Paper towels are able to absorb water because of cellulose’s natural gaps and spaces:

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Most bacteria are about 1 micron, and these gaps range from around 1-10 microns.

Especially if damp, it can be argued that they don’t stop the transfer of bacteria. It’s possible that your bacteria transfers through it and vice versa. This is all before the fact that paper towels can already harbor bacteria on their own.

That being said, paper towels do block some. You just shouldn’t think of them as sterile or a magic blocker for bacteria.