Microorganisms, too embarrassed to ask others about！（Part 15）

Part 3 Sterilization

3. Microoraganisms will never be eliminated even after a number of extermination procedures

"In the previous article, I clearly stated, ""As long as you state that you performed sterilization, you will need to eliminate all those microorganisms"". Now I need to take those words back. 

Death of a microorganism means, ""The ability of growing has been lost irreversibly"". In many cases, regularity is seen in microorganisms being killed that ""The rate of being killed under certain conditions is proportionate to the viable cell count at that time"". In other words, the larger the viable cell count, the larger the number of microorganisms to be killed in a certain length of time　will be. For instance, when microorganisms are in an environment such as heat treatment which will kill microorganisms, that doesn't mean that all the microorganisms will be killed at once and at some point, or when the temperature reaches a certain degree. It may be hard to imagine but the microorganism has their nature that a certain percentage of them will be killed in a certain length of time."

That can be explained by using a model as follows:

Another explanation with specific numbers when the initial count is 1,000 will be as follows:

This is an easy part. However, you may feel the rest is troublesome:

It may be hard to imagine that you have 0.1 of one animal. What does it mean when you have 0.1 microorganisms?

This should be interpreted that the survival rate of one microorganism is 10 % = 1/10. In fact, we generally would have around 10 product units with surviving organisms out of 100 after processing them under conditions with 10% of the survival rate.

That explanation can be shown in the following figure. The figure would help you understand that microorganisms will be killed at a certain rate (90% in this case) instead of a certain count in every 5 minutes. The conditions in which 90% of microorganisms will be killed are called D-value or D10 value.

This means that the longer the process time gets, the closer and closer the viable cell count will be to 0, and at the same time it will never be 0. The definition of sterilization, "Exterminating or removing all the microorganisms" will never be possible.

Note) The rate of killing differs largely among bacterium species, environmental conditions, and so on.