I removed a tank screen filter for the first time a few weeks ago when stripping a 1983 300D. It came out easy and the screen was perfectly clean as was the insides of the fuel tank. Might be typical for inland California, but I don't know the history and that car had been well taken care of, as evidenced by other things I found.

Well its always nice to see a well maintained car, but the bacteria will come from the place the fuel is obtained and one fill up from an infested tank can much things up pretty comprehensively.

Here is a photo of a w126 fuel strainer clogged with this growth. The tank I found that in had been sitting in Portland for years at about 1/8 full.

FUNGUS In Diesel Fuel
 

Agreed, you need to hit this hard and often until the clear plastic intake fuel screen remains clean of debris .

I began with Startron, it's good stuff but isn't really strong enough to clean out the fuel sender once it's gunked up so I too now recommend Bibor JF, I find the best price on Amazon and buy it by the gallon .

It's smart to always carry a spare clean clear plastic intake screen and change it the *instant* you see it filling with black crud (dead fungus) .

I discovered that they're pretty easy to clean and stopped buying new ones a while back ~ every time I fill up I look at it and if there's crud I change it, check the oil, fill up and when I wash my hands I take a moment or three to wash the dirty filter too then set it up right in a clean place and it's dry in 24 hours .