Examples
NB #1: Unfortunately it is common diction to say "the kill rate is log6" or "the germ reduction is log6", which strictly speaking is not only wrong but nonsensical. By saying this one means that the germ reduction is 6 orders of magnitude or the survival probability of each single germ is 10−6. (This wrong diction originates from a misunderstanding of the mathematical expression log 106 = 6)
NB #2: Strictly speaking it is also wrong to talk about single germs or the like. It's correct to use the item cfu or colony forming unit. The main problem is not inevitably the presence of germs (bacteria, spores, ...) but their ability of fast fissiparous, which gives an exponential increase of the number of the germs with time. If one tries to count "a number of germs" one has to, simply spoken, cultivate them on an agar plate, let them grow for a few days and count the macroscopic colonies which have formed. Each of these colonies is resulting from 1 cfu (= 1 "augmentable germ").
Example #1: One item which has to be sterilized carries a contamination of 107 germs prior to sterilisation. The germ reduction capability of the sterilisation process is 6 orders of magnitude (=106 or "log 6"), which means the survival probability of the germs is 10−6. If such items are sterilised the average number of "surviving germs" or, correctly spoken, cfu's which are found on the items after sterilisation is: 107 / 106 = 10 or 107 * 10−6 = 10.
Example #2 (statistically equivalent to #1): A lot of items which have to be sterilized are carrying a contamination of 10 germs each prior to sterilisation. The germ reduction capability of the sterilisation process is 6 orders of magnitude (=106 or "log 6"), which means the survival probability of the germs is 10−6. If one sterilises a statistically significant number of these items the average number of cfu's which is found on the items after sterilisation is: 10 / 106 = 10−5 or 10 * 10−6 = 10−5. This means that, in average 1 cfu is found per 105 = 100000 items.
Read more about this topic: Dry Sterilisation Process
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