial number of microorganisms
N = number of surviving microorganisms after time t
𝐅𝐨𝐫 𝐄𝐱𝐚𝐦𝐩𝐥𝐞:
t=15 minutes
N₀=10^6
N=10^°
D=15/log10^6-10^°=15/6=2.5 minutes
The concept you are referring to relates to the D-value (Decimal Reduction Time) and
sterilization cycle design.
The D-value is the time required to reduce the microbial population by 90% (i.e., 1 log reduction) at a specific temperature. For Geobacillus stearothermophilus (a highly resistant
spore-forming bacterium often used as a biological indicator in autoclaving), the D-value at
121°C is 2.5 minutes. This means that it takes 2.5 minutes at 121°C to achieve a 1-log reduction (i.e, reduce the microbial count by factor of 10)
In sterilization, we often aim for a 6-log reduction (reducing the population by 10^6). To
achieve this, we multiply the D-value by 6:|
2.5 minutes× 6 log = 15 minutes
This calculation shows that 15 minutes at 121°C is sufficient to achieve a 6-log reduction in
Geobacillus stearothermophilus spores, providing a high level of assurance of sterilization.
Assume you start with a microbial population of 1,000,000 microorganisms (or ).
After 2.5 minutes at 121°C, 90% of the population is killed, leaving 10% (or ):
1,000,000 × 0.1 = 100,000
90% of the remaining population is killed again, reducing the population to (or 10,000
microorganisms):
100,000 × 0.1 = 10,000
15 minutes at 121°C is sufficient to reduce the microbial population from 1,000,000 to almost
zero, based on the D-value of 2.5 minutes.