What bacteria am i ingesting when eating or drinking something that a nasty fly or a gnat has landed on? at what point should i be concerned?

Little or no risk: The chance of catching any infection from a single event like this is extremely low, zero for all practical purposes. You each such food all the time. I would guess that for every time you saw an insect on food you ate, there were a hundred other times when it happened but you didn't see it. This is part of life and not a cause for worry.