How is 16s rRNA used to identify unknown bacteria?
With PCR the 16S rRNA is amplified and sequenced, but how can you identify the bacteria with this 'piece of knowledge' if the 16S rRNA is conserved and so the same in all bacteria?
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It is not conserved, its sequence is not the same in all bacteria. Indeed, it has a slow rate of evolution (mutation) that make it perfect to build phylogenetic trees.
As correctly suggested by David in his comment, here are some references about the 16s rRNA and on how it is used to identify known and unknown bacteria.
There are two different kinds of conserved here that people are perhaps using a bit carelessly.
The 16S rRNA is conserved, meaning that it's present in all bacteria (and archea, mitochondria and chloroplasts). However, it's not conserved in the way that the sequence is 100% identical in all bacteria, it just evolves quite slowly (it has a low rate of mutations).