There are bacteria everywhere, indeed. Inside canned food, there are dead corpses of bacteria only, thankfully.
Hypothetically, if you sterilized your mouth somehow to ideal cleanliness, it’d get contaminated next time you inhale unsterilized air.
Let’s give a moment of appreciation to our immune systems. Otherwise, we’d be like https://en.m.wikipedia.org/wiki/Severe_combined_immunodeficiency
Corpses of dead bacteria and dead tuna :)