Given that it's an oral colonist it seems likely that the author has tried the free sample as well, even if they were to live in a no-kissing houshold.
The title is a lovely garden path sentence: "A genetically modified bacterium that outcompetes bacteria causing tooth decay" -- a straightforward parse leads you to the plausible assumption that the genetically modified bacterium causes tooth decay. The insertion of a "the" before "bacteria" would remove the ambiguity.