Mogas and microbial growth

Folks,

I have a C-182 with an auto-fuel STC, which I've been operating on Mogas
(along with an occasional fill of 100LL) for several years. Mogas being
unavailable at my home airport, I use a pair of 6 gallon Gerry cans
(plastic) to transport the fuel from a local gas station to the
airplane.

I've taken the usual precautions to keep dirt and moisture out of the
Gerry cans but lately encountered something a bit unexpected: as I was
preparing to fill the cans, I happened to glance inside one and noticed
several patches of what appeared to be mold or fungus. These were dark
brown in color, from 1/8 to 1/2 inches in diameter; there were 4 to 7 or
so of these on the bottom inside surface of each can.

Has anyone else encountered this, and have any ideas on how such
contamination can be prevented? Beyond the obvious concern of fuel line
blockage from this stuff (I've since started filtering the fuel when I
transfer it the cans to the airplane), I'm worried the plane's entire
fuel system may be contaminated. (I peered inside my Bladders with a
flashlight but can't see enough to tell anything.) Is there any sort of
anti-microbial additive that can be used with Mogas, that can
"disinfect" my Gerry cans and/or fuel system? Does this problem exist
with metal containers, or just peculiar to the plastic cans?

I've heard that 100LL doesn't have this problem, due to the toxicity of
Tetraethyl Lead. Perhaps occasional use in the Gerry cans would
disinfect 'em?

Any advice much appreciated! Thanks in advance...

EGM