Bacterial growth is typically black sledge found in the lowest points of the fuel tank or trapped areas of the tank collecting water. The fungus is typically found when the aircraft is not being used often. Bacterial growth sometimes can be found above the fuel but generally lives in the area where moisture deposits in the tank. Bacterial growth does not actually live in the fuel, it lives in the water trapped in the fuel tank. This is one good reason to sump your fuel tank to remove the water so the bacterial growth has no place to grow. You may be asking yourself, how is bacterial growth a problem? First of all, the biomass will clog your filters and affect your sensitive engine equipment. Secondly, the byproduct of the bacterial growth is very corrosive and damaging the aluminum skins and sealant of your fuel tanks. Typically if bacterial growth is found early enough, major damage can be avoided. Bacterial growth and fungus can leave behind spores and the entire fuel system must be treated at one time meaning the fuel must be discarded or properly treated prior to re-using.