Sort of. Part of the reason that antibiotics work so well is that the amount to kill a bacterium is way smaller than the amount to kill a person.

The problem with mercury (and heavy metals in general) is that there's no way to flush them from the body.* We don't usually worry about salt buildup, because our bodies naturally dilute the salt and flush it out the kidneys. Things like that don't happen with the heavy metals.

* Yes, chelation therapy exists as a treatment for heavy metal poisoning. There are pretty strict guidelines, mostly because one of the side effects of chelation therapy can be death. "The therapy was a success but the patient died."